POWER POINT PRESENTATION
ON
HVDC Transmission
2017 - 2018
IV B. Tech II semester (JNTUH-R13)
Ms. B. Manogna, Assistant Professor
ELECTRICAL AND ELECTRONICS ENGINEERING
INSTITUTE OF AERONAUTICAL ENGINEERING(AUTONOMOUS)
DUNDIGAL, HYDERABAD - 500 043
Unit –I
Introduction
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History▪ First commercial application of HVDC between Swedish
mainland and the island of Gotland in 1954.
▪ Underwater link of 90 km and 20 MW.
▪ After the advent of thyristor convertor, New Brunswickand Quebec 320 MW back-to-back DC interconnectioncommissioned in 1972.
▪ With reduced size, cost and improved reliability of powerelectronic converters, has made HVDC transmission morewidespread.
▪ In North America, total HVDC transmission capacity in1987 was 14,000 MW.
3
INTRODUCTION Now a days large blocks of power are needed to be
transmitted.
There arises some technical problems of
transmitting power to such a long distance using ac.
In the view of the draw backs of ac the HVDC
transmission has come into picture.
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Why HVDC? Environmental advantages (lesser right of way requirement)
Lower line losses compared to AC line (no corona &charging
current)
Economical (only two conductor for transmission &lesser
tower height)
Asynchronous connection (enables to connect two different
electrical networks having different frequency& voltage)
Power flow control (enables the stability of electrical network)
Added benefit to the transmission like stability, power quality
etc.
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ADVANTAGES Advantages of dc transmission
a) Technical Advantages
b) Economic Advantages
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Technical Advantages Reactive power requirement.
System stability.
Short Circuit Current.
Independent Control of AC system.
Fast change of energy flow.
Lesser Corona Loss and Radio interference.
Greater Reliability.
No limits in transmitted distance.
Direction of power flow can be changed very quickly.
7
Economic Advantages In DC Transmission, inductance and capacitance of the
line has no effect on power transfer capability.
A DC line requires only 2 conductors where as an AC line
requires 3 conductors in 3-phase AC systems.
8
Comparison With HVACS.No Item HVAC HVDC
1 Power Transmission Capability Low High (e.g. 3000 MW bipole)
2 Distance Limited byStability considerations. Switching Stations required
No limitations
3 System Connection Synchronous Asynchronous
4 Right of Way requirements High Low
5 Power Control No Yes
6 Features – Frequency Control,Reactive Power Control,Damping of Oscillations etc.
Not Available Available
9
Comparison With HVACS.No Item HVAC HVDC
7 Tapping of Power Connection required
Simple Costly, Multi-terminal Scheme
8 Economical Alternative for Bulk Power
Low to Medium distance, Medium Power Range.
Long Distance
9 System SCL (for considerationin developed AC systems dueto high fault currents)
Contributes to System SCL
Does not
10 Pollution effects pronounced Relatively Lesser More
10
So why HVDC rather than HVAC ?
Long distances make HVDC
cheaper
Improved link stability
Fault isolation
Asynchronous link
Control of load flow (DC
voltage can be exactly
controlled)
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Economic advantages DC lines and cables are
cheaper than ac lines or
cables.
The towers of the dc lines
are narrower, simpler and
cheaper compared to the
towers of the ac lines.
Line losses in a dc line
are lower than the losses
in an ac lines.
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Converter station equipment1. Converters
2. Smoothing reactors
3. Harmonic filters
4. Reactive power supplies
5. Electrodes
6. DC lines
7. AC circuit breakers
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Converter station:
14
Components of HVDC Transmission
SystemsConverters
• They perform AC/DC and DC/AC conversion
• They consist of valve bridges and transformers
• Valve bridge consists of high voltage valves connected in a 6-pulse or 12-pulse
arrangement
• The transformers are ungrounded such that the DC system will be able to establish its
own reference to ground
Smoothing reactors
• They are high reactors with inductance as high as 1 H in series with each pole
• They serve the following:
1. They decrease harmonics in voltages and currents in DC lines
2. They prevent commutation failures in inverters
3. Prevent current from being discontinuous for light loads
Harmonic filters
• Converters generate harmonics in voltages and currents. These harmonics may cause
overheating of capacitors and nearby generators and interference with
telecommunication systems
• Harmonic filters are used to mitigate these harmonics15
Components of HVDC Transmission
SystemsREACTIVE POWER SUPPLIER
Under steady state condition, the reactive power consumed by the converter is about 50% of the active power transferred.
