High voltage Alternating Current transmission was the most popular transmission system in whole world since the first transmission link invented and it is the simplest method also. HVAC is a passive system, in the meaning that the current produced by the generator is directly transmitted to the grid via a transformer if a stepping up or down is needed. This system permits a power transmission and operations in the both directions (i.e. Bi-Directional power flow) and direction of power flow is maintained via potential difference between the two end points.
1.1 Components of HVAC:The basic components of a HVAC system are;
1. Power Transformers2. Transmission Cables3. Reactive Power Compensators (SVC)4. Auxiliaries (Control and Protection system etc)
1.1.1 POWER Transformer:To transmit electrical power from the generating unit to a consumer at far end, we need to transform the Voltage level for this purpose. Therefore we need 3-phase step up transformers for voltage conversion. Transformers allow the voltage level of a network to be increased to transfer large amounts of power over long distances, and then bring the voltage level back down to a level useable by consumers. Transformers commonly contain all 3 phases in the same enclosure, so there are 3 primary windings (one for each phase), and 3 secondary windings. The ratio between the number of turns on the primary and secondary windings determines the voltage conversion ratio.
1.1.1.1 Tap Changer:Most of the transformers are equipped with tap changers. Tap changer is a concept that we can change the transformer turns-ratio and hence secondary voltage according to the load conditions. A tap changer can increase or decrease the number of turns on one side of a transformer to change the voltage conversion ratio. The significance of this is that the voltage of the primary network can vary while the transformer regulates the voltage of the secondary network.
1.1.1.2 Step Up Transformer:A step up transformer is made to accept the large currents generated at medium voltage and transfer the power to the transmission system at a high voltage level, and relatively low current. Generator transformers typically have a large voltage ratio and have a delta connection on the generator side to limit the flow of zero sequence current. These transformers are installed on the Generating station output feeders.
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1.1.1.3 Step Down Transformer:Step down Transformers are inverse of step up transformers. There are two major categories of step down transformer
a. Supply Transformer: This steps the voltage down from transmission or sub-transmission to distribution voltage levels
b. Distribution Transformer: It steps the voltage down from distribution high voltage to low voltage
There are other transformers too used in AC transmission systems e.g. Current and Potential Transformers etc, but these transformers are mainly used for protection systems.
1.1.2 Reactive Power Compensation: Reactive power gives the power system the voltage support it needs to transfer real power from generation to loads.Reactive power support can be installed at any point on a network (although substations are commonly chosen) to help stabilise the surrounding network and ensure normal operation. Common types of reactive power support are listed below:
1.1.2.1 Capacitor bank: At times of large loading on a power system at points that are distant to generation, the voltage can drop to undesirable operational levels, and in extreme cases lead to unplanned load tripping. The net effect of installing capacitors is a boost in voltage, and helps alleviate voltage sag during heavy loading.This is caused by a decrease in the reactive power flow in the supply circuit, and a subsequent reduction of the voltage drop along the length of the circuit.
1.1.2.2 Shunt reactor: A shunt reactor can be required to prevent over-voltages from occurring at parts of the network, either during periods of light loading or when a supply point in the network rises with the export of large volumes of power.
1.1.2.3 Static Var Compensator (SVC):An SVC allows a variable amount of reactive power to be supplied or consumed which is consequently used to control the voltage of a point on the network to a desired level over a range of different operating scenarios. An SVC employs the use of heavy duty electronic switches, which switch many times a second to regulate the amount of reactive power produced or consumed.The following figure shows the over view of a HVAC system. In this figure the major components are shown including SVCs, Power Transformers, XLPE cables etc.
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1.1.2.4 Static VAR Compensator:Normally transmission lines are pretty expensive therefore it is important to load them to their capacity. Static VAR Compensation using a thyristor switching helps to provide VARs in a matter of milliseconds in response to transient event, thus providing damping, and helps hold terminal voltage constant by minimizing line losses due to improved power factor.
There are basically two configurations of Static VAR compensators;
a. Thyristor Controlled Shunt Reactance (TCR)b. Thyristor Switched Capacitor (TSC)
In the limit of minimum or maximum susceptance, SVC behaves like a fixed capacitor or an inductor. Choosing appropriate size is one of the important issues in SVC applications in voltage stability enhancement. The following figure shows SVC structure
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1.1.3 THYRISTOR CONTROLLED REACTOR:In this scheme, the capacitors are usually selected to provide the maximum reactive power needed at the point of installation. The required inductive power is dynamically controlled to maintain the desired voltage profile when the demand for reactive power is less than the maximum. The control is performed through phase angle variation. A typical scheme is shown in the following figure;
In this scheme we get a nearly unity power factor and continuous voltage regulation. At maximum leading VAR, the switch is open and the current in the reactor is zero.As the firing angle increases, the harmonic content increases. A 10MVAR unit typically consists of 10MVAR of capacitor bank and 10MVAR of reactor in addition to the thyristor controls. The variation of inductive, capacitive VARS with the system voltage is shown in Figure below.
1.1.4 THYRISTOR CONTROLLED REACTOR AND THYRISTOR SWITCHED CAPACITOR:In this scheme it contains several capacitor sections operating in parallel with a phase controlled reactor. The task of controlling both the reactor and capacitor requires an electronic controller. The overall efficiency of the scheme is less due to losses in the reactor. The number of capacitor branches depends on the amount of kVAR, thyristor ratings, etc. The variation of reactive power due to a two-switched capacitor and one reactor per phase
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is shown in Figure a. For a 10 MVAR SVC, the capacitor bank will be two 5 MVAR (or in some other combination) and the reactor will be 10 MVAR. A typical VAR demand versus VAR output profile of a scheme is shown in Figure b.
