final seminar report sim 158

8/13/2019 Final Seminar Report Sim 158

1/29

A

SEMINAR REPORT

ON

HVDC (HIGH VOLTAGE DIRECT CURRENT)

&

FACTS (FLEXIBLE AC TRANSMISSION SYSTEM)

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF THE

DEGREE OF

BACHELOR OF TECHNOLOGY

(ELECTRICAL ENGINEERING)

SESSION: 2012-2013

SUBMITTED BY:

SAURABH MEENA

ELECTRICAL ENGINEERING

ROLL NO. : 09EBNEE053

BANSAL SCHOOL OF ENGINEERING AND TECHNOLOGY,

JAIPUR


2/29

ABSTRACT

HIGH VOLTAGE DIRECT CURRENT (HVDC)

High Voltage Direct Current (HVDC) technology has characteristics which make it especially attractive in

certain transmission applications. The number of HVDC projects committed or under consideration globally

has increased in recent years reflecting a renewed interest in this field proven technology. New HVDC

converter designs and improvements in conventional HVDC design have contributed to this trend. This paper

provides an overview of the rationale for selection of HVDC technology and describes some of the latest

technical developments.

FLEXIBLE AC TRANSMISSION SYSTEM (FACTS)

The philosophy of FACTS (Flexible AC Transmission Systems) is to use power electronic controlled devices to

control power flow in transmission network to utilize to its full capacity.

FACTS are one of the best ways to reduce the need for construction of new overhead transmission lines is to

increase power flow over existing lines.

Due to high capital cost of erection of new transmission line we have improved this technology called FACTS.

By using this technology we can improve the power transfer capability of the transmission line and these

devices have high switching capacity even at high frequencies as these devices have no moving parts we can

achieve high sensitivity and better control. There are many types of FACTS controller devices out of them

TCSC (Thyristor Control Series Capacitor) has more advantages than other types


3/29

AN INTRODUCTION TO HVDC TRANSMISSION

One of the most exciting new technical development in electric power system in the last three decades has been

High Voltage Direct Current transmission. From the first of HVDC links to the recent, the voltage has

increased from 100 KV to 800 KV, the rated power from 20 MW to 6300 MW and the distance from 96 km to

1370 km.

Preceding and accompanying this rapid growth of Direct Current Transmission were developments in High

Voltage, High power valves, in control and protection system, in DC cables and in insulation for overhead DC

lines.

In India three HVDC projects are in operation.

(i) The Rihand-Delhi HVDC transmission project having 1500 MW capacity and 500 KV DCvoltage is the first commercial long distance DC transmission project in India.

(ii) Vindhyachal 2x250 MW Back to back DC converter station which asychronously connect the

Northern and Western regions for exchange of power.

(iii) The Nation HVDC experimental line project, which links Lower Sileru in A.P. to Barsoor in

M.P. Phase 1 of this project is capable of transmitting 100 MW at 100 KV DC.

The main advantages of High Voltage Direct Current transmission are

(1) Asynchronous operation

(2) Controllability

(3) Stability

(4) Reliability

(5) Low right of way requirement

(6) Economy on overall basis

(7) Greater power per conductor

(8) Simple line construction

(9) No skin effect, charging current and less corona loss and interference

(10) Ground return can be used

(11) Cables can be worked at a higher voltage gradient

(12) May inter connect AC systems of different frequencies

(13) Low short circuit current on DC line.


4/29

APPLICATIONS OF DC TRANSMISSION

1. For cables crossing bodies of water wider than 32 km. [Ex Sweden-Got land link, a 20 MW, 100 KV

DC single conductor submarine link to supply power to the island of Got land.]

2. For inter connecting AC systems having different frequencies or where asynchronous operation is

desired.

3. For transmitting large amounts of power over long distances by overhead lines.

4. In congested urban areas where it is difficult to acquire right of way for overhead lines and where

lengths involved make AC cables impracticable.

Economic Factors

The cost per unit length of a DC is lower than that of an AC line of the same power capability with comparable

reliability, but the cost of the terminal equipment of a DC line is much more than that in an AC. A graph is

plotted between the cost of transmitting an amount of power by onemethod and the distance over which it is

transmitted, below:

The vertical intercept of each curve is the cost of the terminal equipment alone. The slope of each curve is the

cost per unit length of the line and of that accessory equipment which varies with length. The curve for AC

transmission intersects that for DC transmission at an X axis which is the break even distance, Transmission by

DC is cheaper than AC for distance above 500 km.


