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Home / Technical Articles / Lessons I learned during the start-up of an MV switchgear

General about start-up

In the world of electrical engineering, a term start-up assumes all works which are necessary to be done for putting in operation already tested switchgear, as close as possible installed in position and ambient defined by design documentation and employer’s requirements.

Lessons I learned during the start-up of an MV switchgear
Lessons I learned during the start-up of an MV switchgear

In order for the start-up to be as more valid as possible, working parameters shall also be close to or equal to designed values. The start-up procedure usually begins with the power-up of the equipment and continues with signal checking, parameter fine adjustment, measurements control, etc.

For more details, you can see the previous article called ‘Most important precautions during commissioning and start-up of LV switchgear’.

If we are talking about the start-up of the MV/LV substation, one of the most important parts of that procedure is, for sure, the start-up of MV switchgear. All omissions and mistakes during the design and installation stage related to this equipment can lead to potential faults and other problems during start-up.

These problems may be very expensive and/or require a lot of time to be solved, so in that way, the project time schedule or budget may be significantly jeopardized.

Moreover, in some cases, problems related to MV switchgear can cause problems related to other, downstream equipment, such as power transformer or LV switchgear.

Table of contents:


Example of MV switchgear start-up problems

Within this article, a real example of 20kV switchgear, which belongs to (1000+1000) kVA industrial substation, will be shown. Experience gained through troubleshooting of problems that occurred during start-up will be shared, with adequate descriptions in the form of wiring diagrams.

A simplified block diagram of major electrical equipment inside the substation is given in Figure 1.

Block diagram of MV/LV substation example
Figure 1 – Block diagram of MV/LV substation example (click to expand diagram)

MV switchgear (=14+1L) is of 8DJH type, made by Siemens, with major characteristics as presented in Table 1.

Table 1 – MV switchgear (=14+1L) rated values

8JDH CharacteristicsValue
Rated voltage of equipment24 kV
Rated working (service) voltage20 kV
Rated busbar current630 A
Rated feeder current200 A
Rated short-time withstand current16 kA × 1 s

MV switchgear consists of the following cubicles listed in Table 2 below.

Table 2 – MV switchgear (=14+1L) cubicles

MV switchgear (=14+1L) cubiclesQty.
Incoming feeder cubicle, with switch disconnector and earthing switch2
Measuring cubicle, with voltage and current measuring transformers1
Transformer feeder cubicle, with protection fuses, switch disconnector, and earthing switch2

Feeders with fuses are much less expensive than feeders with circuit breakers and relays. On the other hand, later are much more flexible and able to provide more comprehensive and sophisticated protection.

Besides, if not chosen correctly, fuses may cause problems during the start-up of the equipment with high initial currents, such as no-load power transformers or power capacitors.

If current – time curve of the fuse is not in compliance with start up characteristics of protected equipment, the fuse will blow, and replacement will be necessary.

Switch disconnectors inside transformer feeders can be opened in three possible ways:

  • Manually, by inserting lever into opening on the cubicle and rotation in desired direction
  • By opening mechanism excited by striker pin, which is released when the fuse is blown. In that way, disconnection of transformer from MV grid is provided in the case of severe overload or short circuit
  • Electrically, by applying control voltage to shunt opening release

In this application, shunt opening release is designed to be excited when DGPT transformer relay is activated. Since activation of this relay implies serious transformer issue, it makes sense to disconnect it from primary supply.

Initial protection logic is realized inside main LV switchgear (+1N) by wiring diagram given in Figure 2.

Transformer switch disconnector opening logic
Figure 2 – Transformer switch disconnector opening logic (click to expand wiring diagram)

If all trip contacts on DGPT relay are inactive (closed), auxiliary relays (-1KT1, -1KT2, -1KT3) are excited, their output contacts are closed, and therefore auxiliary relay (-1KT5) is also excited, i.e. output terminals (11) and (12) are open.

Shunt release (+1L4-Y1) is disconnected from control voltage wires (L+, L-). If any of DGPT relay contact is activated (opened), relay (-1KT5) will be disconnected from control voltage, output contacts (11) and (12) will be connected, and shunt release will be excited, i.e. switch disconnector will be opened.

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Miodrag Kokotovic - Author at EEP-Electrical Engineering Portal

Miodrag Kokotovic

Graduated from Faculty of Electrical Engineering, within University of Belgrade, in the field of electrical power systems. Expert in electrical part of tender preparation, design, procurement, construction and commissioning of treatment plants and pumping stations, electrical power quality and energy management.
Profile: Miodrag Kokotovic

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