The art of relay protection applied to high voltage shunt reactors

Typical Shunt Reactor Control Schemes

Typically, shunt reactors are designed for natural cooling, with the radiators directly mounted on the tank. At times, it may be necessary to implement control measures in the cooling circuit based on the condition of the shunt reactor circuit breaker. The control action can be initiated by either the circuit breaker auxiliary contact or by operating an overcurrent relay set to 50% of the reactor’s rated current.

When the reactor is energized, the overcurrent relay ensures secure control action, regardless of the status of the circuit breaker’s auxiliary contact.

Recently, electrical utilities have been increasingly requesting the implementation of automatic shunt reactor switching to enhance power system performance. This involves closely monitoring the voltage level of the busbar. Integrating this functionality into a multifunctional, numerical relay is a straightforward process.

However, it is important for the user to thoroughly evaluate the performance of the relay based on the following factors:

1. Over/under voltage relay with reset ratio or 1% or better is required for such application
2. Typically more than one over/under voltage level with independently settable time delays are required within the relay
3. Over/under voltage relay shall be capable to operate only when all three voltages are above/below set operate level or relay must be capable to measure and operate on the value of the positive sequence voltage

Traditional Shunt Reactor Protection And Control Schemes

Typically, multifunctional numerical protection relays are employed for the protection of power transformers and shunt reactors. Nevertheless, it is still common to find outdated protection schemes for shunt reactor protection that only include a limited number of protection functions.

Here are two examples of traditional protection arrangements, as depicted in the following figures.

Figure 1 – Typical Shunt Reactor Protection Scheme No1

One possible protection scheme involves using restricted ground fault protection (i.e. 87N) as a means of reactor unit protection. This protection will trip immediately for any internal phase to ground faults. Overcurrent protection (i.e. 50/51) is used for internal phase-to-phase fault detection.

Ground overcurrent protection, such as 50G/51G, serves as a secondary safeguard against ground faults and as the primary defense against circuit breaker pole disagreement.

Figure 2 – Typical Shunt Reactor Protection Scheme No2

The second protection scheme uses differential protection (i.e. 87) for reactor unit protection. This protection will trip immediately for any internal phase to phase or phase to ground faults.

Overcurrent protection, specifically 50/51, serves as a secondary safeguard against internal phase-to-phase faults.

Please consult the following chapter for details on the proposed shunt reactor protection scheme, which includes a multifunctional numerical protection relay.

Conclusions

The paper has provided a comprehensive analysis of HV shunt reactors and their protection and control schemes. Figure 3 presents an example of a possible application of a multifunctional numerical relay that utilizes DFT filtering technique.

This can help the end user in properly selecting and applying the relay for HV shunt reactor protection and control.

Figure 3 – Example of complete HV shunt reactor protection and control scheme with multifunctional, numerical relay

All the protection or control functions shown in Figure 3 are commonly found in multifunctional numerical transformer protection relays. It is important to thoroughly evaluate the suitability of a specific relay for use in a shunt reactor application.

Table 1 provides a summary of each function from Figure 3, along with some common setting values. Please note that the suggested settings should be treated as general recommendations.

This paper aims to offer guidance to individuals seeking help with HV shunt reactor protection and control issues.

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