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Home / Technical Articles / Primary rated values for medium voltage switchgear often mixed by engineers

Selecting and rating MV switchgear

It’s not unusual to see that engineers mix terms of primary ratings. If they are not understood well, yes, it’s possible to mix some of them. But it’s essential to differentiate them. This technical article will try to shed some light on the fundamental primary rated values involved in correct selecting and rating the medium voltage switchgear assembly and its main components.

Primary rated values for medium voltage switchgear
Primary rated values for medium voltage switchgear (on photo: Medium voltage switchgear, type NXAIR by SIemens; credit: Maximilian Huelsing via Linkedin)

Just to mention that the process of designing secondary systems with devices for protection, control, measurement, and counting purposes is not covered here.

Table of contents:

  1. Rated voltage (Ur)
  2. Rated insulation level
  3. Rated short-circuit currents
    1. Rated short-circuit breaking current
    2. Rated short-circuit making current and rated peak current
    3. Rated short-time withstand current
  4. Rated duration of short-circuit (tk)
  5. Rated normal current (Ir)

1. Rated voltage Ur

Definition – The maximum operating voltage of the network (UNmax) – at the upper tolerance limit – and the required insulation level of the equipment determine the rated voltage.

Rating rule: Ur ≥ UNmax and Ur corresponds with the required insulations level (see 2. below)

For switchgear, the next highest standard value above the maximum operating voltage is set as the rated voltage. The value of the maximum operating voltage must cover effects of temporary overvoltages, for example caused by: harmonics, load shedding (generators for emergency supply, customer generation etc.) and capacitive equipment, such as capacitor banks, filters.

If a particular insolation level is pre-determined, the rated voltage must correspond with it.


Recommendations:

Recommendation №1 – The rated voltage of the switchgear may differ from that of voltage transformers, surge arresters, and HRC fuses. This is because these devices are subject to additional rating criteria .

Recommendation №2 – Some switching duties require the switching devices to have an even higher rated voltage, although this is rare. In certain cases, the entire switchgear system must be designed for higher voltages.

Recommendation №3 – Due to the reduced insulating properties tt altitudes of above 1000 m above sea level, a higher rated value may be selected – where applicable (see 2. below).

Recommendation №4 – A higher rated voltage can be set to fulfil the requirements of a higher insulation level in order to cope with special ambient conditions, for example pollution or humidity. (see 2. below).

Go back to Contents Table ↑


2. Rated insulation level

The insulation level comprises the lightning impulse withstand voltage (Up) and the short-duration power-frequency withstand voltage (Ud).

Rating rule: Ur ↔ Ud and Up (see table in IEC 62271-1)

The rated insulation level is assigned to the rated voltage in accordance with table 1a, IEC 62271-1 [1] the applicable standard and is, therefore, set at the same time as the rated voltage. Different allocations must be specified separately.

In medium-voltage systems, the standard insulation levels do not differ with respect to the neutral point earthing

Table 1 – Rated voltages and insulation levels in the medium voltage range

Rated voltage Ur/kV
(r.m.s. value)
Rated short-duration power-frequency withstand voltage
U/ kV (r.m.s. value)
Rated lightning impulse withstand voltage
U/ kV (peak value)
Line-earth, line-line, across contact gapAcross isolating distanceLine-earth, line-line, across contact gapAcross isolating distance
(1)(2)(3)(4)(5)
3.610 1220, 4023, 46
7.2202340, 6046, 70
12283260, 7570, 85
17.5384575, 9585, 110
24506095, 125110, 145
367080145, 170160, 195
5295110250290

Go back to Contents Table ↑


Comments:

Recommendation №1 – Operators may specify a higher insulation level in accordance with operational requirements. Reasons for this include foreseeable reduction in insulation due to moisture or pollution or high dielectric stresses during standard operation due to connected overhead systems, traction supply systems etc.

Recommendation №2 – At altitudes of at least 1000 m above sea level, the insulation level decreases caused by the
lower air density.

There are alternatives to compensate this decrease:

  • A higher rated voltage is set.
  • Surge arresters are installed to limit possible overvoltages below the actual insulation capability. Based on the voltage withstand value at the relevant altitude (see Figure 1) the required protection level of the arresters is determined.
  • Gas-/solid-insulated switchgear is used with shielded, earthed connections (cable plug connectors, insulated bars), where the HV conductors are nor surrounded by ambient air.

For low-voltage auxiliary and control equipment no precautionary measures need be taken up to 2000 m altitude. Regarding installation sites at higher altitude see IEC 60664-1 [18].

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

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