STATIC SYNCHRONOUS SERIES COMPENSATOR (SSSC) The Voltage Sourced Converter (VSC) based series compensators - Static Synchronous Series Compensator (SSSC) was proposed by Gyugyi in 1989. The single line diagram of a two machine system with SSSC is shown in Figure 3.10. The SSSC injects a compensating voltage in series with the 41 line irrespective of the line current. From the phasor diagram, it can be stated that at a given line current, the voltage injected by the SSSC forces the opposite polarity voltage across the series line reactance. It works by increasing the voltage across the transmission line and thus increases the corresponding line current and transmitted power.

Figure 3.10 Simplified diagram of series compensation with the phasor diagram. The compensating reactance is defined to be negative when the SSSC is operated in an inductive mode and positive when operated in capacitive mode. The voltage source converter can be controlled in such a way that the output voltage can either lead or lag the line current by 90o . During normal capacitive compensation, the output voltage lags the line current by 90o . The SSSC can increase or decrease the power flow to the same degree in either direction simply by changing the polarity of the injected ac voltage. The reversed (180o ) phase shifted voltage adds directly to the reactive voltage drop of the line. The reactive line impedance appears as if it were increased. If the amplitude of the reversed polarity voltage is large enough, the power flow will be reversed. The transmitted power verses transmitted phase angle relationship is shown in Equation (3.1) and the transmitted power verses transmitted angle as a function of the degree of series compensation is shown in Figure 3.11.

Figure 3.11 Transmitted power verses transmitted angle as a function of series compensationStatic Var Compensator is a shunt-connected static Var generator or absorber whose output is adjusted to exchange capacitive orinductive currentso as to maintain or control specific parameters of the electrical power system (typically bus voltage). SVC is based on thyristors without gate turn-off capability. The operating principal and characteristics of thyristors realize SVC variablereactive impedance. SVC includes two main components and their combination: Thyristor-controlled and Thyristor-switched Reactor (TCR and TSR); and Thyristor-switched capacitor (TSC).In Figure 1 shows the diagram of SVC:Figure 1 - Static VAR Compensators (SVC): TCR/TSR, TSC, FC and Mechanically Switched Resistor

TCR and TSR are both composed of a shunt-connected reactor controlled by two parallel, reverse-connected thyristors. TCR is controlled with proper firing angle input to operate in a continuous manner, while TSR is controlled without firing angle control which results in a step change in reactance.TSC shares similar composition and same operational mode as TSR, but the reactor is replaced by a capacitor. The reactance can only be either fully connected or fully disconnected zero due to the characteristic of capacitor.With different combinations of TCR/TSR, TSC and fixed capacitors, a SVC can meet various requirements to absorb/supply reactive power from/to thetransmission line

SVG Static Var GeneratorIntroduction:SVG (also known as STATCOM) is a kind of important device in the Flexible AC transmission system (FACTS) components, which faces enormous research opportunities and prospects. In recent years, UNT electric adjusts his development strategy according to the national power network development plan, he seizes the development opportunity, by cooperating closely with the internationally renowned research institutions and domestic universities, to produce successfully the SVG serialization products with below 66kv and 100Mvar of the capacity with his own high-level R&D team, these products obtain the good application.

SVG(SVG-Static Var Generator)is the device of stable voltage and reactive power compensation in the Flexible AC transmission system (FACTS), which can also be used for flow controlling in transmission power system. As the core of the high-power three-phase voltage inverter, , SVG could keep the same frequency and phase with the side voltage of the system after connecting, it can ensure the nature of the output power by adjusting the relationship between the inverter output voltage amplitude with the system voltage amplitude. When the inverter output voltage amplitude is more than the system side voltage, SVG improves capacitive reactive power, conversely, the inductive reactive power.

System Principle1)Coupling Transformer

2)Directly Connected to Busbar

StructureUNT-SVG system mainly consists of control cabinet, starting cabinet, power cabinet, connect reactor and coupling transformer.

The control cabinetComposed of master controller, pulse distribution unit, touch screen, communication management, PLC etc., the control cabinet is used for the real-time controlling of SVG, monitoring the operating state of the system, real-timely calculating the reactive power of the power network required, the dynamic tracking and compensation, and communicating with the host device and the control center.

The master controllerWith the combination of, the master controller has a fast calculation speed and a high surplus. They all have powerful digital signal processing capability, programming independent of each other, shared data though internal communications. The master controller includes logic controlling, status analysis and communication with coordinated controlling of multi-controller.

The pulse distribution unit

The pulse distribution unit mainly controls each pause power module SPWM pulse with monitoring reactive flows on each power modules, state detection and malfunction protection, etc. And it transmits all working data of power modules to the master controller.

The starting cabinetWith a simple structure, the starting cabinet mainly consists of vacuum switch and charging resister, etc.

After the main circuit breaker closing, the system voltage charges the DC supporting capacitor of the power module though charging resister. The charging resistor can avoid the damage of IGBT module or DC supporting capacitor because of high current. After charging, the control system closes the power network vacuum switch.

