Ch-2 Voltage Sags and Interruptions

7/27/2019 Ch-2 Voltage Sags and Interruptions

1/7

Voltage Sags and InterruptionsA voltage sag is a short-duration (typically 0.5 to 30 cycles) reduction in rmsvoltage caused by faults on the power system and the starting of large loads,such as motors. Momentary interruptions (typically no more than 2 to 5 s)cause a complete loss of voltage and are a common result of the actions

taken by utilities to clear transient faults on their systems. Sustainedinterruptions of longer than 1 min are generally due to permanent faults.Sources of Sags and Interruptions:

Fig 2.1 Fault locations on the utility power system

Voltage sags and interruptions are generally caused by faults (short circuits)on the utility system. Consider a customer that is supplied from the feedersupplied by circuit breaker 1 on the diagram shown in Fig. 2.1. If there is afault on the same feeder, the customer will experience a voltage sag duringthe fault followed by an interruption when the breaker opens to clear thefault. If the fault is temporary in nature, a reclosing operation on the breakershould be successful and the interruption will only be temporary. It willusually require about 5 or 6 cycles for the breaker to operate, during whichtime a voltage sag occurs. The breaker will remain open for typically a

minimum of 12 cycles up to 5 s depending on utility reclosing practices.Sensitive equipment will almost surely trip during this interruption. A muchmore common event would be a fault on one of the other feeders from thesubstation, i.e., a fault on a parallel feeder, or a fault somewhere on thetransmission system (see the fault locations shown in Fig. 2.1). In either ofthese cases, the customer will experience a voltage sag during the periodthat the fault is actually on the system. As soon as breakers open to clearthe fault, normal voltage will be restored at the customer.


2/7

Note that to clear the fault shown on the transmission system, both breakersA and B must operate. Transmission breakers will typically clear a fault in 5or 6 cycles. In this case there are two lines supplying the distributionsubstation and only one has a fault. Therefore, customers supplied from thesubstation should expect to see only a sag and not an interruption. The

distribution fault on feeder 4 may be cleared either by the lateral fuse or thebreaker, depending on the utilitys fuse saving practice.Figures 2.2 and 2.3 show an interesting utility fault event recorded for anElectric Power Research Institute research project. The top chart in each ofthe figures is the rms voltage variation with time, and the bottom chart is thefirst 175 ms of the actual waveform. Figure 3.3shows the characteristic measured at a customer location on an unfaultedpart of the feeder. Figure 2.3 shows the momentary interruption (actuallytwo separate interruptions) observed downline from the fault. Theinterrupting device in this case was a line recloser that was able to interruptthe fault very quickly in about 2.5 cycles. This device can have a variety of

settings. In this case, it was set for two fast operations and two delayedoperations. Figure 2.2 shows only the brief sag to 65 percent voltage for thefirst fast operation. There was an identical sag for the second operation.While this is very brief sag that is virtually unnoticeable by observing lightingblinks, many industrial processes would have shut down.Figure 2.3 clearly shows the voltage sag prior to fault clearing and thesubsequent two fast recloser operations. The reclose time (the time therecloser was open) was a little more than 2 s, a very common time for autility line recloser. Apparently, the faultperhaps, a tree branchwas notcleared completely by the first operation, forcing a second. The system wasrestored after the second operation.

Figure 2.2 Voltage sag due to a short-circuit fault on a parallel utility feeder Figure 2.3 Utility short-circuit fault event

with two fast trip operations of utilityline recloser

Estimating Voltage Sag Performance:


3/7

The following is a general procedure for working with industrial customers toassure compatibility between the supply system characteristics and thefacility operation:1. Determine the number and characteristics of voltage sags that result fromtransmission system faults.

2. Determine the number and characteristics of voltage sags that result fromdistribution system faults (for facilities that are supplied from distributionsystems).3. Determine the equipment sensitivity to voltage sags. This will determinethe actual performance of the production process based on voltage sagperformance calculated in steps 1 and 2.4. Evaluate the economics of different solutions that could improve theperformance, either on the supply system (fewer voltage sags) or within thecustomer facility (better immunity).

a) Area of vulnerability:

Figure 2.4 Illustration of an area of vulnerability.

minimum voltage sag ride-through capability defined as the minimumvoltage magnitude a piece of equipment can withstand or tolerate withoutmisoperation or failure. An area of vulnerability is determined by the totalcircuit miles of exposure to faults that can cause voltage magnitudes at anend-user facility to drop below the equipment minimum voltage sag ride-through capability. Figure 2.4 shows an example of an area of vulnerabilitydiagram for motor contactor and adjustable-speed-drive loads at an end-userfacility served from the distribution system. The loads will be subject to faultson both the transmission system and the distribution system. The actual

number of voltage sags that a facility can expect is determined by combiningthe area of vulnerability with the expected fault performance for this portionof the power system. The expected fault performance is usually determinedfrom historical data.

b) Equipment sensitivity to voltage sags:

Equipment sensitivity to voltage sags is very dependent on the specific loadtype, control settings, and applications. The most commonly usedcharacteristics are the duration and magnitude of the sag.


