Voltage sags and Interruptions

7/30/2019 Voltage sags and Interruptions

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Voltage Sags and InterruptionsVoltage Sags and InterruptionsVoltage Sags and InterruptionsVoltage Sags and Interruptions

1 M.SURESH AP/EEE/KEC/PERUNDURAI


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Voltage sags and interruptions are related power qualityproblems. Both are usually the result offaults in the power

system and switching actions to isolate the faulted sections.

They are characterized by rms voltage variations outside thenormal operating range of voltages.

Sag :If the RMS Voltage is below the nominal voltage

by 10% to 90% for 0.5 Cycles to 1 Min.



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Sag and InterruptionsSag and InterruptionsSag and InterruptionsSag and Interruptions

A voltage sag is a short-duration (typically 0.5 to 30 cycles)reduction in RMS voltage caused by faults on the power

system and the starting oflarge loads,such as motors.

Momentary interruptions (typically not 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 ontheir systems. Sustained interruptions of longer than 1 minare generally due to permanent faults.



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Fault locations on the utility powerFault locations on the utility powerFault locations on the utility powerFault locations on the utility power

systemsystemsystemsystem

5 or 6 cycles for the breaker to operate.

Remains open for12 cycles up to 5 s



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Example of fault locations that caused misoperation of sensitive production equipmentat an industrial facility (the example system had multiple overhead distribution

feeders and an extensive overhead transmission system supplying the substation).



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Voltage sag due to a shortVoltage sag due to a shortVoltage sag due to a shortVoltage sag due to a short----circuit faultcircuit faultcircuit faultcircuit fault

on a parallel utility feederon a parallel utility feederon a parallel utility feederon a parallel utility feeder

Characteristic measured at a customer location on an

unfaulted part of the feeder6 M.SURESH AP/EEE/KEC/PERUNDURAI


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Utility shortUtility shortUtility shortUtility short----circuit fault event with two fast tripcircuit fault event with two fast tripcircuit fault event with two fast tripcircuit fault event with two fast trip

operations of utility line recloser.operations of utility line recloser.operations of utility line recloser.operations of utility line recloser.

Momentary interruption (actually two separateinterruptions) observed downline from the fault



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Estimating Voltage Sag PerformanceEstimating Voltage Sag PerformanceEstimating Voltage Sag PerformanceEstimating Voltage Sag Performance 1. Determine the number and characteristics of voltage sags that

result from transmission system faults.

2. Determine the number and characteristics of voltage sags thatresult from distribution system faults (for facilities that are suppliedfrom distribution systems).

3. Determine the equipment sensitivity to voltage sags. This will

determine the actual performance of the production processbased on voltage sag performance 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

the customer facility (better immunity).



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Area of vulnerabilityArea of vulnerabilityArea of vulnerabilityArea of vulnerability

The likelihood of sensitive equipment being subjected to voltage

lower than its minimum voltage sag ride-through capability.

Voltage sag ride-through capability- Minimum voltagemagnitude a piece of equipment can withstand or tolerate without

misoperation or failure. (Equipment voltage sag immunity or

susceptibility limit.) Area of vulnerability is determined by the total circuit miles of

exposure to faults that can cause voltage magnitudes at an end-user

facility to drop below the equipment minimum voltage sag ride-through capability.



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Illustration of an Area of VulnerabilityIllustration of an Area of VulnerabilityIllustration of an Area of VulnerabilityIllustration of an Area of Vulnerability



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EXISTING VOLTAGE DIP IMMUNITY TESTINGEXISTING VOLTAGE DIP IMMUNITY TESTINGEXISTING VOLTAGE DIP IMMUNITY TESTINGEXISTING VOLTAGE DIP IMMUNITY TESTING

STANDARDSSTANDARDSSTANDARDSSTANDARDS Existing standards for testing equipment voltage dip immunity focus primarily on verifying a minimum

immunity requirement for equipment response to voltage dips. IEC Standard 61000-4-11 [3] is for

equipment below 16 amps. Usually, these will be single phase devices where the testing is fairly

straightforward because multiple phase dips are not an issue and phase shift is usually of secondary

importance. The most important consideration in this case is the actual immunity characteristic that isrequired. The immunity test levels from 61000-4-11 and 61000-4-34 are compared with the immunity

requirements from SEMI F-47 in Figure 1.

Figure 1. Immunity testing levels from IEC 61000-4-11 and 61000-4-34 compared with immunity requirements in SEMI F47.11 M.SURESH AP/EEE/KEC/PERUNDURAI


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Testing three phase equipment is more complicated. Both IEC 61000-4-11 [3] and

IEC 61000-4-34 [4] cover single-phase and three-phase equipment in a similar way, butthe former only applies to equipment up to 16A per phase, whereas the latter covers

larger equipment.

Figure 2a. Vectors recommended by IEC [3] [4] for phase-to-neutral testing of three-phase equipment.

Figure 2b. Vectors recommended by IEC [3][4] for phase-to-phase testing of three-phase equipment. Note that the vectors

in method A are preferred, the vectors in method B are

acceptable, and the vectors in method C are not acceptable.12 M.SURESH AP/EEE/KEC/PERUNDURAI


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An Adjustable Speed Drive (ASD) is selected as an example industrial loadfor testing because it is very common in many industrial facilities, is often part

ofprocesses sensitive to voltage dips, and is a three phase load that illustratesthe importance of the voltage dip characteristics on the device immunity.