5/29

WHY WE USE DC TRANSMISSION?

The question is often asked, Why we use DC transmission? One response is that losses are lower. But this is

not correct the level of losses is designed in to a transmission system and is regulated by the size of the

conductor selected. DC and AC conductors either as overhead transmission lines or submarine cables can have

lower losses but at higher expense since the larger cross-sectional are will generally results in lower but cost

more.

When converters are used for DC transmission, it is generally by economic choice driven by one of the

following reasons.

1. An overhead DC transmission line with its towers can be designed to be less costly per unit of

length than an equivalent AC line designed to transmit the same level of electric power. However the

DC converter stations at each end are more costly than the terminating station of an AC line and so

there is a breakeven distance above which the total costs of DC transmission is less than its AC

transmission alternative. The DC transmission has lower visual profile than an equivalent AC line and

so contributes to a lower environmental impact. There are other environmental advantages to a DC

transmission line through the electric magnetic fields being DC instead of AC.

2. If transmission is by submarine or underground cable, the breakeven distance is much less than

overhead transmission. It is not practical to consider AC cable systems exceeding 50km but DC cable

system are in service whose length is in the hundreds of kilometres and even distances of 600km or

greater have been considered feasible

3. Some AC power systems are not synchronized to the neighbouring networks even though their

physical distances between them os quite small. Thais occur in Japan where half the country is a 60hz

network and other is 50hz system. It is physically impossible to connect the two together by direct AC

methods in order to exchange electric power between them. However if a DC converter station is

located the required power flow even though the AC systems so connected remain asynchronous.


6/29

TYPES OF DC LINKS

HVDC back to back link: This link is used to connect two AC grids, each AC grid can have its own load

frequency control. Direction of power flow can be controlled by adjusting the characteristics of convertor

valves. There is no increase in fault level and cascade trippings in the network are avoided. [Ex. Vindhyachal

Back to Back HVDC link].

Monopolar link: This links has one conductor, usually of negative polarity, and ground or sea return.

Bipolar link: This link has two conductors one positive, the other negative. Each terminal has two convertors

of equal rated voltages in series on the DC side. The neutral points (junction between convertors) are grounded

at one or both ends. If both neutrals are grounded, the two poles can operate independently. Normally operate

at equal currents: then there is no ground current. In the event of fault on one conductor, the other conductor

with ground return can carry up to half the rated load.

Homopolar link: This links has two or more conductors all having the same polarity, usually negative, and

always operate with ground return. In the event of a fault on one conductor, the entire convertor is available for

connection to the remaining conductor or conductors, which, having some over load capability, can carry more

than half of the rated power and perhaps whole rated power, at the expenses of increased line loss. In a Bipolar

scheme reconnection of the whole convertor to one pole of the line is more complicated and is usually not

feasible because of graded insulation. In this respect a Homopolar line is preferable to a bipolar line in cases

where continuous ground current is not objectionable. An additional advantage, through minor is less corona

loss and negative polarity is preferable to have less radio interference.


7/29

Figure shows HVDC Bipolar system in which there are two poles one is negative and the other is positive.

Each pole consists of one 12 pulse covnertor at both ends in which sending end will act as rectifier and

receiving end will act as invertor. The 12 pulse convertor consists of two series connected 6 pulse bridges


8/29

HVDC Bipolar System Layout

HVDC Rectifier and invertor station in HVDC Bipolar systems consists of following parts

1. AC switchyard

2. AC filter area

3. Convertor transformers

4. Valve hall and control room

5. DC switchyard and smoothing reactor

6. Electrical and mechanical ausiliaries

AC Switchyard

The AC switchyard is generally at 400 KV or 760 KV voltage level corresponding to the standard ofEHV/UHV transmission voltage. The AC yard is of one half breaker bus system. The advantage of one and half

a breaker system is it permits use of only three breakers for two circuits. In one and half a breaker system the

circuits one and two can take supply either from Bus I or Bus II, thus in the event of fault on any bus the supply

is maintained in the circuits by unfaulty bus. Hence, high security against loss of supply.

One and Half Breaker Scheme

The insulation coordination of the AC yard is correlated with that of DC yard and over voltages approaching

from DC side. Metal oxide arrestors are used in AC yard and DC yard. The AC yard is designed in similar

principles like usually EHV AC switchyards with following additional considerations:

A large area on AC yard is covered by AC harmonic filter bank.

More space is provided for movement of large convertor transformers and cranes.

No. of surge arrestors are provided at strategic locations in AC yard.

Protection and control of an eneterprise with valves and DC yard.

