FACTS技术_CIGRE报告

FACTS TECHNOLOGY for OPEN ACCESS. CIGRE JWG 14/37/38/39-24.

FINAL DRAFT REPORT.

AUGUST 2000.

14 August 2000/EGC

CONTENTS.

Introduction.

1FACTS Technology.

2FACTS Applications.

3Planning Methods and Simulation Tools.

4FACTS and Open Access Networks – Case Studies. 5Open Access and Large Systems.

6Economic Evaluation of FACTS Devices.

7Conclusions.

Appendices.

Bibliography.

INTRODUCTION.

A number of CIGRE and IEEE Reports (See the Bibliography – which is not intended to be an exhaustive list of FACTS publications) have been published on FACTS devices, their technology, systems applications and modelling. In this context, FACTS has been defined by the IEEE as “ alternating current transmission systems incorporating power electronic –based and other static controllers to enhance controllability and increase power transfer capability ”

The CIGRE Committees, which have been active in this area over the past few years, are Study Committees 14,37 and 38. Two CIGRE Symposia, one in Tokyo in 1995 and the other in Tours in 1997 also addressed FACTS technology developments and their potential applications in the emerging open access transmission networks.

From this wealth of publications one aspect seemed worthy of preparing, namely a planning guide which summarised the key technological developments in the FACTS area and addressed, in particular, the open access aspects of networks and the scope for the application of FACTS devices therein.

The present Joint Working Group was established in 1997, sponsored by four Study Committees i.e. SC’s 14,37,38 and 39 with its membership nominated from all these Study Committees. The Joint Working Group held its first meeting in France in March 1998 and this Report presents its findings. The interim Report phase has been omitted.

The approved Terms of Reference and Membership List are given in Appendix 0. At its first meeting the definition of FACTS was confirmed by the JWG to include conventional devices such as phase shifting transformers, and HVDC Links.

CHAPTER 1.

FACTS TECHNOLOGY.

1.1Introduction

Flexible AC Transmission Systems (FACTS) controllers originally included only recently developed devices, mainly using semiconductor components. However, in order to provide a complete list of devices for controlling power flows, increasing stability, increasing transfer capability and providing access between different areas in the power system, the definition of FACTS has been broadened in this Report to include traditional devices, some of them without semiconductor control.

1.2FACTS Devices Summary

Terminology

SVC Static Var Compensator

STATCOM Static Synchronous Compenator

MSC Mechanically-switched Capacitor*

TCBR Thyristor Controlled Braking Resistor

TCPST Thyristor Controlled Phase Shifting Transformer

PST Phase Shifting Transformer*

IPC Interphase Power Controller

SC Series Capacitor*

MSSC, TSSC Mechanically/Thyristor Switched Series Capacitor

TCSC Thyristor Controlled Series Capacitor

SSSC Static Synchronous Series Compensator.

UPFC Unified Power Flow Controller

HVDC High Voltage Direct Current

CSC Convertible Static Compensator.

* Not strictly a FACTS device.

Figure 1 tabulates the various FACTS and conventional devices, the system conditions or issues that the devices can be used to address and their key features. Most of the devices are discussed in more detail in the later sections of this Chapter.

