Single Phasing and Phase Unbalance
Wisconsin Public Service electrical engineers document common power quality issues and solutions in these fact sheets:
What Is Single-Phasing?
Loads using three-phase power sources are subject to loss of one of the three phases from the power distribution system. This condition is known as "single-phasing." The loss of a single phase on a three-phase line may be due to a downed line or a blown pole top fuse on the utility system. Loss of a single phase may also result from a single-phase overload condition causing one fuse to blow, or an equipment failure within the end-user's facility.
The loss of one phase, or "leg," of a three-phase line causes serious problems for induction motors. The motor windings overheat due primarily to the flow of negative-sequence current, a condition that exists anytime there is a phase voltage imbalance. The loss of a phase also inhibits the motor's ability to operate at its rated horsepower.
If single-phasing occurs when a motor is rotating, the torque produced by the remaining two positively rotating fields continues to rotate the motor and develop the torque demanded by the load. The negatively rotating field, the field associated with the lost phase, produces currents in inductive loads resulting in voltages in the faulted leg of the three-phase supply. These voltages may be nearly equal to the phase voltage that was lost. Therefore, detecting a single-phasing condition by measuring the voltages at the motor terminals is usually unproductive.
Three-phase motors may continue to run, but they are not capable of starting on a single phase. If after the overload devices on the energized phases isolate the motor, the motor is not then isolated from the lost phase, later attempting a restart on that single-phase supply will cause the motor to draw locked rotor current.
Phase Unbalance
Unbalance of a three-phase system is less extreme than a complete loss of phase, but may have similar consequences. On new installations of three-phase power systems, careful attention is given to balancing the loads on each phase. However, as single-phase loads are added to these originally balanced systems, an unbalance may occur. Thermal overloads, magnetic breakers, and other such devices will not detect this gradual unbalance and therefore will not provide adequate protection.
Voltage unbalance of a three-phase system is expressed as a percentage value, and is often defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents. This voltage unbalance is calculated as shown below:
With phase-to-phase voltages of 230, 232, and 225, the average would be 229 volts.
The maximum deviation from the average is 4 volts.
Therefore, the unbalance is 1.75 percent.
Phase voltage unbalance causes three-phase motors to run at temperatures greater than their published ratings. This excessive heating is due mainly to negative-sequence currents attempting to cause the motor to turn in a direction opposite to its normal rotation. These higher temperatures soon result in degradation of the motor insulation and shortened motor life. The percent increase in temperature of the highest current winding is approximately two times the square of the voltage unbalance. For example, a 3 percent voltage unbalance will cause a temperature rise of about 18 percent.
3² x 2 = 18
The greater the unbalance, the higher the motor winding temperature and the sooner the insulation will fail. NEMA standards recommend a maximum voltage unbalance of 1 percent without derating the motor. The motor can be derated down to 75 percent for a maximum of a 5 percent voltage unbalance. If the voltage unbalance exceeds 5 percent it is not recommended that the motor be operated. A rule of thumb states that for every 10°C a motor is operated over the rated temperature rise, insulation life (and therefore motor life) is reduced by half.
Protecting Motors from Single-Phasing
There are a number of ways to protect machines from single-phasing and voltage unbalance. The diagram below shows a simple protection scheme that has been used to protect industrial equipment from damage caused by single-phasing. However, as has already been discussed, regeneration on the missing leg in inductive loads may make it impossible to detect the loss of phase based on voltage alone.
Three-phase motor single-phasing protection can be provided by time-delay overcurrent protection fuses sized at 125 percent of the motor running current. To produce rated torque under single-phasing conditions, motors will draw a line current of 173 to 200 percent of normal. The overload devices will open to protect the machine in this case. However, this will occur only where the motors are being operated at or near their nameplate ratings.
Loss of a single phase to a three-phase motor reduces the power output of that motor to approximately 57 percent. If the motor is lightly loaded, circulating currents may damage or destroy the motor windings without the overload devices removing the motor from the line. This will also occur where motors are oversized for their application. For example, a 5 horsepower motor is used where the load is only 3 horsepower. To provide adequate protection from single-phasing conditions, the overload devices must be sized to the actual full-load RMS current. This may be determined with a clamp-on meter while the motor is running at its normal full load. For applications where the load is variable, another means of single-phasing protection will be required.
An alternate means of single-phasing protection should also be considered where multiple critical three-phase loads are supplied by a single main service with ground fault protection. A ground fault in one of the loads may cause the time-delay overload protection fuse to clear the overcurrent condition on the faulted phase. However, the overload protection will not clear the ground fault. If the remaining time-delay overcurrent protection fuses do not open before the ground fault protection relay operates, power to the remaining critical loads will be lost.
Integrated circuit technology may provide cost-effective solutions for some phase protection problems. These modules provide a contact closure when voltages of the proper magnitude and phase are present on the monitored line. The relay contacts can be wired into the control logic of the protected load to remove primary power or to prevent attempted restarts during single-phasing conditions. These units are small and relatively inexpensive, and may include sensitivity adjustments for various nominal line voltages.
The diagram below shows a typical application with a single three-phase motor load. Note that the input to the phase monitor module is taken from the final set of motor fuses. Connecting the power monitor in this manner allows:
- installation of the power monitor without disturbing existing protective devices, and
- detection of any failure inside the system that may cause single-phasing.
Outputs may be wired into a control circuit to trip the motor contactor should a failure occur.
An alternative would be to use the module to trip an audible alarm circuit or automatic dialer as shown in this diagram:
Sensing a single-phase condition is meaningless without a reliable source of tripping control power. It is common practice to derive the control power from control power transformers, which are themselves fed from the bus likely to be affected by the single-phasing condition. The most reliable source of control power is DC supplied by a station battery. If a reliable alternate source of control power is not available, a control power transformer configuration must be designed that will assure sufficient voltage for tripping regardless of which phase has been lost.
For additional reading on this subject see:
- ANSI/IEEE Standard 141-1986 (Red Book)
- ANSI/IEEE Standard 241-1990 (Gray Book)
- ANSI/IEEE Standard 242-1986 (Buff Book)
- Bussman, 1990, SPD Electrical Protection Handbook - Selecting Protective Devices Based on the National Electrical Code, St. Louis, MO.
- Robert W. Smeaton, Editor, 1969, Motor Application and Maintenance Handbook, McGraw-Hill Book Company, New York,
- Jerry C. Whitaker, 1991, AC Power Systems Handbook, CRC Press, Boston, MA
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