Under transient conditions it could be much higher.
For a strong AC power system, this reactive power is provided by a shunt capacitor.
ELECTRODES
Electrodes are conductors that provide connection to the earth for neutral and they have large surface to minimse current and surface voltage gradients.
DC LINES:
They may be overhead lines or cables.
DC lines are very similar to AC lines.
AC CIRCUIT BREAKER:
They used to clear the faults in transformer and for taking DC link out of service.
16
Simplified Analysis of Graetz Circuit
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Without Overlap At any instant, two valves are conducting in the bridge, one
from the upper and one from the lower commutation group.
As the next valve of a commutation group fires, the preceding valve turns off.
This assumption, that no overlap between two valves(meaning no two valves are “on” at the same instant), is incorrect.
However, this assumption provides a simpler analysis into the operation of a converter.
The firing of valves are numbered in sequence, with 60°intervals, and conducts for 120°.
Consecutive firing pulse is 60° in steady state.
18
Without Overlap To further simplify analysis, the following assumptions are taken into
consideration1. The DC current is constant2. The valves can be modelled as ideal switches with zero impedance
when on or conducting, and infinite impedance when off or not conducting.
3. The AC voltages at the converter bus are sinusoidal and remain constant.
A period of an AC voltage supply can be divided into 6 intervals, corresponding to the firing of a pair of valves.
The DC voltage waveform repeats for each interval, thus simplifying the calculation of the average DC voltage, since we only have to consider one interval.
Assuming the firing of the 3rd valve is delayed by an angle α (α° after the crossing of the commutation voltage for valve 3 – voltage eba ) the instantaneous DC voltage vd during the interval is given by
vd = eb – ec = ebc, α ≤ ωt ≤ α + 60°
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With Overlap
Due to the leakage inductance of the converter
transformers and the supply network’s impedance, the
current in the valve will not suddenly change. An example
is when commutation from valve 1 to 3, there is a finite
period of time when both valves are conducting. That is
called overlap and its duration is measured by u called
overlap(commutation) angle.
The three modes are:
1. Mode 1 – two and three valve conduction u<60°
2. Mode 2 – Three valve conduction u=60°
3. Mode 3 – Three and four valve conduction u<60°
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Unit –II
Introduction
21
HVDC system configurations HVDC links can be broadly classified into:
1. Monopolar links
2. Bipolar links
3. Homopolar links
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Mono Polar System: One pole, one conductor for transmission and current
return path is through earth.
Mainly used for submarine cable transmission.
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Bipolar System:
Two poles, two conductors in transmission line, one positive
with respect to earth & other negative.
The mid point of Bi-poles in each terminal is earthed via an
electrode line and earth electrode.
In normal condition power flows through lines & negligible
current through earth electrode. (in order of less than 10
Amps.)
24
Homo Polar System:
• Two poles at same polarity & current return path is through ground.
• This system was used earlier for combination of cable &over head transmission.
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Back to Back HVDC Coupling:
• Usually bipolar without earth return.
• Converter & inverters are located at the same place.
• No HVDC Transmission line.
• Provides Asynchronous tie between two electrical Network.
• Improves system stability Power transfer can be in either
direction.