FACTS devices

Issue Device Comment See

Section

Steady-state voltage control MSC Stepwise, infrequent

control

Steady-state voltage

control

SVC, Statcom Continuous control 1.4, 1.5

Steady-state voltage

control

SC Continuous control

Dynamic and post-

contingency voltage

control

SVC, Statcom Fast acting control 1.4, 1.5

Improvement of steady-state load sharing PST Environmental

impact

1.9

Improvement of steady-

state load sharing

SC Low losses

Post-contingency load

sharing

PST 1.9 Post-contingency load

sharing

TCSC Fast control action 1.3

Transient stability

improvement

SC Self regulating

Transient stability

improvement

SVC, Statcom High dynamic 1.4, 1.5

Power Oscillation Damping TCSC Insensitive to

localisation

1.3

Power Oscillation Damping SVC, Statcom Needs robust control

algorithms

1.4, 1.5

Power Quality

Improvement

SVC Voltage fluctuations 1.4

Power Quality Improvement Statcom Voltage fluctuations,

very fast control

1.5

Figure 1. FACTS controllers and their applications

1.3Thyristor Controlled Series Capacitors, TCSC

TCSC is used for Power Oscillation Damping (POD), and/or Sub Synchronous Resonance (SSR) mitigation. The series capacitor is provided with a parallel branch using a reactor and a thyristor valve, see Figure 2. This arrangement provides a continuously controllable reactance since the parallel thyristor reactor branch produces a current that adds up to the line current through the capacitor thereby increasing its capacitive size beyond its physical reactance obtained by the line current only.

Figure 2. TCSC

Further development, of the present design, is focusing on cost reduction and increased current handling capability.

1.4Static Var Compensator, SVC

A typical shunt - connected static var compensator, composed of thyristor - switched capacitors (TSCs) and thyristor - controlled reactors (TCRs) is shown in Figure 3.

Figure 3. SVC

The compensator is normally operated to regulate the voltage of the transmission system at a selected terminal. The V - I characteristic of the SVC indicates the regulation with a given slope around the nominal voltage can be achieved in the normal operating range defined by the maximum capacitive and inductive currents of the SVC. The voltage support capability of the conventional thyristor controlled static var compensator rapidly deteriorates with decreasing system voltage.

In addition to voltage support, SVCs are also employed for transient (first swing) and dynamic stability (damping) improvements.

The future trends of the SVC development are focusing on reducing cost and increasing performance, as an example increasing the short time overload capacity. The SVC equipment needs to be re-locatable and the filters (if needed) designed in such a way so resonance with the power system is avoided. The SVC plays an important role in providing dynamic Mvars, especially in heavily loaded meshed networks since the generators will be controlled off and on depending on despatch.or plant economics. The installation of new generating plant or high voltage overlays may greatly strengthen weak points and render some SVCs redundant after only a few years service. In order to simplify relocatability, SVC installations need to be compact, avoiding permanent buildings; outdoor equipment needs to be arranged in-groups of components that are capable of being carried, by road or rail, with a minimum of dismantling. Packaged substations have been used for many years and the use of transportable cabins to provide a weatherproof housing for sensitive components is a similar concept. Thus the thyristor valves, controls, protection, auxiliary power sources etc. need to be inside a cabin, whereas reactors, capacitors, switchgear, auxiliary/earthing transformers, etc, can be mounted outdoors on easily transportable skids or frames, complete with many of their interconnecting busbars.

1.5Static Compensator, STATCOM

Static Synchronous Compensator (STATCOM) employing forced switching type of semiconductors in a converter that functions as a controllable synchronous voltage source, has been introduced for reactive shunt compensation.

The basic principle of reactive power generation by the STATCOM is analogous to that of the conventional rotating synchronous compensator.

Figure 4. STATCOM

From a DC input voltage source, provided by the charged capacitor, the converter produces a set of controllable three - phase output voltages with the frequency of the AC power system. By varying the amplitude of the output voltages produced, the reactive power exchange between the converter and the AC system can be controlled. If the amplitude of the output voltage (V) is increased above that of the AC system voltage (V T), then the current flows (I q) through the tie reactance from the converter to the AC system, and the converter generate reactive (capacitive) power for the AC system. If V is decreased below V T, then the converter absorbs reactive (inductive) power. The STATCOM converter itself can keep the capacitor charged to the required voltage level.

The V – I characteristic indicates that the STATCOM can provide both capacitive and inductive compensation and is able to control its output current over the rated maximum capacitive or inductive range independently of the AC system voltage. That is, the STATCOM, in contrast of the SVC, can provide full capacitive output current at any system voltage, practically down to zero. The STATCOM may have an increased transient rating in both the inductive and capacitive operating regions.

The ability of the STATCOM to produce full capacitive current at low system voltage also makes it more effective that the SVC in improving the transient stability (first swing). The inherent capability of the STATCOM to generate as well as to absorb reactive power makes it eminently suitable for power oscillation damping.

