Q17Power Quality and FACTS

Role of Current Control in DSTATCOM

A DSTATCOM (Distribution Static Compensator) is a shunt-connected VSC-based device used at distribution level to inject a compensating current into the point of common coupling so that the source current becomes sinusoidal, balanced, and in phase with the supply voltage, thereby compensating for harmonics, reactive power, and load unbalance. The performance of a DSTATCOM depends heavily on the current control technique used to force the actual injected current to track the reference compensating current.

Hysteresis Band Current Control

In hysteresis control, the actual injected current is continuously compared with the reference current, and a fixed tolerance band is defined around the reference. The VSC switches are turned on/off whenever the actual current crosses the upper or lower boundary of the hysteresis band, forcing the current to track the reference within the band. This method is simple, robust, and provides fast dynamic response, but it produces a variable switching frequency, which complicates output filter design and can generate a wider harmonic spectrum.

PI-Based Control with PWM

In this technique, the error between reference and actual current is processed through a proportional-integral (PI) controller, whose output is compared with a fixed-frequency triangular carrier wave to generate PWM switching pulses. This gives a fixed switching frequency, which simplifies filter design and reduces electromagnetic interference, but the dynamic response is generally slower than hysteresis control and the PI gains must be tuned for the operating conditions.

Predictive Current Control

Predictive control uses a mathematical model of the VSC and system to predict the future value of the current for each possible switching state, and selects the switching state that minimizes the predicted error at the next sampling instant. This provides fast and accurate tracking with fixed sampling period, though it requires more computational resources and accurate system modeling.

• Hysteresis control: simple, fast, variable switching frequency
• PI + PWM control: fixed switching frequency, easier filtering, slower response
• Predictive control: accurate tracking, needs system model and higher computation