Fig. a TCR with TSC Fig. b VAR output Vs VAR demand
1.1.5 Thyristor Switched Capacitor (TSC):In this case, all the required capacitors are switched in and out using SCRs. To reduce the number of capacitors, binary grouping is sometimes employed. This scheme is suitable for both balanced and unbalanced loads. A typical scheme is shown in figure below. For a 10MVAR size, bank 1 is 4 MVAR, bank 2 is 3 MVAR, and bank 3 is 3 MVAR. Some designs use binary-based steps. In this scheme, only the capacitor banks are used.
With this scheme, the reactive compensation is corrected on a cycle-by-cycle basis. Each phase is compensated independently and correction to unbalance is made.
1.2 TRANSMISSION CABLES:IN HVAC, cables play a vital role. They are indeed the back bone of transmission system. There are following 3 kinds of HVAC cables employed;Mass Impregnated Cable
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Low Pressure Filled Cable andCross Link Polyethylene CableIn these days XLPE is given more importance than the rest and major AC transmission systems are utilizing this cable vastly since 1940s.
1.2.1 CROSS LINK POLYETHYLENE CABLE (XLPE):It has high electric strength and poor dielectric properties but it is easy to process and low cost therefore it is the perfect client for power cable. However the low melting point of polyethylene, around 110°C, limited the maximum current rating, overload and short-circuits temperatures. But cross-linking the polyethylene, using electron beam processing, permits to increase the thermal properties of the polyethylene. The cross-linked polyethylene is called XLPE. New thermal properties allow conductor temperature around 90°C with emergency capacities up to 140°C.
Underground and Sea Cables (XLPE)
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A XLPE also has excellent dielectric properties for insulation applications. A XLPE cable has a similar design than a Mass Impregnated cable. Due to the XLPE intrinsic properties, Polymeric Insulating cables are lighter than Paper Insulating cables. Furthermore, the very good insulating properties of this material lead to less insulation material, so smaller dimensions of the cable. In DC applications, the breakdown strength of the cable is around 75kV/mm with a working stress of 25kV/mm whereas, for a paper insulated cable, the breakdown strength is around 40kV/mm with a working stress of 10 to 15kV/mm. Another advantage of this kind of cable is the mechanical properties of the XLPE. The bending capabilities and the mechanical resistance are higher than other cables, which permits an easier installation and less maintenance. Likewise the absence of oil circulation requires fewer joints along the cable and no risk of contamination for the environment. XLPE insulating cable can handle lower temperatures than paper insulating cable, and so can be used in cold conditions, for example in cold waters, like in Norway.
1.3 Auxiliaries:The auxiliary components in an AC transmission system include Towers, Protection systems and control. In protection systems there are alot of equipments like Differential Relay, Impedance Relay and many other devices. The control of Ac transmission is mainly done via SCADA (supervisory and data acquisition system) which employs different monitory or instrumentation devices and computer control units.
1.4 Flexible AC transmission systems (FACTS):
Flexible AC transmissions systems are actually a revolution in AC transmission and involve electronic equipments. The first FACTS installation was at the C. J. Slatt Substation in Northern Oregon. This is a 500 kV, 3-phase 60 Hz substation. FACTS insure to have lower transmission losses than usual AC transmission systems. There are basically two types of power that have to be dealt with in power transmission viz. the active power and the reactive power. The active power is said to be “useful power”, the power that home appliances, lighting etc use. But reactive power is a necessary evil. It does no useful work but is essential for transmission of active power through the lines. It is by proper matching up of these two quantities FACTS system allows for higher power transfers. By use of sophisticated solid state devices, FACTS system is able to add or remove precise amount of reactive power from the line by using Static VAR compensators etc. With the aid of FACTS the transmission line capacity has increased and more power can be transferred by the old transmission lines.
2 HVDC transmission system:HVDC was first used commercially 50 years ago. Since then a growing number of transmission schemes have been constructed around the world. HVDC differs from high voltage alternating current HVAC that the voltage is not alternating 50 or 60 cycles per second but is constant. The advantage of HVDC is that long distance transmission is more efficient as there is no need to charge the capacitance of a transmission line with the alternating voltage. The drawback of HVDC is that one needs more expensive terminals at the line ends. The HVDC converter station is made up of a number of equipment known
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from ac transmission schemes. It also contains some special features of which the most important one is the converter valves. In the beginning the converters were equipped with mercury arc valves. Later Thyristor and IGBT valves have been developed and have made the design of HVDC more flexible and also increased the power that is possible to transfer.The basic structural layout of standard HVDC system is shown in the figure below and is briefly discussed.
2.1 Converter Station:Converter stations are situated at each end of DC transmission system and may consist of the following components;
1. Thyristor or VSC valve:Thyrisor valves are used to convert AC to DC and reverse (Inverter). Normally 12-pulse grouped to form quadruple valves. Each thyrisor valve consists of cascaded thyristors. Similarly VSC valves are built using IGBTs in multi-level manner.
2. Transformers:The converter transformers are used to couple AC and DC sides and it is single phase three winding transformer usually.
3. Reactive Power Compensators and Filters:On the AC side of a 12-pulse HVDC converter, current harmonics of the order of 11, 13, 23, 25 and higher are generated. Filters are installed in order to limit the amount of harmonics to the level required by the network. Capacitor banks, STATCOM or SVCs are used to compensate reactive power because HVDC can’t transmit reactive power. DC filters are used to minimize dc disturbances.
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2.2 Transmission Medium:The transmission medium for bulk power transfer over land is over-head lines. Normally these lines are bipolar (2 conductors with opposite polarity). Most economic cable used in HVDC is Impregnated Mass (IM) but XLPE (Polymeric) is extensively used because if its good insulation.
2.3 Advantages of HVDC Systems
It is important to remark that HVDC system not only transmit electrical power from one point to another, but it also has a lot of value added which should have been necessary to solve by another means in the case of using a conventional AC transmission.Some of these aspects are:
1. No limits in transmitted distance. This is valid for both OH lines and sea or underground cables.
2. Very fast control of power flow, which implies stability improvements, not only for the HVDC link but also for the surrounding AC system.