The circuit breakers used in HVDC substation have reinsertion resistors to reduce switchin over


9/29

AC Harmonic Filter Area

A large portion of the area in AC yard is covered by AC filter bank. The filters are required to filter out

harmonics generated due to the operation of 3 phase AC/DC conversion which generates kp 1th harmonic on

AC side, p is the integer and K is the number of pulses of convertor valve. This is derived using fourier

analysis.

Harmonics in AC for 12 pulse system for which K = 12 are 1, 11, 13, 23, 25th

, 5th

and 7th

harmonics are of 10%to 25% which are generated due to the formation of 12 pulse by series connection of sixpulse connection.

Each filter bank has the following components.

1. AC filter capacitor bank

2. Reactor

3. Resistor bank

4. Current transformers

5. Circuit breakers

These AC harmonic filters are essential to reduce the harmonic contenet in the AC voltage within the limits.

AC filter capacitor also provide the leading reactive power consumed by the convertor (shunt compensation).

AC harmonic filters comprise RLC series circuit connected in shunt with the AC busbard. Separate branches

are provided for predominant 5th, 7th, 11th and 13th Harmonic and a high phase filter for higher than 23rd

harmonic and above.

A C Harmonic Filter Circuit

Reactive power demand and compensation: - The operation of the convertor requires a certain

amount of reactive power. This is due to

The manner of controlling HVDC convertors introduces a phase shift between the fundamentals of AC

current and voltage. The magnitude of this phase shift is strongly dependant on the firing angle and inrectifier and extinction angle y in invertor.

The commutation process, in which the DC current is connected from one valve to another, also


10/29

Convertor consumes reactive power both when it operates as rectifier as well as invertor. Besides the reactive

power demand is also due to magnetizing current of convertor transformer. Considering normal valves of a

(rectifier) firing angle and extinction angel y (Invertor) the reactive power demand usually in the range of 50%

- 60% of the transmitted active power.

Convertor Reactive Power Demand

This reactive power in the range of 50% to 60% of the transmitted active power (each convertor station) is

compensated by several ways depending on the quality of the connecting AC network.

The different possibilities for suitable reactive power production are mentioned here.

I. AC filters

II. Shunt capacitors

III. Excessive reactive power from the AC network

IV. Static compensation

V. Synchronous condensers

Valve Hall and Control Room

The valve hall and control room are located between AC yard and DC yard. The valve hall houses quadruple

thyristor valves, air core reactors, terminal bushings associated bus bars and surge arrestor. The control room

building houses control panels for AC yard, DC yard and valves etc. in Bipolar HVDC substation there are two

valve halls, each valve hall houses three quadruple valves. The control room is in between two valve halls. The

valve hall is provided with uniform earthing mat in the flooring and uniform earthed screen in the wall and the

roof. The screen protects the control circuits from the electromagnetic interference produced by the operation

of thyristor valves. The valve hall is provided with air conditioning system. The temperature inside the valve

hall is high due to valve losses and the lowest temperature of valve hall maintained is 10C and the highest

temperature of valve hall maintained is 55C.


11/29

The control room houses the following control panels:

Protection, metering and control panels for AC yard, DC yard and convertor transformers.

control panels for valves

PLC communication and tele control panels etc.

Monitoring panels.

The auxiliary switchgear, low voltage switchgear, DC supply systems is generally installed in a separate

floor of the control room building. The convertor valves are either supported on the valve hall floor on insulator

columns or are under hung from the roof by means of insulators.

Thyristor Valves

Since the individual thyristors has a limited voltage ratings nearly 7 KV, several thyristors are connected in

series to achieve desired rated voltage. The assembly formed is called a thyristor valve. A thyristor valve for an

HVDC convertor comprises of the following:

Several thyristors connected in series to achieve the required insulation level. Each thyristor has its

associated thyristor control unit.

Snubber (voltage grading) circuit for equal distribution of voltage across thyristors and protection of

thyristors in the string.

Cooling system to removal heat from the cathode silicon wager. In HVDC system pure deionised

water is circulated in a closed cycle to remove heat from heat sinks.

A valve is made up of stacking four valve modules in a vertical formation is called a quadruple valve. The


12/29

A typical Bipolar twelve pulse convertor substation has two valve halls, one for each pole. Each valve hall

houses three quadruple valves.