STATCOM uses switched components of thyristor type such as Gate Turn-Off Thyristor, GTO Thyristor, Gate Commutated Thyristor, GCT or Transistor type of components such as the Insulated Gate Bipolar Transistor, IGBT. The Voltage Source Converter, VSC, might be built using series connection of the devices, multi-pulse configuration or multi-level configuration. Using series connection 2 or 3 level design, pulse width modulation (PWM) can be used. PWM simplifies the main circuit design.

GCT is a further developed GTO with gate designed for hard-drive making the component better suited for series connection. One type of the GCT components could also be used for PWM but without series connection. The DC side, the DC capacitor bank, could either be single-phase DC bank if un-symmetrical control is needed and PWM cannot be used.

STATCOM using IGBT is interesting since, as an example, a three-level design and series connection, using high frequency (1- 2 kHz) PWM, makes the device filterless and the technology could be, in addition to the traditional generation or absorption of reactive power, used for active filtering of harmonics. This type of converter also can handle large asymmetries in the system.

Due to the higher performance, the PWM type of STATCOM could find a broader market than the traditional SVC due to increased performance and possibilities for using the STATCOM for active filtering and easier re-locatability (weight and footprint).

1.6 Unified Power Flow Controller, UPFC.

The UPFC is an extremely powerful and versatile concept for power flow control. The UPFC can control, individually or in combination, three effective transmission parameters - voltage, impedance, and angle -, or directly, the active and reactive power flow in the line.

The UPFC is a combination of a STATCOM (converter 1) for the shunt part, and a SSSC (converter 2) for the series part, both connected by a common DC link. See Figure 5.

Figure 5. UPFC

Converter 1 is used primarily to provide the active power demand of converter 2 at the common DC link. Converter 2 itself generates the reactive power demand corresponding to series voltage injection (V

) and, therefore, the transmission system is not burdened by

pq

reactive power flow due to the operation of the UPFC.

Actually, since converter 1 can also generate or absorb reactive power at its AC terminal, independently of the active power it transfers to (or from) the DC terminal it follows that,

with proper controls, it can also fulfil the function of an independent STATCOM. That is it can provide reactive power compensation for the transmission line and thus execute an indirect voltage regulation at the input terminal of the UPFC.

In addition, the UPFC can operate as a series impedance compensator when the shunt element (STATCOM) is out of service and a static var source when the series element (SSSC) is out of service.

Future development needs to focus on less complexity and reduced cost.

1.7SSSC.

The Static Synchronous Series Compensator, SSSC, offers an alternative to conventional series capacitive line compensation. The SSSC is a synchronous voltage source that internally generates the desired compensating voltage – in series with the line – independent of the line current. See Figure 6.

Figure 6. SSSC

The SSSC can be considered functionally as an ideal generator. The SSSC can produce a set of (three) alternating voltages at the desired fundamental frequency with controllable amplitude and phase angle. Further the SSSC can generate or absorb reactive power when tied to an electric power system to function like a synchronous condenser (compensator) and convert the active power it exchanges with the AC system into a DC voltage that is compatible with an electric energy source or storage. The transmitted power becomes a parametric function of the injected voltage. The SSSC can control both reactive and active power with the AC system, simply by controlling the angular position of the injected voltage with respect to the line current.

With the appropriate combinations of SVSs unique FACTS controller arrangements able to control independently real and reactive power flow in individual lines, balance real and reactive flows among line, can be devised. From the standpoint of practical applications, steady state flow control or stability improvements, the SSSC clearly has considerably wider control range then the controlled series capacitor of the same MVA rating.

1.8Switched Series Capacitors, MSSC or TSSC.

Mechanically Switched Series Capacitors, MSSC, are mainly used for load flow control. The capacitor bank is divided into segments each possible to be bypassed by means of breakers. In this way a stepwise controllability of the inserted capacitive reactance is achieved. In case of a large number of switching are needed the breakers can be replaced by thyristor valves acting as electronic switches thus forming a Thyristor Switched Series Capacitor, TSSC.

1.9Phase Shifting Transformer, PST, IPC, TCPAR.

