Shunt reactors (SRs) are used in high voltage energy transmission systems to control the voltage during load variations. Depending on the voltage requirement needs, shunt reactors are switched on or off to provide reactive power compensation. With increasing load variations in today’s system, variable shunt reactors (VSR) are developed as a means to provide more controllability for grid operators in reactive power management by continuously adjusting the compensation according to the load variation. This technology uses a tap changer, of the same type used in power transformers, to vary the inductance by changing the number of electrical turns in the reactor windings. It is now possible to finetune the system voltage and provide regulation capability. The transmission system benefits from improved power quality, optimised grid operation and the possibility of interaction with other regulation devices, such as SVCs (Static Var Compensators).
Shunt reactors can be classified in two types according to the fixed or variable nature of the rating:
- Fixed rating SRs, either dry or oil-filled, and variable shunt reactors (oil-filled).
- Fixed rating SRs is a traditional technology with no means of regulation. Controllability is ensured by a switched in and out to follow the load variations, which can result in step changes in the system voltage level and induce more stress on system components. This drawback can be mitigated either by the combined use of several smaller SRs, smoothing these step variations and facilitating controllability, or by the use of VSRs that enable a continuous compensation of reactive power through the use of a tap changer to change the inductance of the power line or cable it is connected to.
The regulation of a variable reactor is accomplished by a separate regulating winding, or windings, located outside the main winding. The regulating range is limited by the maximum step voltage and voltage range of the tap changer in combination with the specific design concept used. The regulation range typically varies between 50 – 100% of rated reactive power, e.g. a VSR with a rating of 150 MVAr at 300 kV can today be regulated between 80 MVAr and 150 MVAr.
Components & enablers
Typical components of a VSR are:
- One or three phase, iron-core with fixed air gap
- Tap changer
- Insulation material
- Insulating oil
- Cooling system
Advantages & field of application
VSRs combine the proven design of SRS and tap changers that have been used successfully for decades in power transformers.
They are used in high voltage transmission to compensate reactive power and thereby secure voltage stability according to the load variations: VSR enable grid operators to optimise reactive power compensation and benefit from improved voltage control. Main technical benefits of variability compared to a fixed reactor include the smoothing of the voltage jumps, the flexibility to the load, the ability to interact with a SVC, the possibility of relocation to another part of the grid, and the footprint reduction of a VSR replacing several fixed rating shunt reactors and circuit breakers.
Typical network conditions which favour the application of VSRs are:
- Networks with distributed generation (e.g. solar, wind, etc.) may not always provide full control over their electrical output which may create problems of increased flow of reactive power due to the varying reactive power of both generation and consumption.
- Strongly varying loads powered through relatively long overhead lines or underground cables. The application of a VSR will relieve the source line from reactive current and thereby mitigate the line losses and improve the voltage quality.
- Changing networks as additional transmission infrastructure is being installed to improve overall system reliability and support the loss of base load generating facilities (e. g. coal, nuclear).
- Grids where in- and out- switching of a fixed shunt reactor will lead to power quality problems in terms of voltage steps.
Technology Readiness Level
TRL 9 – System ready for full scale deployment
Research & Development
Current fields of research: Analysis and studies on the dynamic behaviour of the shunt reactor are being performed as well as on the ‘ageing’ of such equipment. Moreover, tests on geomagnetically induced currents (GICs) are being done to compensate reactive power and control the voltage level.
Innovation Priority: Manufacturers are seeking improvements in their production processes to address the challenge of the general layout of the winding arrangement, the lead concept to the tap changer and the huge shunt reactor dimensions.
Best practice performance
- They include the 550 kV range and commercial products which are available at ratings up to 300 MVAr
- Large regulation ranges from 20% to 100%
For the sake of comparison, SRs are able to compensate reactive power up to 300 MVAr and be operated at voltage levels up to 765 kV, whereas reactive power compensation for single-phase shunt reactors reaches up to 320 MVAr and maximum voltage levels of 1000 kV.
Best practice application
The objective of the SINCRO.GRID - Phase 1 project is to provide for more efficient use of the existing electricity grid in Slovenia and Croatia, which will enable the existing infrastructure to accept larger quantities of electricity from renewable sources and ensure more reliable electricity supply. In June 2020, the ELES substation in Divača equipped with a variable shunt reactor, successfully made a trial connection of the VSR to the electricity grid in the substation.
VSR manufactured by Siemens in Austria.
This will solve problems of overloads in the long-term and with a positive effect on grid stability and security of supply for customers.
Located off the Yorkshire coast, Hornsea One project will span a huge area of approximately 407 square kilometres. The offshore wind farm will use 7 MW wind turbines, with each one 190 metres tall.
The powerful shunt reactor used within the project features a rating of 120–300 MVAr and a rated voltage of 220 kV. The relatively low sound emissions of less than 84 dB(A) at 300 MVAr also adds to the environmental compatibility of the units.
VSR will cater for the fluctuating demand of reactive power compensation, resulting from the volatile nature of wind power and improve environment compatibility (low sound emission).
An extensive program aimed to invest in deploying VSRs (as well as SVCs) in the grid to compensate for loss of reactive power compensation resulting from the capacitive generation at low power flow from the installed power lines (Mid-Norway grid reinforcement).
Out of the VSRs in operation, 10 are of voltage class 420 kV, 90/120-200 MVAr and two of voltage class 300 kV, 80-150 MVAr with the goal of reactive power compensation.
Less voltage drop/rise with low short circuit capacity and slow tuning of the reactor guaranteed that reactive reserves in SVC and rotating synchronous compensator are secured and given optimal headroom.
A large variable shunt reactor has been developed and applied with a regulation range of 80% at 400 kV Germany transmission grid.
Tap changer is designed with 33 tappings to cover a rating from 50 to 250 MVAr for a 400 kV three-phase unit.
Improved control of voltage, reduced reactive power loading of the grid which results in decreased losses in the lines and in the VSR itself as compared to a fixed shunt reactor. Expensive SVC equipment is also reduced.
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