Unit II: VOLTAGE SAGS AND INTERRUPTIONSSources of sags and interruptions – Estimating voltage sag performance – Motor starting sags – Estimating the sag severity – Mitigation of voltage sags – Active series compensators – Static transfer switches and fast transfer switches

Part A1. List the sources of sag and interruptions.

Rural location remote from power source Long distance from a distribution transformer with interposed loads Unreliable grid system Power distributors tolerances not suitable for voltage sensitive equipment Switching of heavy loads Unbalanced load on a three phase system Equipment not suitable for local supply

2. Mention the methods to improve voltage sags in utility system.a. Reduce faults: Tree trimming, tree wire, animal guards, arresters, circuit patrolsb. Trip faster : Smaller fuses, instantaneous trip, faster transmission relaysc. Support voltage during faults: Raising the nominal voltage, current limiting fuses,

larger station transformers, line reactors3. Define the depth of the voltage dip.

The depth of the voltage dip is defined as the difference between the reference voltage and the residual voltage during voltage dip often expressed as a value in volts or as a percentage or per unit value of the reference voltage.4. Define the duration of the voltage dip.

The duration of a voltage dip is the time between the instant at which the voltage at a particular point on a supply system falls below the voltage dip start threshold and the instant at which it rises to the voltage dip end threshold.5. Explain the area of vulnerability.

The concept of an area of vulnerability has been developed to help evaluate the likelihood of sensitive equipment being subjected to voltage lower than its minimum voltage sag ride-through capability(the minimum voltage magnitude a piece of equipment can withstand or tolerate without misoperation or failure).6. What are the factors affecting equipment sensitivity to the voltage sag?

The Factors affecting equipment sensitivity are specific load type, control settings, and applications.

7. What are the three categories of equipment sensitivity? Equipment sensitive to only the magnitude of voltage sag. Equipment sensitive to both the magnitude and duration of voltage sag. Equipment sensitive to characteristics other than magnitude and duration.

8. What is the use of estimation of voltage sag?The estimation of voltage sag performance is used to determine the need for power conditioning components at sensitive loads in the plant.

9. List the devices used to reduce the voltage sag.

a. Ferroresonance Transformerb. Magnetic Synthesizersc. On-line UPSd. Stand by UPSe. Hybrid UPSf. Motor-Generator Setg. Super Conducting Magnetic Energy Storage Device.h. End user equipment with suitable specifications.

10. Mention the types of compensations.a. Surge impedance compensationb. Line length compensationc. Compensation by sectioning

i. Active compensationii. Passive Compensation

11. What is STS (Static Transfer Switch)?STS is an electrical device that allows instantaneous transfer of power sources to the load. If one power source fails, the STS switch to back up power source so quickly that the load never recognizes the transfer made.

12. What are the classifications of STS?a. Low Voltage STS ( Upto 600V, 200A-4000A)b. Medium Voltage STS ( 4.16kV – 34.5kV)

13. Define about Fast Transfer Switch (FTS).Fast transfer switch is used to obtain the minimal time of switch between two sources of power. This can be achieved by analysing the phase shift between sine waves of two power sources. The components of FTS are,

a. Zero detector unitsb. Phase lock loop systems (PLL).

PART B1. Discuss about the sources of sags and interruption.(16) Voltage sags and interruptions are generally caused by faults (short circuits) on the utility

system.

Fig.2.1: Fault locations on the utility power system.

Fig.2.2: Voltage sag due to a short-circuit fault on a parallel utility feeder.

Fig.2.3: Utility short-circuit fault event with two fast trip operations of utility line recloser Consider a customer that is supplied from the feeder supplied by circuit breaker 1 on the

diagram shown in Fig.2.1. If there is a fault on the same feeder, the customer will experience a voltage-sag during the fault followed by an interruption when the breaker opens to clear the fault. If the fault is temporary in nature, a reclosing operation on the breaker should be successful and the interruption will only be temporary. It will usually require about 5 or 6 cycles for the breaker to operate, during which time 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 much more common event would be a fault on one of the other feeders from the substation, i.e., a fault on a parallel feeder, or a fault somewhere on the transmission system (see the fault locations shown in Fig.2.1). In either of these cases, the customer will experience a voltage-sag during the period that the fault is actually on the system. As soon as breakers open to clear the fault, normal voltage will be restored at the customer.

