The purpose of phase unbalance protection is to prevent motor overheating damage. Motor overheating occurs when the phase voltages are unbalanced. A small voltage unbalance produces a large negative-sequence current flow in both synchronous and induction motors.
The per-unit negative-sequence impedance of either motor is approximately equal to the reciprocal of the rated voltage per-unit locked-rotor current. When, for example, a motor has a locked-rotor current equal to six times rated current, the motor has a negative-sequence impedance of approximately 0.167 per unit (16.7%) on the motor rated input KVA base. When voltages having a 0.05 per-unit negative-sequence component are applied to the motor, negative-sequence currents of 0.30 per unit flow in the windings. Thus, a 5% voltage unbalance produces a stator negative-sequence current equal to 30% of full-load current. This situation can lead to a 40% to 50% increase in temperature rise. A special form of unbalance is the complete loss of one phase at the same voltage level as the motor. Under starting conditions, a three-phase motor is unable to start. If the single phasing occurs during full-load running conditions, the current in the other two phases increases to approximately 173% of normal full-load current. In each case, adequate protection is required. For large facilities, a bus phase-balance (negative-sequence) overvoltage relay (Device 47) could be installed to alarm in a sensitive manner. This alarm would be set in conjunction with individual large motor (phase-balance) negative-sequence over current relays (Device 46). For small installations, a single phase-balance (negative-sequence) over current relay may suffice for a large, important motor; or alternatively one phase-balance (negative-sequence) bus overvoltage relay could be set to protect several motors, by alarming and/or tripping.
When a motor is supplied from a delta-wye or wye-delta transformer, single phasing on the supply voltage (primary) side of the transformer results in currents to the motor in the ratio of 115%, and 230% of normal. In two phases, the current is only slightly greater than prior to single phasing, while it is approximately doubled in the third phase. This situation requires a properly sized overload relay or time-delay fuse in each phase if the motor does not have suitable phase unbalance protection.
Many motors, especially in the higher horsepower ratings, can be seriously damaged by negative-sequence current heating, even though the stator currents are low enough to go undetected by overload (overcurrent) protection. (The standard service factor for large motors is 1.00.)
Therefore, phase unbalance protection is desirable for all motors where its cost can be justified relative to the cost and importance of the motor. Phase unbalance protection should be provided in all applications where single phasing is a strong possibility due to factors such as the presence of fuses, overhead distribution lines subject to conductor breakage, or disconnect switches (which may not close properly on all three phases).
A general recommendation is to apply phase unbalance protection to all motors 750 kW and above. For motors below 750 kW, the specific requirements should be investigated. Phase unbalance protection should also be considered for certain important motors such as hermetic refrigeration chiller motors and similar motors having a service factor less than 1.25.
Unbalanced voltages accompany unbalanced system faults. Therefore, phase unbalance protection should include sufficient delay to permit the system overcurrent protection to clear external faults without unnecessary tripping of the motor or motors.
Delay is also necessary to avoid the possibility of tripping on motor starting inrush.
Therefore, unbalance protection having an inherent delay should be chosen. Another (high risk) scheme is to use an auxiliary timer (Device 62). Its selection is important because the timer probably has a higher failure rate than the protective relay. If a time delay of more than 2 s or 3 s is used, the motor designer should be consulted.
Two types of phase balance relays are normally applied: current balance and negative sequence
overcurrent. The current balance relay operates when the difference in the magnitude of root-mean-square (rms) currents in two phases exceeds a given percentage value. The negative-sequence current relay operates on magnitude of negative-sequence current, but is set in terms of I22
t, the thermal energy produced by this current. In order to set the negative-sequence relay, the I22
t characteristic (or K factor) of the machine should be specified.
The electromechanical relay consists of two or three induction disk elements, each havingtwo current coils. These coils are connected to different phases so that a closing torque is produced on the disk that is proportional to the difference or unbalance between the currents in the two phases. The amount of unbalance current required to close the contacts may be a fixed percentage, typically 25%, or it may be a variable percentage, as shown by the operating characteristic in Figure.
The static relay performs similarly to the electromechanical design. This relay typically provides two set points, which allows an alarm signal at a sensitive, pretrip value ofI22
tin addition to the trip setting. Figure below shows typical characteristics of this relay.