Switchgear interlocking system and arc protection that you MUST consider in the design

Safety in the first place

There are at least two safety requirements that medium-voltage switchgear MUST fulfil: an interlocking system and an arc protection system. Yes, these two systems are crucial in terms of safety because they protect not only the operator and other substation personnel but also the equipment in the substation itself.

There are hundreds and hundreds of incidents worldwide that usually involve wrong breaker manipulation that leads to catastrophic consequences and injuries.

This article sheds some light on the most common interlocking and arc protection systems installed in a medium voltage switchgear.

Table of Contents:

1. Switchgear interlocking systems
   1. Interlocking methods:
      1. Interlock scheme #1: Two incomers and bus coupler interlocking
         1. Electrical interlocking
         2. Mechanical interlocking
      2. Interlock scheme #2: Incomer circuit breaker and earth switch interlocking
      3. Interlock scheme #3: Feeder circuit breaker and earth switch interlocking
2. Internal Arc Classification (IAC)
   1. Relevant standards and testing
      1. IAC certification example
   2. Causes of internal arc
   3. Minimizing the effects

1. Switchgear Interlocking Systems

Interlocking between different switchgear apparatus and enclosure access covers and doors enhances personnel safety, as well as improving operational convenience. If a switching device can cause serious damage in an incorrect position, this must also have a locking facility.

Interlocks consists of the rules. Interlocks MUST ensure that the disconnector cannot be moved or operated unless the circuit breaker is open. Interlocks shall ensure that the circuit breaker cannot be closed unless the disconnector is fully in the ‘closed‘, ‘isolated‘ or ‘earth‘ position.

Interlocking uses electrical and mechanical methods or a combination of both. IEC 62271-200 states mandatory rules for switchgear interlocking:

For metal-enclosed switchgear with removable switching apparatus:

Rule #1 – Switching device must be in the open position before it can be withdrawn.

Rule #2 – Switching device can only be operated in the positive service or test position.

Rule #3 – Switching device cannot be closed unless the auxiliary control circuits required to open the switch are connected. Auxiliary control circuits cannot be disconnected with the switching device closed in the service position.

For metal-enclosed switchgear with disconnectors:

Rule #1 – Disconnector cannot be operated under conditions other than those for which it is intended to be used.

Rule #2 – Disconnector cannot be operated unless the main switching device is open.

Rule #3 – Operation of a main switching device is prevented unless its associated disconnector is in a positive service, test or earth position.

Rule #4 – Disconnectors providing isolation for maintenance and servicing must have a locking facility.

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1.1 Interlocking Methods

The illustration in Figure 1 shows a common switchgear arrangement for a medium voltage power distribution system. This switchgear arrangement uses three separate interlocking methods which are marked with 1, 2 and 3 in the red squares.

Let ‘s now start with describing of each scheme.

Figure 1 – Typical MV power distribution switchgear arrangement with interlocks

Where:

• Q-IL – Circuit breaker (left incomer)
• E-IL – Earth switch (left incomer)
• TXR_L – Supply transformer (left bus)
• Q-IR – Circuit breaker (right incomer)
• E-IR – Earth switch (right incomer)
• TXR_R – Supply transformer (right bus)
• Q-BC – Circuit breaker (bus coupler)
• Q-FL – Circuit breaker (left feeder)
• E-FL – Earth switch (left feeder)
• Q-FR – Circuit breaker (right feeder)
• E-FR – Earth switch (right feeder)

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Interlock Scheme #1

Two Incomers and Bus Coupler Interlocking

The two incomers and the bus coupler circuit breakers use a standard “2 out of 3” interlocking system to prevent a parallel feed from the two incomers onto a common bus. Interlocking allows the following conditions:

Condition #1 – The two incomer circuit breakers closed (Q-1L and Q-1R) with the bus coupler circuit breaker open (Q-BC).

Condition #2 – Left incomer and bus coupler circuit breakers closed (Q-IL and Q-BC) with right incomer circuit breaker open (Q-IR).

Condition #3 – Right incomer and bus coupler circuit breakers closed (Q-IR and Q-BC) with left incomer circuit breaker open (Q-IL).

Typically, these interlocking conditions are met using both electrical  and mechanical method.

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Electrical Interlocking

Normally closed auxiliary contacts from the two incomers and the bus coupler circuit breakers are used to electrically interlock the close command of each circuit breaker.

Left incomer circuit breaker (Q-IL) has a normally closed auxiliary contact from the right incomer circuit breaker (Q-IR) and a normally closed contact from the bus coupler circuit breaker (Q-BC) connected in parallel to allow a close command.

Right incomer circuit breaker (Q-IR) has a normally closed auxiliary contact from the left incomer circuit breaker (Q-IL) and a normally closed contact from the bus coupler circuit breaker (Q-BC) connected in parallel to allow a close command.

Bus coupler circuit breaker (Q-BC) has a normally closed auxiliary contact from the left incomer circuit breaker (Q-IL) and a normally closed contact from the right incomer circuit breaker (Q-IR) connected in parallel to allow a close command.

This control method only allows for any two circuit breakers to be closed at the same time.

Figure 2 – Interlock scheme #1: Two incomers and bus coupler interlocking

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Mechanical Interlocking

Mechanical locks must be used to prevent unauthorized access to areas of the switchgear panel that contain live components. Unless all live parts have been rendered safe – either by a clearly applied earth connection or by being positively disconnected and screened from the other live parts – access to such parts is not possible.

To guarantee effective and comprehensive protection against malfunction, mechanical interlocks must be supplied. Mechanical interlocks must be built and engineered to ensure reliable fail-safe performance.