3. Direction of power flow can be changed very quickly (bi-directional).4. HVDC can carry more power for a given size of conductor5. The need for ROW (Right Of Way) is much smaller for HVDC than for HVAC, for the
same transmitted power. The environmental impact is smaller with HVDC.6. VSC technology allows controlling active and reactive power independently without
any need for extra compensating equipment (STATCOM or SVC).
3 LCC Composition: (HVDC Light or HVDC Classic)LCC HVDC is the oldest of this technology and uses Line Commutated Current source converter (Thyristors).
3.1 Overview and Basic Components of LCC HVDC:LCC consists of HVDC components utilising Power Electronics equipments and basic building blocks of LCC are mentioned in the followings;
1. Converter Transformer2. Smoothing reactor3. Static Synchronous Compensators (STATCOM) or Capacitors4. AC-DC Converters (Consisting of Thyristor Valves)5. AC and DC Filters for removing harmonics6. Protection and Control devices7. DC Cable and return Path
3.1.1 Smoothing Reactors:In every HVDC there are some inevitable harmonics produced by the converters and other auxiliaries in the system that need to be reduced. These harmonics are of higher order (12th,
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24th, 36th) and need AC and DC filters along with some large inductances called ‘Smoothing Reactors’. In HVDC different Smoothing Reactors are used for example
a. Oil insulated Reactor (nominal 360mH)b. Air Insulated Reactor (nominal 180mH)
Oil Insulated Reactor in New Dehli
The dc reactor contributes to the smoothing of the dc current and provides harmonic voltage reduction in the dc line. The dc reactor also contributes to the limitation of the crest current during a short-circuit fault on the dc line. It should be noted that the inductance of the converter transformer also contributes significantly to these functions.Reason of using Reactors:
1. Prevention of intermitted currents2. Limitation of DC fault currents3. Prevention of DC resonance in circuit4. Reducing harmonic currents and telephone interferences5. Reduce level of Voltage and current harmonics on a DC link and transfer of non-
harmonic frequencies between two interconnected AC systems
3.1.2 Converter Transformer:The HVDC (high voltage direct current) converter transformer is a key component in an HVDC transmission system. In addition to its normal application to provide transfer of power between two voltage levels, it serves a number of additional functions like galvanic separation between the AC and DC systems. A fairly large tapping range permits optimum operation also for a large variation in load without loss of efficiency. A converter transformer is placed on the core location to link the AC network with the valve bridge. Owing to expensive component cost and complicated manufacture technology, the converter transformer is one of most important components.
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Usually, modern HVDC systems employ the configuration of one 12pulse converter for each pole. A converter transformer provides 30º phase shift between two 6pulse converters to obtain the configuration of 12pulse converter; if the short circuit occurs on the valve arm or DC busbar, the impedance of converter transformer can restrict the fault current, in order to protect converter valve.A converter transformer employs single phase arrangement or three phase arrangement. Therefore, for a 12pulse converter, the standard configurations of converter transformer banks can be: six single phase two winding transformers; three single phase three winding transformers; two three phase two winding transformers and one three phase three winding transformer.
Types of Converter Transformer
3.1.3 CONVERTER STATION:The layout for Converter Station is shown below and its components are also discussed.
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3.1.4 CONVERTER FOR LCC:In Direct Current transmission system (HVDC), generated power is Alternating (i.e. AC) and to transmit this AC power over a DC link needs rectification. Rectification in old times was done by Mercury Arc Rectifiers (for High Voltages) and they were the primary method of rectification before the advent of Power Electronics equipments like SCRs, GTOs and IGBTs.In case of Line Commutated Converter HVDC systems rectification is done by using Thyristors (SCR). But for High Voltage conditions Thyristors need to withstand high currents. Therefore Thyristors are cascaded to form a Bulk entity called ‘VALVE’.
3.1.5 Thyristor Valve:An SCR (Silicon Controlled Rectifier) or Thyristor is a four layer electronic device used for controlled switching. It is a PNPN junction silicon device with 3 terminals Anode, Cathode and Gate. When there is a current pulse on Gate, SCR will start conduction otherwise remain in non conducting mode. Following figure shows its detail
3.1.6 12-Pulse Converter:In HVDC we normally utilize 12-pulse converter using two 6-pulse bridges connected in series on the DC end. The firing delay angles (α) for the Thyristors are controlled by a Controller. In case of Rectifier action, firing angle (α) is less than 90 degrees while for inversion operation its more than 90 degrees. Following single line diagram depicts the connection of converter to the ‘Converter Transformer’.
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12-pulse arrangement is helpful in reducing the large harmonic components in the DC circuit. Normally the Thyristor Valves are suspended to the ceiling and are water cooled. The following figure shows their layout and physical dimensions,
The Thyristors are triggered by electrical gate pulses generated in a small electronic thyristor control unit (TCU) located near each thyristor. All these equipments are kept in-door.
3.1.7 STATCOM:STATCOM or Static Synchronous Compensator is a shunt device of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow and improve transient stability on power grids. The STATCOM regulates voltage at its terminal by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage is low, the STATCOM generates reactive power (STATCOM capacitive). When system voltage is high, it absorbs reactive power (STATCOM inductive). STATCOM has no long term energy support in the DC Side and cannot exchange real power with the ac system; however it can exchange reactive power.
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Where, V1 is Line to Line voltage of source 1V2 is Line to Line voltage of source 2X is Reactance of interconnection transformer and filtersδ is Phase angle of V1 with respect to V2
The change in reactive power (Q) is performed by means of a Voltage-Sourced Converter (VSC) connected on the secondary side of a coupling transformer. The VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage V2 from a DC voltage source.