The active part of thyristor is a semiconductor mono crystalline silicon wafer with a thickness of half a

millimetre and an area in the range 8 to 60 cm. The wafer has been treated to obtain P-N-P-N with desired

current and voltage properties. The junction temp. with stand capability is 100 to 125C. the water cooled

wafer has 45 cm area and a threshold voltage drop of 0.8 to 1.0 V. the thyristors are mounted on heat sinks.

The modules are cooled in parallel with two cooling circuits in each module giving equal coolings. As the

water should be insulating a special water processing unit is installed to deionise the water to limit the amount

of oxygen in the water. The valve losses are about 0.5 percent of DC power transfer.

Cooling System for Water Cooled Thyristor Valve


13/29

DC Yard

The DC yard has the following essential equipment:-

DC smoothing reactor

DC filters

DC bus bars and isolators, earthing switches, current transducers, voltage dividers, surge arrestors.

Switchgear for switching from ground return to the metallic return.

Smoothing Reactor: HVDC smoothing reactors of a 0.4 henrys to 1 henry are generally used. There are oil

filled reactors. Smoothing reactor is connected in series with the convertor bridges in order to reduce the

current harmonics in the direct current and to reduce valve stresses due transients such as DC line faults and

commutation failures by limiting the fault current and the rate of rise current.

A DC smoothing reactor is located on the low voltage side and air core reactors on the line side of the

convertors. The later to limit any steep front surger entering the station from the DC side. Additional air core

reactors are installed in each phase on the AC side to reduce the rate of rise of current during thyristor turn on.

DC Harmonic Filters

Using Fourier analysis we can evaluate the harmonicas on DC side for 12 pulse connector which is Kp, p is

the integer and K is the pulse number. For 12 pulse system the harmonics generated on DC side are 0, 12, 24,

etc. a high pulse DC filter turned to 12

th

harmonic is usually provided on DC side.

Single Line Diagram of Single Pole Giving Details of DC Yard

1. Surge arrestor 6. Direct voltage measuring device

2. Converter transformer 7. DC filter

3. Air core reactor 8. Current measuring transducer

4. Thyristor valve 9. DC line

5. Smoothing reactor 10. Electrode line

The measuring equipment i.e. a voltage divider, current measuring transducer and current transformer, provide


14/29

Earth Return

In monopolar configuration, the return path is usually through earth or sea. Earth return or sea return reduces

the cost of transmission system.

In Bipolar system the normal power flow is through pole conductors and only negligible out of balance current

flow through earth. The mid points of convertors at both ends are earthed. In monopolar the return is through

earth. The earth electrode station is usually built 10 to 25 km from main HVDC substation to avoid galvanic

corrosion of pipes, foundation structures, cable theatres, earthing material due to cathodic corrosion.

The connection between mid-point of convertor valve and a distant earth electrode is an electrode line.

Electrode line is insulated from earth and connected to the earth electrode.

1. Neutral Bus Switch

2. Switch for Metallic to Ground Transfer

3. HVDC Breaker of Ground to Metallic Return Transfer

DC circuit breaking is difficult due to non-availability of current zero in the DC. Hence links do not

have any provision of DC circuit breakers. HVDC links do not have parallel lines and T off lines due to lack of

HVDC circuit breaker. HVDC circuit breaker using artificial current zero is produced by discharging a

precharged capacitor bank through the breaker contacts has been developed but it is complex and not

economical.

Metallic Return: In the case of fault on a pole the power transfer taken place in Monopolar mode using

ground return in addition to this the line of the pole which is out of order can be used for return path. This type

of current return is called Metallic return.


15/29

ENVIORMENTAL CONSIDERATIONS

The electrical environmental effects from HVDC transmission lines can be characterized by field and ion

effects as well as corona effects (4), (5). The electric field arises from both the electrical charge on the

conductor and for a HVDC overhead transmission line from charges on air ions and aerosols surrounding the

conductor. These gives rise to DC as well as due to ion current density flowing through the air. A DC magnetic

field is produced by DC current flowing through the conductors. Air ions produced by HVDC lines from

clouds which drift away from the line when blown by the wind and may come in contact with humans, animals

and plants outside the transmission lines right-of way or corridor. The corona effects may produce low levels

of radio interference, audible noise and ozone generation.

Field and corona effects

The field and corona effects of transmission lines largely favor DC transmission over AC transmission. The

significant considerations are a follows

1. For a given power transfer requiring extra high voltage transmission, the DC transmission line will

have a smaller tower profile than the equivalent AC transmission carrying the same level of power, this

can also lead to less width of right- of-way for DC transmission option.