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Control characteristics
Objectives of Control
1. Efficient and stable operation.
2. Maximum flexibility of power control without compromising the safety of equipment.
3. Principle of operation of various control systems.
4. Implementation and their performance during normal and abnormal system conditions.
27
Starting and Stopping of dc links/
Energisation
It is to be noted that to avoid operation at high delay or extinction angles, the deenergisation of a bridge at the rectifier(or inverter) station is accompanied by the deenergisation of a bridge at the inverter(or rectifier) station
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Start up dc link▪long gate pulses (120)
▪short gate pulses (60)
Long pulse bring
▪ De-block the inverter at about γ = 90
▪ De-block the rectifier at about α= 85
▪ to establish low direct current3. Gradually ramp up the voltage by
inverter control and the current by rectifier control.
Short pulse firing
In this case, the problem of current extinction during start up is
present as the valve with forward bias is not put into conduction when
the current in that transiently falls below holding current. The starting
sequence is as follows:
▪ Open the bypass switch at one terminal
▪ De-block that terminal and load to minimum current in the rectifier
mode
▪ Open bypass switch at the second terminal and commutate current
to the bypass pair.29
Unit –III
Introduction
30
Harmonic filters:The filter arrangements on the
AC side of an HVDC converter
station have two main duties:
To absorb harmonic currents
generated by the HVDC
converter and thus to reduce
the impact of the harmonics
on the connected AC systems,
like AC voltage distortion and
telephone interference.
To supply reactive power for
compensating the demand of
the converter station.
Each filter branch can have one
to three tuning frequencies.
Figure shows different harmonic
filter types with their impedance
frequency characteristics.31
Design Criteria for AC Filters:Reactive Power Requirements:
The reactive power consumption of an HVDC converter depends on the active
power, the transformer reactance and the control angle. It increases with increasing
active power.
Harmonic Performance Requirements:
HVDC converter stations generate characteristic and non-characteristic harmonic
currents. For a twelve-pulse converter, the characteristic harmonics are of the order
n = (12 * k) ± 1 (k = 1,2,3...). These are the harmonic components that are generated
even during ideal conditions, i.e. ideal smoothing of the direct current, symmetrical
AC voltages, transformer impedance and firing angles. The characteristic harmonic
components are the ones with the highest current level, but other components may
also be of importance. The third harmonic, which is mainly caused by the negative
sequence component of the AC system, will in many cases require filtering. The
purpose of the filter circuit is to provide sufficiently low impedances for the relevant
harmonic components in order to reduce the harmonic voltages to an acceptable
level.
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Design Criteria for AC Filters:Network Impedance:
The distortion level on the AC busbar depends on the grid impedance as
well as the filter impedance. An open circuit model of the grid for all
harmonics is not on the safe side. Parallel resonance between the filter
impedance and the grid impedance may create unacceptable amplification
of harmonic components for which the filters are not tuned. For this reason,
an adequate impedance model of the grid for all relevant harmonics is
required in order to optimize the filter design.
There are basically two methods to include the network impedance in the filter
calculations:
to calculate impedance vectors for all relevant harmonics and grid
conditions
to assume locus area for the impedance vectors
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Requirements to Ratings: Steady-State Calculation
Transient Calculation
To calculate the highest stresses of both lightning and switching
surge type, different circuit configurations and fault cases should
be studied:
Single-Phase Ground Fault
Switching Surge
Filter Energization
Fault Recovery after Three-Phase Ground Fault
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DC Filter Circuits:Harmonic voltages which occur on the DC side of a converter station cause
AC currents which are superimposed on the direct current in the transmission
line. These alternating currents of higher frequencies can create interference in
neighbouring telephone systems despite limitation by smoothing reactors. DC
filter circuits, which are connected in parallel to the station poles, are an
effective tool for combating these problems. The configuration of the DC
filters very strongly resembles the filters on the AC side of the HVDC station.
There are several types of filter design. Single and multiple-tuned filters with
or without the high-pass feature are common. One or several types of DC filter
can be utilized in a converter station.
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Design Criteria for DC Filter
Circuits:The equivalent disturbing current combines all harmonic currents with the aid of weighting factors to a single interference current. With respect to telephone interference, it is the equivalent to the sum of all harmonic currents. It also encompasses the factors which determine the coupling between the HVDC and telephone lines:
Operating mode of the HVDC system (bipolar or monopolar with metallic or ground return)
Specific ground resistance at point x. The intensity of interference currents is strongly dependent on the operating condition of the HVDC. In monopolar operation, telephone interference is significantly stronger than in bipolar operation.