In normal operation, voltage V2 generated by the VSC is in phase with V1 (δ=0), so that only reactive power (Q) is flowing only. If V2 is lower than V1, Q is flowing from V1 to V2 (STATCOM is absorbing reactive power). On the reverse, if V2 is higher than V1, Q is flowing from V2 to V1 (STATCOM is generating reactive power). The amount of reactive power is given by
Q = (V1 (V1 – V2)) / X
3.1.8 STATCOM V-I curve:
As it was mentioned in above paragraph that STATCOM can absorb as well as generate Reactive power (Q), the V-I characteristic curve can be drawn to show these properties of STATCOM,
STATCOM is a better option than a SVC because of its quicker response time. STATCOM can control Active Power where as SVC couldn’t and STATCOMs require lesser space than SVCs (Big Capacitors).
3.1.9 AC AND DC FILTERS (Harmonics Elimination):Since the commutation reactance is low in relation to the DC smoothing reactance, an HVDC converter acts as a source of harmonic currents (AC point of view) and as a source of harmonic voltage in DC point of view these harmonics are of higher order (i.e.11th,24th,36th etc). We need to reduce or minimize the impact of harmonics by utilising AC and DC filters.
3.1.10 AC Filters:Converter operation generates harmonic currents and voltages on the ac and dc sides, respectively. On the ac side, a converter with a pulse number of p generates characteristic harmonics having the order of np±1 (n=1, 2, 3,). AC filters are installed to absorb those harmonic components and to reduce voltage distortion below a required threshold. Tuned filters and high pass filters are used as ac filters. These filters are divided into following types an there combination with resistances and impedances gives very low resistive path to the harmonic components for elimination.
Reference: www.abb.com
3.1.11 DC Filters:On the Dc side of HVDC converters we need DC filters. Usually a DC filter is connected between the pole busbar and the neutral busbar. The structure of passive DC filter is similar to that of AC filter, such as single tuned, double tuned and triple tuned circuits with or without high pass characteristic. A capacitor is installed between the neutral busbar and ground, thereby providing low impedance path for harmonic currents of order 3n (i.e., 3rd, 6th, 9th, etc).
3.2 Protection and Control:The control system for HVDC has multiple functions and it comprises of the following few topics
1. Converter Control2. DC System Control3. Tap Changer Control4. Power Flow and Frequency Control
Similarly there is a protection system for overcoming AC system faults, DC line faults, over current and converter disturbances protections. There are individual protection modules for valve-group and filters too. All these protection and control modules are highly sophisticated electronic devices.
3.3 CABLE and Return Path:In case of offshore wind farms we need a transmission medium for efficient transfer of power to onshore grid. For this purpose cables are submerged in the sea bed. There are different types of cables used for HVDC transmission;
3.3.1 XLPE:All kinds of cables have the same composition. There are three major parts in a cable:- The core.- The semiconductor screen.- The insulation.The core of a cable is the part which carries the current. It is composed of threaded wires which forming a circular section. This part of the cable is usually made with copper, even if construction with aluminium is possible. Aluminium has advantages that it is lighter and cheaper than copper. Nevertheless, this material has not as good conduction and thermal proprieties as the copper.This leads to higher losses in the cable. A core made of aluminium requires a bigger cross-section area than copper to conduct the same amount of current. Furthermore, aluminium corrodes more easily than copper in presence of water, which is a problem for undersea applications. Typically, cables with core made of aluminium fit more with small voltage applications onshore whereas cables with copper core fit better in medium and high voltage applications, and particularly in offshore conditions. For example the 600MW cable use in the Swe-Pol Link has a copper cross-section of 2100mm2 for a voltage of 450kV. The figure below shows its construction.
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4 CONFIGURATION of HVDC:HVDC systems can be configured in different ways. Some of the commonly used configurations are listed below
a. MONOPOLARb. BIPOLARc. BACK TO BACK
4.1 Monopolar:Monopolar HVDC systems have either ground return or metallic return.
4.1.1 Monopolar HVDC System with Ground Return:It consists of one or more six-pulse converter units in series or parallel at each end, a single conductor and return through the earth or sea, as shown in the followings. It can be a cost-effective solution for a HVDC cable transmission and/ or the first stage of a bipolar scheme. At each end of the line, it requires an electrode line and a ground or sea electrode built for continuous operation.
4.1.2 Monopolar HVDC System with Metallic Return:It usually consists of one high-voltage and one medium voltage conductor as shown in followings. A monopolar configuration is used either as the first stage of a bipolar scheme, avoiding ground currents, or when construction of electrode lines and ground electrodes results in an uneconomical solution due to a short distance or high value of earth resistivity.
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4.1.3 Bipolar HVDC Systems:A Bipolar HVDC System consists of two poles, each of which includes one or more twelve-pulse converter units, in series or parallel. There are two conductors, one with positive and the other with negative polarity to ground for power flow in one direction. For power flow in the other direction, the two conductors reverse their polarities. A Bipole system is a combination of two monopolar schemes with ground return, as shown in following figure. With both poles in operation, the imbalance current flow in the ground path can be held to a very low value.
This is a very common arrangement with the following operational capabilities:1. During an outage of one pole, the other could be operated continuously with ground
return.2. For a pole outage, in case long-term ground current flow is undesirable, the bipolar
system could be operated in monopolar metallic return mode, if appropriate DC arrangements are provided, as shown in figure below. Transfer of the current to the metallic path and back without interruption requires a Metallic Return Transfer Breaker (MRT B) and other special-purpose switchgear in the ground path of one terminal. When a short interruption of power flow is permitted, such a breaker is not necessary.
3. During maintenance of ground electrodes or electrode lines, operation is possible with connection of neutrals to the grounding grid of the terminals, with the imbalance current between the two poles held to a very low value.
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4. When one pole cannot be operated with full load current, the two poles of the bipolar scheme could be operated with different currents, as long as both ground electrodes are connected.
5. In case of partial damage to DC line insulation, one or both poles could be continuously operated at reduced voltage.
6. In place of ground return, a third conductor can be added end-to-end. This conductor carries unbalanced currents during bipolar operation and serves as the return path when a pole is out of service.
