Power Quality / Utility Voltage Standards

APS maintains service voltage within the ranges recommended by the American National Standard ANSI C84.1 - 1989. These voltage ranges apply to steady-state voltages, and do not apply to momentary voltage fluctuations, caused by switching operations, motor starting, fluctuating loads, and other normally occurring electrical operations.

ANSI C84.1 divides voltages into two ranges. Range A is the optimal voltage Range. Range B is acceptable, but not optimum. APS strives to design its distribution system to operate within Range A. The occurrence of service voltage outside Range A is infrequent, but may occur due to unusual operating conditions.

Range A minimum voltage is 95% of nominal voltage.
Range A maximum voltage is 105% of nominal voltage.

Range B minimum voltage is 91.7% of nominal voltage.
Range B maximum voltage is 105.8% of nominal voltage.

The following table shows Range A voltages for some common, three-phase, service voltages:

Besides variations in the steady state voltage, momentary voltage sags and overvoltages occur on all utility systems. These short duration voltage variations can adversely affect equipment, particularly variable speed drives, computers, programmable logic controls, and motor contactors. Normally-occurring voltage sags and overvoltage transients were not a major concern before the wide-spread use of electronic equipment. The increased use of sophisticated electronic equipment has made it necessary for utility customers to take steps to mitigate the affect of voltage transients.

Over voltage transients can occur when capacitor banks are switched on, when circuit breakers operate, or when lightning hits a feeder. Transient voltage surge protection should be installed by the utility customer to help prevent electrical equipment from being damaged, or failing to operate properly.

One way to deal with the majority of voltage transients is for the utility customer to specify equipment that can ride through a voltage sag to 60% of normal voltage for 200 milliseconds, and an overvoltage transient of 120% of normal voltage for 100 milliseconds. 200 milliseconds is less than a snap of the fingers, so these are very short events, and yet these voltage transients can cause electrical equipment to malfunction. 

An uninterruptible power supply, or a motor-generator set are examples of devices that utility customers can install to provide ride through capability for the majority of voltage sags.

One of the common causes of voltage sags is an adjacent feeder fault. There are usually several 12.47kV feeders connected to one 69kV to 12.47kV transformer. When a short circuit occurs on one of the feeders, all customers on the faulted feeder will experience a complete loss of voltage. Customers connected to adjacent feeders may see the voltage sag to 60% of normal voltage.
 
What causes short circuits? The list is a long one and includes mylar balloons shorting overhead lines, vehicles hitting utility poles, dig ins, insulation failure, and storms.

More power quality for business answers