Development of HVDC Technology - Semantic ScholarHVDC transmissionsystems, wheninstalled, often form...

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Development of HVDCTechnology

1.1 Introduction

The development of HVDC (High Voltage Direct Current) transmission system dates back to

the 1930s when mercury arc rectifiers were invented. In 1941, the first HVDC transmission

system contract for a commercial HVDC systemwas placed: 60MWwere to be supplied to the

city of Berlin through an underground cable of 115 km in length. In 1945, this systemwas ready

for operation. However, due to the end of World War II, the system was dismantled and never

became operational. It was only in 1954 that the first HVDC (10MW) transmission systemwas

commissioned in Gotland. Since the 1960s, HVDC transmission system is now a mature

technology and has played a vital part in both long distance transmission and in the

interconnection of systems.

HVDC transmission systems, when installed, often form the backbone of an electric power

system. They combine high reliability with a long useful life. Their core component is the

power converter, which serves as the interface to the AC transmission system. The conversion

from AC to DC, and vice versa, is achieved by controllable electronic switches (valves) in a

3-phase bridge configuration.

AnHVDC link avoids some of the disadvantages and limitations of AC transmission and has

the following advantages:

. No technical limit to the length of a submarine cable connection.

. No requirement that the linked systems run in synchronism.

. No increase to the short circuit capacity imposed on AC switchgear.

. Immunity from impedance, phase angle, frequency or voltage fluctuations.

. Preserves independent management of frequency and generator control.

. Improves both the AC system’s stability and, therefore, improves the internal power-

carrying capacity, by modulation of power in response to frequency, power swing or line

rating.

HVDC Transmission Chan-Ki Kim, Vijay K. Sood, Gil-Soo Jang, Seong-Joo Lim, and Seok-Jin Lee� 2009 John Wiley & Sons (Asia) Pte Ltd

Figure 1.1 shows example applications ofHVDC transmission systems inwhich the labeling

is as follows:

1. Power transmission of bulk energy through long distance overhead line.

2. Power transmission of bulk energy through sea cable.

3. Fast and precise control of the flow of energy over an HVDC link to create a positive

damping of electromechanical oscillations and enhance the stability of the network by

modulation of the transmission power by using a Back-to-Back.

4. Since an HVDC link has no constraints with respect to frequency or to phase angle between

the two AC systems, it can be used to link systems with different frequencies using an

Asynchronous Back-to-Back.

5. When power is to be transmitted from a remote generation location across different

countries or different areas within one country, it may be strategically and politically

necessary to offer a connection to potential partners in the areas traversed by using a multi-

terminal DC link.

6. An HVDC transmission system can also be used to link renewable energy sources, such as

wind power, when it is located far away from the consumer.

7. VSC (Voltage-Source Converter) based HVDC technology is gaining more and more

attention. This new technology has become possible as a result of important advances in the

development of Insulated Gate Bipolar Transistors (IGBT). In this system, Pulse-Width

60Hz

60Hz

IslandingArea

Plant

50Hz

Plant

Island

Plant Complex

Wind Power

Wind Power

Figure 1.1 Various applications of an HVDC system.

2 HVDC Transmission

Modulation (PWM) can be used for the VSC as opposed to the thyristor based conventional

HVDC. This technology is well suited for wind power connection to the grid.

8. Since reactive power does not get transmitted over a DC link, two AC systems can be

connected through an HVDC link without increasing the short circuit power; this technique

can be useful in generator connections.

1.2 Advantages of HVDC Systems

The classical application of HVDC systems is the transmission of bulk power over long

distances because the overall cost for the transmission system is less and the losses are lower

than AC transmission. A significant advantage of the DC interconnection is that there is no

stability limit related to the amount of power or the transmission distance.

Long Distance Bulk Power Transmission.When large amounts of power are to be delivered

over long distances, DC transmission is always an alternative to be considered. AC transmis-

sion becomes limited by:

. Acceptable variation of voltage over the transmission distance and expected loading levels.

. Need to maintain stability, that is, synchronous operation across the transmission, after a

disturbance, both transiently and dynamically.. Economic effects of additions necessary to correct the above limitations.

The DC line, requiring as few as two conductors (one only for submarine with earth return)

compared to the AC line’s use of three, requires a smaller right of way and a less obtrusive

tower. Figure 1.2 shows schematically the tower configurations for 1200MW(two circuits AC,

Figure 1.2 Tower configurations for AC and DC transmission.

Development of HVDC Technology 3

bipolar DC) and 1500–2000MW transmission at EHVAC single circuit or monopolar DC by

alternative tower designs. (Note: a single circuit or a single pole above 1600MW capacity has

not been built to date (2008) because of the effect of the potential loss of such a high capacity

circuit on the system.)

As anAC line reaches either the limit imposed by system stability or its thermal capacity and

if adding a parallel line is impossible, it may be possible to convert it to DC. Applying DC up to

three times the AC capacity should be possible for transmission by altering the tower head

configuration, but not the foundations, tower size nor the right of way. Running AC and DC

lines on the same tower are also possible. At present, no example of these being put into effect

can be reported.

Interconnection by AC or HVDC. If two or more independent systems are to be inter-

connected by a synchronous AC link, the common rules concerning security, reliability,

frequency control, voltage control, primary and secondary control of reserve capacity and so on

need to be respected. When the basis for synchronism is established, it depend on the structure

and the strength of the power systems, the number of interconnecting lines, and whether or not

stability problems, for example, inter-area oscillations, may occur. In most cases, more than

one AC link is necessary for reliability; however, there are examples of single-circuit

interconnections for energy and reserve exchange, where limited reliability of the link is

accepted.

By contrast, interconnecting the systems with DC removes any constraints concerning

stability problems or control strategies. The common rules listed above concerning security

and so on can largely be left within the jurisdiction of the separate AC systems, remaining

independent of the agreement to link. The interconnection can bemade byHVDCback-to-back

stations along the border or by interconnecting load and generation centers within the systems

by long distance transmission.

For submarine interconnection, as distance increases, AC cables generate an increasingly

wide variation of voltage with power flow until the rating of the cable is fully taken up by its

charging current. Since intermediate, reactive compensation units cannot be installed, the

maximum practical distance was 50 km until recently. In recent years, the advent of the XLPE

cable (cross-linked polyethylene) for submarine use, with a lower shunt capacitance than

earlier types, has increased this limit to about 100 km. Beyond this distance, DC is the only

technically viable solution.AnHVDCconnection requires only positive and negative (pole and

return) conductors, or in some cases a single conductor with sea return and there is no practical

technical limit to length except cost.

HVDCMulti-Terminal Systems.When power is to be transmitted from a remote generation

location across different countries or different areaswithin one country, itmay be economically

and politically necessary to offer a connection to potential partners in the areas traversed.

Multi-terminal DC is a possibility for this type of application.

HVDCmulti-terminal systems allowmore participants. They have proved to be feasible, for

example, the SACOI 3-terminal cable system between Italy, Corsica (France), and Sardinia

(Italy) and the Quebec–New England 3-terminal overland system in Canada/USA. The Pacific

Intertie and the Nelson River DC links are examples of multi-terminal DC put to practical use.

These are examples of parallelmulti-terminal systems. Seriesmulti-terminal systems have also

been proposed but no practical applications exist at present.

A further example for interconnecting more systems via long distance HVDC links is the

planning of the East–West High Power Trans, connecting Russia, the Baltic States, Belarus,

4 HVDC Transmission

Poland and Germany, where a multi-terminal HVDC system is under consideration. The

advantages of interconnection can be exploited without establishing common rules (for

example, of frequency control) and AC systems can continue to operate and develop

independently. If, in the longer term, the requirements for AC interconnection are fulfilled

and it is agreed to synchronize, the HVDC transmission becomes a strong backbonewithin the

interconnected system and brings considerable stability advantages.

A control choice is available to operate multi-terminal systems with either a coordinated

master power controller, or with each terminal having its own power controller and the voltage-

controlling terminal supplies the balance of power. New control concepts may become

available to overcome the need for a master controller and to allow expansion with more

terminals, each convertor operating with locally available information.