4.1.4 Back-to-Back HVDC Links:Back-to-back HVDC links are special cases of monopolar HVDC interconnections, where there is no DC transmission line and both converters (Rectifier and Inverter) are located at the same site. For economic reasons each converter is usually a twelve-pulse converter unit and the valves for both converters may be located in one valve hall. The control system, cooling equipment and auxiliary system may be integrated into configurations common to the two converters. DC filters are not required, nor are electrodes or electrode lines, the neutral connection being made within the valve hall. It is important to note that AREVA T&D has developed a solution for a back-to-back HVDC link which does not require a smoothing reactor; hence, there is no external DC insulation. Following Figure shows two different circuit configurations used by AREVA T&D for back-to-back HVDC links.
Back to Back configuration
Back to back configuration let us couple two Asynchronous systems (e.g. two grids with different frequency).
5 HVDC with Voltage Source Converters (HVDC Plus):HVDC with Voltage Source Converter controller is a new concept in high voltage dc transmission system. VSC-HVDC system which employs fast-switching devices and PWM techniques, offers a number of advantages over traditional transmission systems, such as no need of external voltage source for commutation, rapid regulation of reactive and active power, feeding weak AC systems or even passive loads, providing high quality power, etc. The first power transmission system based on this technology was built in 1997 by ABB between Hellsjön and Grängesberg in Sweden. ‘ABB’ call it HVDC PLUS. The extension 'plus'
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stands for Power Link Universal Systems and represents economical solutions to the most challenging requirements on power transmission and distribution. Some important highlights are:
1. Feeding AC systems with low short circuit power or even passive networks with no local power generation.
2. STATCOM functionality, i.e. continuously adjustable reactive power support to the AC system to control AC bus voltage and improve system stability. Active and reactive power exchange can be controlled independently from each other within the total power rating of a station.
5.1 Components of HVDC Plus:The principle scheme of a VSC system converter station is shown in Figure below with bipolar configuration scheme.
The operational range for VSC HVDC is shown in the figure below. It gives a relationship between reactive and active power ranges for VSC HVDC. The active power and reactive power ranges are +1 to -1 pu.
A VSC-HVDC transmission system is composed of two main parts, the converters, at both ends of the system, and the cable between them. The main components of these two parts are similar to the LCC-HVDC system, with some differences:
1. AC and DC side harmonic filters2. Converter transformers3. Converters based on IGBT valves
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4. Phase reactors5. Protection and control devices
Figure below gives an overview of the VSC system.
5.2 CONVERTERS:VSC-HVDC converters use Insulator-Gate Bipolar Transistors (IGBT) as switching device.IGBT is a recent technology and is in developing stage.Due to the process of production similar to printed circuit, IGBTs have a small size (around1cm2). Thus many IGBTs connected in parallel, gives to IGBT modules the capability to handle current up to 2.4kA with blocking voltage up to 6.5kV.
5.3 IGBT Valve:When different IGBTs are packed together to form a Bulk, its named IGBT Valve.
Insulated Gate Bipolar Transistor
An IGBT can be compared to a switch, its conduction and blocking states are commanded by the gate. When the voltage between the gate and the emitter (VGE) is inferior to threshold tension (VGE (TH)), the IGBT is in blocking state. Thus the voltage between the collector and the emitter (VCE) is positive and the current flowing in the IGBT (ICE) is null. If VGE become superior to VGE (TH), then the IGBT goes to the conductive mode and ICE flows through it. This capability of control permits to switch at high frequency which allows the use of Pulse Width Modulation (PWM). High frequency switching also reduces harmonics and thus the number of filters used. A project like Tjæreborg in Denmark is designed with a switching frequency of 1950Hz. As for the thyristors, the blocking voltage of an IGBT module does not allow the operation within high-voltage levels. To avoid this problem, many modules are connected in series in valves
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Equivalent Circuit
A basic VSC-HVDC system comprises of two converter stations built with VSC topologies. The simplest topology is the conventional two-level three-phase bridge.Typically, many series connected IGBTs are used for each semiconductor in order to deliver a higher V blocking voltage capability for the converter and therefore increase the DC bus voltage level of the HVDC system. It should be noted that an anti parallel diode is also needed in order to ensure the four-quadrant operation of the converter. The DC bus capacitor provides the required storage of the energy so that the power flow can be controlled and offers filtering for the harmonics.
The converter is typically controlled through sinusoidal PWM (SPWM) and the harmonics are directly associated with the switching frequency of each converter leg. Following figure presents the basic waveforms associated with SPWM and the line-to-neutral voltage waveform of the two-level converter. Each phase-leg of the converter is connected through reactor to the AC system. Filters are also included on the AC side to further reduce the harmonic content flowing into the AC system.
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Following diagram shows the production of PMW by using a comparator and Triangular wave generator.
Simplified diagram of VSC HVDC
5.3.1 Shunt Connected VSCIn this case, the VSC is connected to the power system via a shunt connected transformer, as in the STATCOM configuration of Figure 1. By varying the amplitude and the phase of the output voltages produced, the active power and the reactive power exchange between the converter and the a.c. system can be controlled in a manner similar to that of a rotating synchronous machine. The reactive power exchange between the VSC and the power system can be controlled by varying the amplitude of the output voltage. If the amplitude of the output voltage is increased above that of the ac system voltage, the VSC generates reactive power to the power system. If the amplitude of the output voltage is decreased below that of the ac system voltage, the VSC absorbs reactive power from the power system. The real power exchange between the VSC and altering the phase angle between the output voltage and the ac system voltage can control the power system. If the output voltage is made to lead the ac system voltage, the VSC supplies real power to the ac power system. If the output voltage is made to lag the ac system voltage, the VSC absorbs real
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power from the ac power system. An energy supply or absorb device is required for the real power exchange. This role is played by another VSC or dc energy storage device like a super-conducting magnet or a battery. The exchange of real and reactive power is implemented individually. The product of the power system voltage and the maximum output current determines the VA rating of the VSC.
