Facts Hvdc Controllers

Training Workshop on FACTS Application, Energy, AIT

FACTS & HVDC Controllers by Issarachai Ngamroo, Ph.D.

Sirindhorn International Institute of Technology Thammasat University December 16, 2004

Why FACTS & HVDC ?

Training Workshop on FACTS Application, Energy, AIT

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

Three Gorges HVDCs, China

Why FACTS & HVDC ?

Training Workshop on FACTS Application, Energy, AIT

2. Connection of isolated loads

HVDC or FACTS

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. The HVDC link to Gotland, Sweden

Why FACTS & HVDC ? 3. Interconnection

Training Workshop on FACTS Application, Energy, AIT

HVDC or FACTS 50 Hz

50 Hz

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.

Why FACTS & HVDC ?

Training Workshop on FACTS Application, Energy, AIT

3.2 Between grids (different frequencies)

HVDC 50 Hz

60 Hz

Between grids

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.

Why FACTS & HVDC ?

Training Workshop on FACTS Application, Energy, AIT

4. Increasing existing grid utilization new transmission lines are expensive and not permitted

FACTS FACTS solutions is an attractive means to raise the capability or enhance the reliability of the grid. 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.

Training Workshop on FACTS Application, Energy, AIT

Flexible AC Transmission Systems (FACTS) are the name

given to the application of power electronics devices to 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.

FACTS Concepts V1∠δ1 Training Workshop on FACTS Application, Energy, AIT

Bus 1

V2 ∠δ 2

jx

Bus 2

P12 + jQ12 Active Power Flow Reactive Power Flow

V1V2 P12 = sin (δ1 − δ 2 ) x V12 V1V2 Q12 = − cos (δ1 − δ 2 ) x x

Control Variables 1. Phase Difference : δ1-δ2 2. Voltage : V1, V2 3. Line Reactance : x

Training Workshop on FACTS Application, Energy, AIT

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 1. Expensive

2. Controller interactions are possible

Types of FACTS Controllers

Training Workshop on FACTS Application, Energy, AIT

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

Shunt • Static Var Compensator (SVC) • Static Synchronous Compensator (STATCOM)

Thyristor-based FACTS Controllers: TCSC, SVC ect.

SeriesShunt

SeriesSeries

• Unified Power Flow Controller (UPFC)

• Interline Power Flow Controller (IPFC)

VSC-based FACTS Controllers SSSC, STATCOM, UPFC, IPFC

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

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.

Static Var Compensator (SVC)

Training Workshop on FACTS Application, Energy, AIT

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. • 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)

Thyristor Controller Reactor (TCR)

Training Workshop on FACTS Application, Energy, AIT

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

V

V - ΔV

IL

L α

L

∆QSVC

TCR: Bus voltage and current

Training Workshop on FACTS Application, Energy, AIT

BTCR (α) = 2(π – α) + sin 2α πXL =

σ – sin σ πXL

α : firing angle σ = 2(π-α) σ : conduction angle

Training Workshop on FACTS Application, Energy, AIT

Control Characteristic of the TCR Susceptance, BTCR

• BTCR is maximum at full conduction ( α = 90° or σ = 180° ) BTCR(MAX) = 1/XL

• BTCR is minimum at α = 180 ° or σ = 0° BTCR(MIN) =

0

Thyristor Switched Capacitor (TSC)

Training Workshop on FACTS Application, Energy, AIT

A shunt-connected, thyristor-switched capacitor whose effective reactance is varied in a stepwise manner by full-or zeroConduction operation of the thyristor valve.