Phase shifting transformer, PST, using tap-changers or thyristor switches for control. See Figure 7.

Figure 7. TCPST

In order to reduce cost some units could be equipped with parallel inductor and this solution is also named Interphase Power Controller, IPC. If the PST is provided with a fast acting switching device, i.e. thyristor switches, the PST is renamed in to TCPST, Thyristor Controlled Phase Shifting Transformer, and can be used for Power Oscillation Damping, POD.

1.10High Voltage Direct Current, HVDC

HVDC is mainly used for the coupling of asynchronous AC systems (Back-to-Back HVDC), for sea-cable transmission with distances of more than 50 – 80 km and for long distance bulk power transmission. Due to the ability to control the converters very fast and almost independent from the AC system conditions, the HVDC technology offers a number of advantages especially for the increased power exchange in a deregulated environment:

?Coupling of AC systems with different frequencies or different rules concerning security, reliability, frequency control, voltage control, primary- and secondary-

control reserve capacity, etc.

?system interconnection independent from stability requirements

?transmission over very long distances without stability problems

?power infeed without increase of short circuit current.

?feeding power from remote generation centres direct into load centres.

?load flow control

?frequency control

?voltage control

?substitution of secondary reserve (under certain conditions)

?stabilizing the AC system by fast DC-power-ramping or –modulation

?economical and ecological advantages at very long distance high power transmission compared to AC

Conventional HVDC use line commutated converters (thyristors) which require inertia of the AC systems. Capacitor commutated converters allow also operation in AC systems with lower short circuit capacity. DC transmission (Back-to-Back or long distance) with self commutated converters (GTO, IGCT or IGBT) allows also for feeding into systems without generation.

Today’s HVDC links at high power level use conventional technology. DC links at the medium and lower power level using self-commutated converters are today available and under further development and test operation, also in medium voltage distribution systems. The near future may see multi-terminal HVDC high power links to exchange large amount of power between remote partners and to integrate the countries along the route. Low power DC-links in the low and medium voltage systems allow the customer to purchase power from different partners according to cost situation without restrictions to electrical parameters like voltage amplitude or phase.

1.11Thyristor Controlled Braking Resistor (TCBR)

Braking resistors have been applied as a means of controlling potentially destabilizing system disturbances. They are designed to provide speed control by dissipating power in a power resistor. Stability limits of synchronous generators can be improved by reducing the imbalance between the machine mechanical power and the generator electrical power due to system faults.

Conventional braking resistors are limited in performance due to the dead time of the breaker and due to the limited number of operations possible. TCBR uses electronic switches thus enhancing the above functions. Also,electronic switching enables braking resistors to provide a dynamic response and a variable amount of braking resistance for improved damping control.

The improved performance of TCBR has been shown in different system studies.

1.12Concluding Remarks.

A number of FACTS devices have been described in the chapter and are the result of 10 years or more of development.

It seems certain that development of FACTS technology will continue, aimed at providing flexible control devices to meet deregulated systems needs whilst continuing to strive for lower costs.

CHAPTER 2.

FACTS APPLICATIONS.

2.1 Introduction.

This Chapter is not intended to be an exhaustive list of FACTS installations, but rather to present details of some major examples of FACTS devices that are presently in service or that are planned to be commissioned in the very near future, and the reasons for their installation. As will be appreciated from the various descriptions the older installations were generally –although not always - installed to ensure compliance with security standards whilst the “newer” installations have rather more commercial rationales, often to facilitate open access.

2.2 TCSC – Slatt, USA

The TCSC system, commissioned in 1993, is installed on Bonneville Power Administration's transmission system and located at BPA's C.J.Slatt substation on the Slatt-Buckley 500 kV line in Northern Oregon. At the substation, six identical thyristor controlled capacitor modules are applied to each of the three phases. The capacitors, current limiting reactors, thyristor switchers and protective varistors are located on three platforms, which are at the potential of the 500 kV line and insulated for a BIL of 1550 kV. In addition, each phase has line disconnects and a bypass breaker. The advanced digital control and protection system, located in a building at ground potential, consists of a master controller and a controller for each capacitor module. Communication between platform and ground is accomplished by fibre optics. The thyristors are liquid-cooled via a ground-based heat exchanger. A water glycol mixture is used for this outdoor application.