Note that to clear the fault shown on the transmission system, both breakers A and B must operate. Transmission breakers will typically clear a fault in 5 or 6 cycles. In this case there are two lines supplying the distribution substation and only one has a fault. Therefore, customers supplied from the substation 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 the breaker, depending on the utility’s fuse saving practice.

Any of these fault locations can cause equipment to misoperate in customer facilities. The relative importance of faults on the transmission system and the distribution system will depend on the specific characteristics of the systems (underground versus overhead distribution, lightning flash densities, overhead exposure, etc.) and the sensitivity of the equipment to voltage sags.

Figure 2.2 shows the characteristic measured at a customer location on an unfaulted part of the feeder. Figure 2.3 shows the momentary interruption (actually two separate interruptions) observed downline from the fault. The interrupting device in this case was a line recloser that was able to interrupt the fault very quickly in about 2.5 cycles.

2. Explain how the voltage sag performance is estimated? (6)

It is important to understand the expected voltage sag performance of the supply system so that facilities can be designed and equipment specifications developed to assure the optimum operation of production facilities. The following is a general procedure for working with industrial customers to assure compatibility between the supply system characteristics and the facility operation:

i. Determine the number and characteristics of voltage sags that result from transmission system faults.

ii. Determine the number and characteristics of voltage sags that result from distribution system faults (for facilities that are supplied from distribution systems).

iii. Determine the equipment sensitivity to voltage sags. This will determine the actual performance of the production process based on voltage sag performance calculated in steps 1 and 2.

iv. Evaluate the economics of different solutions that could improve the performance, either on the supply system (fewer voltage sags) or within the customer facility (better immunity).

3. Describe the mitigation of voltage sag. (16)There are many solutions to prevent damage due to voltage dips.

Typically, these solutions can be categorized into three classes: Solutions in the manufacturing process itself; Solutions between the process and the public electric grid; Solutions in the grid.

a. Reduction of the Number of FaultsShort circuits cannot be entirely eliminated. The actions taken are:

replacing overhead lines with cables; the use of insulated conductors on overhead lines; regular tree cutting in the area of the transmission line; fencing against animals; shielding overhead conductors with additional shield wires; increased insulation levels; increased frequency of overhaul and periodic maintenance, cleaning insulators, etc.b. Reduction of the Fault Clearance Time

The duration of a voltage dip is largely determined by the speed at which short circuits are cleared. A necessary feature of short-circuit protection is the graduation of the operating times of switches, fuses, etc., in order to ensure that a short circuit is cleared at the most appropriate point in the supply system. This means that the clearance time and, consequently, the duration of voltage dips and short interruptions depend on the location where the short circuit has occurred. A reduction in fault-clearance time does not mean a decrease in the number of faults but only a mitigation of their effects. It also does not influence the number or the duration of supply interruptions, for the duration depends solely on the speed of voltage recovery. Fast fault clearing does not influence the number of voltage dips, but can significantly reduce their duration.

The basic method for reducing fault duration consists of the use of current-limiting fuses. These are capable of clearing a fault in a very short time. Decreasing the short-circuit current and shortening its duration significantly limit the duration of a voltage dip to rarely exceeding one cycle.c. Modification of the Supply System Configuration

These operations allow for a reduction in the severity of the phenomenon, but at a high cost, particularly in HV systems. The basic method of preventing voltage dips is to install elements of redundancy, as follows: Installing generators close to sensitive loads. They support the voltage

during distant dips. The voltage reduction equals the percentage share of the generator current in the short-circuit current.