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Active harmonic filter: Active filters can be a supplement to passive filters due to their superior performance. They
can be installed on the DC side or on the AC side of the converter. The connection to the high-
voltage system is achieved by means of a passive filter, forming a so-called hybrid filter. This
arrangement limits the voltage level and the transient stresses on the active part, so that
comparatively low equipment ratings can be used. Appropriate design allows the exploitation
of the positive characteristics of both passive and active filters. Additionally, the passive part
can be used as a conventional passive filter if the active part is by-passed for maintenance
purposes.
Main Components:
GBT converter
Reactor for inductivity adapting
Thyristor switch for converter overvoltage and overcurrent protection
Transformer
Low-pass filter
Vacuum switch
ZnO arrester
Isolators and grounding switches
LC branch for deviating the 50-Hz current component
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Single-line diagram of the active AC
filter:
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Unit-IV
Introduction
39
Definition of “Facts” & “Facts
Controller”:
FACTS:(IEEE Definition)
▪ Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability.
FACTS Controller:
A power electronic-based system and other static equipment that provide control of one or more AC transmission system parameters.
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FACTS: Flexible AC Transmission
System (Facts) is a new
integrated concept based on
power electronic switching
converters and dynamic
controllers to enhance the
system utilization and power
transfer capacity as well as
the stability, security,
reliability and power quality
of AC system
interconnections.
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Advantages of FACTS technology: Control of power flow to ensure optimum power flow.
Increase the loading capability of of lines to their thermal capabilities, including short term and seasonal. This can be achieved by overcoming other limitations, and sharing power among lines according to their capability.
Increase the system security by raising the transient stability limit.
Provides greater flexibility in siting new generation.
Reduce reactive power flows, thus allowing the lines to carry more active power.
42
Advantages of HVDC over HVAC
using facts transmission Controlled power.
Very less corona and ferranti effect
Asynchronous operation possible between regions having different electrical parameters(i.efrequency)
No restriction on line length as no reactance in dc lines
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Whether HVDC or FACTS:Both are complementary technologies
The role of HVDC is to interconnect ac systems where a reliable ac
interconnection would be too expensive
Independent frequency and control
Lower line cost
Power control, voltage control and stability control possible
The large market potential for FACTS is within AC system on a value added
basis where
The existing steady-state phase angle between bus node is reasonable
The cost of FACTS solution is lower than the HVDC cost
The required FACTS controller capacity is lesser than the transmission
rating
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FACTS overview:
45
Classification of facts:Depending on the type of connection to the network
The FACTS device can be classified in TWO ways. Serial controller
Derivation controller
Serial to serial controller
Serial derivation controllers
Depending on technological features the FACTS devices can be divided into two generations:
First generation - uses thyristors with ignition controlled by door (SCR).
Second generation - semiconductors with ignition and extinction controlled by door (GTO, IGBT, etc.).
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Serial Controllers: Consist of a variable impedance as a condenser, coil.
Inject a serial tension(variable impedance multiplied by the current) to the line.
Tension is in quadrature with the line current.
Consumes reactive power.
Ex: Serial Synchronous Static Compensator (SSSC).
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Controllers in derivation:
Consist of a variable impedance, variable source or a combination of both.
Inject current to the system in the point of connection. (variable impedance connected to line tension causes variable current flow, thus injecting current to the line).
While the injected current is in quadrature with the line tension.
Consumes reactive power.
Ex: Synchronous Static Compensator (STATCOM).
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Serial - Serial Controllers: Combination of coordinated serial controllers in a
multiline transmission system or can also be an unified controller.
The serial controllers provide serial reactive compensation for each line also transferring active power between lines through the link of power.
The term “unified” means that the DC terminals of the converters of all the controllers are connected to achieve a transfer of active power between each other.