The TCSC's high speed switching capability provides a mechanism for controlling line power flow, which permits increased loading of existing transmission lines, and allows for rapid readjustment of line power flow in response to various contingencies. The TCSC also can regulate steady-state power flow within its rating limits. Transmission loading may be limited by system stability or transient stability of generation. The TCSC is a powerful tool to help relieve these constraints. Its controls can be designed to modulate the line reactance and provide damping of system swing modes.

The TCSC provides a mechanism for greatly reducing a potential subsynchronous resonance problem at thermal generators electrically close to transmission lines with series compensation. In some cases, the inability to mitigate SSR with conventional series capacitors has limited line compensation to levels between 20 and 40 percent. With even a small percentage of TCSC, the total compensation can be increased significantly.

The figure 1,below, shows an elementary one-line diagram of the Slatt-TCSC. It is comprised of six identical TCSC modules connected in series. Each module consists of a capacitor, a bi-directional thyristor valve (with its associated reactor), and a varistor. A bypass breaker (with its associated reactor) is connected across the entire device for use in operational and protective functions. Also, three disconnect switches are used to bypass and isolate the TCSC from the Slatt-Buckley transmission line.

Figure 1. One-line diagram of Slatt TCSC.

Each module can operate either bypassed or inserted. In addition, when the capacitor is inserted, the thyristor valve can be phase-controlled to vary the effective fundamental-frequency impedance of the capacitor. The basic operating principles are explained below. While bypassed, the thyristors are gated for full conduction, and the net reactance of the module is slightly inductive because of the reactor in series with the thyristor valve. Note that some current also flows through the capacitor during bypassed operation, but most flows through the thyristor valve and reactor because it is a much lower impedance path.

Figure 2. TCSC control modes

If the capacitor is inserted by turning off the thyristor valve (that is, blocking all gating signals to the thyristors), the effective capacitance of the module is the same as its nominal value. This is illustrated in figure 2. This mode of operation is essentially the same as for a conventional series capacitor. While the capacitor is inserted, the thyristors can be gated near the end of each half cycle in a manner that can circulate controlled amount of inductive

current through the capacitor, thereby increasing the effective capacitive reactance of the module. This concept is referred to as vernier control. In this mode, the inserted reactance can be controlled in a continuously-variable (vernier) manner from a minimum value of the

capacitor alone (1.33 ohms) to as much as 4.0 ohms. The upper limit for vernier operation is a function of line current magnitude and time spent at the operating point.

The Slatt TCSC consists of six modules. The operation of all six modules is automatically co-ordinated from a higher level control system called the common control. All modules receive "ohms" orders from the common level, and these orders establish the operating mode and vernier level for each individual module.

2.3 TCSC - Kayenta, USA

The Kayenta TCSC installation, commissioned in 1993, consists of two series capacitor

banks, each rated 165 Mvar and 1000 Amps with a single phase 60 Hz impedance of 55

ohms. One bank is operated in a conventional series compensation configuration with the second bank subdivided into a 40-ohm conventional segment and a 15-ohm TCSC segment.

Kayenta substation is in the middle of a 320 km 230 kV transmission line.

With power transfers on the interconnected network approaching the transmission system’s ability to reliably serve increasing loads, and with restrictions in building new transmission lines series compensation became an attractive alternative to increase power transfer

capability. Adding of 330 Mvar of series compensation to the line provides 70% series

compensation and increased the power scheduling capability by 30 % to 400 MW.

The following control modes are used:

?current control (power flow control)

?impedance control (current sharing in the meshed system)

?inductive mode (TSR-mode)

The following control modes have been tested successfully but are not in commercial operation:

?power oscillation damping (POD-mode)

?phase balancing

?impedance control mode with inductive impedance

2.4 SVC Harker – NGC

Two Static Var Compensators with a nominal rating of –75 to +150 Mvar each have been installed at the Harker substation in the north of the UK grid near the Scottish border in 1992. The main task of these SVCs is voltage control, but also a stability controller for damping of system power oscillations is implemented.

