Medium-Voltage Switchgear: Parts, Functions, Testing

Short answer

Medium-voltage switchgear is metal-enclosed assemblies that bring together the busbars, circuit breakers, disconnectors, instrument transformers and relays of each circuit. It is used in distribution substations, industry and solar plants, and its design protects personnel even in the event of an internal arc.

Short answer. A medium-voltage switchgear panel is a metal-enclosed, grounded module that brings together the equipment of one circuit: circuit breaker, disconnector or withdrawable truck, earthing switch, instrument transformers and protection relay. Several panels joined by a busbar system form the switchboard. IEC 62271-200 covers those above 1 kV and up to 52 kV.

What medium-voltage switchgear is

Medium-voltage switchgear, also called MV panels or MV switchboards, is metal-enclosed, grounded switchgear and controlgear. Each panel (or cubicle) contains the equipment for one circuit: circuit breaker or switch-disconnector, disconnector or withdrawable truck, earthing switch, current and voltage transformers, and the protection relay with its control. Several panels joined by a busbar system form the lineup.

IEC 62271-200 covers AC metal-enclosed switchgear and controlgear above 1 kV and up to 52 kV. In North America the IEEE C37.20 series is used.

Main parts

Busbar compartment, with the main busbars that join all the panels.
Switching device compartment: vacuum circuit breaker on a withdrawable truck or fixed, switch-disconnector, or fused contactor for motors.
Cable compartment, with the terminations, current transformers, earthing switch and sometimes surge arresters.
Low-voltage compartment, with relays, meters, terminal blocks and controls.
Pressure relief ducts that carry the hot gases of an internal arc away from the operator.
Mechanical interlocks between breaker, truck, earthing switch and doors.
Capacitive voltage presence indicators.

How it works

The panel separates the equipment into compartments so that a fault or work in one does not affect the others. IEC 62271-200 classifies switchgear with three concepts:

Loss of service continuity (LSC): what stays in service when a compartment is opened. LSC1 requires the whole lineup to be de-energized; LSC2 allows a compartment to be opened with the busbars energized, with variants LSC2A and LSC2B.
Partition class: PM, with grounded metal partitions, or PI, with insulating partitions.
Internal arc classification (IAC): accessibility (A for authorized personnel, B for the public), tested sides (F front, L lateral, R rear), arc current and duration.

The interlocks impose the order of operations: the truck cannot be withdrawn with the breaker closed, the earthing switch cannot be closed with the truck in service, and the cable door cannot be opened without the earth closed.

To select a panel, the following are defined, among other data: the rated voltage and its insulation level, and the rated current of the busbars and of each circuit. The short-time withstand current and its duration, the internal arc classification and the site conditions (altitude, temperature and humidity) are also defined. The installation must also meet the local electrical code.

Types

Air-insulated switchgear (AIS) with withdrawable breaker, common in distribution substations and industry; in North America, the metal-clad type of IEEE C37.20.2.
Gas-insulated switchgear (medium-voltage GIS), more compact and less sensitive to the environment, with SF6 or SF6-free alternatives.
Ring main units (RMU) for underground distribution networks; see the tests of an RMU. The sequence of opening, earthing and restoring an RMU is in RMU switching operation.
Primary and secondary distribution switchgear, according to rated current and short-circuit capacity.
Industry: medium-voltage motor control centers with vacuum contactors and fuses.
Solar plants: switchgear in the inverter stations and incomer panels in the connection substation. A manufacturer example, with KYN28A withdrawable panels and NG7 RMU, is in Chint KYN28A switchgear and NG7 RMU.

Associated protection

Phase and ground overcurrent 50/51 and 50N/51N, on almost every feeder.
Directional 67 and 67N in meshed networks or with generation.
Undervoltage 27, overvoltage 59 and frequency 81 for load shedding and for generating plants.
Thermal overload 49, negative sequence 46 and differential 87M on motors.
Arc detection by light and current, which clears an internal arc in a few milliseconds and reduces damage.
Busbar differential or logic blocking schemes between the incomer and the feeders.

Commissioning and maintenance tests

Power-frequency or VLF withstand voltage of the lineup and the cables; see the VLF test on cables and switchgear.
Insulation resistance before and after the voltage test.
Contact resistance of the main circuit and the busbar joints.
Circuit breaker: timing, contact resistance and vacuum integrity according to the manufacturer.
Current and voltage transformers: ratio, polarity and excitation.
Relays: secondary injection of each function and actual breaker trip.
Mechanical and electrical interlocks, and truck operation.
Torque of bolted joints.
Motors fed from the panel: see what is measured on a medium-voltage motor and why.
Predictive maintenance: thermography through infrared windows, and partial discharge by transient earth voltage (TEV) and ultrasound. See what infrared thermography detects.