2. The steady and direct magnetic field of DC transmission line near at the edge of transmission right-of-

way will be about the same value in magnitude as the earths naturally occurring magnetic field. For

this reason alone, it seems unlikely that this small contribution by HVDC transmission lines to the

background geometric field would be the basis of concern.

3. The static and steady electric field from DC transmission at the levels experienced beneath lines or

edges of the right-of way have no known adverse biological effects. There is no theory or mechanism

to explain how a static electric field at the levels produced by DC transmission lines could effect human

health. Electric fields from ac transmission lines have been under more intense scrutiny than fields

generated from dc transmission lines.

4. The ion and corona effects of dc transmission line lead to a small contribution of ozone production to

higher naturally occurring background concentrations. Exacting long term measurements are required

to detect such concentrations.

5. If ground return is used with monopolar operation, the resulting dc magnetic field can cause error in

magnetic compass readings taken in the vicinity of the DC line or cable. This impact is minimized by

providing a conductor or cable return path in close proximity to the main conductor or cable for

magnetic cancellation. Another concern with continuous ground current is that some of return current


16/29

INHERENT PROBLEMS ASSOCIATED WITH HVDC

1. Expensive Convertors: Expensive convertor stations are required at each end of a DC transmission link,

whereas only transformer stations are required in an AC link.

2. Reactive Power Requirement: Convertors require much reactive power, both in rectification as well as in

inversion. At each convertor the reactive power consumed may be as much at 50% of the active power

rating of the DC link. The Reactive power requirement is partly supplied by the filter capacitance, and

partly by synchronous or static capacitors that need to be installed for the purpose.

3. Generation of Harmonics: Convertors generates a lot of harmonics both on the DC side and the AC side.

Filters are used on the AC side to reduce the amount of harmonics transferred to the AC systems. on the

DC system smoothing reactors are used. These components add to the cost of convertors.

4. Difficulty of Circuit Breaking: Due to the absence of a natural current zero with DC, circuit breaking is

difficult. This is not a major problem in single HVDC link systems, as circuit Breaking can be

accomplished by a very rapid absorbing of the energy back into the AC system.

5. Difficulty of Voltage Transformation: Power is generally used at low voltage, but for reasons of

efficiency must be transmitted at high voltage. Absence of the equivalent of DC transformers makes it

necessary for voltage transformation to carried out on the AC side of the system and prevents a purely DC

system being used.

6. Difficulty of High Power generation: Due to the problems of commutation with DC machines, voltage,

speed and size are limited.


17/29

AN INTRODUCTION TO FACTS

Without electricity, modern society would cease to function. As the volume of power transmitted and

distributed increases, so do the requirements for a high quality and reliable supply. At the same time, rising

costs and growing environmental concerns make the process of building new power transmission and

distribution lines increasingly complicated and time-consuming. Making exciting lines as well as new ones

more efficient and economical then become a compelling alternative. The purpose of the transmission network

is to pool power plants and load canters in order to minimize the total power generation capacity and fuel cost.

The power systems of today, by and large, are mechanically controlled. There is a wide spread use of

microelectronics, computers and high-speed communications for control and protection of present transmission

systems; however, when operating signals are sent to the power circuits, where the final power control action is

taken, the switching devices are mechanical and there is little high-speed control.

Another problem with mechanical devices is that control cannot be initiated frequently, because these

mechanical devices tend to wear out very quickly compared to static devices. In India, for example there has

been a fourfold increase in the value of wholesale transactions during the last decade. At the same time,

however, addition of new transmission capacity to handle this growth has become increasingly difficult.

Fortunately, a new generation of technologies promises to solve the growth-capacity dilemma by offering

unconventional ways to increase transmission capacity with much less need for building new overhead lines.

Today, one such technological area that has the potential to revolutionize utility transmission system is

FLEXIBLE AC TRANSMISSION.FACTS devices are static equipment used for effective transmission. It

means to enhance controllability and increase power transfer capability. It was introduced by Dr N.G.Hingorani

in 1988 from the Electric Power Research Institute (EPRI) in the USA.

DEFINITION:

FACTS is defined by IEEE as power electronic based system and other static equipment that provide control

of one or more AC transmission system parameters to enhance controllability and power transfer capability.

Symbol for FACTS devices


18/29

BASICS OF FACTS:

The main idea of FACTS can be explained by the basic equation of power transmission is shown in the below

figure. Power transmitted between two nodes of the system depends on voltages at both ends of the

interconnection. Different FAC TS devices can actively influence one or more of these parameters for power

flow control and improvement of voltage stability at node of interconnection. Depending on the system

configuration, the tasks of FACTS can be summarized as follows:

Meshed systems & bulk power transmission can cause power flow control.