Care has to be taken when weak systems have to be integrated into a multi-terminal

system, so that faults within them do not cause too widespread a disturbance. Furthermore,

if a multi-terminal system is to develop and grow independently, as AC systems can do, the

integration of a new converter station needs a review and re-coordination of the control

structure and parameters of all converters. However, smaller converters (with current rating

below or equal to the current margin, that is, about 10% of the existing system) may be

integrated at a later date.

AC System Support. An AC load flow depends on the difference in angle between voltage

vectors in different parts of the network. This angle cannot be influenced directly but depends

on the power balance. Secondly, a change in power generation or in the load demandwill cause

a change in system frequency that has to be restored by altering the generation. As this task has

to be fulfilled by the generator speed controllers, the frequency restoration is a slow action.

System stability also depends on there being sufficient flexibility to allow the automatic

adjustment of the voltage vectors.

If stability problems are encountered which can be solved by fast frequency control, HVDC

systems can fulfill this task by drawing the energy from the remote network. Due to the ability

to change the operating point virtually instantaneously, HVDC can feed (or reduce) active

power into the disturbed system to control the frequencymuch faster than a normally controlled

generator. If the feeding AC system is strong enough, the DC link can, within its rating, control

the frequency in the receiving system. A prerequisite for this kind of system support is only the

appropriate mode of control.

Take the case of an AC system containing relatively long transmission lines, where

electromechanical oscillations can be excited by system faults and are weakly damped.

Assume the addition of a DC link (point-to-point or back-to-back) from outside into this

system. Control features for power modulation, with the appropriate phase angle, can actively

introduce damping torque. In general, this valuable feature of an HVDC link is inherent and

requires no significant extra costs. Where the systems at each end of the DC have different

natural frequencies of oscillation, the damping torque can be applied to either/or both systems

simultaneously if necessary.

Two controls are available. Where a terminal’s AC network is part of a large system, the DC

controls can react to swings of power and attempt to mitigate their effect by damping power to

maintain synchronism.Where it is a separate system, applying a slope characteristic similar to

that of a generator can be used to apply frequency control.

Limitation of Faults. Faults causing depression of voltage on power swings do not transmit

across a DC barrier. They may emerge on the other side of a DC link simply as a reduction in


power, but voltage will not affected. Constraining the influence of certain critical faults on AC

systems can be a valuable attribute of DC.

Limitation of Short Circuit Level.When new lines are built to extend AC systems, the short

circuit level of the system will unavoidably be increased. The switchgear apparatus must cope

with the short circuit requirements or an expensive refurbishment has to take place. Since

reactive power is not transmitted over a DC link, it provides ameans to extend the active power

exchanged without increasing the short circuit level.

Power Flow Control. An HVDC link operates at any condition of voltage and frequency of

the two AC systems. An independent control is therefore available to transmit power, leaving

each system’s existing load frequency control to act normally. A valuable strategy then is to

hold in reserve the system control features given above for occasionswhenvoltage or frequency

stray outside the normal bands of operation.

Where a link is contained within one AC system the same applies, but special stability

controls act when system oscillations exceed a certain band of, for example, the rate of change

of bus bar voltage angle.

Voltage Control.AnHVDC link can also be used for voltage control. The converter absorbs

reactive power depending on its control angle, which normally will be compensated for by

filters and/or capacitor banks. By extending the control angle operating range (to a lower

voltage) and additional capacitor banks (to raise voltage) togetherwith a fast acting transformer

tap-changer, the reactive power demand can be used for independent voltage control at both

connection points. This operation, outside the optimum (minimum) control angles, leads to

higher short-time operational losses and stress on components, but these are usually marginal

compared to the operational improvement. If it is to be used as a permanent feature, thismethod

of operation has to be taken into account in the design phase of the DC link.

It is important to realize that the normal constant power regime of a DC link can destabilize

anAC network under distress. A normal feature of theDC link is the voltage-dependent current

limit where DC power is limited when voltage drops below the normal range, so that the

reactive power is made available to the AC system. Under disturbed conditions, it is a good

principle to look after theACvoltage first, and then order the power flow accordingly. There are

substantial AC filters at the converter stations, which can be used to bolster AC voltage if

stability is threatened. The DC control drops DC power, so that the converters absorb less

reactive power and the reactive capacity of the filters is available to the network. Though the

loss of power flow is unwelcome, the boost to AC voltage maybe more valuable.

Self-commutatedVSCs are able to provide independent control of active and reactive power.

Reactive power generation or absorption is possible, within converter ratings, at anyDC power

transfer rate.

System Reserve. The maximum unit site of generation in the system is determined by the

maximum loss of power for which the system frequency can be maintained, within defined

limits. When a large amount of power is fed into an AC system by an HVDC long distance

transmission system, it can also be thought of as generation. The maximum power of one pole

of the HVDC link is in the same way limited by the system parameters.

The largest possible loss of power of an HVDC link, in case of a fault causing line outages,

depends on the DC line tower configuration and on the ability to transmit power via ground or

metallic return. Assuming that the current carrying capacity of a conductor is well above its

nominal current rating, there can be a short-time capacity of overload in the converter and line

on the remaining healthy equipment, to reduce the shock to the system as awhole in case of pole

6 HVDC Transmission

faults. Dimensioning the maximum sudden step-change resulting from a fault can therefore be

precise when DC is used.

Environmental Benefits. As well as the comparison of life cycle costs, the environmental

compatibility of a design alternative needs examination.AnACalternative constitutesAC lines

plus AC substations whilst a DC alternative has DC lines plus converter stations plus

corresponding AC substations. The environmental impacts of the two methods will not be

equal. A qualitative comparison between AC and DC lines with regard to impact on the

environment is as follows:

. Visual impact constitutes an environmental advantage for a DC line, since the tower size for

the same power is lower when compared to the tower size of an AC line.. Right-of-way width of a DC line compared to an AC line is considerably reduced. This

facilitates suitable routes in densely populated areas and in regions with difficult terrain.. Corona phenomenon has a substantially different nature in DC than in AC transmission.

Generally, for a bipolar DC transmission line and an AC transmission line with almost the

same rms conductor voltage to earth and equal transmitting capacity, annual mean Corona

Losses (CL) are more favorable for the DC than the AC case, particularly in poor weather

conditions.. Radio interference (RI) results from Corona discharges, which generate high frequency

currents in the conductors producing electromagnetic radiation, in the vicinity of the lines. RI

measurements have shown that radio noise from a DC line is considerably lower than from

AC lines of similar capacity.. Audible noise (AN) values resulting from comparable DC and AC lines during fair weather

are quite similar. However, during rain, the better performance and the lower interference

levels generated by DC compared to AC lines are considered an advantage.. With regard to magnetic fields, conditions for DC lines are quite different than AC lines.

Since a DC line has an unchanging electric field, it exerts effectively nomagnetic field on the

surroundings. The DC field of a monopolar line is comparable to the strength of the earth’s

magnetic field.. Regarding generation and emission by DC lines of positively charged ions, O3, N2 and free

electrons, research studies and investigations of possible consequences have shown, up to

now, no evidence of hazard from any operating DC line.

1.3 HVDC System Costs

It is verymuch cost effective for a long distanceDCpower transmission compared toACpower

transmission. In case of undersea cables where the intersections of the bold lines are located at a

relatively short distance as shown in Figure 1.3, the DC system is much more economical.

In Figure 1.3, (1) illustrates the initial cost for HVAC power transmission and (2) illustrates

the initial cost of HVDC power transmission with a bigger initial cost due to a higher valve cost

for HVDC transmission. In addition, (3) and (5) represent the cost for transmission line

construction inHVAC andHVDC power transmissions, respectively and they demonstrate that

HVDC power transmission has a lower cost for transmission line construction. In the case of

HVAC power transmission, a shunt capacitor must be installed typically at every 100 km or

200 km because of its electrostatic capacity. In other words, the increase in total cost for power

transmission lines is accompanied by additional costs due to shunt capacitors. In the same


Figure 1.3, (6) and (7) illustrate losses of HVDC and HVAC systems during power transmis-

sion. It is shown that anHVDC system has a smaller loss if the same amount of electric power is

delivered. Therefore, HVAC transmission is favorable for distances less than about 450 km and

HVDC transmission is favorable for distances exceeding 450 km.

