FACTS HVDC Controllers

FACTS & HVDC

Controllers

by

Issarachai Ngamroo, Ph.D.

plication,

Sirindhorn

Sirindhorn

International Institute of Technology

International Institute of Technology

Thammasat

Thammasat

University

University

December 16, 2004

December 16, 2004

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Why FACTS & HVDC ?

1. Connection of generation

Some of power plants (large

hydro and thermal stations)

can be located near the load

and can be connected by

relatively short AC lines to the

grid. But some of them have to

be located far from the load,

particularly hydro plants and

coal plants, and the

transmissions often has to be

HVDC or AC with FACTS.

FACTS or HVDC

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Why FACTS & HVDC ?

2. Connection of isolated loads

With isolated loads we mean loads that due to geographical or other conditions are not connected to a major grid but have to rely on (small) local

generation. Examples are islands and remote towns and villages. The local generation is often expensive and not environmentally sound. If an isolated load can be connected to a main grid the cost of electricity goes down. The transmissions options are often HVDC/HVDC Light or AC with FACTS.

HVDC or FACTS

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Why FACTS & HVDC ?

3. Interconnection

It is increasingly economic to interconnect with neighbouring grids to benefit from the pooling of resources. We have selected to distinguish interconnections within a grid and new interconnections between grids.

3.1 Within a grid (same frequency)

By this we mean const ructing or strengthening a circuit between two points that belongs to the same synchronous grid (or group of grids). If the electrical distance is short or of moderate length, it is often enough to build one or two uncompensated AC-lines or cables. But with increasing distance, the addition of FACTS and utilizing HVDC can be the optimum choice.

HVDC or FACTS

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Why FACTS & HVDC ?

This means linking two separate networks that are

not running in synchronism so that exchange of power

can take place. If they are linked by an AC circuit

assuming the same nominal frequency, then the

combined network becomes one synchronous grid

with common frequency control. But if the power

transfer is on a HVDC link, the networks can maintain

their separate frequencies.

HVDC

Between grids

3.2 Between grids (different frequencies)

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Why FACTS & HVDC ?

4. Increasing existing grid utilization

In many countries new transmission facilities are not

permitted and transmission grids world-wide are as a

consequence of load growth stressed closer to their power

transfer limits. In many cases FACTS solutions appear as an

attractive short term means to raise the transfer limit or to

more generally enhance the reliability of the existing grid.

FACTS solutions is an attractive means to raise the

capability or enhance the reliability of the grid.

new transmission

lines are expensive

and not permitted

plication,

control the power flows and other quantities in power systems.

IEEE Definitions

FACTS: AC transmission systems incorporating the power

electronic-based and other static controllers to enhance controllability and increase power transfer capability.

FACTS Controllers: A power electronic based system &

other static equipment that provide control of one or more AC transmission parameters.

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FACTS Concepts

(

)

sin

V V

P

x

δ δ

=

−

jx

V

∠

δ

V

∠

δ

P

+

jQ

Bus 1

Bus 2

Active Power Flow

Control Variables

1. Phase Difference :

δ

-δ

2. Voltage :

V

, V

3. Line Reactance :

x

(

)

cos

V

V V

Q

x

x

δ δ

=

−

−

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Objectives of FACTS Controllers

1. Solve Power Transfer Limit & Stability Problems

1.1 Thermal Limit

1.2 Voltage Limit

1.3 Stability Limit

1.3.1Transient Stability Limit

1.3.2 Small Signal Stability Limit

1.3.3 Voltage Stability Limit

2. Increase (control) power transfer capability of a line

3. Mitigate subsynchronous resonance (SSR)

4. Power quality improvement

5. Load compensation

6. Limit short circuit current

7. Increase the loadability of the system

Demerits

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Types of FACTS Controllers

FACTS

Series-Shunt

Series-

Series

• Thyristor Controlled Series Capacitor (TCSC) • Static Synchronous Series Compensator (SSSC)

Series

Shunt

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Figure: Basic type of FACTS controllers: (a) general symbol for FACTS controller; (b) Series Controller; (c) Shunt Controller; (d) Unified Series-Series Controller; (e) Coordinated Series and Shunt Controller and (f) Unified Series-Shunt Controller.