Radial systems &parallel lines can cause impedance control.

Weak systems can cause voltage control.


19/29

FACTS CONTROLLERS:

GENARATIONS OF CONTROLLERS:

FIRST GENERATION: This uses a simple logic of capacitance of manually based upon a roughly pre-

manipulated data

SECOND GENERATION: The electronics makes its doubt here, forming a more smooth operation using relays

and semi-conductor switches. This even did not eradicate the demerit of manual presence.

THIRD GENERATION: This uses FACTS devices to control the flow of power in the transmission line, and

also increase the stability in transient conditions.

The power that can be transmitted over a line depends on three factors

1. Series reactance of the line

2. Bus voltages

3. Transmission angle d

v Voltage along the line can be controlled by reactance shunt compensation.

v Series line inductance can be controlled by series capacitive compensation.

v Transmission angle can be varied by phase shifting.

FACTS can be connected

1. In series with the power system.

2. In shunt with the power system.

3. Both in series and shunt with the power system.


20/29

SERIES COMPENSATION

In series compensation FACTS are connected in series with power system. It works as a controllable

voltage source

Symbol for Series Controller

METHODS OF SERIES COMPENSATION:

Static synchronous series compensator (SSSC)

Thyristor controlled series capacitor (TCSC)

Thyristor switched series capacitor (TSSC)

Static synchronous series compensator (SSSC)

As mentioned, Static Synchronous Series Compensator (SSSC) is placed in the group of series connected

FACTS devices. SSSC consists of a voltage source inverter connected in series through a coupling transformer

to the transmission line.

A source of energy is required for providing and maintaining the DC voltage across the DC capacitor and

compensation of SSSC losses. Fig. 2 shows the model of SSSC which consists of a series connected voltage

source in series with an impedance. This impedance represents the impedance of coupling transformer.


21/29

THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC):

The basic thyristor controlled series capacitor scheme proposed in 1986 by Vithayathil

with others as a method of rapid adjustment of network impedance is shown in fig.It consists of the series

compensating capacitor shunted by a thyristor-controlled reactor. In a practical TCSC implementation several

such basic compensators may be connected in series to obtain the desired voltage rating and operating

characteristic

TCSC is a capacitive reactance compensator which consists of a series capacitive bank shunted by a thyristor

controlled reactor in order to provide smoothly variable capacitive reactance.

SINGLE LINE DIAGRAM OF TCSC

BENEFITS OF TCSC:

Study state control of power flow.

Transient stability improvement.

Balancing power flow in parallel lines.

To control line impedance.

Reduce transmission losses


22/29

SHUNT COMPENSATION:

In shunt compensation power system is connected in shunt with the FACTS device .It works as a controllable

current source.

Symbol for shunt Controller

METHODS OF SHUNT COMPENSATION:

Static Synchronous compensator (STATCOM)

Static VAR Compensator (SVC)

Thyristor Controlled Reactor (TCR)

Thyristor Switched Reactor (TSR)

Thyristor Switched Capacitor (TSC)

Mechanically Switched Capacitor (MSC)

Static Synchronous compensator (STATCOM)

The Static Synchronous Compensator (STATCOM) 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

[1]. 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).

The variation of reactive power 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. The principle of operation of

the STATCOM is explained on the figure below showing the active and reactive power transfer between a

source V1 and a source V2. In this figure, V1 represents the system voltage to be controlled and V2 is the

voltage generated by the VSC.


23/29

Operating Principle of the STATCOM

P = (V1V2) sin / X , Q = V1(V1 V2cos) / X

Symbol Meaning

V1 Line to line voltage of source 1

V2 Line to line voltage of source 2

X Reactance of interconnection transformer and filters

Phase angle of V1 with respect to V2

In steady state operation, the voltage V2 generated by the VSC is in phase with V1 (=0), so that only reactive

power is flowing (P=0). 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 (V1V2 )) /X.

A capacitor connected on the DC side of the VSC acts as a DC voltage source. In steady state the voltage V2

has to be phase shifted slightly behind V1 in order to compensate for transformer and VSC losses and to keep

the capacitor charged. Two VSC technologies can be used for the VSC:

VSC using GTO-based square-wave inverters and special interconnection transformers. Typically four

three-level inverters are used to build a 48-step voltage waveform. Special interconnection transformers are

used to neutralize harmonics contained in the square waves generated by individual inverters. In this type of

VSC, the fundamental component of voltage V2 is proportional to the voltage Vdc. Therefore Vdc has to be

varied for controlling the reactive power.

