Practical Power Plant Engineering. Zark Bedalov
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.
| Nominal voltage | 4.16 kV, 60 Hz, 3 ph, 3 w |
| BIL | 60 kV |
| Switchgear, interrupting capacity | 40 kA r.m.s. symmetrical |
| MV controllers, interrupting | 40 kA r.m.s. symmetrical |
| Assembly | Metal‐clad/metal enclosed |
| Incoming and tie breakers | 2500 A, vacuum type |
| Motor controllers (contactors) | 400 A, fused, for up to 2000 kW maximum, NEMA E2 |
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).
2.7.9 Low Voltage Service Voltage
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).
2.7.10 Bus Tie Breaker Switching
2.7.10.1 Incoming Transformer Failure
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/Rem position.
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/Rem position.
Figure 2.11 Switchgear breaker interlocks.
In case of a failure of one of the transformers, the faulty transformer (Assume: T12 on B bus) will be isolated by being tripped automatically or manually (locally). Once the incoming breaker for the transformer T12 opens, a signal is sent to fully isolate T12 by tripping its HV breaker. This will be followed by closing the 13.8 kV bus tie breaker.
Please note, all the B buses from 13.8 kV down to 480 V are temporarily shut down following the isolation of the faulty T12 transformer. When the tie breaker closes, the healthy transformer (T11) now feeds both Buses A and B. The control system now starts restoring the power (incoming and tie breakers) sequentially from the top bus to the lowest bus in that order to operate the plant on the single transformer. The whole process of power restoration is completed within several seconds.
This automatic switching process explained above is more applicable to the power plants, which may have three or four buses operated from 13.8 kV down to 480 V. The industrial plants have fewer plant bus levels, and the switching following a transformer failure may be manual.
The actual switching is arranged on the “Break before Make” principle. This is also called a “Dead Transfer.” Therefore, the two incomers + tie breakers operate on 2 out of 3 closed principle. However, since we are also planning to connect the standby DG to the Bus B in case of a total outage, the operating logic must accommodate an operating condition of 2 out of 4 breakers closed. Should there be a total power outage, both switchgear incomer breakers are opened, a signal is sent to the standby DG to start and close onto the 13.8 kV bus. That done, the tie breaker closes next (2 out of 4 conditions) and the essential power restoration commences across the whole 13.8 kV bus and the plant.
The breaker operating and closing coils as well as the protection and trip circuits operate on 125 Vdc circuits fed from the station battery. By having the DC system available, the main breakers can be operated during a total plant outage.
The plant restoration following a total blackout and DG operation follows by the appearance of voltage on the HV side of the main transformers. HV breakers are closed and a proper safe moment is awaited to initiate the restoration