5.3.2 Series Connected VSCIn this case, the VSC is connected to the power system in series via a series connected transformer, as in the SSSC configuration of Figure 2. By varying the amplitude and the phase of the output voltages produced, the magnitude and the angle of the injected voltage can be controlled. The VSC output voltage injected in series with the line acts as an ac voltage source. The current flowing through the VSC corresponds to the line current. The VA rating of the VSC is termined by the product of the maximum injected voltage and the maximum line current. If the injected voltage is controlled with a quadrature relationship to the line current, the VSC provides only reactive power to the ac power system and there is no need for another VSC for energy storage device on the dc terminal. If the injected voltage is controlled in a four-quadrant manner (360 deg.) to the line current, the VSC provides both real power and reactive power to the ac power system and another VSC or energy storage device is needed for the real power exchange on the dc terminal.
5.3.3 Transformer:As it can be observed in Figure above, the transformers are used to interconnect the VSC with the AC network. The main function of the transformers is to adapt the voltage level of the AC network to a voltage level suitable to the converter. This voltage level can be controlled using a tap changer, which will maximize the reactive power flow.
5.3.4 Phase Reactor:The phase reactors, known also as converter reactors, are used to continuously control the active and reactive power flow. The phase reactors have three main functions:
1. The first one is to provide low-pass filtering of the PWM pattern in order to provide the desired fundamental frequency voltage;
2. the second function is to provide active and reactive power control; the active and reactive power flow between the AC and the DC side is defined by the fundamental frequency voltage across the reactors;
3. The last function is to limit the short-circuit currents.
5.3.5 DC Link Capacitors:On the DC side, there are two capacitor stacks of the same power rating. The main goal of the DC-link capacitor is to provide a low-inductance path for the turned-off current. Moreover, the DC capacitor serves as an energy store and it reduces the harmonics ripple on the DC voltage. Depending on the size of the DC side capacitor, DC voltage variations caused by disturbances in the system (e.g. AC faults) can be limited.
5.3.6 Control of VSC HVDC:VSC-HVDC highlights its capability of four-quadrant operation with no specific interaction between active and reactive power control. Usually, the VSC have four operation modes:
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a) Constant DC voltage controlb) Constant active power control c) Constant DC current control d) Constant AC voltage control
VSC-HVDC features the feasibility of supplying power to either active or passive networks. When supplying power to a passive network, the rectifier takes the onus to control and stabilize the DC voltage. The inverter, on the other hand, is used to ensure the stability of the output voltage of the converter transformer. DC voltage controlling and reactive power controlling are two basic control goals for the rectifier. The principle diagram of the controller is illustrated in Figure below.
Rectifier DC voltage control diagram
Inverter’s AC voltage control principle is derived from the method of space vector to provide stable RMS values of the converter transformer output voltage. The method can be detailed with the example of three-phase voltages as follows.Define the space vector of the three-phase voltages as
The three-phase voltages of the converter transformer voltage are expressed as
Where Us2 is the RMS value of line-to-line output voltage of the converter transformer, vs2a, vs2b, vs2c are the instantaneous values of abc three phase voltages, respectively.
5.3.7 ADVANTAGES OF VSC HVDC:a. One of the main advantages of VSC-HVDC technology is that the controls of active
power and reactive power are independent. The system can operate in the four quadrants of the PQ-plan. In a LCC-HVDC system, a limited control of reactive power can be achieved but it requires additional expensive equipments. The use of PWM in VSC-HVDC systems permits to achieve any amplitude and phase angle, and so the control of active and reactive power can be made independently without additional equipments. The AC voltage in the network can be controlled while the transmitted active power is constant. The reactive power balance can be achieved by using the reactive power generated or consumed by the converters.
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b. Another good point for VSC-HVDC technology is that it is independent of the AC network. The converters can create an AC voltage at any frequency, and without the presence of generator in the network. Thus passive networks can be connected with VSC-HVDC technology.
c. There is no risk of commutation failure of converters.d. It can operate in null powers even because active and reactive powers are
controllable within certain range.e. VSC HVDC needs no STATCOM, Harmonic filters etc. Its design is simple and need
less space.
6 COMPARISON:In the following paragraphs there has been produced a comparison between all the three transmission systems with regards to different aspects:
6.1 POWER and VOLTAGE Limitations:As we can see from the following figure, it is clear that HVDC with voltage source converters (HVDC Light) technology can deliver high voltages with higher rated powers unlike LCC (Classic) and AC transmission networks which have limitations.
Cables needed for Transmission:
The table below gives a detailed overview of cable requirement for different transmission techniques used;
Wind Farm Capacity (MW)
(100km Distance)High Voltage AC
High Voltage DC
LCC
High Voltage DC
VSC
Bipolar Monopolar
300 2 1+1 1 1+1
500 3 2+2 1 2+2
900 5 4+4 2 3+3
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1200 6 5+5 2 4+4
In case of monopolar scheme LCC needs lesser number of cables than HVAC, but in bipolar mode more cables are needed. Normally for higher power transmission systems HVDC are preferred over HVAC because of cable numbers. Apart from this fact, AC lines are normally 3-phase, hence for a single DC line there are 3 lines for a network.
6.2 TRANSMISSION CAPACITY:The following graph depicts the relationship between distance and transmission capacity. If an old system needs up gradation then this graph maybe helpful to decide the system of transmission. For higher transmission capacities (i.e.900MW) HVDC Classic (LCC) is preferred while for 600-900MW both HVDC systems are interchangeable. From this graph, it is clear that HVAC is not a good option for long length transmission system and hence its bound only to a certain narrow distance of 100Km, this is because of losses in long transmission network. Currently it has been stated that HVDC-LCC systems can be used for as high as 7000MW capacity systems while HVDC-VSC are striving their capacity rating which is 400MW.