V

V + ΔV

IC

C α

C

∆QSVC

Training Workshop on FACTS Application, Energy, AIT

SVC Applications

1. Damping of power oscillations

2. Voltage Stability Enhancement Vth

V

X th I

Training Workshop on FACTS Application, Energy, AIT

P + jQ

Voltage Stability Enhancement

3. Maximum Power Transfer Improvement

V

δ

P + jQ

V

No SVC P = (V2/X) sin δ

QSVC SVC

0

jX/2

With SVC P = (2V2/X) sin (δ/2) QSVC = (4V2/X) (1 - cos (δ/2))

6

Q s v c (m a x ) = 4 * P m a x = 5 .3 2 5

4 P (pu)

Training Workshop on FACTS Application, Energy, AIT

jX/2

V

P m a x (c o m p ) = 2 * P m a x = 2 .6 6

3

P m a x (u n c o m p ) = 1 .3 3

2

1

0 0

50

100 a n g le (d e g )

150

4. Transient Stability Margin Enhancement V

Training Workshop on FACTS Application, Energy, AIT

jX/2

jX/2 SVC

Equal-Area Criterion With SVC, decelerating area margin is larger. 3

D e c e le ra tin g A re a

2.5

2

D e c e le ra tin g A re a M a rg in (S h u n t C o m p .)

A c c e le ra tin g A re a

1.5

1

0.5

0 0

20

40

60

80

100

120

140

160

180

Thyristor Control Series Capacitor (TCSC)

Training Workshop on FACTS Application, Energy, AIT

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.

Tunable Parallel LC Circuit

Swedish National Grid TCSC at Stode

TCSC Applications Training Workshop on FACTS Application, Energy, AIT

1. Transient Stability Enhancement

With SC Without SC

V1V2 sin (δ1 − δ 2 ) P12( nc ) = x V1V2 P12( c ) = sin (δ1 − δ 2 ) x − xc

P12(max) with SC > P12(max) without SC

Training Workshop on FACTS Application, Energy, AIT

2. Voltage Stability Enhancement

Decreasing line reactance increases maximum active power demand

Training Workshop on FACTS Application, Energy, AIT

3. Damping of Power Oscillations

Training Workshop on FACTS Application, Energy, AIT

4. Subsynchronous Resonance (SSR) Mitigation

Voltage Source Converter (VSC)-based FACTS Controllers : STATCOM, SSSC, UPFC

Training Workshop on FACTS Application, Energy, AIT

Voltage Source Converter

Vs

Power System

Vs

0

Vc

DC Energy Storage

P + jQ

P + jQ

Xt

Vc

Ө

AC Voltage Source with controllable Magnitude & Phase

Training Workshop on FACTS Application, Energy, AIT

Active & Reactive Power Control by VSC Vs

Vc Ө

Vs

0°

P + jQ

Vc

Ө

Xt

P = (VsVc/Xt) sin Ө Q = Vs (Vccos Ө - Vs)/Xt

Control Variables for Power Flow Direction 1. Active Power Flow => Phase difference Ө 2. Reactive Power Flow => Voltage magnitude Vc

Active & Reactive Power Diagram of VSC Training Workshop on FACTS Application, Energy, AIT

Q Ө

Absorbs P Vs Supplies Q

Vc

Vc Ө

Rectifier

Ө

Supplies P Supplies Q

Vs Inverter Vc

Vs

Ө

Vc Absorbs P Absorbs Q

P

Supplies P Absorbs Q

Vs

Static Synchronous Compensator (STATCOM)

Training Workshop on FACTS Application, Energy, AIT

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.

Vs

Iq

Vs Iq

Q Vc

Xt Vc

Idc +

Vdc

Training Workshop on FACTS Application, Energy, AIT

Control Modes of STATCOM Vs Iq Xt

At AC Terminal

Iq V c > Vs

Capacitive Mode (supplies Q)

Vac

Vc V c < Vs

Inductive Mode (absorbs Q)

-Iq Advantages 1. Voltage Stability Enhancement 2. Angle Stability Improvement 3. Power Quality etc.

