Feb 4, 20223 min read

The Basics of Primary MV Switchgear

Primary Medium Voltage Switchgear

Primary medium voltage switchgear represents an important part of the primary distribution substation functionality. The switchgear works as a connection node between the outgoing distribution feeders and the in-feeding power transformers. The most common construction with the switchgear is an indoor-mounted metal-enclosed one.

The rated voltage, current and short circuit withstand ranges for secondary switchgears typically reach up to 36 kV, 1250 A and 50 kA respectively.

Basics Of Primary Medium Voltage Switchgear (on photo: ABB's MV air-insulated metal-clad switchgear, type 'UNISAFE'; credit: tamintablo.com)

The same switchgear constructions are used in primary distribution by the utilities and in heavier industrial and power plant applications.

On a 36 kV voltage level also outdoor air-insulated constructions are used to some extent. This switchgear is built at the site utilizing individual standard components. This gives the possibility to construct customer-specific solutions like busbar arrangements that are not available with factory-built metal-enclosed switchgear.

Figure 1 – Example of primary medium voltage switchgear – UniGear ZVC feeder unit

With the release of the IEC 62271-200 standard, new definitions and classifications of Medium Voltage switchgear have been introduced.

One of the most significant changes is that classification of switchgear into metal-enclosed, compartmented and cubicle types have been abandoned. The revision of switchgear classification rules has been based on the user’s point of view, in particular on aspects like service and maintenance of the switchgear, according to the requirements and expectations for proper management, from installation to dismantling.

"In this context, Loss of Service Continuity (LSC) has been selected as a fundamental parameter for the user.

Switchgear Construction

The following concentrates on factory-built metal-enclosed indoor switchgear. The primary switching devices typically include switch disconnectors, fused switch disconnectors, contactors and circuit breakers, either fixed or withdrawable. The current and voltage measurement can be done either with traditional instrument transformers (CTs and VTs) or with sensors.

Certain combinations are also possible, depending on the individual switchgear manufacturer.

!!! Two main construction principles exist, namely the air-insulated one and the gas-insulated (SF6) one. The choice between these two alternatives is a result of evaluation of different aspects and differentiating factors during the switchgear’s life cycle.

This evaluation can typically include the following viewpoints:

Space requirements
Service requirements (tools and knowledge)
Reliability and availability
Installed base
Operational safety
Life cycle costs
Performance ratings

Switchgear consists of a number of cubicles. The basic construction of each cubicle is chosen to meet the intended use. This means that for example, the power transformer in-feed cubicle can have different primary and secondary devices from what the outgoing line feeder cubicle has.

The switchgear manufactures provide a selection of typical cubicle constructions. By combining these typical cubicles, one can create switchgear suitable for the intended use.

Figure 2 – Example of typical cubicles for single-busbar solution

The number of different cubicles within the switchgear is coupled together by the busbars.

In an air-insulated construction, these busbars are placed in a metal-enclosed compartment surrounded by normal air. With the gas-insulated construction, the busbars are placed in a hermetically sealed metal enclosure under pressurized insulating gas (SF6).

Depending on the manufacturer, the busbar compartment can be divided into cubicle-wide sections.

Typical busbar arrangements supported by the different switchgear manufacturers are the single-busbar and double-busbar arrangements.

Figure 3 – Example of typical cubicles for double-busbar solution

Protection, Control and Metering

The protection relays are externally powered advanced Intelligent Electronic Devices (IEDs), also referred to as Feeder Terminals. In modern primary switchgear, the bay-dedicated functions like protection, control and measurement are carried out with Feeder Terminals.

The Feeder Terminal performs the assigned protection functions, carries out the local and remote control of switching devices, gathers and processes and displays measured data and indicates the status of the switching devices.

Figure 4 – Air-insulated primary switchgear equipped with Feeder Terminals (type ‘UniGear ZS1’)

The horizontal communication between feeder terminals in each cubicle provides the possibility for station level automation and gateway connections to upper-level systems for complete primary distribution network real-time control and monitoring.