Increasing the number of substations and busbars in order to limit the number of customers, who potentially may be affected by the disturbance.

Installing current-limiting reactors at strategic points of the system in order to increase electrical distance to the fault. It should, however, be remembered that this action may make a voltage dip deeper for other customers.

Supplying sensitive customer’s busbars from several substations. The effects of a voltage dip on one substation will be reduced by the influence of the others. The more independent these substations are, the more effective the measure is. The best reduction effect can be achieved by providing a power supply from two different supplying systems. The second supply increases the number of dips but reduces their duration and depth.

d. Voltage StabilizersA more sophisticated way to eliminate the negative effects of dips is called custom power

technology. This technology is mainly based on power electronics and also, on some occasions, electrical energy storage.

The most common method for mitigating the effects of the considered disturbances is the use of additional equipment, namely voltage stabilizers. They can be installed on both the supplier’s or the customer’s side but, as experience shows, the customer is the one who much more frequently does it, since the improvement in supply conditions and increasing the equipment’s immunity are beyond the customer’s control.These systems can be generally termed as systems of improved power parameters.

Two kinds of technical solution can be distinguished:i. Energy storage systems. The stored energy is utilized to supply a critical load during

the disturbance. These systems can be used in the case of voltage dips with arbitrary residual voltage, as well as during short supply interruptions. The immunity level of equipment depends on the amount of energy stored and on the energy requirements of the protected process. In many cases the reaction time of the compensation equipment should be considered critical. Since the energy storage process is, as a rule, very costly, it is applied only to particularly sensitive equipment. Examples of energy storage systems are: uninterruptible power supplies (UPSs), superconducting magnetic energy storage (SMES), rotating machines with flywheels, motor–generator systems, etc.

ii. Systems having no energy-storing capability. These can only be used to reduce the effects of voltage dips (typically up to a maximum of 50 %) but not of supply interruptions. They differ in depth of the voltage dip, which they are able to compensate. The duration of a dip is not a critical parameter in these systems. Their cost, as a rule, is smaller than that of the energy-storing systems.Example of such solutions are:

o Constant voltage transformer (CVT);o Static fast transfer switching (SFTS);o Static generators of the fundamental harmonic currents and voltages.

e. Improvement in Equipment ImmunityOne of the most advantageous solutions, in both technical and economical terms, is the use of equipment of a sufficient immunity level that is adequate for the intended operational environment. This is an effective method which eliminates unwanted disconnections due to voltage dips (short interruptions to a lesser extent). More and more frequently the immunity to a voltage dip of a specified depth and duration becomes the basis of a manufacturer’s offer, determining its commercial success.The level of compatibility of a sensitive load with the supply network is assessed prior to connection. The possible procedure includes three stages:

i. Acquiring information on system operation. That is, the prospective number of voltage dips. There are a number of ways to get such data: contacting the electric power supplier, monitoring the power supply over an extended period of time, analysis of faults, etc. Obtaining credible information requires the measurements to be performed for a long time. An alternative is the use of statistical methods of prediction.

ii. Acquiring information on equipment sensitivity. This information can be obtained from the manufacturer, by conducting tests or assuming typical sensitivity characteristics. In practice, it frequently happens that the user learns about the limited immunity of the equipment only after installing it.

iii. Determination of the potential effect. If the foregoing information is available, thereis the possibility to assess the potential threat of equipment failure (failure rate) and evaluate the economic effect of its occurrence (Section 4.6.1). On that basis a method of proceeding can be chosen: improvement of supply conditions, better (i.e. less sensitive) equipment and application of a stabilizer or acceptance of the existing situation.

4. Discuss the role of Active Series Compensators in power quality improvement.(8)Advances in power electronic technologies and new topologies for these devices have

resulted in new options for providing voltage sag ride-through support to critical loads. One of the important new options is a device that can boost the voltage by injecting a voltage in series with the remaining voltage during a voltage sag condition. These are referred to as active series compensation devices. They are available in size ranges from small single-phase devices (1 to 5 kVA) to very large devices that can be applied on the medium-voltage systems (2 MVA and larger).

