자려식 SVC용 중성점 전위 변동억제에 관한 연구

Abstract

SVC(Static Var Compensator)s play important roles in larger and more complex electric power system. Rapid and continuous reactive power compensation by SVC contributes to voltage stabilization, power oscillation damping, overvoltage suppression, minimization of transmission losses and so on.

Harmonic contents of the three level inverter are less than those of two level inverter at the same switching frequency and blocking voltage of the switching device is half of the DC-link voltage. So three level inverter topology is generally used in realizing high performance, high voltage AC drive system.

Multi level inverters connected in series are suitable for high voltage system because of their circuit structure. They are capable of reducing harmonic components in the AC source side current without requiring high frequency switching.

However, the main problem of multi level inverters without independent DC voltage sources is the unbalance of DC capacitor voltage. Problems in using SVC are the voltage unbalance at each stage of DC capacitor. And, equalizing DC capacitor voltages is required to ensure the even sharing of voltage stress in the power devices, and to compensate reactive power accurately.

In this paper, a simple SVPWM(Space Vector Pulse Width Modulation) method for neutral point balancing of three level inverter is proposed. By using the new PWM strategy, voltage vector selection is easily done like that of two level inverter. And the neutral point voltage control by changing the sequence of voltage vector is easy to implement.

Instantaneous power vector theory which expresses the instantaneous apparent power vector can be applied for controlling reactive power. The validity of the proposed method is confirmed by simulation studies and experiments.