VSC using IGBT-based PWM inverters. This type of inverter uses Pulse-Width Modulation (PWM)

technique to synthesize a sinusoidal waveform from a DC voltage source with a typical chopping frequency of

a few kilohertz. Harmonic voltages are cancelled by connecting filters at the AC side of the VSC. This type of

VSC uses a fixed DC voltage Vdc. Voltage V2 is varied by changing the modulation index of the PWM


24/29

The figure below shows a single-line diagram of the STATCOM and a simplified block diagram of its control

system.

Single-line Diagram of a STATCOM and Its Control System Block Diagram

The control system consists of:

A phase-locked loop (PLL) which synchronizes on the positive-sequence component of the three-phase

primary voltage V1. The output of the PLL (angle =t) is used to compute the direct-axis and quadrature-axis

components of the AC three-phase voltage and currents (labeled as Vd, Vq or Id, Iq on the diagram).

Measurement systems measuring the d and q components of AC positive-sequence voltage and currents

to be controlled as well as the DC voltage Vdc.

An outer regulation loop consisting of an AC voltage regulator and a DC voltage regulator. The output

of the AC voltage regulator is the reference current Iqref for the current regulator (Iq = current in quadrature

with voltage which controls reactive power flow). The output of the DC voltage regulator is the reference

current Idref for the current regulator (Id = current in phase with voltage which controls active power flow).

An inner current regulation loop consisting of a current regulator. The current regulator controls the

magnitude and phase of the voltage generated by the PWM converter (V2d V2q) from the Idref and Iqref

reference currents produced respectively by the DC voltage regulator and the AC voltage regulator (in voltage

control mode). The current regulator is assisted by a feed forward type regulator which predicts the V2 voltage

output (V2d V2q) from the V1 measurement (V1d V1q) and the transformer leakage reactance.

The STACOM block is a phasor model which does not include detailed representations of the power

electronics. You must use it with the phasor simulation method, activated with the Powergui block. It can be

used in three-phase power systems together with synchronous generators, motors, dynamic loads and other

FACTS and DR systems to perform transient stability studies and observe impact of the STATCOM on

electromechanical oscillations and transmission capacity at fundamental frequency.


25/29

STATCOM V-I characteristic

The STATCOM can be operated in two different modes:

In voltage regulation mode (the voltage is regulated within limits as explained below)

In var control mode (the STATCOM reactive power output is kept constant)

When the STATCOM is operated in voltage regulation mode, it implements the following V-I characteristic.

The V-I characteristic is described by the following equation:

V= Vref+Xs I

Where

V Positive sequence voltage (pu)

I Reactive current (pu/Pnom) (I > 0 indicates an inductive current)

Xs Slope or droop reactance (pu/Pnom)

Static VAR Compensator (SVC):

The SVC regulates voltage at its terminals by controlling the amount of reactive power injected into or

absorbed from the power system.

When system voltage is low, the SVC generates reactive power (SVC capacitive). When system voltage is

high, it absorbs reactive power (SVC inductive).

Thyristor-controlled reactor (TCR): reactor is connected in series with a bidirectional thyristor valve. The

thyristor valve is phase-controlled. Equivalent reactance is varied continuously.

Thyristor-switched reactor (TSR): Same as TCR but thyristor is either in zero- or full- conduction.

Equivalent reactance is varied in stepwise manner.

Thyristor-switched capacitor (TSC): capacitor is connected in series with a bidirectional thyristor valve.


26/29

FACTS CONCEPTS SIMILAR TO HVDC

While some of the relevant technology i.e., Static VAR Compensation is already in wide use, the FACTS

concept has brought to the table a tremendous potential for thyristor based

Controllers which will surely revolutionize the power system.

The technology offers the utilities the ability to:

1. Control power flows on their transmission routes;

2. Allow secure loading of transmission lines to their full thermal capacity.

FACTS technology, while allowing use of transmission to its thermal capacity, does not do away with the need

for additional transmission lines or the upgrading of existing lines where thermal limits have been reached or

when evaluation of losses added to the cost of FACTS technology shows that new lines or upgrading of

existing lines is the most optimum answer. Often, ac transmission systems are thought of as being "inflexible".

Power flow in ac networks simply follows Ohm's law and ordinarily cannot be made to flow along specific

desired paths. As a result, ac networks suffer from parallel-path, or "loop" flows. The power flows from source

to load in inverse proportion to the relative impedances of the transmission paths. Low impedance paths take

the largest fraction of flow, but all lines in the interconnection are a part of the flow path. Thus, utilities not

involved in an interchange power transaction can be affected.

