Parallel Operation of Inverters - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 284

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 tomeet 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, the global settings of the rated voltage
and frequency should be accurate. The cases with R-, L- and C-inverters are discussed, with
experimental results provided.
19.1 Introduction
There are several reasons why inverters are needed to operate in parallel. One obvious reason
is because of the limited availability of high current power electronic devices. Another reason
is that parallel-operated inverters are able to provide system redundancy and high reliability
needed by critical customers. Moreover, the parallel operation of inverters also eases the
difficulties in thermal management and design for high-power inverters.