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Variable Shunt Reactors for Reactive Power Compensation
Variable Shunt Reactorsfor
Reactive Power Compensation
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The flow of reactive power inelectrical networks is a conse-quence of varying reactive powerconsumption and generation.
As the load requirement ischanging, the reactive currentdrawn from the system alsochanges. When reactive currentflows, a voltage drop occursand the system voltage willdecrease with respect to theno-load value. Furthermore,the flow of reactive current willcause additional losses in thenetwork.
On systems, where the varyingreactive power requirements
Introduction
due to changing consumptionand/or generation cannot betolerated, it is common practiceto provide correction or com-pensation means in order tomaintain operation of thepower system within definedand accepted conditions.
In networks, where consumptionand generation are reasonablypredictable and stable, fixedshunt capacitor banks or fixedshunt reactors are used. How-ever, reactive power compensa-tion is then accomplished for aspecific operating condition only.More flexibility can be achievedby providing for mechanically
switching of those units.A special case of voltage regu-lation occurs when loadschange very rapidly (e.g. in el-ectric arc furnaces) - since suchrapid voltage changes (flickerphenomena) can cause distur-bances in lighting systems andin other power equipments. Toimprove the voltage quality insuch cases, Static Var Compen-sator (SVC) installations areprovided. An SVC, consisting ofa thyristor-controlled reactor inparallel to a fixed capacitorbank, provides a fast controll-able reactive load which res-ponds very quickly to voltagechanges.
Fig. 1 Distributed generation - wind farm
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Application
If such a very fast and dynamicresponse is not mandatory, acontinuously Variable ShuntReactor (VSR) may be the opti-mum choice for providing therequired function of reactivepower compensation over awide range of service condi-tions at minimum investmentand operating costs.
Functions which may beachieved by VSRs are:
• maintain steady-statevoltage limit conditions
• keep the reactivepower flow withinpredefined limits
• maintain a desiredpower factor
As part of the overall reactivecompensation scheme, a conti-nuously VSR may provide reac-tive power compensation andsmooth steady-state voltagecontrol in response to the dailyload cycle and possible chan-ges in power generation. Fur-thermore, the flexibility of VSRs
offers the ability to adapt to sy-stem topology changes or newoperational requirements in thefuture.
Typical network conditionswhich favor the application ofVSRs are:
• Networks with distributedgeneration (e.g. solar, wind,combined heat and powerplants, etc.) may not alwaysprovide full control over theirelectrical output.This may create problems ofincreased flow of reactivepower due to the varyingreactive power of both,generation as well asconsumption.
• Strongly varying loadspowered through relativelylong overhead lines or cables(e.g. power supply of islandsor that of remotely locatedindustrial plants). The appli-cation of a continuously VSRin shunt connection with theload will relieve the feedingline from reactive current andthereby mitigating the line losses and improving the vol-tage quality at the customer.
Fig. 2 Variable Shunt Reactor (VSR), 33 kV-15 Mvar
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Equipmentdescription
With more than 3 decadesof successful field experience,Trench is the recognized worldleader in designing and manu-facturing of power reactors forall utility and industrial applica-tions. Our unique design ap-proach for VSRs is the appro-priate answer for continuousadjustable reactive power com-pensation within minutes /hour time scale.
The VSR obtains adjustability ofit’s power output via variationof it’s inductance – achieved byadjusting the air gaps of it’smagnetic circuit. This is reali-zed by means of moveable
plunger cores. Depending onthe position of the twin cores,an air gap of variable length in-side the winding is provided.
The magnetic circuit consistsessentially of cylindrical shapedtwin plunger cores and severalC-shaped yokes, that enclosethe plunger cores as well as thewinding. By rotating the cen-trally arranged spindle (via amotor drive unit) the cores areapproached or separated –hence the air gap between thecores may be adjusted fromnear zero to about the fulllength of the winding.
This results in a wide adjust-ment range of the compensa-tion power of the VSR. The ac-tion of varying the inductancemay take about one minute, atime period which is approxi-mately the operation time oftransformer tap changers, thetraditional means for maintai-ning a constant system voltagelevel.
Fig. 3 Schematic design of a Variable Shunt Reactor (VSR)
cover
A
N
B
worm gear
Coperating shaft
angular gear
motor drive cabinet
movable plunger coresspindlewindingtank
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Design Features
Special Design
• 3 – phase, iron-cored,oil-immersed coil withvariable air gap
• continuously adjustable,via a motor drive unit inon-load condition
• common oil-filled steel tankwith detachable conservator
• with flanged-on radiatorsand throttle-valves
• mobile underbase with bi-directional rollers or skid base
• porcelain or condenserbushings according to vol-tage level and/or creepagedistance requirements- optional: touch-freedesign via plug-in cabletermination system
• motor drive unit, 230/400 V50 Hz, with pushbuttons for coil adjustment,with hand – crank foremergency operation
• cabinet IP 54 withanti-condensation heatercontrolled by thermostat,mechanical position indicator
• double potentiometer forremote indication
• current transformers,10 VA cl. 0,2
• contact thermometerwith signalization
• double float Buchholz relay
• magnetic type oil levelindicator with signalization
• dehydrating silicagel breather
• further protection and moni-toring devices on request
• oil filling: transformer oilon naphtenic basis,inhibited or non-inhibited,acc. IEC 60296:2003
• surface treatment: sandblasted with multiplecoating, top coat colorRAL 7033
• 3 separated singlephase units, mechanicallyconnected via a commongear box
• uniform insulation(non-graded)
• loss optimized design
• extended powerregulation range
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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 Y N
Cooling method O N A N
Fig. 4 Variable Shunt Reactor (VSR), 47 kV - 7 Mvar
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Automatic Controller
Fig. 5 Automatic controller
In order to allow for automaticcontrol of the VSR, Trench is alsoable to provide an automaticcontrol device (based on oursuccessful concept of the EFC 50earth fault compensation con-troller).
The EFC 50 regulates the VSRwithin the given limits of reac-tive power demand – this is par-ticular convenient for unmannedor fully automated stations.
It further allows remote opera-tion of the VSR by means of di-gital inputs or via a substationautomation and control system.The automatic controller further-more provides data logging andcomprehensive recording func-tions.
Subject to change whithout notice09.10
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