4/7

Generally, equipment sensitivity to voltage sags can be divided into threecategories:

i) Equipment sensitive to only the magnitude of a voltage sag: Thisgroup includes devices such as undervoltage relays, processcontrols, motor drive controls,6 and many types of automated

machines (e.g., semiconductor manufacturing equipment). Devicesin this group are sensitive to the minimum (or maximum) voltagemagnitude experienced during a sag (or swell). The duration of thedisturbance is usually of secondary importance for these devices.

ii) Equipment sensitive to both the magnitude and duration of avoltage Sag: This group includes virtually all equipment that useselectronic power supplies. Such equipment misoperates or failswhen the power supply output voltage drops below specified values.Thus, the important characteristic for this type of equipment is theduration that the rms voltage is below a specified threshold atwhich the equipment trips.

iii) Equipment sensitive to characteristics other than magnitude andDuration: Some devices are affected by other sag characteristicssuch as the phase unbalance during the sag event, the point-in-thewave at which the sag is initiated, or any transient oscillationsoccurring during the disturbance. These characteristics are moresubtle than magnitude and duration, and their impacts are muchmore difficult to generalize.

c) Transmission system sag performance evaluation: refer text bookd) Utility distribution system sag performance evaluation: refer text book

Fundamental Principles of Protection:

Figure 2.5 Approaches for voltage sag ride-through.

Several things can be done by the utility, end user, and equipmentmanufacturer to reduce the number and severity of voltage sags and toreduce the sensitivity of equipment to voltage sags. Figure 2.5 illustrates


5/7

voltage sag solution alternatives and their relative costs. As this chartindicates, it is generally less costly to tackle the problem at its lowest level,close to the load.Following are ideas that can be incorporated into any companys equipmentprocurement specifications to help alleviate problems associated with

voltage sags:1. Equipment manufacturers should have voltage sag ride-through capabilitycurves available to their customers so that an initial evaluation of theequipment can be performed.2. The company procuring new equipment should establish a procedure thatrates the importance of the equipment. If the equipment is critical in nature,the company must make sure that adequate ride-through capability isincluded when the equipment is purchased.3. Equipment should at least be able to ride through voltage sags with aminimum voltage of 70 percent (ITI curve).As we entertain solutions at higher levels of available power, the solutions

generally become more costly. If the required ride-through cannot beobtained at the specification stage, it may be possible to apply anuninterruptible power supply (UPS) system or some other type of powerconditioning to the machine control. This is applicable when the machinesthemselves can withstand the sag or interruption, but the controls wouldautomatically shut them down.At level 3 in Fig. 2.5, some sort of backup power supply with the capability tosupport the load for a brief period is required. Level 4 represents alterationsmade to the utility power system to significantly reduce the number of sagsand interruptions.

Motor-Starting Sags:Motors have the undesirable effect of drawing several times their full loadcurrent while starting. This large current will, by flowing through systemimpedances, cause a voltage sag which may dim lights, cause contactors todrop out, and disrupt sensitive equipment. The time required for the motorto accelerate to rated speed increases with the magnitude of the sag, and anexcessive sag may prevent the motor from starting successfully. Motorstarting sags can persist for many seconds, as illustrated in Fig. 2.6


6/7

Figure 2.6Typical motor-starting voltage sag.

a. Motor-starting methods:

Autotransformer starters have two autotransformers connected in opendelta. Taps provide a motor voltage of 80, 65, or 50 percent of systemvoltage during start-up. Line current and starting torque vary with the squareof the voltage applied to the motor, so the 50 percent tap will deliver only 25percent of the full-voltage starting current and torque. The lowest tap whichwill supply the required starting torque is selected.

Resistance and reactance starters initially insert an impedance in series withthe motor. After a time delay, this impedance is shorted out. Startingresistors may be shorted out over several steps; starting reactors areshorted out in a single step. Line current and starting torque vary directlywith the voltage applied to the motor, so for a given starting voltage, thesestarters draw more current from the line than with autotransformer starters,but provide higher starting torque. Reactors are typically provided with 50,45, and 37.5 percent taps.Part-winding starters are attractive for use with dual-rated motors (220/440V or 230/460 V). The stator of a dual-rated motor consists of two windingsconnected in parallel at the lower voltage rating, or in series at the higher

voltage rating. When operated with a part-winding starter at the lowervoltage rating, only one winding is energized initially, limiting startingcurrent and starting torque to 50 percent of the values seen when bothwindings are energized simultaneously.Delta-wye starters connect the stator in wye for starting and then, after atime delay, reconnect the windings in delta. The wye connection reduces thestarting voltage to 57 percent of the system line-line voltage; startingcurrent and starting torque are reduced to 33 percent of their


7/7

values for full-voltage start.

b. Estimating the sag severity during full-voltage starting:starting an induction motor results in a steep dip in voltage, followed by agradual recovery. If full-voltage starting is used, the sag voltage, in per unit

of nominal system voltage, is

Figure 2.7 illustrates the results of this computation for sag to 90 percent ofnominal voltage, using typical system impedances and motor characteristics.If the result is above the minimum allowable steady-state voltage for theaffected equipment, then the full-voltage starting is acceptable. If not, thenthe sag magnitude versus duration characteristic must be compared to the

voltage tolerance envelope of the affected equipment.

Figure 2.7Typical motor versus transformer size for full-voltage starting sags of 90

percent.

Documents

Ch-2 Voltage Sags and Interruptions