Table 1.1 represents the relationship between the capacity and the DC voltage of most

commonly used HVDC systems. The general DC voltage may not be determined by the

capacity of the HVDC system below 400MW but instead it should be solely determined by

the manufacturer or electric power company because of the economical trade-offs between the

insulation level and the losses in an HVDC system.

Cost

Total AC Cost(8)

Approx.450 km

ACLosses

(6)

Total DC Cost(9)

DCLosses

(7)

DC LineCost (5)

DC TerminalCost (2)

AC TerminalCost (1)

SC Distance

AC LineCost (3)

Shunt CapacityCost (4)

Figure 1.3 Transmission distance and investment costs for AC and DC power transmission lines.

Table 1.1 Relationships between the capacity and the voltage of HVDC systems.

Capacity (MW) AC voltage (kV) DC voltage (kV) (PTP) DC voltage (kV) (BTB)

200 115 — 2� 60

400 115–230 — 2� 80

500 230–345 �250 2� 100

1000 345–500 �400–500 2� 150

1500 345–500 �500 —

2000 500 �500–600 —

2500 500 �500–600 —

3000 500 �600 —

8 HVDC Transmission

The most unique characteristic of the data presented in Table 1.1 is the fact that since the

concept of earth return does not exist in a BTB connection, the negative voltage may not

exist accordingly and that the DC voltage could be lower than in a PTP connection since it

has no transmission line and no transmission line resistive loss needs to be considered. The

lower DC voltage in BTB connection implies that the number of serially connected

thyristors in the valve of the HVDC system and the insulation level of the peripheral

devices will be reduced. Therefore, the manufacturing cost for BTB–HVDC systems is less

than PTP–HVDC systems if the net cost excluding the cost for the transmission line at the

same capacity is considered. (According to some reports, the total manufacturing cost for

BTB is less than PTP by 20%.)

Station Costs. In any economic assessment of either overall project viability or comparison

of competing options, the base capital costs are invariably the most significant items and hence

the most important in terms of accuracy. Throughout this exercise, and indeed throughout the

process of defining a transmission project, the accuracy of a cost estimate will improve, as the

parameters (technical and commercial) become better known. In recent years, several reviews

have been done on the capital costs of HVDC equipment supply.

A cost breakdown analysis for recent DC stations is presented in Table 1.2. It obviouslymust

be treated with caution, as quoted costs of DC stations are subject – like anything else – to the

vagaries of the marketplace. Though they have recently been dropping, it cannot be predicted

whether such conditions will continue or reverse. In Table 1.2, the ‘total’ level gives typical

turnkey costs of the vendor’s HVDC supply and installation. These costs cover both terminals

(of a two-terminal scheme), and are based on some simplifying assumptions. They assume that

the DC bipole is made up of one valve group per pole and also that no special measures are

required for reactive power compensation and/or voltage control to incorporate a DC scheme

Table 1.2 HVDC turnkey cost division cost values given in year 2000US$/kW (both ends inclusive) for

one valve group per pole.

Back-to-BackMonopole

500 kV,

Bipole

�500 kV,

Bipole

�500 kV,

Bipole

�600 kV,

200

MW

(%)

500

MW

(%)

500MW

(%)

1000MW

(%)

2000MW

(%)

3000MW

(%)

Valve groups 19 19 21 21 22 22

Convertor

transformers

22.5 22.5 21 22 22 22

DC switchyard

and filtering

3 3 6 6 6 6

AC switchyard

and filtering

11 11 10 9.5 9 9

Control/prot./comm. 8.5 8.5 8 8 8 8

Civil/mech. works 13 13 14 14 13.5 13.5

Aux. power 2 2 2.5 2.5 2.5 2.5

Project eng.

and admin.

21 21 17.5 17 17 17

100 100 100 100 100 100

Total per kW $130 $90 $180 $170 $145 $150


into a weak AC system. These costs also do not include any costs by the purchaser entity itself,

taxes, interest during construction or other money borrowing costs. In certain applications, the

purchaser’s costs can be substantial.

If �500 kV is selected in place of �600 kV for a 3000MW bipole, a station cost

approximately 5 to 10% lower applies. In view of market volatility, the above estimates

should be treated as having accuracy no better than �20%. These costs can be used to

explore development options but confirmatory figures obviously need to be obtained from

manufacturers. Each power system is different with respect to voltage, system strength,

harmonic and reactive power limits. Each HVDC scheme is therefore unique and caution is

needed when utilizing the turnkey costs and facility cost variations discussed above for

comparing options. Additional shunt capacitors cost about $10/kvar. More sophisticated

control devices, such as SVCs or STATCOMs, cost from about $30 to $50/kvar (total

installed cost). In the case of very weak systems where additional system strength in the

form of synchronous compensators is applied, larger costs from about $70/kvar to $90/kvar

may be implied.

Overhead Line Costs. Depending on the degree of system reliability required and the

sensitivity tolerated for transient and permanent line faults, various types of HVDC overhead

lines can be constructed with different remaining transmission capacity after line faults. An

increase in reliabilitymeans an increase of cost of transmission lines. The figures in parenthesis

in Table 1.3 assume that the two station poles can be switched to operate in parallel (at a small

increase in the station cost) and that the line conductors have the thermal capacity for twice the

current. Thismay be inherently available as conductor sizes are selected to satisfy design limits

for corona discharge.

Line designers were asked to calculate, for a typical situation familiar to them, the ratio of

costs per kmof eachDC line, using the correspondingAC line cost as 1.0 per unit per km length.

Suitable design parameters, including conductor sizes, with which they were familiar for each

case, could be used, assuming a simple bipolar tower without a metallic neutral return. The

results are given in Table 1.4. The intent was to compare the cost of towers, conductors and

construction only, without taking into account other parts of the system.

Stages in Expansion of HVDC Transmission. HVDC transmission can be fitted more

readily than AC to a gradual expansion plan for transfer of power. In this way, unnecessary

investments can be avoided or a delay of investments can be obtained. AC transmission

often has to be built from the start with a high capacity to maintain stability, but DC can be

tailored to discrete stages. The most common staging in DC transmission is first to build a

monopole and later a bipole. To develop further from this stage, a new bipole can be added

or the convertor stations can be upgraded in current and/or voltage by adding converters in

series or parallel. In many applications HVDC is chosen for large power transfers on a long-

term basis. The transfer may, however, be low in the initial stage and higher after a certain

period. Based on the build-up timing and having the investment costs for convertor stations

in mind, it is natural to evaluate different approaches of a stepwise implementation of the

total HVDC transmission scheme.

The major alternatives are:

1. Stage 1 Pole 1

Stage 2 Extension with Pole 2

10 HVDC Transmission

Table

1.3

HVDCoverheadlineconfigurations.

Variant

Tower

configuration

Rem

aining

transm

ission

capacity

Relative

cost(%

)

Loss

ofonepole

groundreturn

Permitted

Notpermitted

tower

breakage

Single

monopolarline

00

085

Single

bipolarline

50(100)

00

100

Double

bipolarline

100

100

0114

Twomonopolarlines

50(100)

050(100)

126

Twolines

(bipolarorhomopolar)

100

100

100

136


Usually the two poles have equal power and voltage ratings. The transmission linemight be

designed as a bipolar line from the beginning and additional converters are added at both

ends.

2. Stage 1 Bipole 1 at reduced voltage

Stage 2 Upgrading by increasing the DC voltage

This requires additional converters in series.

3. Stage 1 Bipole 1

Stage 2 Upgrading by increasing the DC current

This requires additional converters in parallel. Both alternatives 2 and 3 should have the

transmission line designed for the higher voltage or current capacity from the beginning.

