Variable Shunt Reactors
for
Reactive Power Compensation
Introduction
The flow of reactive power in
electrical networks is a consequence of varying reactive power
consumption and generation.
As the load requirement is
changing, the reactive current
drawn from the system also
changes. When reactive current
flows, a voltage drop occurs
and the system voltage will
decrease with respect to the
no-load value. Furthermore,
the flow of reactive current will
cause additional losses in the
network.
On systems, where the varying
reactive power requirements
Fig. 1 Distributed generation - wind farm
2
due to changing consumption
and/or generation cannot be
tolerated, it is common practice
to provide correction or compensation means in order to
maintain operation of the
power system within defined
and accepted conditions.
In networks, where consumption
and generation are reasonably
predictable and stable, fixed
shunt capacitor banks or fixed
shunt reactors are used. However, reactive power compensation is then accomplished for a
specific operating condition only.
More flexibility can be achieved
by providing for mechanically
switching of those units.
A special case of voltage regulation occurs when loads
change very rapidly (e.g. in electric arc furnaces) - since such
rapid voltage changes (flicker
phenomena) can cause disturbances in lighting systems and
in other power equipments. To
improve the voltage quality in
such cases, Static Var Compensator (SVC) installations are
provided. An SVC, consisting of
a thyristor-controlled reactor in
parallel to a fixed capacitor
bank, provides a fast controllable reactive load which responds very quickly to voltage
changes.
Application
If such a very fast and dynamic
response is not mandatory, a
continuously Variable Shunt
Reactor (VSR) may be the optimum choice for providing the
required function of reactive
power compensation over a
wide range of service conditions at minimum investment
and operating costs.
Functions which may be
achieved by VSRs are:
• maintain steady-state
voltage limit conditions
• keep the reactive
power flow within
predefined limits
offers the ability to adapt to system topology changes or new
operational requirements in the
future.
• maintain a desired
power factor
As part of the overall reactive
compensation scheme, a continuously VSR may provide reactive power compensation and
smooth steady-state voltage
control in response to the daily
load cycle and possible changes in power generation. Furthermore, the flexibility of VSRs
Typical network conditions
which favor the application of
VSRs are:
• Networks with distributed
generation (e.g. solar, wind,
combined heat and power
plants, etc.) may not always
provide full control over their
electrical output.
This may create problems of
increased flow of reactive
power due to the varying
reactive power of both,
generation as well as
consumption.
• Strongly varying loads
powered through relatively
long overhead lines or cables
(e.g. power supply of islands
or that of remotely located
industrial plants). The application of a continuously VSR
in shunt connection with the
load will relieve the feeding
line from reactive current and
thereby mitigating the line
losses and improving the voltage quality at the customer.
Fig. 2 Variable Shunt Reactor (VSR), 33 kV-15 Mvar
3
Equipment
description
With more than 3 decades
of successful field experience,
Trench is the recognized world
leader in designing and manufacturing of power reactors for
all utility and industrial applications. Our unique design approach for VSRs is the appropriate answer for continuous
adjustable reactive power compensation within minutes /
hour time scale.
The VSR obtains adjustability of
it’s power output via variation
of it’s inductance – achieved by
adjusting the air gaps of it’s
magnetic circuit. This is realized by means of moveable
plunger cores. Depending on
the position of the twin cores,
an air gap of variable length inside the winding is provided.
The magnetic circuit consists
essentially of cylindrical shaped
twin plunger cores and several
C-shaped yokes, that enclose
the plunger cores as well as the
winding. By rotating the centrally arranged spindle (via a
motor drive unit) the cores are
approached or separated –
hence the air gap between the
cores may be adjusted from
near zero to about the full
length of the winding.
A
B
N
cover
This results in a wide adjustment range of the compensation power of the VSR. The action of varying the inductance
may take about one minute, a
time period which is approximately the operation time of
transformer tap changers, the
traditional means for maintaining a constant system voltage
level.
C
worm gear
operating shaft
angular gear
motor drive cabinet
tank
winding
spindle
Fig. 3 Schematic design of a Variable Shunt Reactor (VSR)
4
movable plunger cores
Design Features
• 3 – phase, iron-cored,
oil-immersed coil with
variable air gap
• continuously adjustable,
via a motor drive unit in
on-load condition
• motor drive unit, 230/400 V
50 Hz, with push
buttons for coil adjustment,
with hand – crank for
emergency operation
• double float Buchholz relay
• magnetic type oil level
indicator with signalization
• dehydrating silicagel breather
• common oil-filled steel tank
with detachable conservator
• cabinet IP 54 with
anti-condensation heater
controlled by thermostat,
mechanical position indicator
• further protection and monitoring devices on request
• with flanged-on radiators
and throttle-valves
• double potentiometer for
remote indication
• oil filling: transformer oil
on naphtenic basis,
inhibited or non-inhibited,
acc. IEC 60296:2003
• mobile underbase with bidirectional rollers or skid base
• current transformers,
10 VA cl. 0,2
• porcelain or condenser
bushings according to voltage level and/or creepage
distance requirements
- optional: touch-free
design via plug-in cable
termination system
• contact thermometer
with signalization
• surface treatment: sand
blasted with multiple
coating, top coat color
RAL 7033
• uniform insulation
(non-graded)
• extended power
regulation range
Special Design
• 3 separated single
phase units, mechanically
connected via a common
gear box
• loss optimized design
5
Technical Characteristics
System voltage
up to 123 kV
Rated 3-phase power
up to 50 Mvar
Power regulation range
20 – 100 %, stepless adjustable
Rated frequency
50 Hz (60 Hz on request)
Adjustment time from min to max. power
< 300 s
Connection symbol
YN
Cooling method
ONAN
Fig. 4 Variable Shunt Reactor (VSR), 47 kV - 7 Mvar
6
Automatic Controller
In order to allow for automatic
control of the VSR, Trench is also
able to provide an automatic
control device (based on our
successful concept of the EFC 50
earth fault compensation controller).
The EFC 50 regulates the VSR
within the given limits of reactive power demand – this is particular convenient for unmanned
or fully automated stations.
It further allows remote operation of the VSR by means of digital inputs or via a substation
automation and control system.
The automatic controller furthermore provides data logging and
comprehensive recording functions.
Fig. 5 Automatic controller
7
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