Static Synchronous Series Compensator (SSSC)

Training Workshop on FACTS Application, Energy, AIT

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

Vq

Vq

Xt

I

Q

At AC Terminal

Vq

Inductive Mode (absorbs Q)

I

-Vq

Capacitive Mode (supplies Q)

Control Modes of SSSC

Training Workshop on FACTS Application, Energy, AIT

- Injected voltage (Vq) emulates an inductive or a capacitive in series with the transmission line. Vs

Vq Vs

Vq I

Vr

Xt

VL δ

VL Vr

Vq

Vs

δ

Vr

Training Workshop on FACTS Application, Energy, AIT

Unified Power Flow Controller (UPFC) 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.

V

I

Vpq

V + Vpq Vpq

P

Q

Q V STATCOM

V + Vpq

SSSC

• Independent reactive power exchange between shunt/series converters and power system. • Active power constraint : Pshunt = Pseries

Training Workshop on FACTS Application, Energy, AIT

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 AC

Converter DC line or cable

Limitations of HVAC Transmission 1. Reactive Power Loss 2. Stability 3. Current Carrying Capacity 4. Ferranti Effect

AC

Solve by HVDC Transmission

Advantages of HVDC Training Workshop on FACTS Application, Energy, AIT

1. Total investment cost of HVDC transmission is lower. Investment Cost

Terminal Cost

Transmission Line Cost

Tower Cost

Land Cost

Training Workshop on FACTS Application, Energy, AIT

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 Total DC cost < Total AC cost

Training Workshop on FACTS Application, Energy, AIT

Tower Cost & Land Cost

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

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.

4. HVDC transmission for asynchronous connection Converter

Training Workshop on FACTS Application, Energy, AIT

AC

Converter DC line or cable

AC

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

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

Interconnection with Different Frequencies

Other advantages of HVDC 5. Require less space compared to ac for same voltage rating and size

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

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.

Types of HVDC Links

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

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.

Training Workshop on FACTS Application, Energy, AIT

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 Hydro Québec – New England transmission

Training Workshop on FACTS Application, Energy, AIT

Main Components of an HVDC System

Converter

Training Workshop on FACTS Application, Energy, AIT

3 phase converter arrangement (thyristor and arresters):

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

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

HVDC Converter Transformers

Training Workshop on FACTS Application, Energy, AIT

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

1 phase / 3 winding /354 MVA

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

Training Workshop on FACTS Application, Energy, AIT

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 FennoSkan HVDC transmission

Oil-insulated smoothing reactor in the Rihand - Dehli HVDC

AC/DC Filters

Training Workshop on FACTS Application, Energy, AIT

1. 2. 3.

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. Filters are used to provide low impedance path to the ground for the harmonic currents. They are connected to the converter terminals so that harmonics should not enter to the AC system.

Two three-phase AC filter banks for 400 kV at the Tjele HVDC converter station, Denmark.

500 kV DC-filter with Suspended capacitor

Training Workshop on FACTS Application, Energy, AIT

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.

Capacitor Bank

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

Training Workshop on FACTS Application, Energy, AIT

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

3.

VSC => Active Power & Reactive Power are controllable.

Training Workshop on FACTS Application, Energy, AIT

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.

Rectifier

Inverter

Conventional HVDC & HVDC Light

Training Workshop on FACTS Application, Energy, AIT

Conventional HVDC

HVDC Light main circuit

Whether HVDC or FACTS ?

Training Workshop on FACTS Application, Energy, AIT

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 transmission rating

Training Workshop on FACTS Application, Energy, AIT

Costs of HVDC & FACTS Throughput

HVDC 2 terminal

FACTS

200 MW

$ 40-50 M

$ 5-10 M

500 MW

75-100 M

10-20 M

1000 MW

120-170 M

20-30 M

2000 MW

200-300 M

30-50 M

FACTS

Cost (US$)

Shunt Capacitor

8/kvar

Series Capacitor

20/kvar

SVC

40/kvar

TCSC

40/kvar

STATCOM

50/kvar

UPFC (series portion)

50/kvar

UPFC (shunt portion)

50/kvar

Training Workshop on FACTS Application, Energy, AIT

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.