A fundamental notion behind FACTS is that it is possible to continuously vary the apparent impedance of

specific transmission lines so as to force power to flow along a "contract path". This is a brand-new concept for

many system planners. As illustrated in Figure 1.3, with precise control of the impedance of transmission lines

using FACTS devices, it is possible to maintain constant power flow along a desired path in the presence of

continuous changes of load levels in the external ac network, and to react in a planned way to contingencies.

Just as in HVDC applications, FACTS controls could be designed to enhance the behaviour of the uncontrolled

systems. The flexible system owes its tighter transmission control to its ability to manage the interrelated

parameters that constrain today's systems, including series impedance, shunt impedance, phase angle, and the

occurrence of oscillations at various frequencies below the rated frequency. By adding to in this way, the

controllers enable a transmission line to function nearer its thermal rating. For example, a 500-kV line may

have a loading limit of 1000-2000MW for safe operation, but a thermal limit of 3000 MW. It is often not

possible both to overcome these constraints and maintain the required system reliability by conventional

mechanical means alone, such as tap changers, phase shifters, and switched capacitors and reactors

(inductors).Granted, mechanical controllers are on the whole less expensive, but they increasingly need to be

supplemented by rapidly responding power electronics controllers.


27/29

Applying Flexibility to the Electric Power System:

The power industry term FACTS (Flexible AC Transmission Systems) covers a number of technologies that

enhance the security, capacity and flexibility of power transmission systems. FACTS solutions enable power

grid owners to increase existing transmission network capacity while maintaining or improving the operating

margins necessary for grid stability. As a result, more power can reach consumers with a minimum impact on

the environment, after substantially shorter project implementation times, and at lower investment costs - all

compared to the alternative of building new transmission lines or power generation facilities. The two main

reasons for incorporating FACTS devices in Electric power systems are:

Raising dynamic stability limits

Provide better power flow control

FACTS technologies deliver the following benefits

A Rapidly Implemented Installations: FACTS projects are installed at existing substations and avoid the

taking of public or private lands. They can be completed in less than 12 to 18 months a substantially shorter

timeframe than the process required for constructing new transmission lines.

Increased System Capacity: FACTS provide increased capacity on the existing electrical transmission system

infrastructure by allowing maximum operational efficiency of existing transmission lines and other equipment.

Enhanced System Reliability: FACTS strengthen the operational integrity of transmission networks, allowing

greater voltage stability and power flow control, which leads to enhanced system reliability and security.

Improved System Controllability: FACTS allow improved system controllability by building intelligenceinto the transmission network via the ability to instantaneously respond to system disturbances and gridlock

constraints and to enable redirection of power flows.

Seamless System Interconnections: FACTS, in the form of BTB dc-link configurations, can establish

seamless interconnections within and between regional and local Networks, allowing controlled power

transfer and an increase in grid stability


28/29

CONCLUSION:

HVDC Transmission system:

HVDC transmission system is a very superior type of transmission system topology, which serves for power

transmission and thus contributes the advantage like Use of ground return possible, Skin effect, Tower size etc.

Although HVDC possess some disadvantages. The extent of advantages makes it a very suitable one for the

transmission. For long distance transmission of electricity HVDC transmission is the best one than Extra High

Voltage AC transmission.

FACTS :

It is envisaged that in future FACTS devices could be installed on wide scale by electrical utilities in an attempt

to control the power flows through their networks. Concern has been expressed that such wide scale application

of FACTS devices could cause conflict between the control systems of the different devices Using the

advanced solid state technology, FACTS controllers offer flexibility of system operation fast and reliable

control. They better utilization of existing power generation and transmission facilities without comprising

system availability and security .The planner has to select controller out of the set of FACTS controllers, for

improving the system operation based on cost benefit analysis.


29/29

REFERENCES

HVDC

1. A Refined HVDC Control System- Ekstrom. A and Liss. G (IEEE)

2. Rapid City Tie-New Technology Tames The East, West Interconnection- M. Brahman, D. Dickson, P.

Fisher, M. Stolz.

3. Engineering book

FACTS

1. S.Nilsson, Special Application Considerations for Custom Power systems.

2. www.IEEE.com

3. www.siemens.org
http://www.ieee.com/http://www.siemens.org/http://www.siemens.org/http://www.ieee.com/

final seminar report sim 158

Documents