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Series Controllers

1. It could be a variable impedance such as capacitor,

reactor, etc or power electronics based variable

source of main frequency, sub-synchronous or

harmonic frequencies ( or a combination).

2. All series controller inject voltage in series with line.

3. If voltage is in phase quadrature with the line

current, it only supplies or absorbs the variable

reactive power.

Shunt Controllers

1. It could be a variable impedance, variable source or

a combination of these.

2. All shunt controllers inject current into the system

at the point of connection.

3. If injected current is in quadrature with the line

voltage, it only supplies or absorbs the variable

reactive power.

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Combined Series-Shunt Controllers

1. It could be a combination of separate shunt and series controllers as coordinated or unified. UPFC is example of this.

2. Combined series and shunt controller injects current

into the system with shunt part and voltage in series with series part of controller.

3. When shunt and series controllers are unified there can be a real power exchange between shunt and series controllers via DC power link.

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Static Var Compensator (SVC)

• Regulate the line

voltage by connecting an inductor or a

capacitor in shunt with the transmission line

• Thyristor Controlled

Reactor (TCR)

• Thyristor Switched

Capacitor (TSC)

A shunt-connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control the bus voltage.

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Thyristor Controller Reactor (TCR)

L

I

α

V

L

V

-

Δ

V

∆Q

A shunt-connected, thyristor-controlled inductor whose effective reactamce is varied in a continuous manner by partial-conduction control of the thyristor valve.

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TCR: Bus voltage and current

σ = 2(π-α)

σ : conduction angle

α : firing angle

B

(α) = 2(π – α) + sin 2α

πX

= σ – sin σ

πX

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Control Characteristic of the TCR Susceptance, B

• B

is maximum at full

conduction ( α = 90° or

σ = 180° )

B

= 1/X

• B

is minimum at α =

180 ° or σ = 0°

B

= 0

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Thyristor Switched Capacitor (TSC)

C

I

α

V

C

V

+

Δ

V

∆Q

A shunt-connected, thyristor-switched capacitor whose effective reactance is varied in a stepwise manner by full-or

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SVC Applications

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Voltage Stability Enhancement th V V I th X P+ jQ

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3. Maximum Power Transfer Improvement

No SVC

P = (V

_{/X) sin δ}

With SVC

P = (2V

_{/X) sin (δ/2)}

Q

= (4V

_{/X) (1 - cos (δ/2))}

jX/2

P + jQ

V

0

V

δ

jX/2

SVC

V

Q

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4. Transient Stability Margin Enhancement

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 0 0 . 5 1 1 . 5 2 2 . 5 3 A c c e le ra ti n g A re a D e c e le ra ti n g A re a D e c e le ra tin g A re a M a rg i n (S h u n t C o m p .)

Equal-Area Criterion With SVC, decelerating area margin

is larger.

jX/2 jX/2

SVC V

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Thyristor Control Series Capacitor (TCSC)

Tunable Parallel LC Circuit

Swedish National Grid TCSC at Stode

A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance.

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TCSC Applications

1. Transient Stability Enhancement

(

)

sin

V V

P

x

δ δ

=

−

(

)

sin

V V

P

x

x

δ δ

=

−

−

plication,

2. Voltage Stability Enhancement

Decreasing line reactance increases maximum active

power demand

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Voltage Source Converter (VSC)-based

