02-11-2012, 06:01 PM
Simulation and controller design of an Interline Power Flow
Controller in EMTP RV
Simulation and controller design of an Interline Power Flow.pdf (Size: 355.62 KB / Downloads: 64)
INTRODUCTION
Flexible AC Transmission Systems (FACTS), based on either
Voltage or Current Source Converters (VSC/CSC), can be used to
control steady-state as well as dynamic/transient performance of
the power system. Converter-based FACTS controllers, when
compared to conventional switched capacitor/reactor and thyristor-
based FACTS controllers such as Static Var Compensator
(SVC) and Thyristor-controlled Series Capacitor (TCSC), have
the advantage of generating/absorbing reactive power without the
use of ac capacitors and reactors. In addition, converter-based
FACTS controllers are capable of independently controlling both
active and reactive power flow in the power system [1].
Series connected converter-based FACTS controllers include
Static Synchronous Series Compensator (SSSC), Unified Power
Flow Controller (UPFC), and Interline Power Flow Controller
(IPFC). A SSSC is a series compensator with ability to operate in
capacitive/inductive modes to improve system stability [3,4]. The
UPFC includes a Static Synchronous compensator (STATCOM)
and a SSSC that share a common dc-link. The IPFC consists of
two or more SSSC with a common dc-link; so, each SSSC contains
a VSC that is in series with the transmission line through a
coupling transformer, and injects a voltage - with controllable
magnitude and phase angle - into the line. IPFCs provide independent
control of reactive power of each individual line, while
active power could be transferred via the dc-link between the
compensated lines. An IPFC can also be used to equalize active/
reactive power between transmission lines, and transfer power
from overloaded lines to under-loaded lines [2].
METHODOLOGY
An IPFC (Fig. 1) uses two or more VSCs that share a common
dc-link. Each VSC injects a voltage - with controllable
amplitude and phase angle - into the power transmission line
through a coupling transformer. Each VSC provides series reactive
power compensation for an individual line and it can also
supply/absorb active power to/from the common dc-link [1,2].
Thus, an IPFC has an additional degree of freedom to control
active power flow in the power system when compared to a traditional
compensator. This capability makes it possible to transfer
power from over- to under-loaded lines, reduce the line resistive
voltage drop, and improve the stability of the power system.
The coupling transformer primary windings of the master and
slave converters are (pseudo) star-connected while their secondary
windings are connected in series with each phase of the transmission
line. In addition, the transformer leakage reactance
allows regulation of the output voltage magnitude and phase
angle, with respect to the transmission line current, and offers stable
control of the VSC power output.
CONTROL SCHEME OF IPFC
The IPFC is designed to maintain the impedance characteristic
of the two transmission lines. The IPFC consists of two converter
systems: (a) a master converter system that is capable of
regulating both resistive and inductive impedances of Line 1;
and, (b) a slave converter system that regulates Line 2 reactance
and keeps the common dc-link voltage of the VSC at a desired
level. So, each VSC is independently controllable.
Balancing the dc voltages Vdc1 and Vdc2 on the capacitors C1
and C2 respectively, is an important concern in multi-level converters
(Fig. 2). Uneven voltage charging on the capacitors can
cause over-voltages on the switching devices and that could be
destructive for them. The problem may be solved by either (a) a
modified PWM switching pattern [11], (b) by a voltage regulator
for each level using an additional charge balancing leg [12], or ©
separate dc sources. In order to maintain an equal voltage in the
dc-side, the voltage of the neutral point must be regulated. Here,
based on [15], the zero sequence current i0 is used to equalize
voltages on the dc-link capacitors of the VSC.
SIMULATION RESULTS
The simulated power system (Fig. 8), modelled with EMTP
RV, consists of two identical transmission systems; their characteristics
are given in the Appendix. In the master system, the controller
is designed to compensate 0.4 pu of the transmission line
reactance and 0.2 pu of transmission line resistance. In the slave
system, the controller is designed to compensate 0.4 pu of the
transmission line reactance and to keep the dc-link voltage of the
IPFC constant. Three test results are presented below to evaluate
the performance of the related control systems.
CONCLUSION
The study of an IPFC system with two parallel lines has demonstrated
the flexible control of active/reactive power to assist in
the transmission system. The behavior of the system under various
transient and load changes at the receiving-end of the transmission
system are presented and analyzed. The results of an
IPFC system with two 3-level NPC VSCs in EMTP-RV have validated
the conceptual design presented in [1,2]. The simulation
results demonstrate the capability of the IPFC in compensating
both resistance and reactance of the transmission line, and maintaining
the dc-link voltage of the IPFC. A balancing circuit based
on zero sequence current is employed to equalise the dc link
capacitor voltages.