N2 Electrical Trade Theory|The Easy Way! 89
Consider a transformer rated at 60 kVA. Tis means that each phase is rated at 20 kVA. If, for example, two of the phases are delivering 18 kVA and the third phase 24 kVA, the transformer is delivering 60 kVA but the one phase is obviously overloaded by 4 kVA. Te overload protection of the transformer will trip in this case. Normal overload protection is effective in preventing phase imbalance of this nature.
With a rigidly balanced three-phase load such as a three-phase motor, any imbalance in the supply current will indicate a fault. Single-phasing is a severe case of phase imbalance.
6.3.1 Specific phase imbalance protection
Balance protection is based on the principle that the current entering a healthy circuit is equal to the current leaving it. If a fault develops, the current difference (current imbalance) can be used to trip the appropriate circuit-breakers. Tis type of protection is used on transmission lines.
Phase imbalance protection is sometimes used on feeders supplying three-phase motors as there is always a possibility of one phase opening as a result of a fuse failure or a conductor open-circuiting. A connector failure or the short-circuiting of a few turns of a winding can also lead to a phase imbalance.
Many contactors which are used to control motors are fitted with relays which offer protection against overcurrent (including locked rotor condition), phase failure (single-phasing), and phase imbalance (current imbalance).
Tese relays are fitted with a current transformer ( a core transformer similar to the core of an earth leakage unit) around each phase conductor. Tese current transformers compare the current in one phase against the currents in the other phases. When the imbalance exceeds a predetermined value, the relay operates and opens the circuit.
As the current transformers are inside the relays, the phase conductors pass through them internally and the supply cable is simply connected to the contactor in the usual manner.
Since the three phases of three-phase circuit-breakers are mechanically connected to each other, they protect motors from single-phasing when a fault occurs in only one phase. In the case of a fuse failure somewhere in the circuit of a three-phase motor, the circuit-breaker also offers some protection from single-phasing as it is very likely that the other two phases will trip on overload.
▲ 6.16.5.1.3 Te overcurrent protection device protecting a three-phase motor must prevent the motor from continuing to operate under load if single-phasing occurs.
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