Fig 2.4: Active Series CompensatorA one-line diagram illustrating the power electronics that are used to achieve the

compensation is shown in Fig. 2.4. When a disturbance to the input voltage is detected, a fast switch opens and the power is supplied through the series-connected electronics. This circuit adds or subtracts a voltage signal to the input voltage so that the output voltage remains within a specified tolerance during the disturbance. The switch is very fast so that the disturbance seen by the load is less than a quarter cycle in duration. This is fast enough to

avoid problems with almost all sensitive loads. The circuit can provide voltage boosting of about 50 percent, which is sufficient for almost all voltage sag conditions.5. Write notes on Static transfer switches and Fast transfer switches.

There are a number of alternatives for protection of an entire facility that may be sensitive to voltage sags. These include dynamic voltage restorers (DVRs) and UPS systems that use technology similar to the systems described previously but applied at the medium-voltage level.

Another alternative that can be applied at either the low-voltage level or the medium-voltage level is the automatic transfer switch. Automatic transfer switches can be of various technologies, ranging from conventional breakers to static switches. Conventional transfer switches will switch from the primary supply to a backup supply in seconds. Fast transfer switches that use vacuum breaker technology are available that can transfer in about 2 electrical cycles. This can be fast enough to protect many sensitive loads. Static switches use power electronic switches to accomplish the transfer within about a quarter of an electrical cycle. The transfer switch configuration is shown in Fig. 3.28. An example medium-voltage installation is shown in Fig. 3.29. The most important consideration in the effectiveness of a transfer switch for protection of sensitive loads is that it requires two independent supplies to the facility. For instance, if both supplies come from the same substation bus, then they will both be exposed to the same voltage sags when there is a fault condition somewhere in the supply system. If a significant percentage of the events affecting the facility are caused by faults on the transmission system, the fast transfer switch might have little benefit for protection of the equipment in the facility.

Fig.2.5: Configuration of a static transfer switch used to switch between a primary supply and a backup supply in the event of a disturbance. The controls would switch back to the primary supply after normal power is restored.

6. Write notes on ferroresonant transformer. (8)

Ferroresonant transformers, also called constant-voltage transformers (CVTs), can handle most voltage sag conditions.

Fig.2.6. Ferroresonant Transformer

CVTs are especially attractive for constant, low-power loads. Variable loads, especially with high inrush currents, present more of a problem for CVTs because of the tuned circuit on the output.

Ferroresonant transformers are basically 1:1 transformers which are providing an output voltage which is not significantly affected by input voltage variations

Figure 2.7 shows the voltage sag ride-through improvement of a process controller fed from a 120-VA ferroresonant transformer. With the CVT, the process controller can ride through a voltage sag down to 30 percent of nominal, as opposed to 82 percent without one.

Fig.2.7. Voltage Sag improvement using ferroresonant transformer

From the above figure, it is clear that the ride-through capability is held constant at a certain level. The reason for this is the small power requirement of the process controller, only 15 VA. Ferroresonant transformers should be sized significantly larger than the load.

Fig.2.8. Voltage sag versus ferroresonant transformer loading Figure 2.8 shows the allowable voltage sag as a percentage of nominal voltage (that will

result in at least 90 percent voltage on the CVT output) versus ferroresonant transformer loading, as specified by one manufacturer. At 25 percent of loading, the allowable voltage sag is 30 percent of nominal, which means that the CVT will output over 90 percent normal voltage as long as the input voltage is above 30 percent.

This is important since the plant voltage rarely falls below 30 percent of nominal during voltage sag conditions. As the loading is increased, the corresponding ride-through capability is reduced, and when the ferroresonant transformer is overloaded (e.g., 150 percent loading), the voltage will collapse to zero.