Transmission distance limitation:
In case of HVAC transmission system there is always a check over the length of transmission line and cable used for this purpose. Inductive and capacitive elements on the cable, which are proportional to the length, lead to charging current in the cable. These last, as well as useful currents are quarried in the cable and the transmission capabilities of the cable are reduced. The creation of large amount of reactive power is also an important limiting factor. This implies large and expensive compensation system at the ends of the cable and losses increase consequently. Furthermore, the number of cables is more important with HVAC system as the connection should be done with three phases.The following figure indicates the transmission line distance with its capacity as well as the compensation of reactive power at sending and receiving end.
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In case of HVDC there is no limit over transmission line length because there is no existence of capacitance ad inductance with virtue of Direct Current. The only source of loss is resistance which remains almost constant.
6.3 REACTIVE POWER COMPENSATION: The long HVAC overhead lines produce and consume the reactive power, which is a serious problem. If the transmission line has a series inductance L and shunt capacitance C per unit of length and operating voltage V and current I, the reactive power produced by the line is
QC=ωCV2
And consumer reactive power isQL=ωLI2 per unit length, if QC=QL
V/I= [L/C] 1/2=Zs=surge impedance of linePower of the line is Pn=VI=V2/Zs and is called Natural Load. So the power carried by the line depends on the operating voltage and the surge impedance of the line.The power flow in an AC system and the power transfer in a transmission line can be expressed
E1 and E2 are the two terminal voltages, δ is the phase difference of these voltages, and X is the series reactance. In case of AC transmission system, we use STATCOM and SVC (synchronous VAR Compensators) to compensate for reactive power needed by power converters; similarly STATCOMs are also used for LCC DC transmissions. But for VSC HVDC systems we don’t need to use these compensation tools.
6.4 ENERGY TRANSMISSION COST:This cost includes cost of components used in the system as well as the maintenance cost. The data for different capacity Wind Farms depicted in the following graphs clearly differentiate between HVAC and HVDC and shows that the energy cost is lower for long distance transmission in case of HVDC-LCC and highest in HVAC case. Following data shows 600 and 900Mwatt systems with average wind speeds of 11m/s.
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Energy Transmission Cost Vs. Distance
7 Environmental Effects:There are some environmental issues which must be considered for the converter stations, such as: audible noise, visual impact, electromagnetic compatibility and use of ground or sea return path in monopolar operation. In general, it can be said that a HVDC system is highly compatible with any environment and can be integrated into it without the need to compromise on any environmentally important issues of today.The possible influences on the environment caused by High Power Electricity Transmission Systems can include: • The effects of electric fields• The effects of magnetic fields• Radio interference • Audible noise • Ground currents and corrosion effects• The Right of Way • Visual impactsAll these issues are discussed individually in the followings;
7.1 EFFECT OF ELECTRIC FIELD:
It is well known that the electric fields produced by a HVDC transmission line are the combination of the electrostatic field created due to the line voltage and the space charge field that is due to the charge produced by the line’s corona. This means that presence of a charge between the conductors and the ground has an impact on the total electric field produced by the DC line. There were alot of researches done in this field and it is concluded that the HVAC has much discharge streaming currents than HVDC. Using the experimental line section, measurements of the ionic current streaming through a human standing under a HVDC line at voltage level of ±1000 kV (kilo-Volts), and of the capacitive current under a HVAC line at a voltage 1150 kV were performed. These experiments indicated that the difference in current between the two technologies was approximately 100-fold (2-3 μА for the HVDC line, versus 0.2 mA for the HVAC line).
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7.2 EFFECT OF MAGNETIC FIELD:
According to various estimates, the limit on the magnetic field strength of an AC power transmission system varies from 10 to 50 μТ (micro Tesla). The magnetic fields associated with DC lines produce no perceivable effects. The DC lines’ magnetic field is in the same range of strength as that of the Earth's natural magnetic field.
7.2.1 Radio Interference:The radio interference caused by electric power transmission lines is the result of the corona discharge around conductors, which is generated only at positive voltages. As a result, on a HVDC line radio interference is generated only by positive pole conductors, whereas with a HVAC transmission line radio interference is generated by all of the three AC phases. The radio interference of HVDC is usually about 6-8db lower than HVAC.
7.2.2 AUDIBLE NOISE:
Audible noise is one of the important design parameters for both overhead lines and substations. All known measures to decrease audible noise from these sources are quite costly. The main source of audible noise in DC transmission is ‘Converter Transformer’. Noise levels from a DC line will usually decrease during foul weather, unlike the noise levels on AC lines. As a rule, the audible noise from transmission lines should not exceed, in residential areas, 50 dB during the day, or 40 dB at night.
7.3 GROUND CURRENTS AND CORROSION EFFECTS:
For cable monopole HVDC transmission systems, current return is performed through the ground. In the case of an overhead line operating after an emergency outage on one pole, it is possible to use the wire of an emergency pole as the return circuit. Even in this case, however, it is necessary to provide the opportunity for the current to pass though the earth for some time. In several cases a special additional conductor, which normally serves as the lightning guard for the line, has been used to enable monopole operation of a HVDC overhead transmission line. When the “metallic return” is utilized, HVDC power transmission does not introduce any additional environmental impact in comparison with HVAC transmission lines.
If there are pipelines or other underground metal objects near the grounding installation, it is recommended that additional cathodic protection of such objects be provided to allow prevent rapid corrosion.
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7.4 Right of Way Cost:Improved energy transmission possibilities contribute to a more efficient utilization of existing power plants. The land coverage and the associated right-of-way cost for a HVDC overhead transmission line is not as high as for an AC line.This reduces the visual impact. It is also possible to increase the power transmission capacity for existing rights of way.
