High voltage motor

Frequently switchgear is used for motor control, particularly for large high-voltage motors. Switchgear (Fig, 29-6) must be used for motors... 

Ganz. A. and Jakob. R.. Switching high voltage motors with SF,t self-extinguishing circuit breakers. Brown Bnveri Review, 110-714. November (1979),... 

Lightning and switching surges that damage motors are related to high-voltage motors. The insulation level of motors is below that of many other types of apparatus such as transformers, switchgear, cables,... 

Motors are controlled by circuit breakers or contactors. With high voltage motors it is necessary to ensnre that the main terminal box and the terminals inside can withstand the effects of a major three-phase fanlt inside the box. This applies especially to motors that are to be used in a hazardous area. As a guide to the level of safeguard, Table 5.13 may be used. 

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

Switchgear is available up to at least 400 kV, whereas motor control centres are only designed for voltages up to approximately 15 kV because this is the normal limit for high voltage motors. 

Hence the circuit breakers and the busbars in the switchboard may have to be derated for the breaking duty. The amount of DC component, or off-set as it is often called, depends upon the point in time set by (po when the fault is applied. The occurrence of 100% off-set is seldom but cannot be ignored. The design and selection of the switchboard should be based on 100% off-set, especially if it is fed by generators and feeds a group of high voltage motors. 

Note 4 Occasionally used for high voltage motors and variable speed drives. 

For high voltage cables, and low voltage feeder cables between switchboards, the disconnection time can be reduced from 5 seconds to not less than 0.2 seconds. The time of 0.2 seconds is determined from the total clearance time of a circuit breaker protected by a fast acting overcurrent relay. For high voltage motor circuits in which the short-circuit protection is provided... 

A preliminary estimate of the load is Si, which may be assumed to consist of a certain amount of motor load and some static load. The motor load can be assumed to be connected to the main high voltage switchboard and at lower voltage switchboards. Let the high voltage motor load be Sh and the lower voltage motor load be Sim Assume the static load Sis to be connected to the lower voltage switchboards. Typical power factors for these loads are 0.87, 0.85 and 0.97 respectively. The total active and reactive power estimates are,... 

The high voltage motor unit transformers have a total capacity 8 slightly higher than that of the motors Shm,... 

The contribution from the high voltage motors is found as follows. 

For typical high voltage motors the starting power factor is between 0.15 and 0.2 lagging, hence assume 0.2. The sub-transient impedance becomes,... 

Include all high voltage motors and their driven machinery. 

Figure 12.15 shows the application of the above functions to small and large high voltage motors. 

Most high voltage motors used in the oil industry operate at steady loads between 75% and 90% of their rated power capability. Should the motor suddenly find itself underloaded then it is possible that the driven machine has inadvertently lost its load, e.g. a pump loses liquid at its suction port. This... 

In general a large amount of the theoretical aspects of the protection of high voltage motors applies to low voltage motors. However, some functions are not normally required, in particular, core balance earth faults (50 N and 51 N) see the note below -... 

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