The power cabinet The power cabinet is the most important part of the Static Var Generator with consisting of power modules.

SVG applies cascaded H-bridge multi-level structure; each phase constitutes by many power modules with high-power IGBT module, which follows international trend of technical development, with its features as following:

The main circuit adopts chain tandem structure, star type connection, each pulse constitutes of the multiple chain modules. Modular design, the power module is interchangeable with simple installation and easy maintenance. Redundant design, with a large margin and high reliability Imported IGBT with good quality Output voltage of the multiple modules superposes to achieve the high switch frequency. High quality of output voltage waveform Compacted structure with small size High power density The connect reactor To connect SVG and the power network and achieve the energy buffer To decrease the switching wave of the output current of SVG and reduce common mode interferenceThe coupling transformer To transform the power network voltage into the suitable working voltage of power cabinet To achieve the electrical isolation of the high-voltage and the low-voltage and improve the system reliability

FeaturesBy adopting the combination of SPWM technology and SHE calculation, the total harmonic distortion of the output current is no more than 3%

Apply multi-processor parallel running, to reduce the sampling control time, to improve dynamic response speed, the response time is no more than 5ms.

Though FPGA controlling gate drive pulse of IGBT, the control accuracy of the device is up to 0.005 degrees. Reactive power output is smooth and continuous, low percentage of uneven module DC voltage and harmonic distain.The control methods designed for electrical system include fast voltage controlling, constant voltage controlling; voltage priority constant reactive controlling, voltage priority constant power factor controlling and other customer customized controlling methods.

Each H-bridge unit applies advanced laminated busbar design with module structure and good interchange.Primary circuit connects with secondary control interface by optical fiber with strong anti-jamming capability and good stability.

Having LVRT capability, and in line with the regulation on LVRT capability of State Grid Corporation.Having master-slave control capability with convenient branch tied expansions

The communication manager is the node of the SVG system and the background control data, and it is compatible with many communication conventions and facilitates the comprehensive self-system data integration.With forced air cooling, it has a good cooling effect.

SVG equips temperature and humidity control system with high intelligence. In the low temperature operating environment, the control system supports the best operating environment for SVG with heating heat distribution and superior dehumidification performance.

With independent intellectual property right, SVG provides technical service and a full range of technical support for users.

Be suitable for the field of high cold and hot temperature, high altitude, etc.

ApplicationLong distance power transmissionGlobal electricity industry is heading to a high-power power network; large energy loss is happened in the long-distance power transmission. The reliability of the power network is becoming the most outstanding problem.

Traction power substation for electric locomotiveThe electric locomotive is the single-phase power supplied, rectified part of traction converter produces much harmonic wave and the load of traction power substation changes quickly when time changes.

Wind photovoltaic power generationPhotovoltaic and wind power generation stores the energy and inverts, and the power is supplied by cluster grid integration. The output power changes according to light and wind speed, which need stable voltage, harmonic filter and compensatory reactive.

Electric arc furnace and rolling millDuring the operation of electric arc furnace and rolling mill, it happens that each phase current unbalance, the voltage flicker, low power factor, reactive and active current variation, a large number of harmonic will seriously affect the power network quality.

Elevator at port and coal minesThe load of elevator with large capacity changes quickly and frequently, when the elevator works, the starting current of the motor is large and can reach to the maximum value instantaneously and produces much reactive power, while when elevator doesnt work, there is no load and DC speed regulating rectifier produces reactive power and harmonic wave.

High frequency welding machine and IF furnace (Intermediate frequency furnace)High frequency welding machine and IF furnace are the typical rectifier-inverter devices, which can produce a large number of high-order harmonic because of impact load to seriously affect the power energy and quality of the power network.

Auto production lineDC speed regulating is used on gear system, electrical welding devices and painting devices, the rectifier produces reactive power and harmonic wave. The reactive power transfer changes quickly according to the transmission load.

Drilling platformThe drilling platform is usually powered by 6-pulse rectifier, 5, 7, 11, 13 harmonic wave is more serious resulting in current increasing in system and low efficiency of the motor.

Intelligent buildingA lot of fluorescent lamps, project lamps, computers, elevators and other electrical equipment can produce serious distorted voltage wave , which seriously affects the power quality.

For the generation of controllable reactive power by the converter can be a voltage source type (VSC) (Fig.1.7a) or a current source type (CSC) (1.7b). However, converters presently employed in FACTS Controllers are based on voltage source converter [3]. The major reasons for this preference are as follows:i) Current source converters require power semiconductors with bidirectional voltage blocking capability. The available high power semiconductors with gate turn-off capability (GTOs, IGBTs) either cannot block reverse voltage at all or can only do it with detrimental effort on other parameters (e.g., increased conduction losses)ii) Dc-link reactor of CSC is practically more lossy than complementary dc-link capacitor of VSC. iii) The CSC requires capacitors at its ac terminals while VSC requires reactors, which may be naturally provided by the leakage inductance of the coupling transformer.

STATCOM based on (a) Current Source Converter (CSC) and (b) Voltage Source Converter (VSC