4. Stage 1 Bipole 1

Stage 2 Bipole 2

The two bipoles need not be equally rated, but additional security can be gained by

paralleling poles under line outage conditions. If the transmission includes submarine

cables, it is often economical to install the required rating and no more at each stage.

Environmental Aspects. Increasing environmental awareness throughout the world is

impacting both the implementation and the cost of transmission projects. Environmental

objections to projects can lead to lengthy delays in construction, and thus to increased cost.

Interconnection projects may span different jurisdictions, each with its own controls and

application procedures.More directly,measures tomitigate environmental impact lead directly

to cost increases. On the other hand, these cost increases have to be compared to the cost of

delay and especially to the income not received from a scheme that had proved to be

economical. The environmental issues which can give rise to increased costs include the

following:

. As with AC overhead transmission lines, there is increasing resistance to the construction of

newDC transmission lines. The objections are typically based onvisual impact and concerns

about electromagnetic fields. Objections to overhead lines led to a decision to underground

the land portions of the KONTEK scheme (Denmark–Germany) and to prolonged delays to

the Italy–Greece interconnection. These objections can affect associated AC connections

and system reinforcements as much as the DC transmission scheme itself. This has been the

Table 1.4 Cost ratios for DC and AC transmission line constructiona.

Case AC equivalent line Cost pu HVDC bipolar line ratings Range of costs pu

1. 230 kV, double circuit 1.00 �250 kV, 500MW 0.68 to 0.95




aNote these figures are not derived from a statistically significant sample, but fromonly three approximate

estimates by practitioners.


case with the Moyle interconnection (N. Ireland–Scotland), where a description of the

necessary process has been published.. Many submarine HVDC schemes were conceived as monopolar schemes, thus minimizing

cable costs. However, the use of a sea/earth return path leads to questions of corrosion of

other metallic objects (pipelines, cable sheaths, etc.), production of chlorine gas and impact

on fish populations.. Monopolar submarine HVDC schemes cause magnetic compass deflections, depending

on cable orientations, water depth and current magnitude. In some jurisdictions there are

limits to magnetic compass deflections, which may require a return cable alongside the

pole cable, or the use of coaxial cable with integral current return, where ratings are

appropriate.. Burial of submarine cables may be necessary to minimize the risk of mechanical damage to

the cables by trawling or shipping. However, the disturbance of the seabed caused by

trenching and cable lying may have adverse impacts on marine life.. Stringent acoustic noise limitations may be placed on convertor station installations.

Measures to limit noise from reactors, valve cooling systems, filter banks and so on can

have an impact on the cost of the convertor station.. Direct voltage across line insulators tends to attract and polarize airborne dust particles. Anti-

fog insulators are usually needed and creepage distance requirements are greater than on an

AC line.. An objection to any higher-voltage transmission linemay be that it is difficult to supply small

loads to villages along the right-of-way.

1.4 Overview and Organization of HVDC Systems

HVDC transmission refers to that the AC power generated at a power plant is transformed

into DC power before its transmission. At the inverter (receiving side), it is then transformed

back into its original AC power and then supplied to each household. Such power

transmission method makes it possible to transmit electric power in an economic way

through up-conversion of voltage, which is an advantage in existing AC transmission

technology and to overcome many disadvantages associated with AC power transmission

as well. The overall structure of an HVDC system is as shown in Figure 1.4 and its basic

components are described below.

AC Breaker. This is used to isolate the HVDC system from the AC system when the HVDC

system is malfunctioning. This breaker must be rated to carry full load current, interrupt fault

current, and energize the usually large converter transformers. The purposes of this breaker are

for the interface between AC switch yards or between AC busbar and HVDC system

(Figure 1.5).

AC Filters and Capacitor Bank. The converter generates voltage and current harmonics at

both the AC and DC sides. Such harmonics overheat the generator and disturb the communi-

cation system. On the AC side, a double tuned AC filter is used to remove these two types of

harmonics. In addition, the reactive power sources such as a capacitor bank or synchronous

compensator are installed to provide the reactive power necessary for power conversion

(Figure 1.6).


Current (i)

Zero crossing 60/s (60 Hz)

Time (t)

Figure 1.5 Blocking at the zero crossing of AC current.

ACACSystem2System1

ACFilter

Controls, Protection, Monitoring

DCFilter

DCFilter

321 654

Pole1

Pole2

To FromOtherTerminal

Figure 1.4 Basic structural diagram of a bipolar HVDC system.

Figure 1.6 Double tuned AC filter for the 11th and 13th harmonics (Reproduced by permission of

AREVA).


Converter Transformer. This transforms the voltage from the AC system to be supplied to

theDC system. It also provides a separation between theACandDC system. Specifically, when

the two units of 6 pulse converters are serially connected to generate a 12 pulse output, a

3-winding converter transformer is used (Figure 1.7).

Thyristor Converter. A converter, which is an essential component of HVDC power

transmission, is developed using power electronics. It is one of many research areas dealing

with the transformation and control of power by switching devices in the power converter. It

performs the conversion from AC to DC or from DC to AC. It is mainly comprised of a valve

bridge and a transformer with a tap converter. Figure 1.8 shows the thyristor converter installed

and operating in Cheju Island. Its thyristor stack is configured with 6-pulses or 12-pulses and it

is connected to the voltage valve (Figure 1.9).

Smoothing Reactors andDCFilters.The smoothing reactor reduces theDC ripple current to

prevent it from becoming discontinuous at low power levels. Also, the smoothing reactor forms

an integral component, together with the DC filter, to protect the converter valve during a

commutation failure by limiting the rapid rise of current flowing into the converter.

HVDC Controller Structure. Figure 1.10 shows the basic control diagram of an HVDC

system. An HVDC system can be divided into several levels. Master control determines the

power order or frequency order and calculates the current order for both poles. Then, the current

order that was received from master control is modified by control functions and limits in pole

control. Valve group control consists of a converter control and a valve firing control. The

converter control includes the current controller. The valve firing control distributes the firing

signal to all thyristors.

Line Commutated Current Source Converter and Voltage Source Converter. Line Com-

mutated Current Source Converter (LCC), as shown in Figure 1.11, consists of a 12-pulse

converter, AC filter and synchronous compensator. LCC depends on the AC system voltage for

its proper operation. LCC operates at a lagging power factor, because the firing of the converter

has to be delayed relative to the voltage crossing to control the DC voltage.

Figure 1.7 Three-winding converter transformer (Reproduced by permission of AREVA).


Figure 1.12 shows the concepts of Voltage Source Converter (VSC). VSC is based on forced

commutated devices that is, IGBTs or GTOs, which allows converter operation in all four

quadrants of the P–Q plane. Since commutation can be achieved quickly and independently of

theAC system voltage, an entirely different type of operation compared to the LCC converter is

possible.

Figure 1.9 Thyristor stack (Reproduced by permission of AREVA).

Figure 1.8 Thyristor converter (Reproduced by permission of AREVA).


Point to Point System. Most HVDC systems fall under this category. It consists of either

cable or overhead lines or a combination of these two. This type of system has one of the forms

shown in Figure 1.13, depending on the number of overhead lines and the polarity.

Monopolar HVDC. This type of HVDC link consists of a single conductor and a return path

either through the ground or sea. This method is mostly used for power transmission using

cables. Use of this type of system is dictated by the costs of installing the cable. A metallic

return path is preferred instead of through the ground when the ground resistance is too high or

the underground/undersea metallic components may cause some interference (Figure 1.14).

Bipolar HVDC. It consists of two poles, one positive polarity and the other negative polarity,

and with their neutral points grounded. In steady state operation, the current flowing in each

pole is the same and hence no current flows in the grounded return. The two poles may be

operated separately. If either polemalfunctions, then the other pole can transmit power by itself

with ground return. In a bipole the amount of power transmission is increased by a factor of two

compared to the monopolar case. This creates fewer harmonics in normal operation as

compared to the monopolar case. Reverse power flow can be controlled by converting the

polarities of the two poles (Figures 1.15 and 1.16).

