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.

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 (U_r)
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 (t_k)
5. Rated normal current (I_r)

1. Rated voltage U_r

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

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

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).

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2. Rated insulation level

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

Rating rule: U_r ↔ U_d and U_p (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

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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].

Example for diagram above: a lightning impulse withstand voltage of Up = 120 kV is required at an altitude of 1800 m. A system with a rated voltage of 36 kV achieves this, while a 24 kV system is not suitable because it can only maintain a lightning impulse withstand voltage of 125 kV up to 1000 m.

Recommendation №3 – In environments with high levels of pollution, longer creepage paths may be required. This can be achieved not only with a higher insulation level, but also with alternative methods:

• Avoiding unprotected insulation
• Additional insulation for exposed parts (e.g. heat shrink tube to act as bridging protection)
• Gas-insulated switchgear with shielded, earthed connections (cable plug connectors, insulated bars)

Note that, in many cases, surge arresters fitted at central points can be used as an alternative to increasing insulation levels.

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3. Rated short-circuit currents

Rated short-circuit currents include:

1. The peak current I_p and short-circuit making current I_ma
2. The short-time current I_k and short-circuit breaking current I_sc

The peak current and short-time current are withstand capability variables (steady state), while the short-circuit making and breaking current indicate the switching capacity. The applicable standard defines a fixed ratio between the impulse and short-time current as a basic design requirement for switching devices and switchgear: I_p/I_k = 2.5 (2.6 at 60 Hz).

The same ratio applies to the dynamic variables I_ma/I_sc. The values that actually occur in the network, however, have priority.

Table 2 – Rating Rules

Both conditions (a) and (b) must be fulfilled. For switchgear, the next highest standard value above the actual line variable is set.

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3.1 Rated short-circuit breaking current

The rated short-circuit breaking current (I_sc)is characterised by the r.m.s. value of the AC component and the relative DC component, which results from the opening time of the circuit-breaker and the DC time constant of the short-circuit current. The circuit-breaker may be equipped with more than one release having different tripping times, which in turn means different opening times.

Thus, depending on the release, the DC component can also vary. The selection of the rated value Isc must be based on the shortest opening time. Unless otherwise specified the DC time constant of the short-circuit current is τ = 45 ms (standard value).

Whether or not a circuit breaker at given rated short-circuit breaking current is suitable for other (than the rated) DC components – based on different opening time or different short-circuit current time constant, can be verified by the total breaking current. Keeping I_total constant, higher DC components at lower AC component may be calculated.

However, the total breaking current resulting from the rated values of the circuit-breaker (I_sc, T_op and τ) must not be exceeded.

DC component (β) at the instant of contact separation:

Permissible total current, resulting from the rated short-circuit breaking current (AC component) at τ = 45 ms and shortest opening time T_op-min:

For other short-circuit breaking currents (≠ Isc) and other DC components (τ ≠ 45 ms; Top ≠ Top-min) applies:

Warning! The conversion of breaking currents makes sense only up to 50 % DC component. Higher values require the approval of the circuit-breaker manufacturer.

Where,

• β – Relative DC component at the instant of contact separation
• τ – DC component of the system’s short-circuit current
• f_N – Power-frequency, 50 Hz or 60 Hz
• I_AC(eff) – IAC ≠ Isc; short-circuit breaking current (AC component, r.m.s. value)
• I_sc – Rated short-circuit breaking current (AC component)
• I_total – Total (short-circuit) breaking current
• T_op – Opening time of the circuit-breaker
• T_r – ½ cycle of the rated frequency; 10 ms (50 Hz) or 8.3 ms (60 Hz)

Related reading:

Things you MUST know when commissioning low voltage switchgear and circuit breakers

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3.2 Rated short-circuit making current and rated peak current

The design specification, given by the standard, establishes a fixed ratio between the rated peak and short-time withstand currents: I_p / I_k = 2.5 (2.6 at 60 Hz). The same ratio applies to the dynamic ratings I_ma / I_sc. In pure distribution networks the current almost always keep to this ratio. However, factor 2.5 (2.6 at 60 Hz) may be exceeded in the network, particularly when regenerative loads are connected to the system.

The following can indicate a higher making or peak current that differs from the standard:

• Higher number of motors
• Motors of high rated power
• Generators.

To minimize voltage drops (voltage quality) caused by large, direct-starting motors, networks may be designed for high short-circuit current levels. In those installations, the magnitude of the peak current must be thoroughly taken into account.

Related reading:

Voltage drop calculations and design of urban distribution feeders

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3.3 Rated short-time withstand current

Main and earthing circuits of switchgear assemblies can have different ratings. The rated short-time withstand current may be converted into other current or time values. See the following paragraphs (4. and rule #4).

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4. Rated duration of short-circuit t_k

This refers to the duration for which switchgear can conduct the rated short-time current in a steady state.

Rating rule: t_k ≥ t_{k line}

A key factor here is the total short-circuit duration, which is governed by the settings of the protection relay, the selectivity requirements, the set tripping times, and the opening times of the switching devices. A general value of 100 ms is normally assumed as the shortest possible breaking time (opening + arcing time), using a protection relay and a standard circuit-breaker.

Values of between 3 s and 1 s are normal for switching devices and switchgear.

Recommendation №1 – In accordance with the applicable standard, the preferred value is tk = 1 s, although 3 s has since established itself for most switching devices and switchgear.

Recommendation №2 – Main and earthing circuits can have different rated values.

Recommendation №3 – For cost reasons, current transformers mostly are designed for times of < 3 s.

Recommendation №4 – “Thermal” conversion between the short-time current and the short-circuit duration: The short-time current and the short-circuit duration can be converted by means of the I^²t value: I₁² × t₁ = I₂² × t₂.

A conversion for lower short-circuit currents with longer duration is harmless. A conversion for larger short-time currents with shorter durations may only be carried out within the rated values, so that the peak withstand current and the making and breaking capacity of the circuit-breakers and the switchgear assembly are not exceeded.

Related calculation tool:

Short circuit current calculation at various point of electrical circuits (Isc)

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5. Rated normal current I_r

A key factor for the rated value is the actual operating current I_B, which flows continuously or for a long period in the network, and the prevailing ambient temperature.

Rating rule: I_r ≥ I_Bmax at a given ambient temperature

For switchgear, the next highest standard value above the maximum operating current is set as the rated current. Reserves have to be taken into account in the standard value to be selected if the ambient temperature or the characteristics of the load may change.

Recommendation №1 – Busbars and feeder circuits can have different rated values; the same applies to individual sections of busbars that have been sub-divided.

Recommendation №2 – Reserves for temporary (planned) overload or emergencies must be taken into account.

Recommendation №3 – When busbars are rated, the operating principle of couplings with parallel infeeds must also be taken into account at the planning stage.

Recommendation №4 – The operating current may contain harmonics which considerably contribute to the temperature rise; the total current determines the rating.

Recommendation №5 – The influence of the site altitude on the current carrying capacity in almost negliglible in practice. The dissipation of heat losses, just as the insulation capacity depends on the air density which decays with increasing altitude.

Regarding the current carrying capacity, however, the reduced cooling effect at high altitudes is compensated by lower ambient temperatures.

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Sources: MV Switchgear Application Guide by Siemens