7. Write notes on Magnetic synthesizer. (8)

Magnetic synthesizers can handle three phase and provide improved voltage sag support and regulation for three-phase loads.

They use resonant circuits made of nonlinear inductors and capacitors to store energy, pulsating saturation transformers to modify the voltage waveform, and filters to filter out harmonic distortion.

They are applicable over a size range from about 15 to 200 kVA and are typically applied for process loads.

They supply power through a zigzag transformer. The zigzag name comes from the way the transformer changes the phase angle between voltage and current. The zigzag transformer traps triplen harmonic currents and prevents them from reaching the power source.

Applications of magnetic synthesizers include protection of large computer installations, computerized medical imaging equipment, and industrial processes, like plastic extruders, especially from voltage sags. They protect sensitive loads not only from voltage sags but also from transients, overvoltage, undervoltage, and voltage surges. However, they can be bulky and noisy. The block diagram in Figure 2.9 illustrates the main components of a magnetic synthesizer.

Fig.2.9. Block diagram of Magnetic Synthesizer

8. Explain about power quality improvement using motor generators sets. (8)

Motor-Generator sets are available in various sizes and configurations. This is one of the established technologies for preventing sensitive loads from sags and interruptions.

Fig.2.10. Typical M-G set with flywheel. Fig.2.10 shows the arrangement of M-G set in which the motor is powered by a driver

circuit from line. The motor drive a generator that energize the load. Flywheels on the same shaft provide greater inertia to increase ride-through time.

When the line suffers a disturbance, the inertia of the machines and the flywheels maintains the power supply for several seconds. This arrangement may also be used to separate sensitive loads from other classes of disturbances such as harmonic distortion and switching transients.

Disadvantages:o Losseso Noise and maintenanceo Frequency and voltage drops with the speed. This may not desirable for some

loads. Written-pole synchronous machine are also used. In this machine, the number of poles

getting varied according the speed of the machine to maintain the frequency as well the voltage constant.

Solid state inverters are also preferred for some cases. But the loss and cost associated with this arrangement is high.

9. Discuss about motor starting sags. (12)The motors are drawing more current during starting. This large current will, by flowing

through system impedances, causes a voltage sag.Effects:

Dim lights Contactor drop-outs Disturbance to sensitive equipments.

The time required for the motor to accelerate to rated speed increases with the magnitude of the sag, and excessive sag may prevent the motor from starting successfully. Motor starting sags can persist for many seconds, as illustrated in Fig. 2.11.

Fig.2.11. Typical motor starting voltage sagMotor Starting methods:

Starting Method Reduction on L-L Voltage Reduction in starting current

Autotransformer starting Taps provide a motor voltage of 80, 65, or 50 percent of system voltage during start-up.

Vary with the square of the voltage applied

Resistance and reactance starting Varies with the resistance/ reactance value. Reactors are typically provided with 50, 45, and 37.5 percent taps.

Varies with the resistance/ reactance value.

Part-winding starting Energies one part of winding 50%

Star-Delta starting 57% 33%

Estimation of the sag severity during full-voltage starting:

Voltage sag in pu,

where,V(pu) – Actual system voltage, in per unit of normalkVALR - Motor locked rotor kVA kVASC – System short circuit kVA at motor

Computation for sag to 90 percent of nominal voltage, using typical system impedances and motor characteristics.

If the sag is above the minimum allowable steady state value of the affected equipment, then the full voltage starting is acceptable.

Otherwise, voltage sag – duration characteristics to be compared with the voltage tolerance envelope of the affected equipment.

Such complicated analysis may be left to computer analysis.

Fig.2.12. Typical motor versus transformer size for full-voltage starting sags of 90%. Computer simulation requires the following data:

o Parameter values for the standard induction motor equivalent circuit: R1, X1, R2, X2

and XM.o Number of motor poles and rated rpm (or slip).o WK2 (inertia constant) values for the motor and the motor load.o Torque versus speed characteristic for the motor load.

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