FACTS Controllers : STATCOM, SSSC, UPFC

Voltage Source Converter

V

V

P + jQ

DC Energy

Storage

Power

System

AC Voltage Source

with controllable

Magnitude & Phase

X

P + jQ

V

Ө

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Active & Reactive Power Control

by VSC

X

P + jQ

V

0°

V

Ө

P = (V

V

/X

) sin Ө

Q = V

(V

cos Ө - V

)/X

V

V

Ө

Control Variables for Power Flow Direction

1. Active Power Flow => Phase difference Ө

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Active & Reactive Power

Diagram of VSC

P

Q

Supplies P

Supplies Q

Rectifier Inverter

V

V

Ө

V

V

Ө

Ө

V

V

V

Ө

V

Absorbs P

Supplies Q

Absorbs P

Absorbs Q

Supplies P

Absorbs Q

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Static Synchronous Compensator

(STATCOM)

V

V

I

Q

V

+

-

I

V

V

X

I

STATCOM is the voltage-source converter, which converts a DC input voltage into AC output voltage in order to compensate the active and reactive needed by the system.

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I

V

> V

V

< V

V

Inductive Mode

(absorbs Q)

-I

Capacitive Mode

(supplies Q)

At AC Terminal

V

V

X

I

Control Modes of STATCOM

Advantages

1. Voltage Stability Enhancement

2. Angle Stability Improvement

3. Power Quality

plication,

Static Synchronous Series Compensator

(SSSC)

X

I

V

I

V

Q

SSSC is the solid-state synchronous voltage source employing an appropriate DC to AC inverter with gate turn-off thyristor used for series compensation of transmission lines.

I

V

Capacitive Mode

(supplies Q)

Inductive Mode

(absorbs Q)

-V

At AC Terminal

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Control Modes of SSSC

X

I

V

V

V

- Injected voltage (V

) emulates an inductive or a capacitive

in series with the transmission line.

V

V

V

V

δ

V

V

V

V

δ

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Unified Power Flow Controller (UPFC)

V

V + V

V

• Independent reactive power exchange between shunt/series

converters and power system.

• Active power constraint : P = P

V

I

V

V + V

Q

Q

P

STATCOM SSSC

UPFC is a combination of STATCOM and SSSC, which are coupled via a common DC link, to allow bi-directional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM, and are controlled to provide concurrent real and reactive series line compensation without an external electric energy source.

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HVDC

The High Voltage Direct Current (HVDC) technology

is used to transmit electricity over long distances by

overhead transmission lines or submarine cables. It is

also used to interconnect separate power systems,

where traditional alternating current (AC) connections

can not be used.

Converter Converter DC line or cable

AC AC

Limitations of HVAC Transmission

1.

Reactive Power Loss

2.

Stability

3.

Current Carrying Capacity

4. Ferranti Effect

Solve by

HVDC

Transmission

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Advantages of HVDC

1. Total investment cost of HVDC transmission is lower.

Investment Cost

Terminal

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Terminal Cost & Transmission Line Cost

1.1 A HVDC transmission line costs less than an AC line for the same transmission capacity.

1.2 DC terminal cost is more

expensive than AC terminal cost. 1.3 But above a certain distance, the

so called "break-even distance", the HVDC alternative will always give the lowest cost.

1.4 Break even distance: 600 ~ 800 km

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2. HVDC cable transmissions for long distance water

crossing

In a long AC cable transmission, the reactive power flow due to the large cable capacitance will limit the maximum possible transmission distance. With HVDC there is no such limitation, why, for long cable links, HVDC is the only viable technical alternative. The longest HVDC submarine cable presently in operation is the 250 km Baltic Cable

transmission between Sweden and Germany. Several HVDC submarine cables of 500 km or more are currently being planned in Europe and elsewhere.

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3. HVDC transmission has lower losses.

An optimized HVDC transmission line has lower

losses than AC lines for the same power capacity. The losses in the converter stations have of course to be added, but since they are only about 0.6 % of the transmitted power in each station, the total HVDC transmission losses come out lower than the AC losses in practically all cases. HVDC cables also have lower losses than AC cables. The diagram below shows a comparison of the losses for overhead line transmissions of 1200 MW with AC and HVDC.

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4. HVDC transmission for asynchronous connection

Many HVDC links interconnect incompatible AC systems. Several HVDC links interconnect AC system that are not running in synchronism with each other. System frequencies of both areas may be same or different.

