Power Supply with regard to Selectivity Evaluation (Examples)

Example 1: Supply from one transformer

Fig. 43/1 shows a supply option using a transformer with 630 kVA / ukr = 6 %. Protection is ensured by means of HV HRC fuses at the medium-voltage side.

In practice, this configuration with a switch-disconnector plus HV HRC fuse assembly is used for transformer output

< 1,000 kVA. Full selectivity to low voltage cannot be attained here. In particular in coupling connections to SPS networks, selectivity is mandatory. Selectivity is attained by using a low-cost medium-voltage circuit-breaker with protective device.

For transformer outputs > 1,250 kVA (10 kV) or 2,000 kVA (20 kV), circuit-breakers with an appropriate protective

device are used as standard to ensure protection at the medium-voltage side.

With lower transformer outputs, circuit-breakers are only used if a high switching frequency is required, for example, or higher nominal voltages (e.g. 36 kV) are applied.

In this example, it is possible to configure a selective installation at the low-voltage side by using LV HRC fuses up to 425 A and, if sub-distribution systems are lined up, by grading the fuses with a factor of 1.6.

The use of circuit-breakers in subordinate distributions or a combination of circuit-breaker and fuse could be critical when selectivity is required. Successful protection depends on the type of circuit-breaker used (air circuit-breaker (ACB), molded-case circuit-breaker (MCCB)) and the network configuration.

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In terms of selectivity evaluation it is always recommend-able to perform a network calculation at an early planning stage.

Assessment of a worst case scenario in the power system (feeders close to the transformer, remotest feeders, device combinations (circuit-breaker / fuse …) is often sufficient to get a rough idea.

What’s important to know is that a selectivity evaluation and its results are only true for the devices considered in the calculation. When different products or device combinations are then used for project implementation, the calculation must be performed again, as devices from different manufacturers may deviate from the original results in their tripping characteristics or tolerance bands of the characteristic curve.

Example 2:

Supply from two transformers and tie breaker The following example for power supply (Fig. 43/2) demonstrates a higher power demand which is covered by two transformers with 630 kVA each. The busbars at the level of the low-voltage main distribution are isolated by means of a tie breaker.

The advantage is that if one busbar system is faulted, parts of the installation can still be used.

A tie breaker also makes sense for limiting short-circuit currents for the busbar system when higher transformer outputs are involved (in normal operation, the tie breaker is open). This layout is economical (cheaper devices / systems), provided that it can be ensured that systems are not operated in parallel. In addition, network dimensioning in terms of a selective grading of devices is enhanced.

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Example 3:

Redundant power supply / safety power supply with generator

If a redundant power supply system is desired or stipu-lated, the networks are coupled by cables or busbars (Fig. 43/3). For safety reasons, installation components must be erected separately, surrounded by suitable fire barriers. The regional expert (TÜV, expert, Association of Property Insurers, etc.) should be involved in the imple-mentation planning at an early stage.

The connection to the public power supply system (NPS = normal power supply) is made by a circuit-breaker feeder in the redundant power supply system (SPS = safety power supply), which provides protective functions and enables separate operation of installation components.

A corresponding feeder circuit-breaker on the NPS side allows for disconnecting and protecting the cable / busbar line. Two circuit-breakers are required as a coupling be-tween the NPS and SPS networks.

The design of the transformer circuit-breakers, tie breakers and generator circuit-breakers (3-pole or 4-pole) depends on the power system design (distance of NPS / SPS), the grounding concept and the power system philosophy of the inspecting body (TÜV, expert, etc.). Siemens recom-mends the use of 4-pole devices when distances between NPS and SPS are greater than 50 m. This way, the power systems are decoupled (also refer to section 9.1, Electro-magnetic Compatibility (EMC)).

The configuration of the SPS should be kept as simple as possible in view of the selectivity requirement. Complex power supply systems, combinations of circuit-breaker and fuses as well as line-ups of sub-distribution systems should be avoided, if possible.

Note:

A network calculation at the planning start is highly recommended. The following should be verified:

1. During normal operation, the normal power supply system feeds the safety supply equipment (transformer breaker ON, tie breaker ON, generator circuit-breaker OFF).

Is the transformer output sufficient to cover the power demand of the NPS and SPS?

2. When the switchgear is operated as described in para-graph 1, the maximum short-circuit current applied on the SPS busbar is shaped by the transformers.

Is the breaking capacity of the devices connected in the SPS sufficient?

3. In case of a fault on the NPS busbar, the generator takes over power supply of the connected equipment (trans-former circuit-breaker ON or OFF (depending on the fault), tie breaker OFF, generator circuit-breaker ON).

In this operating condition, the minimum short-circuit current of the generator is critical.

Is the minimum short-circuit current sufficient to meet the tripping times required in case of a fault (5 seconds for stationary loads (machines), 0.4 seconds for

non-stationary loads (equipment connected to power outlets), alternatives for protection according to DIN VDE 100 Part 410 (local equipotential bonding, touch voltage

< 50 V) have not been taken into account in this case)?

4. The power system may also have to be rated and verified to meet the maximum required voltage drop specification.

Circuit-breaker 1.1 Circuit-breaker 1.2

Cable / Line

Chapter 5

Dimensioning of the Main