Zark Bedalov - Practical Power Plant Engineering

LV connection to 12/15 MVA, 13.8 to 4.16 kV transformers: A cable bus from the transformers to the switchgear: 2100 A, 3 ph for 15 MVA, 133% cable insulation for ungrounded systems, with top entry from the transformer bushings into 4.16 kV switchgear, also as top entry.

HV connection to 2/3 MVA, load center transformers at 13.8 kV: 150 A cable drop from overhead line, 133% cable insulation for ungrounded systems to the HV load interrupter on the dry type transformers. Lightning arresters are required at the cable/line transitions.

At 4.16 kV: 400 A cable from 4.16 kV switchgear, 133% cable insulation for ungrounded systems. Load interrupter is not required if the load center is in the same room as the 4.16 kV switchgear. Lightning arresters may not be required. Those arrester used on the transformer HV side are considered also effective for the LV side. However, arresters are inexpensive at this voltage level. It will not hurt to install them in here.

LV connection at 480 V from 2/3 MVA transformers to LV Switchgear: This connection is assumed to be butted directly from the transformers to the LV (480 V) switchgear buses.

Overcurrent Settings for the Incomers: An electrical inspector in USA will likely verify and insure that the interrupting device (breaker) always has a lower setting than the cable it is protecting. In the case of the 13.8 kV incomers, even though the cable bus may have a rating of 2000 A and it connects to a 2000 A frame breaker, the breaker would likely have the protective relay overcurrent trip set lower than the cable bus rating, for instance 1700 A.

Or to be even more selective, the relay settings of the incomers could be arranged to have two groups of settings: 900 and 1700 A. The Group 1 setting of 900 A is for the two incomer breaker operation and as soon as one transformer fails and the full 13.8 kV load is transferred to one breaker only, the relay setting change would be initiated to Group 2 (1700 A) for the 40 MVA transformer rating by the breaker status interlock of the failed breaker. Similarly, the same can be arranged for the 4.16 kV incomers.

The 4.16 kV connected load is estimated to be around 15 MW at 0.8 pf (18.7 MVA). The 4.16 kV loads are critical loads for the plant production, requiring adequate availability of power supply. To establish the highest level of availability of the plant, the MV load board will be supplied from two 13.8 kV feeders from the main substation. The MV (5 kV) control assembly will be used to feed the plant MV motors rated over 200–2000 kW and 2/3 MVA, 4.16 kV to 480 V unit substations ( Figure 2.9). The 4.16 kV loads will be controlled by the MV controllers, employing MV‐fused 400 A contactors.

The MV motor control assembly will be located close to the plant. It will have two incoming 4.16 kV switchgear feeder cells and a tie breaker. Each incomer breaker will be rated 2500 A to be able to carry the full MV load from a single incoming transformer of 12/15 MVA transformers.

The 4.16 kV motor control board is a metal enclosed assembly, NEMA class E2 (fused) with integral 120 V accontrol transformers. The four motor controller cells on each side will be attached to the regular 4.16 kV metal‐clad switchgear in the middle. The incoming breakers will automatically interact with the tie breaker on a 2 out of 3 switching principle to carry the full load in case of a transformer outage.

Figure 2.9 4.16 kV motor controller assembly.

The incomers and tie breakers are assembled in their metal‐clad, single cell enclosures. Motor and feeder controllers can be stacked up as two units per vertical section. The controllers for feeding the unit substation feeders are of latched type to ensure they are immediately energized and restored following a restoration from a power failure. The unit substation radial overhead feeders are protected by GE Multilin feeder protective relays, or equal, capable of automatic one shot reclosing. Each motor controller includes a set of medium voltage fuses, a vacuum contactor, and GE Multilin motor protector relay or equal. Fuses for the motors are of R‐rated type.

Fuses for the transformer and outgoing feeders are of current limiting L‐rated type (see Chapter 3).

All the breakers and controllers are provided with means of operating from local and remote positions. The means of communication for the breaker operation, interlocks, and status are by Ethernet from the control room (see Chapter 17).

A simplified one‐line diagram of one of the LV switchgear is shown in Figure 2.10. It is mostly used to distribute power to plant MCCs. The switchgear breakers are rated from 800 to 3200 A frame sizes. The breaker trip unit can be lower than the frame sizes to suit the load. This is shown as 800AF/600AT, as frame and trip rating for the breaker. L and S are the characteristics for the long and short breaker sensor trip element, respectively.

Figure 2.10 Part of LV switchgear.

The common LV switchgear voltages are as follows:

In USA: 480 V, 3 ph, 60 Hz. The lowest voltage used is 120 V, 1 ph.

Canada: 600 V, 3 ph, 60 Hz. The lowest voltage is 120 V, 1 ph.

Europe, Asia, South America, Australia: 400 V, 3 ph, 50 Hz. The lowest plant voltage is 231 V, 1 ph, which is the line to ground voltage from 400 V, 3 ph.

These voltages are used to feed low voltage, three‐phase motors up to 200 kW, and auxiliary feeders.

Smaller motors are fed at 1 ph, 120 V in USA/Canada, or 231 V in the IEC countries.

Larger motors up to 500 kW can also be fed at LV (400–600 V), if controlled by VFDs or SoftStarts (see Chapter 15).

According to the overall key one‐line diagram, a part of which we repeat here in Figure 2.11, the incoming transformer T11 feeds Bus A, while the T12 feeds the Bus B of the main 13.8 kV switchgear. The switchgear bus tie breaker is held open during the normal plant operation and its operating control switch on its front panel is held on Loc/ Remposition.

The incoming breaker transfer switching can be arranged by the plant control system (automatic) or hard wire logic (manual) on the switchgear. The bus tie breaker is not allowed to be closed while both incomers are closed. Once either incoming circuit breaker opens, the tie breaker closes immediately behind, if its control switch is also placed on Loc /Rem. Generally, all the main breakers should have their switches on placed on Loc/ Remposition.

Figure 2.11 Switchgear breaker interlocks.