Power Quality Solution - Static Var Generator

1. Abstract

Static Var Generator is an important basic component in FACTS (flexible AC transmission systems) technology. It is the latest emerged reactive compensating device based on big power inverter, also an important part of flexible AC transmission systems with advanced science & technology in the world in power industry. Static var generator applied electric power technology, computer technology and modern control technology to power system to control output voltage phase, control power system network parameters and network structure flexibly and fast, then adjust reactive power continuously from capacitive to inductive, which realize reactive power compensating fast, then suppress voltage fluctuation and increase system stability. SVG can dynamically compensate reactive power current and harmonic current, which reduced circuit loss, increased the active power transmission capability, and finally improved power quality. SVG have some advantages, for example: fast respond rate, consecutive reactive power absorbing and low power consumption. 

2. Working principle

2.1 Main circuit

From the above diagram, for the circuit applied to three-phase four-wire connection system, the Bridge arms I_{ca ,} I_cb I_cc of SVG will compensate reactive power and harmonics on I_la, I_lb,I_lc of system circuit, then for zero-sequence current on Neutral Line caused by unbalanced load, it is compensated solely by the fourth Bridge arm I_cn so that its value is 0. For output filer section, it mainly filters high frequency noise generated by on/off of switch components so that there is no any interference on output signal of the device. Between main circuit and Low-pass Filter, they are input inductances, which convert voltage signals from main circuit to output current signal. At DC side of inverter, there is a capacitance with large capability; the voltage of the capacitance must keep constant. (Because big voltage fluctuation at DC side will influence output instruction signal of inverter)

2.2 Control circuit

This part mainly realized instruction signal generation and all kinds of protection functions; the related circuit is as followings:

The instruction signal current is calculated by TTA arithmetic, once get the synchronized signal of power network voltage, the instruction signal of circuit can be gotten, then use converter output current acquired by sensor to compare with instruction signal, the difference between converter output current and instruction current go across PI adjustor and limiter, then compared with triangle wave, then after modulation, PWM drive signal is gotten from main circuit. For DC side, the voltage is stable; its value was obtained through sensor to compare with 750V, and then sent to PI adjustor (at the same time, must acquire synchronized Sine Cosine Signal of power network) and limiter, finally we get the active power current signal with Fundamental-Positive Sequence, it superpose with current instruction through TTA arithmetic together to compare and modulate with output current of inverter.

2.3 Technology characteristic

2.4  Specification description

2.5  Application scope

It is very suitable for AC motor reactive power compensating, rectification, frequency conversion, power frequency, Medium Frequency Induction Heating Furnace, High Frequency Induction Heating Furnace, Welding etc mixed loads; It is also suitable for low voltage big power electrolysis, galvanization, electric arc furnace, oil field pumping unit, steel rolling factory, chemical industry, subway, machine making factory, wind power plants, pump station, port, automobile factory, Conference and Exhibition Venues, office building and so on.