7.5 VISUAL IMPACTS:When transmission lines cross populated areas and especially national parks, resorts and other territories where conservation of the natural landscape is important, special demands are placed on the transmission line’s dimensions. For example, it is sometimes necessary to limit the height of the towers with the height of the trees in woodland, so that the transmission line itself is largely obscured. Special demands are then placed on the aesthetics of the design of the line.HVDC overhead transmission lines offer several advantages from the point of view of visual impact relative to HVAC lines of the same capacity. Bipolar HVDC transmission lines have two conductors and already because of that it is simpler in design in comparison with the three-phase structure of a HVAC line. HVDC lines require shorter tower heights in comparison with HVAC lines of equal capacity and comparable voltage levels.
7.6 Transmission Losses:Current and voltage limits are the two important factors of the high voltage transmission line. The AC resistance of a conductor is higher than its DC resistance because of skin effect, and eventually loss is higher for AC transmission. The switching surges are the serious transient over voltages for the high voltage transmission line, in the case of AC transmission the peak values are two or three times normal crest voltage but for DC transmission it is 1.7 times normal voltage.
7.7 CORONA Discharge LOSSES:Corona effects on the surface of high voltage overhead power transmission lines are the principal source of radiated noise. The ion and corona effects on the DC transmission lines lead to a small contribution of ozone production. The natural concentration of ozone in the clean air is approximately 50 ppb (parts per billion) and in the city area this value may reach 150 ppb. The limiting values for persons risk is around 180-200 ppb. The HVDC overhead transmission line produces 10 ppb as compared with naturally occurring concentration. The losses due to corona effect are lower in HVDC case than HVAC transmission system. The higher the transmission voltage for HVAC, significant is the corona discharge.
8 CONCLUSION:The HVDC transmissions can be compared with the HVAC transmission basically from two points of view: the transmission costs point of view and the technical point of view respectively.Analyzing the two systems regarding the transmission costs, the next advantages of HVDC transmission systems over the HVAC transmission systems can be found
1. considering similar insulating requirements for peak voltage levels, a DC line/cable will carry the same amount of power with two conductors as an AC line/cable with three
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conductors therefore for the same power level, an HVDC transmission system will require smaller Right-of-Way, simpler towers and also the conductor and insulator costs will be reduced, in comparison with a classical HVAC transmission
2. the power transmission losses (conductor losses) are reduced by about two-thirds when the DC option is used instead of the AC one
3. furthermore, when a HVDC transmission is used, the absence of the skin effect can be noticed and also the dielectric and corona losses are kept at low level, thus the efficiency of the transmission is increased
4. However, the disadvantage of the HVDC transmissions regarding the costs comes from use of the converters and filters
As a conclusion it can be said that the HVAC transmissions are more economical than HVDC transmissions when used for small distances. Once the breakeven distance is reached the DC alternative becomes more economical fact which may be observed from Figure below.
In the case of the overhead lines the breakeven distance can vary between 400 to 700 km depending on the per unit line costs while, if a cable system is used the breakeven distance vary between 25 and 50 km. The typical breakeven distance for overhead lines is 500 km.Analyzing the two transmission systems, from the technical point of view, the HVDC transmissions overcome some of the problems which are usually associated with the AC transmissions. Thus, the stability limits are overcome when an HVDC transmission is used due to the fact that the power carrying ability of DC lines is not affected by the transmission distance. In the case of the HVAC transmission the power transfer in the AC lines is dependent on the phase angle which increases with the distance and thus the power transfer is limited.The second problem which is solved by using the DC transmission instead of the AC transmission is the line charging. In the case of an HVAC transmission, line compensation (using STATCOMs, SVCs etc) is used in order to solve the line charging issue, while in the case of DC lines such compensation is not required. Due to this issue, in the case of HVAC transmission the breakeven distance is reduced to 50 km. Therefore HVDC is more suitable and efficient transmission system than HVAC. There is research going on in HVDC system performance and it is expected that HVDC future is bright.
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9 References:
HVDC transmission: power conversion applications in power systems / by Chan-Ki Kim, Oxford Publishers;2009
Flexible power transmission: the HVDC options / J. Arrillaga, Y.H. Liu, N.R. Watson. Chichester ; Hoboken, NJ : John Wiley, c2007
HVDC and FACTS Controllers: Applications of Static Converters in Power Systems,Vijay K Sood.
HVDC Power Transmission Systems: Technology And Systems Interactions; K R Padiyar,2005; New age International Publishers
Wind Power In Power Systems: Thomas Ackermann,2005; John Wiley and Sons
MODELING AND CONTROL OF A LINE-COMMUTATED HVDC TRANSMISSION SYSTEM INTERACTING WITH A VSC
STATCOM, Paulo Fischer de ToledoDynamic Modelling of Line and Capacitor Commutated Converters for HVDC Power Transmission, 2003, Germany
Grid Connection of Large Offshore Wind Farms Using HVDC: Lie Xu; 2005, Wiley Intersciencewww.abb.com , www.ieee.org.comwww.siemens.co.uk , www.mathworks.com
Topologies and Control of VSC-HVDC Systems for Grid Connection of Large-Scale Off Shore Wind Farms: Keliang Zhou, Ming Cheng; Southeast University ChinaPower-Electronic Systems for the Grid Integration of Renewable Energy Sources: Juan Manuel Carrasco, 2006, IEEE
GRID INTEGRATION OF WIND FARMS USING SVC AND STATCOM: S. Foster, L. Xu and B. Fox; Queens University, UKHVDC Transmission Overview: M. P. Bahrman, P.E., Member, IEEE
Analysis and Control of Wind Farm Incorporated VSC-HVDC in Unbalanced Conditions: 2005, Ming Yin, Gengyin Li, Member, IEEE, Ming Zhou, Student Member, IEEE, Yong Liu
HVDC Connection of Offshore Wind Farms to the Transmission System: 2007, Paola Bresesti, Member, IEEE, Wil L. Kling, Member, IEEE, Ralph L. Hendriks, Member, IEEE, and Riccardo Vailati
Economic Comparison of HVAC and HVDC Solutions for Large Offshore Wind Farms under Special Consideration of Reliability: Master’s Thesis, Lazaros P. Lazaridis, 2005; Royal Institute of Technology Stockholm
