21-11-2012, 06:17 PM
Performance study of a Continuously Controlled Shunt Reactor for Bus voltage Management in EHV systems
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Abstract
In EHV substations, it is a common practice to use
breaker switched bus reactors to maintain the bus voltage within
permissible limits under varying load conditions. With the
development of Controlled Shunt Reactor (CSR) which is a
thyristor controlled high impedance transformer, a stable bus
voltage can be maintained by providing variable reactive power
based on the bus voltage deviations due to the load variations.
The high impedance transformer which is also known as reactor
transformer (RT) can be made to any size without any limitation
unlike gapped core shunt reactors. As a single CSR of large
capacity can be realized with suitable control mechanism, this
approach proves to be technically superior and economical
compared to the existing practice of breaker switched bus
reactors.
A CSR with a detailed control system is modeled along with a
typical EHV system in PSCAD/EMTDC environment. The study
includes the effectiveness of filters introduced in the tertiary of
the reactor transformer in controlling the harmonics generated
during partial conduction of thyristors. The transient and steady
state performance of the CSR system for varying system
conditions is studied and the same is compared with the
conventional practice. The paper presents and discusses the
results of the study.
INTRODUCTION
the application of shunt reactors for controlling the over
voltages in power systems is a well known practice. In the
context of EHV systems, this has special significance due to
the large amount of reactive power generation. The effect of
the same, results in dynamic power frequency over voltages
during line charging and sudden load throw off conditions.
During light load conditions also, the capacitive reactive
power generated in the line results in steady state over
voltages. For the above reasons, the presence of shunt reactors
is a mandatory requirement for the operation of EHV lines.
INDIA.
Presented at the International Conference on Power Systems
Transients (IPST’07) in Lyon, France on June 4-7, 2007
But the permanent connection of the shunt reactors leads to
reduced voltage levels and decreased transmission capacity
of the lines during full load conditions. This problem is
addressed by breaker controlled shunt reactors in many parts
of the world. In such cases, the dynamic over voltages and
problems evolving from breaker switching of reactors are
solved by other technical innovations. The Controlled Shunt
Reactor1 (CSR) which is a thyristor controlled equipment
offers a good solution with a fast response time to take care of
dynamic conditions. Also the switching problems associated
with breakers are completely avoided. An ON/OFF type CSR
is in operation at Itarsi substation in India for last five years.
MODELING OF CSR
CSR is a group of equipment connected in a scheme to
realize the desired functionality. The CSR scheme designed for
bus voltage control application is as shown in fig.1.The main
equipment is the RT which is a three winding transformer. The
primary (HV), the secondary (LV) windings are star connected
and the tertiary is in delta to facilitate the circulation of triplen
harmonics. This is realized from the standard library of
PSCAD with appropriate voltage ratings and impedances. The
key issue is the 100% impedance between the primary and the
secondary windings which has a significant influence on the
full load losses of the transformer. The impedance between the
secondary and tertiary is important for effective control of
harmonics during partial conduction of thyristors. In other
words, the coupling between secondary and tertiary is stronger
compared to the one between primary and tertiary. The
parameters of RT are chosen in line with the practical
experience of building the first prototype CSR.
CONCLUSIONS
The simulation studies prove that the CSR developed can
be effectively used for the management of bus voltage in a
EHV sub station. The main equipment RT being a simple
transformer type, can be designed as a single three phase unit
or as three single phase units. There is no restriction for
selecting the suitable capacity of reactor for any specific
location.
The application of CSR for this purpose provides economic
benefits in terms of space and equipment. As a single large
equipment compared to individual shunt reactor units, CSR
occupies less space and the individual switchgear, protection
and other substation equipment reduce in quantity. The filters
required along with CSR are effective in mitigating the
harmonics produced during partial conduction of thyristors.
The CSR control system being automatic and local bus
voltage dependent, is simple, reliable and fast. Thus it is
technically superior to manual switching of shunt reactors
which is the existing practice in most of the EHV substations.
The problems associated with reactor switching can be averted
with the use of CSR.