Fixed Shunt Reactors have been traditionally used in transmission and distribution systems for many years. The reason that it is called Fixed Shunt Reactor is that its rated reactive power consumption is approximately constant; in other words, it has a fixed reactance (XR). The inductive reactive power which is consumed by Reactor can be calculated as follows:
QIR is the 3‐phase inductive reactive power consumed by the Shunt Reactor in VAR
V is the phase‐to‐phase voltage at the point the reactor is connected to the network in Volts
XR is the total inductive reactance of the Shunt Reactor in Ohms
LR is the equivalent inductance of the Shunt Reactor in Henry,
Iis the flowing current into the Reactor’s branch in Amperes
As seen in equation 6-1, the consumed reactive power is only a function of the line voltage and is not affected by the load current of the line. The Reactor is characterized by the following.
- Rated power
- Rated system voltage
- Power frequency
- Number of phases
- Insulation level
- Temperature rise
- Sound Level
- Linearity characteristic
In order to design a Shunt Reactor, the required rated power (QIR) and the rated system voltage (V) are specified; so according to equation 6-1, all that should be done is to implement a certain reactance (XR) to have the desired reactive power consumption.
There are two general types of Shunt Reactors. First one is dry-type Reactor of an air-core or core-less design. Second one is oil-immersed Shunt Reactor.
Dry-type Shunt Reactors are limited to voltages up to 34.5kV and are often installed on the tertiary of a transformer (Figure 6-5) or are used in SVC installations.
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Figure 6-5: An
old design air-core Shunt Reactor (A) and a modern air-core Shunt Reactor (B),
connected to the tertiary winding of a large transmission network transformer
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Figure 6-6 illustrates a three-phase core-less Shunt Reactor (dry-type) which is installed in a distribution substation. As seen, there is no iron at all in the core-less concept.
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| Figure 6-6: A 3-phase core-less or air-core Shunt Reactor |
On the other hand, oil-immersed Shunt Reactors are used for higher voltage levels (higher than 40kV) and are the most compact and cost efficient Reactors. Regarding the core design, two different ways has been used in building oil-immersed Shunt Reactors. First one is referred to as core-type or gapped core, and the second one is called shell-type design.
It is desired to implement a certain reactance (XR) in the reactor in order to consume the required level of reactive power. Since it is needed to take a relatively large current in Shunt Reactors to have considerable reactive power consumption, the equivalent reactance in Shunt Reactors should be small compare to a power transformer magnetizing impedance. Therefore the magnetic permeability of the core (μ) should be reduced in Reactors. A good way to reduce the magnetic permeability and the resulting reactance (XR) is to create air-gap in the core. One way is to create one big air-gap and adjust the gap length in order to achieve a certain level of reactance. However, having one big gap results in large losses.
Basically, the choice between single or 3-phase shunt reactors is exactly the same as transformers. It should be a balance between cost savings and risk costs when large 3-phase reactors are used instead of three single phase units.
One 3-phase Shunt Reactor has cost advantages compare to three single phase units. To explain, the price of a 3-phase reactor is lower than three single phase units. In addition, the total power loss is also lower in a 3-phase unit. Moreover, the size of substation decreases and additional savings can be made in civil works and related substation equipment.
On the other hand, when using three single phase shunt reactors, it is economically reasonable to keep a fourth single phase unit as spare. However utilities seldom keep a spare 3-phase Reactor due to cost reasons; for this reason, the reliability of 3-phase Shunt Reactors is very important. A well-designed and well-manufactured Shunt Reactor should have the same or better reliability as a transformer and should also have low levels of sound and vibrations. It should be noted that the reliability of a 3-phase Shunt Reactor should be the same as a single phase one.