Back-to-Back System. In this type of system, the rectifier and the inverter are located in the same

station. Ingeneral, it isusedforprovidinganasynchronousinterconnectionfor twoACsystems.The

amplitudeofDCvoltage isgenerallysmall, around150kVtooptimize thevalvecosts (Figure1.17).

Id

idVrdV

Tap Changer Tap Changer

CEA CEA

niMniM

FiringPulses

Valve GroupControls

Firi

ng C

ontr

ols

Firi

ng C

ontr

ols

CC CC

VDCOL VDCOL

PoleControls

Σ

SupplementaryControl Signals

SupplementaryControl Signals

ΣP/U

Power Order

MasterControls

(Via Telecom)

VmesIdrefIdref

IdrefIdrefId Id

ΔIdαocc

α0

αocea

γ ref γ ρεφγ γ

α0

Figure 1.10 Basic control diagram of an HVDC system.


HVDC Multi-Terminal. This refers to an HVDC system that consists of three or more

transforming stations. Its architecture is more complex compared to that of a two terminal

point-to-point system. It requires a significant complexity to facilitate communication and

control between each transforming station. However, it is considered to be a relatively new

technology and has potential for a wide range of applications in the future. There are two types

of multi-terminal links – a parallel or serial type, as shown in Figure 1.18.

V

I

V I1

IH

V

IC

I1

I2

CB

CB

Filter

CB

CB

CBI

CompensationTransformer

ACNetwork

Tr.

Reactor

Thyristor

Vdc

(a)

(Inverter)(Rectifier)

Udr Udia b

Commutation voltage atrectifier

Commutation voltage atinverter

cba

acb

a

acVp

Vn

Uvi

I vi

ULi

Xc

ULr

X c

Uvr

Ivr

Vp

Vn

Id

(b)

β

β

γ

γ

α

α μ

μ

Figure 1.11 Operational characteristics of a line-commutated current-source HVDC system.


1.5 Review of the HVDC System Reliability

Reliability and availability deal with the degree to which system performance is limited by

system failures. And several simple models are available for calculation HVDC system

performance indices, as follows:

. MTTF (Mean Time To Failure).

. MTBT (Mean Time Between Failure).

0

0

0

0

0 0 0

0

0

0

0ms 10ms 20ms 30ms 40ms0ms 10ms 20ms 30ms 40ms0ms 10ms 20ms 30ms 40ms0ms 10ms 20ms 30ms 40ms

0ms 10ms 20ms 30ms 40ms


0ms 10ms 20ms 30ms 40ms0ms 10ms 20ms 30ms 40ms0ms 10ms 20ms 30ms 40ms0ms 10ms 20ms 30ms 40ms



0 0

+

+

+

+

X

U V W

Y Z

# 2

# 3

# 4

U

V

W

WVU

U V W

U V W

Figure 1.12 Operational characteristics of a voltage-source HVDC system.

Monopolar

TransmissionLine

TerminalA

TerminalB

Bipolar

TransmissionLineTerminal A Terminal B

Pole 1

Pole 2

Figure 1.13 Point-to-point system.


. MTTR (Mean Time To Repair).

. MTBSD (Mean Time Between System Down).

. MTTSD (Mean Time To System Down).

Reliability. “The probability that an item will perform its intended function for a specified

interval under stated conditions.”

Generally, this means that the probability is that the scheme will be capable of transmitting

its rated load at any point in time under normal operational conditions. Specifically, for the

purposes of an HVDC project, ‘reliability’ is expressed in terms of the number of forced

outages of the scheme per year, normally termed “Forced Outage Rate” (FOR), after which

emergency repair work would be necessary in order to restore the equipment to normal

operation.

Availability. “A measure of the degree to which an item is in the operable and committable

state at the start of the mission, when the mission is called for at an unknown (random) point in

time.”

SE AC System

Zsys

AC

Filt

ers

DC

Filt

ers

DC Line

Zsys

RE AC System

DC

Filt

ers

DC Line

-ve Pole

+ve Pole

Figure 1.15 Bipolar HVDC system.

SE AC System

ZsysA

C F

ilter

s

DC

Filt

ers

Electrode Line Impedance

DC Line

Zsys

RE AC System

Figure 1.14 Monopolar HVDC system.


In the case of anHVDC scheme, pure “availability” is not commercially important since, for

example, equipment failures at the times of zero loads do not affect the amount of energy being

transmitted by the scheme. It is more appropriate to consider “energy availability.”

Energy Availability. “Ameasure of the energywhich could have been transmitted except for

limitations of capacity due to outages of equipment items within the project plant.”

This is the maximum amount of energy capable of being transmitted by the scheme,

expressed as a power–time area, averaged over a year in percentage or per-unit terms of the

maximum energy that could be passed, if the scheme worked continuously at rated full load

without shut-downs. For example, in the case of a single-pole converter that is fully operational

for 99% of the year, the energy availability would be 99%. In the case of a bipole or dual-pole

converter, however, there are two possible operational states – 100% load (both poles fully

operational) and 50% load (only one pole fully operational) and availability will be some

combination of these states. For example, if an HVDC scheme is operational on both poles for

99% of the year and operational on one pole only for 1% of the year the overall energy

availability would be: 99%� 1þ 1%� 0.5¼ 99.5%.

Maintainability. “The probability that an item will be retained in or restored to a specified

conditionwithin a given period of time, when themaintenance is performed in accordancewith

prescribed procedures and resources.”

The maintenance requirements of equipment can result in stoppage of its mission’ (energy

output) and therefore have to be allowed for in the reliability analysis since maintenance

outages are classed as ‘Scheduled Outages’. In the maintenance interval, it is necessary to

SE AC System

Zsys

AC

Filt

ers

DC

Filt

ers

DC Line

Zsys

RE AC System

DC

Filt

ers

DC Line

-ve Pole

+ve Pole

By-Pass Switch

Figure 1.16 Bipolar HVDC system with bypass switch.


complete all ‘off line’ preventatives, diagnostic and corrective maintenance. Normally it is

possible to schedule themaintenance of the converter stations at either end of anHVDCscheme

to coincide, thereby reducing the overall down-time (Outage Time) of the scheme. It is

sometimes not economically practical; however, to have two teamsof trainedmaintenance staff

available for such short periods, hence a more extended maintenance outagemay be necessary.

TwoMajor Parameters of RAMAnalysis.All the above parameters (Reliability, Availability

and Maintainability) can be reduced to two parameters by which the performance of a power

transmission plant may be measured.

Energy Availability. This will affect the revenue generated by the scheme.

Forced Outage Rate.This will affect the continuity of the energy supplied to a customer and

will also influence the cost of maintenance of the scheme.

Reliability Study Model. A reliability study model is built by grouping together smaller

models, known as ‘subsystems’. A subsystem is a collection of components (or smaller

subsystems) whose individual reliabilities can be combined together on the basis of their inter-

relationships (dependencies) to provide an overall measure of subsystem reliability. The

subsystem is then treated as a single component with its own failure and repair characteristics.

System 1 System 2

AC AC

(a)

(b)

SE AC System

Zsys

ACFi

lters

Zsys

RE AC System +ve Pole

(DC Filters are absent)

-ve Pole

(Smoothing Reactor is sometimes absent)

Figure 1.17 Back-to-back HVDC system.


In this way, a reliability study model can be simplified by consideration of its reliability in

modular fashion into a smaller quantity of representative subsystem modules. There are no

fixed rules regarding theway in which components are combined together to form subsystems;

the choice is based on the nature of the plant and the experience of existing installations.

Terminals in Parallel Terminals in Series

(a)

SE AC System

Zsys

AC

Filt

ers

Zsys

RE AC System

Zsys

Zsys(b)

SE AC System

Zsys

AC

Filt

ers

Zsys

RE AC System

Zsys

SE AC System

(c)

Figure 1.18 HVDC multi-terminal.