Examples

4.1 Interconnected System with same frequency

a) UCTE (Union for the Co-ordination of transmission of Electricity) and Nordel

(websites: www.ucte.org and www.nordel.org) b) US Eastern & Western

4.2 With different frequency: In Japan 50-60 Hz systems

Converter Converter DC line or cable

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Interconnection with Same Frequency

The Nordel power system in

Scandinavia is not synchronous with the UCTE

grid in western continental Europe even though the nominal frequencies are the same.

The power system of eastern USA is not synchronous with that of western USA. The reason for this is that it is sometimes difficult or impossible to connect two AC

networks due to stability reasons.

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Other advantages of HVDC

5. Require less space compared to ac for same voltage rating and size

6. Ground can be used as returned conductor

7. Less corona loss and radio interference

8. No charging current

9. No skin and Ferranti effect

10. No switching transient

11. An HVDC transmission limits short circuit currents

12. HVDC transmission for controllability of power flow 13. Environmental benefits.

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Disadvantages of HVDC

1. High cost of terminal equipments

HVDC transmission system requires converters at both ends and those are very expensive than ac equipments

2. Introduction of harmonics

Converter generate considerable amount of harmonics both on ac and dc sides. Some harmonics are filtered out but some harmonics still enter into the system and affect the apparatus These harmonics may also interfere with communication system.

3. Blocking of reactive power

DC lines block the flow of reactive power from one end to another end. These reactive powers are required by some load that must be fulfilled by the inverters.

4. Point-to-point transmission not possible.

It is not possible to tap dc power at several locations in the line. Wherever power is to be trapped, a control

station is required and coordinated with other terminals. This increases the complexity and cost of the systems.

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Types of HVDC Links

1. Monopolar & Bipolar

Monopolar - Having one conductor and ground is used as return path. Bipolar

-There are two

conductors (Poles). One operates at +v polarity and other is on –v polarity. -During fault in

one pole, it works as monopolar.

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2. HVDC back-to-back station : Japan 50/60 Hz systems

2.1 To create an asynchronous interconnection between two AC networks, which could have the same or different frequencies.

2.2 Both the rectifier and the inverter are located in the same station

2.3 The direct voltage level can be selected without consideration to the optimum values for an overhead line and a cable, and is therefore normally quite low, 150 kV or lower. The only major equipment on the DC-side is a smoothing reactor.

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3. HVDC multi-terminal system

3.1 A multi-terminal HVDC transmission is an HVDC system with more than two converter stations.

3.2 A multi-terminal HVDC transmission is more complex than an ordinary mono/bi-polars transmission. In particular, the control system is more elaborate and the telecommunication requirements between the stations become larger.

3.3 There is only one large-scale multi-terminal HVDC system in operation in the world today. It is the 2000 MW

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Converter

1. Sending end converter works as rectifier (converts AC power to DC power), however converter at receiving end works as inverter

(converts DC power to AC power).

2. Several thyristors are connected in series/ parallel to form a valve to achieve higher voltage/current ratings.

3. Line-commutated converter: use thyristor as switch

Self-commutated converter: use Gate-turn off (GTO) thyristor etc, as switch

3 phase arrangement inside a valve hall (500 kVdc / 825MW).

3 phase converter arrangement (thyristor and arresters):

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HVDC Converter Transformers

1. Voltage transformation between the AC supply and the HVDC system.

2. Supply of AC voltages in two separate circuits with a relative phase shift of 30 electrical degrees for reduction of low order harmonics, especially the 5th and 7th harmonics.

3. Act as a galvanic barrier between the AC and DC systems to prevent the DC potential to enter the AC system.

4. Reactive impedance in the AC supply to reduce short circuit currents and to control the rate of rise in valve current during commutation.

For six-pulse converter, a conventional 3-phase or three single phase transformers is used. Converter transformers serve several functions.

