Abstract—This paper describes a theoretical and experimental
study on a control strategy for the parallel operation of three-phase
voltage source inverters (VSI), to be applied to UPS. The proposed
control system for each inverter consists of two main loops, which
both use instantaneous values. The first (parallelism control) employs
the feedback of the inductor currents from the output filter
to modify the input voltages of the same filter and thereby control
the power flow of each VSI to the load. Additionally, the second
loop (voltage control) is responsible for controlling the output voltages
of the LC filter, which coincides with the output voltages of
the VSI. The proposed control strategy ensures the proper sharing
of the load current and avoids current circulation among the inverters
during transient and steady-state operation. The VSI and
the proposed control strategy are analyzed in an orthogonal
stationary frame, and as a result, simple and effective models were
achieved. The proposed control system was digitally implemented
in a TMS320F2812 DSP and was verified through experimental results
with a 10 kVA prototype, which has the parallel operation of
two three-phase VSIs.
II. INTRODUCTION
ININTERRUPTIBLE power supply (UPS) devices are employed
to feed critical loads which, at high power values,
utilize a three-phase system. Moreover, inmany applications the
total load consists of a set of single and three-phase loads, which
requires the employment of three-phase UPS, capable of feeding
all types of load. In addition, critical loads also require a power
supplywith high-reliability and redundancy that can be obtained
with the parallel-connection ofUPSs.As iswell-known, the parallelism
of UPS is a problem related to the parallel operation of
voltage source inverters (VSIs).
The parallel operation of three-phase VSIs has a greater complication
due to the complexity and the greater number of variables
involved in a three-phase system. Recent publications in
the literature [1]–[3] broach this problem and they point out
that there is still a need for new solutions to the parallelism of
three-phase VSIs.
It is also well-known that the parallel operation of VSIs
requires a control system to ensure proper operation of the
structure. A traditional solution is the strategy based on the frequency
and voltage droop [4]–[18]. This strategy controls the
average active and reactive power flow from the VSI to the load
and it does not require communication among the inverters. It
provides increased reliability and redundancy but it has errors
associated with load sharing, poor transient response, reduced
voltage regulation and it does not control the division of the
harmonic currents [1], [3]. In the literature there are interesting
studies reported [8], [9], [11], [18]–[22] which minimize the
disadvantages, in most cases addressing single-phase systems.
On the other hand, there are strategies related to communication,
such as central control [23]–[25], master-slave control
[26]–[29] and distributed control [2], [3], [30], [25]. These
strategies are most effective in terms of load sharing, but high
reliability and redundancy are not available due to the communication
between units. In recent years, these strategies have
been based on the instantaneous current control [2], [3], [24],
[34], [35], in which the parallelism control receives information
on the instantaneous current supplied by all units. Good transient
response and the appropriate load sharing among VSIs,
including the harmonics of load current, are advantages introduced
by instantaneous control.
The evolution of these strategies is associated with the
use of microcontrollers, DSP and FPGA in power converters
[36]–[44]. The digital control provides the means to propose
new solutions to complex problems as in the parallel operation
of three-phase voltage inverters.
In this context, this paper proposes a distributed control
strategy for the parallel operation of three-phase VSIs. Each
VSI has its own control unit, responsible for regulating the
output voltages and ensuring its parallel operation.
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