Busbars sections

Supply source Busbar section, Test circuit Cable... 

Finger between the two busbars, section a-a, which may shear off from its roots. 

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow. 

High voltage switchboards are available with busbar ratings up to 5000 A. Consider for example an 11 kV switchboard that is fed by four 25 MVA generators, two connected to the left-hand side busbar section and two on the right-hand side section. The total rated current from a pair of generators is 2624 A, which is the maximum current that can flow across the busbar section circuit breaker. Hence the busbars can be adequately rated at 3000 A or 3150 A for this plant. 

Suppose the generator is connected to a nearby switchboard. The generator and busbar section circuit breakers will need to at least withstand the fault current given in (7.1). The equation consists of three essential parts -... 

Device Generator incoming Transformer incoming Busbar section Motor outgoing Transformer outgoing... 

The following description of integrated motor control systems (IMCS) is based upon Reference 7, for which permission to use the material therein was kindly given by Switchgear and Instrumentation Ltd. The principles described can be used for low and high voltage switchgear that contain plain feeders, interconnectors, incomers and busbar section circuit breakers, in addition to... 

Four main units are used in the IMCS, which are the motor control unit (MCU), the feeder control unit (FCU), the circuit breaker control unit (CBCU) and the central control unit (CCU). A MCU is a microprocessor (micro-computer) based module which has integrated control, monitoring, protection functions, and a communication interface for the motor starter. An FCU is very similar to a MCU and interfaces communication for the plain feeder contactor or circuit breaker. A CBCU is also similar to a MCU but is used for incomers, interconnectors and busbar section circuit breakers. A CCU provides the facility to communicate simultaneously with MCUs, FCUs, CBCU, a distributed control system (DCS), system control and data acquisition (SCADA) and other digital information systems. Other discrete devices such as special protective relays can also be addressed by the CCU provided the software and porting systems are compatible. 

Circuit breakers are invariably used for the incomer, busbar section and switchboard interconnector switching devices, because the currents that they need to switch are too high for contactors to handle properly. Outgoing circuits can be static loads or motor loads, and these are usually limited... 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

Each switchboard in the main distribution network can have its own one-line diagram displayed, and for the more complicated switchboards each busbar section can be displayed. The active and reactive power flows and current flows can be shown for each item where suitable analogue transducers have been fitted. This is particularly beneficial for the main power generation switchboard. AU names and tag numbers can be shown for clarity. A diagram similar to Figure 16.1 can be displayed. 

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