Examples of Subsystems are given in the following:

1. Converter Valve: consisting of thyristors, gate units, monitoring units, ground-level

electronics, cooling components, and so on.

2. Harmonic Filter: consisting of inductors, capacitors, resistors, CTs, isolators, AC breakers,

and so on.

A reliability study model of a complete system is built up by relating together all the

subsystemswhich it contains in terms of the effect of their failures on the other subsystems. For

example, failure of a pump in a thyristors valvewater cooling system could affect the reliability

of the converter in which the valve is situated. The relationship between affected subsystems is

termed ‘dependency’.

Types of Dependency. The dependency relationship between two or more components is

usually expressed in two ways:

1. Series Dependency. In this case, the concept used is analogous to that of series-connected

fuses in an electrical circuit where circuit continuity is dependent on the health of each and

every individual fuse. Failure of one ormore fuses results in a failure of the circuit to conduct

current. In the same way, in terms of reliability, the failure of one or more series dependent

components results in complete failure of the equipment (or subsystem) to operate as

designed. For example, failure of either a convertor transformer or a thyristors valve in one

pole will result in shut-down of the complete pole. The transformer and valve are thus

considered to be in series dependency.

2. Parallel Dependency. In this case, the concept used is that of parallel-connected fuses in an

electrical circuit. Failure of one fusewill not result in failure of the circuit to conduct current

although the circuit’s current carrying capacity may be reduced. Similarly, in terms of

reliability, the failure of one or more parallel-dependent components may not result in

equipment failure but may limit the equipment’s rated capacity or functionality. For

example, if a scheme contains two harmonic filters, the failure of one filter may not result

in converter shut-down since the converter may be able to function with only one filter in

circuit (although, in some situations, at a reduced rating). The filters could thus be

considered to be in parallel dependency.

3. Redundancy. This is a version of parallel dependency but in this case a greater number of

components are provided than the minimum necessary to achieve the required equipment

rating. Hence, one or more components are able to fail without causing a reduction of the

rated capability of the equipment.

Example of MTTF. To check the reliability of an HVDC system, assume that the ratio of

occurrence of failure in themain components such as valves, converters, controllers, protection

circuit and filters are given by Table 1.5.

The relationship between the ratio of occurrence of failure (l) and the reliability can be givenas Figure 1.19(a) where Ri(t) denotes the reliability. For convenience, assume that l1¼ l4¼l5¼ l6 and l2¼ l3, the following equation can be derived:

Rsys1 ¼ exp½ � ð4l1 þ 2l2Þt� ð1:1Þ


In this case, the MTTF can be expressed as in the following equation:

MTTF ¼ 1

4l1 þ 2l2ð1:2Þ

If one assumes that the relationship between l1 and l2 is described by (l2/l11)k1, one shouldsimply decrease l2 in order to increase the MTTF. If one considers a system, Rsys consisting of

two modules l1, l2 in parallel to each other in order to draw a explanatory comparison of the

concept of multiplexing, then the MTTF can be expressed as follows:

RsysðtÞ ¼ expð� l1tÞ � expð� l2tÞ ð1:3Þ

Table 1.5 Components of an HVDC system and the ratio

of occurrence of failure.

Valve l1Converter l2Controller l3AC protection l4DC protection l5HPF, BPF l6

Rλ1

λ1

λ1

λ1

λ1

λ1

λ1

λ2

λ2

λ2 λ6

λ6

1(t) = exp(- 1t)

R6(t) = exp(- 6t)

Rsys1 = Ri(t) = exp(- it)6

i=1

(a)

(b)

: Rsys3(t)

: Rsys2(t)

Figure 1.19 Relation between the ratio of occurrence of failure and the reliability.


MTTF ¼ 1

l1 þ l2ð1:4Þ

In this case, l1 is the dominant factor in minimizing theMTTF. Therefore, l1 should be simply

reduced in order to increase the MTTF.

If only one of the multiplexed l1 in Figure 1.19(b) is normal then assume that it is operating in

the normal condition and set R1(t)¼ exp(�l1t), R2(t)¼ exp(�l2t) Now, the expression

becomes

Rsys2ðtÞ ¼ ½1�ð1� expð� l1tÞÞ2� � expð� l2tÞ ¼ ½2R1ðtÞ�R21ðtÞ�R2ðtÞ ð1:5Þ

Rsys3ðtÞ ¼ ½1�ð1� expð� l1tÞÞ3� � expð� l2tÞ ¼ ½3R1ðtÞ� 3R21ðtÞþR3

1ðtÞ�R2ðtÞ ð1:6Þ

Assumes that l1¼ 2.1831� 10�5 (failure of unit/hr), l2¼ 1.0� 10�6 (failure of unit/hr), then

the equation becomes:

RsysðtÞ ¼ expð� 2:2831� 10� 5tÞ ð1:7Þand the MTTF is given as:

MTTFsys ¼ 43800½hr� ¼ 5½years� ð1:8ÞCalculating Reliability/Availability [3]. There are several ways in which reliability and

availability can be evaluated. One of the more popular methods is the ‘Monte Carlo’ method of

random sampling. The drawback to this method is that when failure rates are very low, a finite

quantity of random samples may result in zero failure occurrences being found. Hence

components with very low failure rates cannot be represented accurately. The reliability

software which is used is based on the ‘Continuous Markov’ method. This considers all

components to be in continuous transition between ‘working’ and ‘failed’ states. Because all

the transitions between states are not time-dependent, they can be expressed as linear

simultaneous equations which can then be solved by matrix arithmetic.

Case Study:A 100MWHVDC power link is required to connect a power station to a major

load center. The power station and local center are separated by a body of water. As this power

link will be supporting critical load the following design targets have been set for the convertor

stations:

(i) Energy availability of the two convertor stations (excluding the cable): 99.5%.

(ii) Number of forced outages in 5 years: 1.

It will be permitted to use the body of water as the neutral DC connection. A cost

optimization exercise has been carried out and it has been found that the most economic

interconnection voltage would be 100 kV DC. It will be permitted to close down the link on a

schedule basis once every year, for repair and maintenance activities to be undertaken.

Stage 1: Monopole. If one first considers a basic scheme which is only designed to meet

the power transfer requirements stated above, one arrives at the design shown in


Figure 1.20. Each end of the basic design consists of:

1. A 12-pulse convertor where each valve contains 14 ‘active’ thyristors in series.

Associated control and auxiliary equipment is lumped into this subsystem.

2. One AC harmonic filter connected to the AC bus.

3. One set of DC yard equipment including; the smoothing reactor, the DC

measuring equipment, and so on.

The system can be represented by the simplified dependency diagram shown in

Figure 1.20 (note that the HVDC cable has been ignored). By evaluating this

equipment dependency using the standard software the results shown in Table 1.6

and Table 1.7 are obtained.

It can be seen that this design is inadequate as over a five year period this scheme

could be expected to trip (that is, suddenly stop transmitting power) over twenty six

times. It is therefore necessary to increase the redundancy in the scheme.

Stage 2: Redundant Filter. Considering the stage 2 results it can be seen that the biggest

contribution to Forced Outage and Energy Availability is the one AC harmonic filter

on each AC bus. The addition of a parallel filter would mean that the failure of one

filter would not require the HVDC scheme to be tripped. A further advantage of

F

Single Line Diagram

F

Dependency Diagram

AC Filter AC FilterConvertor ConvertorDC Yard DC Yard

Figure 1.20 HVDC system composed of a monopole.

Table 1.6 Calculated FOR and energy availability.

Stage Number of forced

outages in 5 yr

Energy

availability (%)

Capital

cost (%)

1: Monopole 26 98.26 100

2: Redundant filter 8 99.64 104

3: 100% rated bipole 1 99.38 156


connecting a second filter in parallel during normal operation is that theACharmonic

filter losses (I2R losses) will be a quarter. From Table 1.8 it can be seen that the AC

harmonic filters, once duplicated make only a very small contribution to the forced

outages and energy unavailability (Figure 1.21).