The National Grid Company (NGC) owns and operates the 400 and 275 kV transmission system in England and Wales. The NGC transmission system is interconnected with the power system in Scotland by means of two double circuit tie lines.

Figure 3. UK grid and system response to a 3-phase fault with and without SVC-POD-control

The SVCs at Harker were specifically installed for the purpose of increasing the transient and dynamic stability margins to meet the appropriate planning standards when designing for an increase in the power transfer from Scotland to England.

The basic control requirement for the Harker SVCs was for constant voltage control with the SVCs to be operated at zero output, so as to have the full capability of the SVCs available for transient stability enhancement. The system damping with only constant voltage control was just adequate as per the planning standards. However it was considered that investing in an additional power oscillation damping (POD) control loop at the same time could be justified economically for the future.

The real power flow across the tie-lines between Scotland and England was chosen for the input signal for the POD controller, based on the experience that there are light damped power oscillations with a frequency of about 0.5 Hz, which is the lowest electro-mechanical

frequency seen in the system. This mode of oscillation is strongly present in the tie-line power flow.

The curves in Figure 3 show the calculated system response following a 3-phase fault east of Harker cleared by line tripping. The power flow in the tie-line as well as the output of one SVC are shown for the case, that no SVC is in operation, that the SVCs operate in voltage control mode only and the POD controller is active additionally. While improving the stability performance also by controlling the voltage a distinctly more rapid damping can be achieved with the POD control activated.

2.5 MSSC - Kanawha River, USA

Kanawha River Mechanically Switched Series Capacitor, commissioned in 1991, is used in order to adopt the compensation level of a backbone 345 kV line in order to have sufficient stability margins during an outage of the parallel 765 kV system. Kanawha River is operated manually from a central dispatch. Depending on loading conditions of 765 kV system the compensation level of the Kanawha River 345 kV line is selected from 0 to 60% compensation in 10 % increments. Thereby always having the necessary capacity of the line in case of 765 kV outage but reducing the compensation level to minimum necessary in order to reduce system losses.

2.6 TCSC - St?de, Sweden

The St?de TCSC, commissioned in 1998, is implemented in order to obtain the desired level of compensation of a long 400 kV line from the northern part of Sweden down to the central part where the load is located. In its southern part the line is also connected to a large nuclear unit. Without the TCSC there is a risk for SSR which would have limited the compensation level of the line below what is desired with respect to system power transfer capability. The installation has a fixed part and a thyristor controlled part. (See Figure 4).

Existing SC Added TCSC

Figure 4. One-line diagram St?de TCSC

St?de TCSC uses a local controller using local measured variables such as line currents and capacitor voltage. The controller calculates the thyristor triggering instants and measures the capacitor voltage and thyristor currents in order to operate the TCSC at a stable boost level of 20 %. (See Figure 5).

Figure 5. TCSC control system.

The installation is unmanned and operated from a central dispatch some 400-km from the installation. A station control and monitoring system collect and stores all events and the installation uses dial up facilities in order to read all information from remote. Operation of breakers and disconnectors are done in a traditional way using RTU’s.

2.7 STATCOM - EAST CLAYDON, England

East Claydon 400kV substation is located approximately 40 miles to the north-west of London. This substation was selected as the site for the installation of an SVC to provide additional compensation in the South. It is necessary for the SVC to fulfil the defined system need at its initial location at East Claydon and also to be arranged for possible subsequent relocation to other NGC substations at either 400kV or 275kV. The STATCOM is expected to be commissioned in late 2000/early 2001.

A STATCOM-based SVC is required to perform at least as well as an equivalent conventional SVC employing TCR/TSCs. The favourable time-scale associated with this project provided the opportunity for NGC to purchase a STATCOM-based device, which implements its stated intention of encouraging new technologies in order to improve the long term performance of the system. NGC placed an order in January 1997 for a 0 to +225 Mvar SVC utilising GTO technology (a STATCOM-type SVC) at East Claydon 400kV substation.