Common faults

Poorly made cable terminations.
Moisture and condensation that cause surface discharges and treeing.
Loose bolted joints that heat up.
Misadjusted truck insertion mechanisms, with plug contacts that do not seat fully.
Ferroresonance in voltage transformers with an isolated neutral.
Bypassed or damaged interlocks.
Relays not tested since commissioning, with settings that no longer match the network.

What the manual of a gas-insulated switchgear teaches

Manufacturers’ installation and operation manuals contain design decisions that do not appear in general guides. As an example, the installation and operating manual of the Siemens 8DJH gas-insulated medium-voltage switchgear (revision 10, 2017) shows how the concepts of IEC 62271-200 translate into a real panel:

Series data. For rated voltages of 7.2 to 24 kV, the manual gives 630 A busbars, 400 or 630 A ring feeders, 250 or 630 A circuit-breaker feeders and 200 A transformer feeders. The short-time withstand current goes up to 25 kA for 1 s. The rated filling pressure is 150 kPa absolute, with a minimum functional level of 130 kPa. These are values of one specific series, not of switchgear in general.
Modifications and type tests. Type tests and the internal arc classification are valid only while the switchgear is kept as tested. The manual warns that modifications not coordinated with the manufacturer, non-original spare parts or adjustments outside the specification can void compliance with the IEC 62271-200 type tests and the warranty.
Personal protective equipment. For switchgear with an internal arc classification tested to IEC 62271-200, no personal protection is required to operate it; for switchgear without it, it is. If covers must be removed to work, protection against hot gases is used, and the manual makes clear that protection against an internal arc is still not total.
Pressure relief and room. Relief is downward in the standard design, and as an option to the rear, with a duct and pressure absorber. The manual requires that the room, including the cable basement, withstand the pressure of an internal arc and have relief openings to the outside. The IAC classification of the panel is only useful if the room supports it.
Mechanical interlocks. Separate operating openings for disconnecting and for earthing, so that you cannot go directly from closed to earthed without returning to the open position. The cable compartment cover can only be removed with the feeder earthed. Optionally, closing or leaving earth is blocked while the cover is removed, and all three movements accept padlocks. With the circuit breaker closed, the three-position disconnector cannot be operated; while an operating opening of the disconnector is open, the circuit breaker cannot be operated. Other electrical interlocks are optional and defined by the customer. For the design criteria see interlocks in medium-voltage switchgear and RMU.
Ready-for-service indicator. A magnetic indicator coupled to a measuring cell inside the vessel responds to gas density, not to relative pressure: temperature and altitude do not affect it. If gas is lost, which is what reduces dielectric strength, the indicator shows it; it can have a contact for remote signaling.
Voltage detection. Integrated, pluggable capacitive dividers conforming to IEC 61243-5, with VOIS, CAPDIS or WEGA indicators.
Cable testing. The manual provides for on-site voltage testing of the cables up to 80% of the power-frequency withstand voltage Ud, and requires removing the pluggable voltage transformers beforehand, since they must not be subjected to that test. For the general procedure see on-site withstand voltage test.

What to do with this in a project: ask the manufacturer for the manual of the exact configuration, verify that the room meets what the internal arc report requires. Record the density indicator reading as part of commissioning. Do not allow component changes without documented backing. For thermal monitoring see thermography in substations and switchgear.

Testing and commissioning with Tiza Group

At Tiza Group we test and commission medium-voltage switchgear in substations, industry and solar plants: insulation tests, circuit breakers, instrument transformers and protection. We work in Colombia, or elsewhere on request. See our testing and commissioning service or write to us and we will propose a test plan.

References

IEC 62271-1, High-voltage switchgear and controlgear: common specifications.
IEC 62271-100, Alternating-current circuit-breakers.
IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
IEEE C37.20.2, Metal-clad switchgear.
IEEE 400.2, Field testing of shielded power cable systems using very low frequency (VLF).
IEC 60270, Partial discharge measurements.

Frequently asked questions

What does the IAC AFLR classification mean on a switchgear panel?

It is the internal arc classification of IEC 62271-200. It indicates that the panel was tested to protect authorized personnel (A) on the front (F), lateral (L) and rear (R) sides, with an arc current and duration given next to the classification.

Why do cable terminations fail in switchgear?

A poorly made termination is a common cause of failure in switchgear. Typical defects are cuts in the insulation, misplaced stress control and dirt; loose bolts and moisture also contribute. A VLF test and partial discharge measurement help detect them.

What is the difference between a withdrawable-breaker panel and a fixed one?

In the withdrawable type the breaker moves on a truck to a test or disconnected position, which makes maintenance easier and creates a visible isolation. In the fixed type the breaker stays in place and isolation is provided by a disconnector; it is more compact.