Stage 3: 100% Rated Bipole. In order to gain any further real improvements, it is necessary to

duplicate the link itself. The single line diagram and dependency diagram for this

arrangement is given in Figure 1.22. Consider two poles, each rated at 50% (50MW),

connected in series to form a bipole. The Forced Outage of one pole will reduce the

transmitted power to 50% not 0% as previously. Considering the results in Table 1.9,

duplication of the poles can be seen to havemade a dramatic effect on the FOR. which

is now better than one expected failure in five years. However, the EnergyAvailability

target has still not been achieved (Table 1.9).

Case Study Conclusion.By constructing this scheme from two poles each rated at 100% it is

possible, in this simple example, to achieve the design target of:

. Energy availability of: 99.5%.

. Number of forced outages in 5 years: 1.

From the results it can be seen that the calculations have a significant economic effect. The

customermay, for example, choose to relax the target energy availability figure by 0.12%. Such

a change would mean that the scheme design proposed in stage 3 could be utilized, thereby

reducing the capital cost of the convertor station.

Table 1.8 HVDC system composed of one pole and filters considering redundancya.

Forced outage rate Energy unavailability Energy availability

Duplicated filter 0.010 798 0.004 098 99.9959

Convertor 0.731 940 0.138 605 99.8614

DC yard 0.067 643 0.038 882 99.9611

DC yard 0.067 643 0.038 882 99.9611

Convertor 0.731 940 0.138 605 99.8614


One pole 1.620 764 0.362 717 99.6373

aTrips in 5 years: 8.

Table 1.7 HVDC system composed as a monopolea.

Forced outage rate Energy unavailability Energy availability

AC filter 0.848 800 0.640 173 99.3598

Convertor 1.730 681 0.196 532 99.8035

DC yard 0.067 643 0.038 882 99.9611

DC yard 0.067 643 0.038 882 99.9611

Convertor 1.730 681 0.196 532 99.8035

AC filter 0.848 800 0.640 173 99.3598

One pole 5.294 249 1.740 369 98.2596



F

Single Line Diagram

F

Dependency Diagram

F F

50%

50%

CommonEquipments

DuplicateFilters

DC YardSame as

Other End

DC YardConvertor

Convertor

Figure 1.22 HVDC system composed of a bipole.

F F

Dependency Diagram

DuplicatedFilters Convertor ConvertorDC Yard DC Yard

DuplicatedFilters

F F

Single Line Diagram

Figure 1.21 HVDC system composed of one pole and two filters considering redundancy.


Table 1.6 compares the calculated F.O.R. and energy availability of the various cases

considered with a percentage capital cost, where the simplest scheme discussed in stage 1 is

taken as having a capital cost of 100%. By generating such a table for a particular scheme, the

user can optimize the scheme design based on the capital cost of the equipment versus the

charges the user would incur due to the loss of the HVDC scheme.

Evaluation of the Reliability in an HVDC System

Case 1.The entire system is composed as amono pole. In this case, it is reported that the system

is tripped 26 times in 5 years.

Case 2. The HVDC system is composed of one pole and two filters considering redundancy. In

this case, it is reported that the system is tripped 8 times in 5 years.

Case 3. HVDC system is composed of a bipole. In this case, it is reported that the system is

tripped one time in 5 years.

1.6 HVDC Characteristics and Economic Aspects

The Benefits of Interconnection. By way of a review, the principal benefits from interconnect-

ing two, or more, power systems are set out below:

. Economies of Scale. In general, large generation units are more efficient than small.

However, too large a unit size runs the risk of a major disturbance if it develops a fault.

The larger the interconnected system, the easier it is to withstand the loss of a large unit.. Fuel Economy. The dispatch of generating plant aims to use efficient plant for continuous

load and less efficient for meeting peaks. Wider fuel choices from interconnected systems

offer opportunities to optimize dispatch on a larger system, where more plant options are

likely to be available, and thus to reduce supply costs.. Reduction in Reserve Capacity. A margin of reserve capacity has to be maintained in

operating generation of any system to cater for plant maintenance and/or breakdown. The

interconnection of two or more separate systems enables their standby determined reserve

requirement. Additions of generation plant to meet rising predicted demand can be deferred

by interconnecting, or some of the reserve is freed to supply demand.

Table 1.9 HVDC system composed of a bipolea.

Forced outage rate Energy unavailability Energy Availability


Convertor 0.731 940 0.138 605 99.8614

DC yard 0.067 643 0.038 882 99.9611

DC yard 0.067 643 0.038 882 99.9611

Convertor 0.731 940 0.138 605 99.8614


One pole 1.599 167 0.354 550 99.6454

Common equipment 0.169 400 0.262 838 99.7372

Two poles 0.202 297 0.624 602 99.3754



. Diversity in Demand. Different types of consumer mix, East/West time zone shift, North/

South seasonal shift and even different religious observance (Moslem Friday, Christian

Sunday and different festivals) can all result in non-coincident system peaks, such that the

interconnected system’s maximum demand is considerably less than the sum of demands on

the separate systems.. Fuel Source Diversity. Different types of generating plant have differing operating pre-

ferences. Large coal-fired, combined-cycle gas and nuclear plant are efficientwhen operating

continuously. Hydro and gas turbines can bemore suitable for peaking and reserve duties. If a

system with a high proportion of the former can be interconnected with one with a high

proportion of the latter, higher load factors can be achievedon thermal plant by interchanging

energy. Surplus hydro energy in flood periods can be put to good use, saving fuel in the

thermal system. There may also be strategic advantages in a system having access to the

alternative fuel sources of a neighboring system.. Reliability and Security of Supply. The security and reliability of existing networks will be

increased by an interconnection making available the additional variety of generation type

and standby capacity from outside.. Environmental Benefits. Many of the above factors have consequential environmental

benefits. Improved efficiency of energy delivery is the most obvious. A particular benefit

associated with hydro is the realization of integrated operation that can permit some

reduction of carbon dioxide releases into the atmosphere from fossil sources in addition

to the saving in fuel. Furthermore, the cooperative operation of run-of-river and storage

hydro plants via interconnection enables significantly more energy to be produced from

a given hydrological scenario. This can apply within and/or between river systems, and

be coupled with opportunities of conserving water for other uses or in drought

conditions. It follows that interconnection also provides opportunities to reduce the

scale of new storage hydro projects, and thereby limit the inundation of land and

destruction of habitat.. Financial Participation.The owners of interconnected systems are able to share both the cost

and the benefits of large projects, for example, hydro, which may otherwise not be

economically justified.. Technology Exchange. Shared systems tend to encourage standardization of designs,

operational practices and information exchange.. Pooling Opportunities. In 1997 there were some pooling agreements or directives linking

utilities in theword, from the formal to the informal, to realize the benefits of interconnection.

Most used a traditional central-planning style as the mechanism for gaining the benefits.

However, there is now the alternative of thewholesale competitivemarket, giving third-party

access via a transmission system for bidding energy resources to determine the system

marginal price for dispatch.Aswell as energy, there aremarkets developing in power reserve,

frequency control, black-start capability and voltage control. These will call for a more

precise ability to control the operation of the power systemand the advantages ofDCoverAC

in this respect will be a consideration for planners and developers. Unconstrained access,

under the transmission operators control will be vital in this world.

These benefits apply across the whole system, as long as they meet the objectives that load

demand is reliably served at minimum total system cost. However, they can often only be

obtainedwithin the framework of a power pool that coordinates planning and operations so that


economies of scale, increases in reliability, and other ‘system’ benefits are achieved to the

extent practicable.

