24-08-2012, 02:27 PM
Control and Protection of a Microgrid with Converter Interfaced Micro Sources
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Abstract
This paper describes protection and control of a
microgrid with converter interfaced micro sources. The proposed
protection and control scheme consider both grid connected and
autonomous operation of the microgrid. A protection scheme,
capable of detecting faults effectively in both grid connected and
islanded operations is proposed. The main challenge of the
protection, due to current limiting state of the converters is
overcome by using admittance relays. The relays operate
according to the inverse time characteristic based on measured
admittance of the line. The proposed scheme isolates the fault
from both sides, while downstream side of the microgrid operates
in islanding condition. Moreover faults can be detected in
autonomous operation. In grid connected mode distributed
generators (DG) supply the rated power while in absence of the
grid, DGs share the entire power requirement proportional to
rating based on output voltage angle droop control.
INTRODUCTION
The Interconnection of distributed generators (DGs) to the
utility grid through power electronic converters has raised
concern about system protection and smooth transfer between
the grid connected and standalone modes. Parallel converters
have been controlled to deliver desired real and reactive power
to the system. Local signals are used as feedback to control the
converters, since in a real system, the distance between the
DGs may make an inter-communication impractical. The real
and reactive power sharing can be achieved by controlling two
independent quantities – the frequency, and the fundamental
voltage magnitude [1-2]. As the converter can change the
output voltage instantaneously, the load sharing can be
achieved though output voltage magnitude and angle control
[3]. Moreover, smooth transfer between the grid connected
mode and the standalone or islanded mode is crucial in
microgrid operation [4]. Either in grid connected or
autonomous operation, the protection scheme has a great
impact on system operation by ensuring effective fault
isolation and minimum load shedding.
PROPOSED RELAY CHARACTERISTIC
A new inverse time relay characteristic is proposed based
on admittance measurement of the protected line to avoid the
drawbacks of overcurrent and distance relays. A line segment
as shown in Fig. 1 is considered for this purpose. Let the relay
be located at node R and let K be an arbitrary point on the line.
The total admittance of the protected line segment is denoted
by Yt and measured admittance between the points R and K is
denoted by Ym.
CONVERTER STRUCTURE AND CONTROL
DG1 is assumed to be an ideal dc voltage source supplying
a voltage of Vdc1 to the VSC. The converter contains three Hbridges.
The outputs of each H-bridge are connected to singlephase
transformers and the three transformers are connected in
Y. The VSC is controlled under closed-loop feedback.
Consider the equivalent circuit of one phase of the converter
as shown in Fig. 3. In this, u⋅Vdc1 represents the converter
output voltage, where u is the switching function that can take
on values ± 1. The resistance RT represents the switching and
transformer losses, while the inductance LT represents the
leakage reactance of the transformers. The filter capacitor Cf is
connected to the output of the transformers to bypass
switching harmonics, while Lf represents the output inductance
of the DG source. The converter structures of all the single
phase DG sources are same.
SIMULATION STUDIES
Simulation studies are carried out with a six bus microgrid
system as shown in Fig. 4. The micro-grid has three converter
interfaced DGs at BUS-2, BUS-4 and BUS-6. The loads are
represented by Load_1 and Load_2, which are connected at
BUS-3 and BUS-5 respectively. The measured admittance
based protective relays, R1, R2 and R3, are located at BUS-1,
BUS-3 and BUS-5 respectively. Each of the DG is connected
to the microgrid through a circuit breaker which provides the
protection for the DG. The system parameters chosen for the
study are shown in Table-I. The simulation studies
demonstrate the protection of the microgrid in grid connected
as well in autonomous mode, ensuring proper load sharing
among the DGs.
CONCLUSION
The coordinated control and protection of a microgrid in
grid connected and autonomous mode is discussed in this
paper. The proposed protection scheme ensures maximum
reliability of the system, while the converter control enable the
DGs to share power as desired in grid connected or
autonomous operation. The main challenge of the protection,
due to current limiting state of the converters is overcome by
using admittance relays in the proposed scheme. A stable
operation in various operating condition shows the efficacy of
proposed control and protection scheme.