POWER QUALITY BASICS: INTERRUPTIONS

Interruptions are classified by IEEE 1159 into either a short-duration or long-duration variation. However, the term “interruption” is often used to refer to short-duration interruption, while the latter is preceded by the word “sustained” to indicate a long-duration. They are measured and described by their duration since the voltage magnitude is always less than 10% of nominal.

It is one of the general categories of power quality problems mentioned in the second post of the power quality basics series of this site.

Interruption

Interruption is the power quality problem with the most perceivable effect on facilities. It generally affects the industrial sector, particularly the continuous process industry. In addition, the communication and information processing business is also significantly disturbed.

Short-duration Interruption

Interruption is defined as the decrease in the voltage supply level to less than 10% of nominal for up to one (1) minute duration. They are further subdivided into: Instantaneous (1/2 to 30 cycles), Momentary (30 cycles to 3 seconds) and Temporary (3 seconds to 1 minute).

Interruptions mostly result from reclosing circuit breakers or reclosers attempting to clear non-permanent faults, first opening and then reclosing after a short time delay. The devices are usually on the distribution system, but at some locations, momentary interruptions also occur for faults on the subtransmission system. The extent of interruption will depend on the reclosing capability of the protective device. For example, instantaneous reclosing will limit the interruption caused by a temporary fault to less than 30 cycles. On the other hand, time delayed reclosing of the protective device may cause a momentary or temporary interruption.

Aside from system faults, interruptions can also be due to control malfunctions and equipment failures.

Consequences of short interruptions are similar to the effects of voltage sags. Interruptions may cause the following (but not limited to):

Ø  Stoppage of sensitive equipment (i.e. computers, PLC, ASD) 

Ø  Unnecessary tripping of protective devices

Ø  Loss of data

Ø  Malfunction of data processing equipment.

Sustained Interruption

Sustained Interruption is defined by IEEE 1159 as the decrease in the voltage supply level to zero for more than one (1) minute. It is classified as a long duration voltage variation phenomena. Sustained interruptions are often permanent in nature and require manual intervention for restoration. In addition, they are specific power system phenomena and have no relation to the usage of the term outage. Outage does not refer to a specific phenomenon, but rather to the state of a system component that has failed to function. Furthermore, in the context of power quality monitoring, interruption has no relation to reliability or other continuity of service statistics.

Sustained interruptions are usually caused by permanent faults due to storms, trees striking lines or poles, utility or customer equipment failure in the power system or miscoordination of protection devices. Consequently, such disturbances would result to a complete shutdown of the customer facility.

Interruptions and Voltage Sags

Some interruptions may be preceded by a voltage sag,particularly when these PQ problems are due to faults on the source system. The voltage sag occurs between the time a fault initiates and the protective device operates. On the faulted feeder, loads will experience a voltage sag followed immediately by an interruption. The figure below illustrates a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles, which subsequently drops to zero for about 1.8 s until the recloser closes back in.

Interruption after a Voltage Sag (Courtesy of Electrical Power Systems Quality)

Also, as mentioned, the effects of voltage sags are almost similar to interruptions. Yet, interruptions affect the majority of end-users, while voltage sags only impact the more sensitive end-users. In other words, if other customers on the same circuit are also affected, then, the probability is high that the disturbance is due to interruption and not voltage sag.

Interruption - Prevention and Protection

To prevent interruptions, the utility may do the following:

1.    Reduce incidents of system faults

Ø  Includes arrester installation, feeder inspections, tree trimming and animal guards

2.    Limit the number of affected customers interrupted

Ø  Improve selectivity through single-phase reclosers and/or extra downstream reclosers

3.    Fast reclosing

To protect equipment from interruptions, end-users may use Uninterruptible Power Supply (UPS) and other energy storage systems. Back-up generator or Self-generation is necessary for sustained interruptions. Other solutions include the use of static transfer switch and dynamic voltage restorer with energy storage.

Synopsis:

Magnitude:

Short Interruption - Less than 0.10 per unit

Sustained interruption – 0.0 pu 

Duration:

Short Interruption - ½ cycle to 1 minute

Sustained interruption - More than 1 minute

Source: Utility or facility

Symptoms: Equipment Shutdown

Occurrence: Less than 2 interruptions/year in the US

Protection: Uninterruptible Power Supply (UPS), Self-generation, Energy storage  

References:

Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2^nd ed.). New York: McGraw-Hill.

IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality. New York: IEEE, Inc.

Short, T. (2006). Distribution Reliability and Power Quality. Boca Raton: CRC Press

Utility Systems Technologies, Inc. (2009). Power Quality Basics