Technical Considerations. When independent, asynchronous systems are to be intercon-

nected, there are some configurations for which DC is the only solution or at least an interesting

alternative, for example, sea-cable interconnection. Normally with bulk power transmission,

whether an interconnection or not, there is a choice betweenAC andDC, and the determination

may in some cases be a matter of economics. In the case of DC a large investment is required in

terminal equipment for conversion, and this is mostly independent of the length of transmis-

sion. However, DC overhead lines are cheaper than AC lines for the same power transfer and

DC line losses are less than those for AC lines. Recent trends suggest that overhead line costs

are increasing at a higher rate than the costs of terminal equipment. This means that break-even

distances might reduce, that is the distance at which the overall cost (including losses) of AC

and DC alternatives are equal. However, there are twomore significant effects that increase the

break-even distance. One is that series compensation can reduce the effective length of a line

for stability and FACTS controllers (power electronics applied to the control of voltage and

power flow) can extend the range or capacity of AC systems. The other is the compact AC line,

which draws the phases closer together to lower the reactance and increase the shunt

capacitance and therefore improve the AC system’s capacity.

The break-even points of transmission cost between AC and DC taking into account the

investment and the operational losses depend strongly on the basis of the loss evaluation. From

the technical point of view neither the transmission distance nor the amount of power to be

transmitted is practically limited with DC.

The advantages of DC power transmission over ACpower transmission can be characterized

as follows.

Lower Insulation Class of the Line, so more Economical.Amaximum of DC voltage is only

1=ffiffiffi

2p

of maximum of AC voltage with the same RMS (Root Mean Square) value. So, it has a

significant advantage in terms of insulation. The number of supporting insulators andwires can

be significantly reduced and even the height of iron tower can be lowered, so that the overall

economical benefit is quite significant (Figure 1.23).

In the case of DC, the power factor is always 1 pu. It has great power transmission efficiency

(Figure 1.24).

DC power does not have an alternating imaginary part (Reactive Part) like AC power. So, no

reactive power is generated by the reactance. Since DC power transmission has more of real

power, which is used for the actual power consumption than AC power transmission, it has a

higher power transmission efficiency.

EmPeak

AC

DCtime

Em2

Figure 1.23 Comparison of maximum voltage for AC and DC.


DC Power Transmission can use the earth as a conductor. This is more economical

compared to AC power transmission, which requires at least two wires or more. Thus, if it is

feasible to transmit power to the earth return path, then onemay omit the return path conductor.

Consequently, it is more useful in the region which requires a ROW (Right OfWay; iron tower

passage) (Figure 1.25).

Power Interchagers. BTB or PTP HVDC system have found application for power

interchange between AC systems which operate at different frequencies. An example is the

50 Hz/60 Hz Frequency converter at Sakuma, Japan. And an isolated generation configuration

arises when a generation complex are asynchronously connected to the receiving AC system

via an HVDC system. Also, if the generation complex is dispersed in an isolated generation

area, it is common to provide an power collection system to feed the HVDC system

(Figure 1.26).

krowteN zH05krowteN zH06

Figure 1.26 DC interconnection between systems with two different frequencies.

Cos θ

Apparent Power

Active Power

Reactive Power

(a)

No Apparent Power

Active Power

(b)

Power Angle (Cos θ) =1

0

Figure 1.24 Comparison of power factor for DC and AC.

Conductor/Cable/Earth/Sea Return

Figure 1.25 Mono-polar system using a single-line return path.


Effects of separating AC systems by DC interconnection. Since DC power transmission

provides real power but not reactive power for the opposite system, the current inflow from the

neighboring system does not increase when the AC system breaks down. Consequently, it

creates a virtual effect of separating the two systems. So, by partitioning the existingAC system

into adequate sizes andmaking an interconnectionwith aDC system, then it may be possible to

effectively suppress the short-circuit current and to smoothly operate the entire system

(Figure 1.27).

Improvement of stability. Since it is possible to promptly control the power flow by

controlling the converter, the transient stability may be improved. The transient stability

refers to a degree of capability to maintain its stable condition and to continue a good power

transmission despite the sudden external impacts, such as open and short-circuit of transmis-

sion lines or ground-fault when the AC power system is being operated in a stable condition.

References

[1] HVDC Systems and their Planning, Siemens (1999).

[2] Economic Assessment of HVDC links, ELT_196_4, ELECTRA.

[3] Cheju-Haenam HVDC Manual, AREVA (1996).

[4] Barker, C.D. andSykes,A.M. (1998)DesignHVDCTransmissionSchemes forDefinedAvailability.Proceedings

of Generation and Transmission, IEE, 1(1), 4/1–4/11.

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transmission and hybrid back-to-back HVDC/AC transmission. IEEE Transactions on Power Delivery, 5(2),

1137–1144.

[6] Hammons, T.J., Olsen, A. and Gudnundsson, T. (1989) Feasibility of Iceland/United KingdomHVDC submarine

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[7] Diemond, C.C., Bowles, J.P., Burtnyk, V. et al. (1990) AC–DC economics and alternatives – 1987 panel session

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[8] Andersen, B. and Barker, C. (2000) A new era in HVDC? IEE Review, 46(2), 33–39.

[9] Bakken, B.H. and Faanes, H.H. (1997) Technical and economic aspects of using a long submarine HVDC

connection for frequency control. IEEE Transactions on Power Systems, 12(3), 1252–1258.

[10] Povh, D. (2000) Use of HVDC and FACTS. Proceedings of the IEEE, 88(2), 235–245.

[11] Kuruganty, S. (1995) Comparison of reliability performance of group connected and conventional HV DC

transmission systems. IEEE Transactions on Power Delivery, 10(4), 1889–1895.

[12] Billinton, R., Fotuhi-Firuzabad, M. and Faried, S.O. (2002) Reliability evaluation of multiterminal HVDC

subtransmission systems. Generation, Transmission and Distribution, IEE Proceedings, 149(5), 571–577.

[13] Hingorani, N.G. (1996) High-voltage DC transmission: a power electronics workhorse. Spectrum, IEEE, 33(4),

63–72.

[14] Dialynas, E.N., Koskolos, N.C. and Agoris, D. (1996) Reliability assessment of autonomous power systems

incorporating HVDC interconnection links. IEEE Transactions on Power Delivery, 11(1), 519–525.

[15] Kuruganty, S. (1994) Effect of HVDC component enhancement on the overall system reliability performance.

IEEE Transactions on Power Delivery, 9(1), 343–351.

[16] Dialynas, E.N. and Koskolos, N.C. (1994) Reliability modeling and evaluation of HVDC power transmission

systems. IEEE Transactions on Power Delivery, 9(2), 872–878.

krowteN BkrowteN ADC Power

Figure 1.27 Effects of separating systems by DC interconnection.


[17] Melvold, D.J. (1992) HVDC converter terminal maintenance/spare parts philosophy and comparison with

performance. IEEE Transactions on Power Delivery, 7(2), 869–875.

[18] Baker, A.C., Zaffanella, L.E., Anzivino, L.D. et al. (1989) A comparison of HVAC and HVDC contamination

performance of station post insulators. IEEE Transactions on Power Delivery, 4(2), 1486–1491.

[19] Kuruganty, P.R.S. and Woodford, D.A. (1988) A reliability cost-benefit analysis for HVDC transmission

expansion planning. IEEE Transactions on Power Delivery, 3(3), 1241–1248.

[20] Hingorani, N.G. (1988) Power electronics in electric utilities: role of power electronics in future power systems.

Proceedings of the IEEE, 76(4), 481–482.

[21] El-Amin, I.M., Yacamini, R. and Brameller, A. (1979) AC–HVDC solution and security assessment using a

diakoptical method. International Journal of Electrical Power and Energy Systems, 1(3), 175–179.

[22] Kalra, P.K. (1987) Feasibility study for development of expert systems for power system control. Electric Power

Systems Research, 12(2), 125–130.

[23] Sood, V.K. (2007) HVDC Transmission, Power Electronics Handbook, 2nd Edn, pp. 769–795.

[24] Cochrane, J.J., Emerson, M.P., Donahue, J.A. et al. (1996) A survey of HVDC operating and maintenance

practices and their impact on reliability and performance. IEEETransactions onPowerDelivery, 11(1), 514–518.

[25] Kunder, P. (1996) Power System Stability and Control, McGraw-Hill, New York.

[26] High-Voltage Direct Current Handbook (1994) EPRI TR-104166S.


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