31-01-2012, 02:46 PM
Microgrid Power Quality Enhancement Using a Three-Phase Four-Wire Grid-Interfacing Compensator
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INTRODUCTION
MICROGRIDS can generally be viewed as a cluster of
microgenerators connected to the mains utility grid,
usually through some voltage-source-inverter (VSI)-based interfaces.
Concerning the interfacing of a microgrid to the utility
system, an important area of study is to investigate the impact
of unbalanced utility grid voltages and utility voltage sags,
which are two most common utility voltage quality problems,
on the overall system performance. As a common practice, if
the utility grid voltages are seriously unbalanced, a separation
device, connected between the microgrid and the mains grid to
provide isolation in the event of mains faults
THREE-PHASE FOUR-WIRE GRID-INTERFACING
POWER QUALITY COMPENSATOR
. 1 shows the general layout of the proposed grid-interfacing
power quality compensator. The compensator consists
POWER COMPENSATOR FEATURES AND CONTROL STRUCTURES
of two four-phase-leg inverters, namely inverter A (shunt) and
inverter B (series). The main functions of inverter A are to
maintain a set of balanced sensitive load voltages within the
microgrid even under unbalanced load and grid voltage conditions,
generate and dispatch power, share the power demand
optimally with the other parallel-connected DG systems when
the microgrid islands, and synchronize the microgrid with the
utility system at the instant of connection.
CONTROL OF SHUNT INVERTER A
A. Description of Control Algorithm
As shown in Fig. 2(a), the control system of shunt inverter
A contains a voltage–current regulation block, and external real
and reactive power control blocks. The detailed design of the
power control blocks has already been presented by Li et al. [3]
and is therefore not duplicated here. Instead, this paper focuses
more on the design of the voltage–current regulation block of
shunt inverter A, after giving an introductory description of the
power control blocks for the sake of completeness.
Closed-Loop Transfer Functions
As a common practice, the design of the proposed control
algorithm for the series inverter begins with the inner voltage
loop in the α−β−0 frame. Analyzing Fig. 5© with Kinv =
2/Vdc, the closed-loop transfer function of the inner voltage
loop can be derived as in (12), shown at the bottom of the
page, where RLine and LLine, which normally represent line
resistance and inductance, are here lumped together with the
series transformer winding parameters for convenience. When
performing zero-sequence analysis, neutral line parameters are
also lumped into RLine and LLine.
CONTROL OF COMPENSATOR DURING UTILITY VOLTAGE SAGS (FAULT CURRENT LIMITATION)
The control schemes, presented in the earlier two sections,
regulate the compensator well during normal operating conditions,
but not during utility voltage sags. Referring to Fig. 7,
at the start of a sag, Vsag drops below its nominal value, and
shunt inverter A should now disable its external power control
algorithms and set its voltage references ({V ∗
α, V ∗β} in Fig. 2) tosome appropriate values according to the load sensitivity level.
