The main parts of an air insulated primary medium voltage switchgear and their application

Introduction about medium voltage switchgear:

Medium voltage switchgear is an important part of the energy distribution chain in an alternating current (AC) system from the power generation through the transmission and final distribution to the consumers of the energy. The frame of the specification, terms and definitions, ratings, design and construction, type testing, and production routine testing are described for the European region in the following standards:
* IEC 62271-1 High-voltage switchgear and controlgear – Common specification.
* IEC 62271-200 AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
* IEC 62271-300 Gas-Insulated Metal-Enclosed Switchgear for Rated Voltages above 52 kV.

IEC standards are widely accepted around the world, however, some of the large countries such as the United States, China, and Russia have different standards. Section 3.5 of the IEC 62271-1 specifies all possible parts of switchgear and controlgear.

The combination of the parts together creates a switchgear system with the following functionalities in medium voltage networks:

* Distribution of the energy from the upper-level transmission system down to the energy consumption point.
* Switching the energy.
* Measurements for protection, indication, and billing purposes.
* Protection of loads and equipment against faults.
* Control, blocking, and interlocking depend on the needs and approach of the network operation.
*Communication of data to SCADA or DCS systems.
*Protection of the operational staff in the substation.

Various type of switchgear designs that meets the criteria of the IEC standard can be found on the market, designed and manufactured by many producers. IEC standard clearly differentiates air-insulated technology from gas-insulated technology.
Switchgear design varies depending on the position in the distribution system.

The complexity of the switchgear design is given by the level of energy that must be reliably distributed by the switchgear and the complexity of the protection and control schemes. In general, the higher the electrical parameters of the switchgear are, the higher the protection and control complexity required.

You can find more information about medium voltage switchgear in this category.

The typical architecture of primary air-insulated medium voltage switchgear (AIS) is organized into four basic compartments:

Figure1: cross-section view of ABB company type UniGear medium voltage switchgear 

Medium voltage switchgear basic structure compartment:

The primary structure (marked as section B in Fig.1, 2, and 3) represents metal sheets that give shape, dimensions, stiffness, and robustness to the switchgear and it includes also copper parts. Copper parts carry energy and connect all compartments and apparatuses of the switchgear together. Structure as such,
provides metal-based segregations between compartments defined according to IEC 62271-200 different accessibility and together with arc proof door also arc withstand ability.

Figure 2: Cross-section view of Eaton company medium voltage switchgear

Medium voltage switchgear circuit breaker compartment:

The circuit breaker compartment (marked as section C in Fig. 1, 2, and 3) includes MV switching apparatus. Circuit breaker compartments can be equipped with various types of switching apparatuses such as load break switches, contactors, circuit breakers, etc. Reliable and safe opening and closing of the steady state as well as fault currents and voltages are the elementary function of the switching apparatus in the switchgear. The majority of the MV primary air-insulated panels are equipped with circuit breakers. Vacuum interrupting technology in medium voltage is dominant today.

Medium voltage switchgear cable compartment:

The cable compartment (marked as section D in Fig.1, 2, and 3) beside cable terminations includes sensing devices. The sensing device’s primary purpose is to measure phase currents, phase voltages, residual current, and residual voltage.
Dominant technology for measurement is an instrument transformer (IT) based on the well-known inductive principle for both current and voltage measurement.

Figure 3: Cross-section Seimens company type NXAIR medium voltage switchgear

Medium voltage switchgear low voltage compartment:

The low voltage compartment (marked as section A in Fig.1, 2, and 3) includes all auxiliary devices such as IED (Intelligent Electronic Device), meters, communication equipment, control and indication equipment, terminals, and wires.
All devices in low voltage compartments connected through wires and communication represent intelligent parts of the switchgear, by means of management of the required protection, control, interlocking, blocking, and switchgear data communication functionalities. Communication of the data from the switchgear can be managed by a large variety of principles and communication protocols. The best communication standard and platform today is IEC 61850.

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