The Power Buffer: A Distribution Component for Quality and Performance

1. The Power Buffer: A Distribution Component for Quality and Performance Daniel L. Logue and Philip T. Krein University of Illinois May 11, 2001

2. Presentation Overview • What is a power buffer • Configuration • Control of fast dynamics • Local static var compensation • Example

3. What is a Power Buffer? • A component at the distribution or end-use level that acts as an interface between the distribution system and the load. • Based on “power-electronic building block” (PEBB) concepts for implementation. • Provides static and dynamic control capability.

4. What is a Power Buffer? Problem: • Consider a load operated with power electronics (a motor drive or power supply). • This combination presents a true constant power load to the utility. • The best current techniques use an active rectifier to produce high power quality, but still have constant-power load behavior.

5. What is a Power Buffer? • Instead, use the components to decouple the needs of source and load: • The load needs high-quality consistent power • The load draws a specific amount of energy and has its own power factor • From the source, we would prefer to see constant impedance • The source would benefit from adjustable power factor.

6. Configuration • Here is a motor rectifier-inverter set ready to act as a power buffer. • Notice the internal energy storage – use this as a dynamic buffer element.

7. Configuration • The two blocks use identical circuits (except for control). • The inverter block is becoming standardized as a PEBB.

8. Control of Fast Dynamics • If a power line disturbance occurs, the buffer input should act to enforce constant impedance. • The energy storage makes up the short-term deficit for a sustaining time. • Battery backup can extend the sustaining time if necessary. • When the system recovers, the impedance becomes slightly lower for a time to make up the energy deficit.

9. Control of Fast Dynamics • How? Control the voltage vPWM on a short time-scale to make the input current react as if the load impedance is fixed. • Pulse-width modulation in the rectifier allows direct adjustment of vPWM.

10. Control of Fast Dynamics • Phase, for example: • Consider a line sag. • The input current drops with power buffer control. • The capacitor voltage falls as it supplies the energy shortfall. • Restore it slowly, later.

11. Control of Fast Dynamics • Top: source power • Bottom: internal capacitor voltage

12. Local Static VAR Compensation • We can also adjust the phase of the input current, independent of other effects in the system. • Phase can be adjusted to any leading or lagging value, provided there is sufficient current rating. • We have used cases with parallel loads in which the buffer corrects power factor.

13. Power Buffer Example • Power buffer interfaced constant power load, paralleled with a general RL load. Control L L R R i i pb 1 pb 1 in pb PFC + v v Inverter Rectifier C i pwm in 1 - Power Buffer i 2 L R 2 2

14. Power Buffer Example • The power buffer supplies a voltage regulated inverter that draws 848 W. • Parallel RL load draws 126.1 W and 475.3 VAR. • At t=1 s, the input voltage falls to 90% of nominal for 0.2 s. • During the sag, the buffer input impedance is held constant.

15. Power Buffer Example At t=1 s, the input voltage falls to 90% nominal and recovers0.2 s later. The buffer allows the effect to be spread over 10 s.

16. Power Buffer Example Close up of the input impedance during the source voltage transient.

17. Power Buffer Example Input voltage and current waveforms before and after lock-in of the SVC for power factor correction.

18. Conclusion • A power buffer is a way to control a rectifier-inverter set to produce source and load decoupling. • During transients, impedance can be held fixed. • The power buffer can also absorb or supply reactive power.