Control of Power Inverters in Renewable Energy and Smart Grid Integration

19

Parallel Operation of Inverters

Inverters may have to be operated in parallel directly or indirectly. How to share the load among the inverters operated in parallel is a challenging problem. The synchronverter discussed in Chapter 18 is able to share the load accurately. In this chapter, the inherent limitations of the conventional droop control scheme are revealed at first. It is shown that, when the conventional droop control scheme is adopted, parallel-operated inverters should have the same per-unit output impedance in order for them to share the load accurately in proportion to their power ratings. The droop controllers should also generate the same voltage set-point for the inverters. Both conditions are difficult to meet in practice, which results in errors in proportional load sharing. A robust droop controller is then presented to achieve accurate proportional load sharing without meeting these two requirements and to reduce the load voltage drop due to the load effect and the droop effect. The load voltage can be maintained within the desired range around the rated value. The strategy is robust against numerical errors, disturbances, noises, feeder impedance, parameter drifts and component mismatches, etc. The only sharing error comes from the error in measuring the load voltage. When there are errors in the voltage measured, a trade-off between the voltage drop and the sharing accuracy appears. It is also explained that, in order to avoid errors in power sharing, ...