Ex: Interline Power Flow Compensator (IPFC).
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Serial - Derivation Controllers: Combination of serial and derivations controllers
separated, co-ordinately controlled.
Inject current to the system through the component in derivation of the controller, and serial tension with the line utilizing the serial component.
When the serial and derivation controllers are unified, they can have an exchange of active power between them through their link.
Ex: Unified Power Flow Controller (UPFC)
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UNIT-V
INTRODUCTION
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Classification of FACTS :
FIRST GENERATION OF FACTS Static Compensator of VAR’s (SVC, TCR)
Tyristor Controlled Series Compensation (TCSC, TCSR)
Tyristor Controlled phase shifting Transformer (TCPST)
Tyristor Controlled voltage regulator (TCVR)
SECOND GENERATION OF FACTS Synchronous Static Compensator (STATCOM with and
without storage)
Static Synchronous Series Compensator (SSSC with and without storage)
Unified Power flow Controller (UPFC)
Interline Power Flow Controller (IPFC)
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STATIC VAR COMPENSATOR:
Regulate voltage and stabilise(dynamic) the system.
SVC is an automated impedance matching device, designed to bring the system closer to unity power factor.
If load is capacitive (leading), the SVC will use reactors (in form of TCR)
Under inductive (lagging) ,the capacitor banks are automatically switched in.
SVR may be placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage.
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STATIC VAR COMPENSATOR:
54
STATIC SYNCHRONOUS
COMPENSATOR:
STATCOM is a regulating(poor power factor and poor voltage) device.
Based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power.
If connected to a source of power it can also provide active AC power.
STATCOM provides better damping characteristics than the SVC as it is able to transiently exchange active power with the system.
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STATIC SYNCHRONOUS
COMPENSATOR:
56
STATIC SYNCHRONOUS SERIES
COMPENSATOR:
Works the same way as the STATCOM. It has a VSC serially connected to a transmission line
through a transformer. A SSSC is able to exchange active and reactive power with
the transmission system. Thus SSSC can work like a controllable serial condenser
and serial reactor. The voltage injected through a SSSC is not related to the
line intensity and can be controlled independently. As a result SSSC can give good results with low loads as well
as high loads.
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STATIC SYNCHRONOUS SERIES
COMPENSATOR:
58
UNIFIED POWER FLOW
CONTROLLER: A UPFC system can regulate the active and reactive power
at same time.
It has the ability to adjust the three control parameters(bus voltage, transmission line reactance, and phase angle between two buses, either simultaneously or independently).
The converter 2 has the main function of the UPFC; it injects an AC voltage to the line, where magnitude and phase angle are controllable through a serial transformer.
Converter 1 give or absorb the real power that the converter 2 demands.
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UNIFIED POWER FLOW
CONTROLLER:
60
Technical Benefits of FACTS
Load FlowControl
VoltageControl
Transientstability
DynamicStability
SVC LESS HIGH LOW MEDIUM
STATCOM LESS HIGH MEDIUM MEDIUM
TCSC MEDIUM LESS HIGH MEDIUM
UPFC HIGH HIGH MEDIUM MEDIUM
61
Maintenance of FACTS devices
Is minimal and similar to that required for shunt capacitors, reactors and transformers
The amount of maintenance ranges from 150 to 250 man-hours per year
Operation of FACTS devices
operated automatically
can be done locally and remotely
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APPLICATIONS OF FACTS: Steady state voltage stability
Power flow control
Damping of power system oscillations
Reducing generation costs
HVDC link application
Deregulated power systems
Flicker mitigation
Interconnection of renewable, distributed generation and storages.
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FUTURE ENHHANCEMENTS OF
FACTS: Several FACTS devices have been introduced for
various application world-wide.
A number of new types of devices are in the stage of being introduced in practice.
Many new devices are under research process, such as
HFC (Hybrid Flow Controller)
RHFC (Rotary Hybrid Flow Controller)
DPFC (distributed power flow controller)
C-UPFC (center node) and many more.
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Thank You
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