1 phase / 3 winding /354 MVA

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DC Smoothing Reactors

A DC reactor is normally connected in series with the converter. The main objectives of the reactor are:

1. To reduce the harmonic currents on the DC side of the converter. 2. To reduce the risk of commutation failures by limiting the rate of

rise of the DC line current at transient disturbances in the AC or DC systems.

Air-core smoothing reactor in the

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AC/DC Filters

1. Harmonics generated by converters are of the order of

np±1 in AC side and np in DC side where p is number of

pulses and n is integer.

2. Filters are used to provide low impedance path to the ground for the harmonic currents.

3. They are connected to the converter terminals so that harmonics should not enter to the AC system.

500 kV DC-filter with Suspended capacitor Two three-phase AC filter banks for

400 kV at the Tjele HVDC converter station, Denmark.

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Reactive Power Sources

Conventional HVDC converters always have a demand for reactive power. At normal operation, a converter consumes reactive power in an amount that corresponds to approximately 50 % of the transmitted active power. The least costly way to generate reactive power is in shunt connected capacitor banks.

400kV shunt capacitor at the Dannebo HVDC converter station, Sweden Capacitor Bank

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HVDC Light

1. HVDC Light unit sizes range from a few tens of MW to presently 350 MW and for DC voltages up to ±150 kV and units can be connected in parallel.

2. HVDC Light consists of two elements: converter stations and a pair of cables. The converter stations are Voltage Source Converters (VSCs) employing state of the art turn on/turn off IGBT power semiconductors. (IGBT = Insulated

Gate Bipolar Transistor) => Self-commutated switch

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HVDC Light Applications

1. Infeed of small-scale generation e.g. small hydraulic generators, windmill farms and solar power etc.

2. Feed small local loads, isolated load, and island 3. Asynchronous grid connection etc.

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Conventional HVDC & HVDC Light

HVDC Light main circuit Conventional HVDC

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Whether HVDC or FACTS ?

1. Both are complementary technologies.

2. The role of HVDC is to interconnect ac systems where a reliable ac interconnection would be too expensive.

2.1 Independent frequency and control 2.2 Lower line cost

2.3 Power control, voltage control and stability control possible

3. The large market potential for FACTS is within AC system on a value added basis where

3.1 The existing steady-state phase angle between bus node is reasonable

3.2 The cost of FACTS solution is lower than the HVDC cost 3.3 The required FACTS controller capacity is lesser than the

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Costs of HVDC & FACTS

20-30 M 120-170 M 1000 MW 30-50 M 200-300 M 2000 MW 10-20 M 75-100 M 500 MW $ 5-10 M $ 40-50 M 200 MW FACTS HVDC 2 terminal Throughput 50/kvar UPFC (shunt portion) 50/kvar UPFC (series portion) 50/kvar STATCOM 40/kvar TCSC 40/kvar SVC 20/kvar Series Capacitor 8/kvar Shunt Capacitor Cost (US$) FACTS

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References

1. N.G. Hingorani & L. Gyugyi, Understanding FACTS, IEEE Press. 2. Y.H. Song & A.T. John, Flexible AC Transmission Systems

(FACTS), IEE Power and Energy Series.

3. R.M. Mathur & R.K. Varma, Thyristor-Based FACTS Controllers for Electrical Transmission Systems, Wiley.

4. E. Acha et al, FACTS Modelling and Simulation in Power Networks, Wiley.

5. P.M. Anderson, Series Compensation of Power System,PBLSH! 6. E. Acha et al, Power Electronic Control in Electrical Systems,

Newnes.

7. S.N. Singh, Electric Power Generation, Transmission and Distribution, Prentice-Hall.

8. P. Kundur, Power System Stability and Control, McGraw Hill. 9. Pardiya, HVDC Power Transmission System, Wiley.

10. E.W. Kimbark, Direct Current Transmission, Wiley.

11. E.Uhlmann, Power Transmission by Direct Current, Springer. 12. J.Arrillaga, High Voltage Direct Current Transmission, IEE.