Page 33 - Energize April 2021

TECHNICAL

or based on GE’s design criteria, such as corona rings, top hats, • Design/manufacturing capabilities. The size of a shunt
elevating structures, line/grounding connectors, etc. reactor depends primarily on its inductance and current;
the higher the inductance and/or the current, the bigger
Application of shunt reactors the coils. For very low reactive power, the inductance may
The main functions of shunt reactors in transmission and be extremely high, which may exceed the manufacturing
distribution systems are: capabilities (maximum diameter x height of the winding
• Control of operating voltages machines). For very high reactive power, the current may be
• Support of reactive power compensation extremely high, requiring a huge aluminium mass to achieve
• Reduction of switching transients on transmission lines a desirable temperature increase and/or losses dissipation. In
both cases, the selection of a properly rated voltage for the
Shunt reactors can be installed at both transmission and distribution shunt reactor may eliminate or minimize the impacts on the
grids, being directly connected to substation busbars, transmission reactor’s design and manufacture.
line endings and to the tertiary windings of large power transformers, • Rated voltage and power of the tertiary winding, for tertiary
as shown in Figure 2. reactors. Therefore, the reactive power of a shunt reactor
depends heavily on its operating voltages.

The typical ranges of reactive power (three-phase basis) for each
system voltage that result in feasible and competitive dry type air
core shunt reactors are presented below:
• 15 kV and below: 0,5 Mvar to 25 Mvar
• 25 kV and 38 kV: 2,5 Mvar to 60 Mvar
• 72,5 kV: 5 Mvar to 100 Mvar
Figure 2: Shunt reactor application in power systems
• 138 kV: 7,5 Mvar to 150 Mvar
• 245 kV: 20 Mvar to 200 Mvar
The rated voltage and reactive power of a shunt reactor, as well its • 345 kV and 400 kV: 50 Mvar to 250 Mvar
location, are normally determined by system studies, such as load • 500 kV: 100 Mvar to 350 Mvar
flow and transients. Moreover, the ratings of shunt reactors also
depend on the following factors: For reactive power other than the above-mentioned ranges, the
• Current and voltage capabilities of the switching devices. The manufacturer should be consulted to determine technical feasibility.
introduction of inductive currents may create severe transient
voltages (TRV) over the shunt reactor and across the switching Protection and grounding
devices. The magnitude and rate-of-rise of the TRV depends on In most applications of shunt reactors in EHV and HV systems,
the shunt reactor’s inductance and stray capacitances which the star points of the reactors are connected to earth, whereas
in turn depend on the voltage, reactive power, grounding, and MV shunt reactors are generally not grounded. If the star point of
construction of the shunt reactor. A typical TRV associated with the transformer tertiary winding is not earthed, then grounding the
the switching off of the shunt reactor is around 1,7 to 2,0 p.u. of reactor would assist in detecting earth faults in this zone.
the rated voltage. However, such ground fault detection can also be made by
• Connection and grounding type of the shunt reactor. Most shunt means of voltage measurement, with a grounded star primary and
reactors are connected in star (or wye), being ungrounded for open delta secondary voltage transformer used to detect ground
system voltages of 72,5 kV and below and grounded for system faults on the network supplied by the tertiary winding. Shunt
voltages of 115 kV and above. For line reactors, when single- reactors should be equipped with over-current and earth fault
phase auto-reclosing of transmission lines is required, shunt protection monitoring for the line side current.
reactors can be grounded by a neutral reactor or resistor. The In cases where the reactors are connected to the tertiary
neutral grounding reactor can also be a dry type air core reactor, winding of a transformer, it is most likely that the reactor feeder
with similar construction to the shunt reactors. There are very will be included in the transformer-differential protection.
few cases of delta-connected shunt reactors, mainly related to Differential protection of the reactor can be achieved by splitting
industrial applications (Figure 3). each phase into two legs and monitoring the unbalanced
current in the star point.
This method provides extremely fast and sensitive protection
of the reactor’s windings, especially in terms of inter-turn faults but
requires special coils design (split-phase reactors).

Switching of reactors
IEEE C37.015 provides comprehensive guidelines for the switching
requirements of shunt reactors. One of the most important aspects
of switching reactors is current chopping, caused by forcing the
reactor current to zero before the zero crossing. This could result
in high voltage across breaker poles. The introduction of copper/
chromium contact material in vacuum circuit breakers overcomes
Figure 3 the problem of over-voltages as a result of switching, provided

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