Emerging markets, such as distributed power, are increasing the size of power conversion applications. One solution for generating high-power outputs is to place a number of electronic voltage source inverters (VSIs) in parallel. To date, engineers have used several methods to overcome the difficulties associated with paralleling VSIs for standalone operation. These methods, however, take only the fundamental components of a load current into account. In addition, the transmission rate of commands or power calculations limit the system's bandwidth response, which can cause problems during sudden-load applications or removals, motor hard-starting, and nonlinear rectifier load conditions that are typical in existing power systems. A new alternative — the Ecostar Power Converter (EPC) — acts like one unit. It offers an advanced algorithm that reliably connects up to 8 units (1MW each) in parallel and achieves balanced load-sharing during tough loading conditions.

Synchronization Scheme

To minimize circulating real power and maximize power usage, it's necessary to minimize the phase-angle difference among the parallel EPCs. Fig. 1 shows a parallel system with two EPCs. L1 and L2 represent the output inductance of each EPC, and v1 and v2 indicate the output voltage vectors from the two voltage sources. The phase-angle difference between the two voltage vectors is represented as δ.

You can determine the circulating real power between v1 and v2 by using the following equation:

Circulating Real Power =
V1× V2× sin (δ)/2πf/(L1+L2)

The parameter f is the fundamental operation frequency of the inverters, and V1 and V2 are the rms values of the AC output voltage of v1 and v2, respectively. The circulating power and load sharing is sensitive to the phase-angle difference (δ) between any two modules. Therefore, it's important to minimize the phase-angle difference between inverters when paralleling VSIs.

The EPC's synchronization method is based on a square-wave signal generated by the fundamental operating frequency. This square-wave signal is coupled to all of the units in the system. Each EPC then produces a saw-toothed triangle waveform that contains the data needed to generate AC voltage output.

With this synchronization scheme, the updating rate of each EPC doesn't have to be the same or be synchronized for the phase-angle synchronization among the parallel EPCs. Furthermore, the fundamental operating and sampling frequencies do not need precise synchronization.

This method also eliminates the need for analog-to-digital conversion between the microprocessor and the saw-toothed wave generator because the synch signal source is digital and provides much better noise immunity than analog signals. The digital signal is based on a relatively low frequency. Thus, the system bandwidth requirements for transmission among the parallel EPCs is low.

Standalone Mode Performance

In Fig. 2, you can see how a two-unit system maintains balanced load sharing during the transient response to a large step-load test. Around the time of the trigger point, marked as “T,” the load changes from 40kW to 80kW. Channels 2 and 3, which represent the phase A current of each EPC, remain in close proximity throughout the test.

Fig. 3 shows a dynamic-load test result for two parallel EPCs. At the time of the trigger point, someone hard-started a 20 hp induction motor. Right after the starting, the load current significantly increased because of the high inrush current at zero speed. The motor then accelerated to the rated speed. (Note: the load current was constantly changing during the acceleration.)

The results indicate that a two-parallel system keeps load sharing balanced — even during the transient response to motor hard-starting. This is demonstrated by the almost true synchronization of Channels 1 and 2 throughout the duration of the test.

The last figure, Fig. 4, depicts a redundancy test result for a total load of 100kW for three parallel EPCs. Channels 2, 3, and 4 represent the phase A current of each EPC. At the time of the trigger point, Channel 4 deliberately drops off the system. The other two EPCs immediately pick up the load and maintain balanced load sharing.

Conclusion

If you need high-power output from a power conversion application, paralleling VSIs is a good approach. This technique allows customers to establish a range of power ratings with one basic design or expand a system by adding more modules. Parallel VSI systems also improve reliability because each VSI handles lower currents and dissipates less heat. Finally, and most important, this method synchronizes the phase angle of VSIs to minimize circulating power and achieve fast, dynamic responses to various load-sharing actions.

Mike Wang is a product design engineer with Ballard Power Systems in Dearborn, Mich. You can reach him at michael.wang@ballard.com.

Doug Deng is a principal engineer with Ballard Power Systems. You can reach him at doug.deng@ballard.com.