Power plant auxiliary system is usually pretty complicated and consists of a dozen motors, transformers, capacitor banks, variable frequency drives, PLCs, and other electrical devices. Therefore, protection relaying cannot be simple, and it requires special attention for each component of a power and control system as well as coordination between them.
This technical article will shed some light on how an auxiliary system should be designed in order to sustain the main production facilities in power plants. Special attention is given to the of motors, transformers, and other devices from the point of view of the normal and emergency operation of the entire plant.
Faulted electrical equipment must be removed from service as quickly as possible. For many electrical faults or abnormal events within the plant, this may require that the generator be removed from the system, the excitation system tripped, the turbine valves closed, and the boiler fires extinguished. Often this is not acceptable.
In addition, the auxiliary system must be configured to allow the unit to return to service as soon as possible.
A portion of a typical auxiliary system of a unit-connected generator is shown in Figure 1 below.
The 4 kV auxiliary bus is fed or from the startup transformer and is the source for the major motors. As unit sizes increase, the auxiliary load increases proportionately, requiring higher rated transformers and higher rated, higher voltage motors. This has resulted in higher bus voltages, such as 6.9 kV and 13 kV.
Phase fault currents also increased, requiring switchgear with higher interrupting capacity. In sizing the switch gear there are two contradictory factors that must be considered. The impedance of standard transformers increases as their ratings increase.
Current-limiting reactors can be used either as separate devices or incorporated in the switchgear.
In addition to the 4 kV (or higher) bus, a lower voltage auxiliary bus system is used to feed the dozens or hundreds of smaller motors, heating and lighting loads that are present in the plant. The nominal voltage rating of this lower voltage bus system can be .
The lower voltage buses are energized from the higher voltage bus as shown in Figure 1.
The circuit breakers used on the lower voltage buses are included in the metal-enclosed switchgear and are covered in ANSI standards C37.20-1 and C37.20-3. They may not be draw-out type necessarily, don't have CTs and may be mounted in .
Circuit breakers may be air type or molded case breakers with limited interrupting capacity. Protection is provided by series trip coils or thermal elements
Extinguish the arc in a gap of less than 13 mm (0.5 in) because there are no constituents in the vacuum that can be ionized to support the arc. extinguish the arc using one of two methods: the puffer design blows the arc out with a small amount of gas blasted in a restricted arc space; The rotating arc design uses the electromagnetic effect to rotate the arc through SF6 that cools and extinguishes it.
Nowadays, vacuum and SF6 circuit breakers are more commonly used. More about which one and where to apply, you can learn. These circuit breakers are mainly drawout type, allowing the breaker to be removed for maintenance.
A differential bus relay could be used to provide primary protection and the overcurrent relays provide backup protection for motor relay or switch gear failures. The time delay may then be acceptable. The pickup setting must still recognize the magnitude of starting current of the largest motor.
If it cannot be set above this value, an interlock must be provided which will block the backup relay.
Power plant auxiliary system is usually pretty complicated and consists of a dozen motors, transformers, capacitor banks, variable frequency drives, PLCs, and other electrical devices. Therefore, protection relaying cannot be simple, and it requires special attention for each component of a power and control system as well as coordination between them.
This technical article will shed some light on how an auxiliary system should be designed in order to sustain the main production facilities in power plants. Special attention is given to the of motors, transformers, and other devices from the point of view of the normal and emergency operation of the entire plant.
Faulted electrical equipment must be removed from service as quickly as possible. For many electrical faults or abnormal events within the plant, this may require that the generator be removed from the system, the excitation system tripped, the turbine valves closed, and the boiler fires extinguished. Often this is not acceptable.
In addition, the auxiliary system must be configured to allow the unit to return to service as soon as possible.
A portion of a typical auxiliary system of a unit-connected generator is shown in Figure 1 below.
The 4 kV auxiliary bus is fed or from the startup transformer and is the source for the major motors. As unit sizes increase, the auxiliary load increases proportionately, requiring higher rated transformers and higher rated, higher voltage motors. This has resulted in higher bus voltages, such as 6.9 kV and 13 kV.
Phase fault currents also increased, requiring switchgear with higher interrupting capacity. In sizing the switch gear there are two contradictory factors that must be considered. The impedance of standard transformers increases as their ratings increase.
Current-limiting reactors can be used either as separate devices or incorporated in the switchgear.
In addition to the 4 kV (or higher) bus, a lower voltage auxiliary bus system is used to feed the dozens or hundreds of smaller motors, heating and lighting loads that are present in the plant. The nominal voltage rating of this lower voltage bus system can be .
The lower voltage buses are energized from the higher voltage bus as shown in Figure 1.
The circuit breakers used on the lower voltage buses are included in the metal-enclosed switchgear and are covered in ANSI standards C37.20-1 and C37.20-3. They may not be draw-out type necessarily, don't have CTs and may be mounted in .
Circuit breakers may be air type or molded case breakers with limited interrupting capacity. Protection is provided by series trip coils or thermal elements
Extinguish the arc in a gap of less than 13 mm (0.5 in) because there are no constituents in the vacuum that can be ionized to support the arc. extinguish the arc using one of two methods: the puffer design blows the arc out with a small amount of gas blasted in a restricted arc space; The rotating arc design uses the electromagnetic effect to rotate the arc through SF6 that cools and extinguishes it.
Nowadays, vacuum and SF6 circuit breakers are more commonly used. More about which one and where to apply, you can learn. These circuit breakers are mainly drawout type, allowing the breaker to be removed for maintenance.
A differential bus relay could be used to provide primary protection and the overcurrent relays provide backup protection for motor relay or switch gear failures. The time delay may then be acceptable. The pickup setting must still recognize the magnitude of starting current of the largest motor.
If it cannot be set above this value, an interlock must be provided which will block the backup relay.
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