During a voltage dip or momentary interruption, the diodes in an ASD rectifierbridge will not conduct if the peak line voltage drops below the dc busvoltage. While the ASD is still controlling the motor and its load, energy isdrawn from the dc-bus capacitors, which will cause the dc-bus voltage to

decrease. If the dc-bus voltage falls below the ASDs undervoltage trippoint before the line voltage returns, then the control circuit will respondaccording to the drives program, typically shutting down the drive

ASD Performance to SAGASD Performance to SAGASD Performance to SAGASD Performance to SAG



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Basic diagram of the power section of transistorizedadjustable-speed drive, including PWM ac and BLDC.



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ASDASDASDASD

A typical 5 hp (3.5 kW) ASD is used, tests were conducted to determine how the drive would

respond to voltage dips that were generated using different test methods. The tests were

conducted with the drive loaded to 81 percent of full load, with an input voltage of400V

rms phase-to-phase, 50 Hz. The voltage sag generator used is compatible with SEMI F47-

0200, SEMI F42-0200, and the proposed IEC 61000-4-34 standards.

Figure. ASD voltage dip immunity testing setup.



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Minimum dc bus values during the voltage dipMinimum dc bus values during the voltage dipMinimum dc bus values during the voltage dipMinimum dc bus values during the voltage diptests (refer to Table 1 for test conditions).tests (refer to Table 1 for test conditions).tests (refer to Table 1 for test conditions).tests (refer to Table 1 for test conditions).



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Results of ASD voltage dip immunity testing withResults of ASD voltage dip immunity testing withResults of ASD voltage dip immunity testing withResults of ASD voltage dip immunity testing with

different voltage dip characteristics.different voltage dip characteristics.different voltage dip characteristics.different voltage dip characteristics.



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Equipment sensitivity to voltage sagsEquipment sensitivity to voltage sagsEquipment sensitivity to voltage sagsEquipment sensitivity to voltage sags

The facility comprises of many different equipments which

possesses different sensitivity to voltage sag.

The Sensitivity of the equipment to voltage sag depends upon

The Type of Load

Control Settings

Applications



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Sag CharacteristicsSag CharacteristicsSag CharacteristicsSag Characteristics

Duration of Sag

Magnitude of Sag

Phase Shift

Unbalance Missing Voltage

Three Phase Voltage Unbalance during Sag

The point in which the Sag initiates and terminates



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Equipment sensitivity to voltage sags can be divided into three categories

Equipment sensitive to only the magnitude of a voltage sag

Undervoltage relays, process controls, motor drive controls, and

many types of automated machines (e.g., semiconductormanufacturing equipment). Devices in this group are sensitive to the

minimum (or maximum) voltage magnitude experienced during a

sag (or swell). The duration of the disturbance is usually of secondary

importance for these devices.

Equipment sensitive to both the magnitude and duration of a voltagesag.

All equipment that uses electronic power supplies. Such equipmentmisoperates or fails when the power supply output voltage drops below

specified values. Thus, the important characteristic for this type of

equipment is the duration that the rms voltage is below a specified

threshold at which the equipment trips.20 M.SURESH AP/EEE/KEC/PERUNDURAI


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Equipment sensitive to characteristics other than magnitudeand duration

Some devices are affected by other sag characteristics such as thephase unbalance during the sag event, the point-in-the wave at

which the sag is initiated, or any transient oscillations

occurring during the disturbance. These characteristics are moresubtle than magnitude and duration, and their impacts are much

more difficult to generalize.



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Typical equipment voltage sag ride-

through capability curves

Typical loads will likely trip off when the Voltage is below the CBEMA or ITI Curve

ASD The ride through

capability Of the device is very

sensitive to voltage sag. It trips

for the sag below 0.9 p.u that

last longer for 4 cycle

Motor Contactor - It trips for thesag below 0.5 p.u that last

longer for 1 cycle



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The fault within the shaded portion will cause the end user voltage to drop below 0.5. i.e., the motor

contactors having a minimum voltage sag ride through capability of 0.5 p.u would have tripped out

when a fault causing a voltage sag with duration of more than 1 cycle occurs with in the AOV.

However faults outside this area will not cause the voltage to drop below 0.5 p.u23 M.SURESH AP/EEE/KEC/PERUNDURAI


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Transmission system sagTransmission system sagTransmission system sagTransmission system sag

Customer Facility

Voltage Sag

Transmission System Distribution System

If the transmission System supplies sagto the customer, Voltage sag depends

upon only the transmission system faultperformance

If the Distribution System supplies sag tocustomer, the voltage sag depends upon

fault performance on both transmission &distribution system

Transmission line faults and subsequentopening of protective devices rarely causesany interruptions for any customer becauseof the interconnected nature of ModernPower System

Depends Upon



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Transmission system sagTransmission system sagTransmission system sagTransmission system sag Here the type of fault must also be considered in this analysis. The Single

Line to Ground fault will not produce the same sag as like ThreePhase SCG fault. In 3- Ph. the sag will be even more severe.

The characteristics of the equipment depends upon how the voltages arechanged in the transformer (Star / Delta) and how the equipments areconnected . i.e., Ph-Gnd ; Ph-Ph

Equipment connected line-to-line would experience a minimum

voltage of 33 percent.

Equipment connected line-to-neutral would experience a minimumvoltage of 58 percent.

The transformer connections and the equipment connections determine the actualvoltage that equipment will experience during a fault on the supply system



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Voltage sag types at end-use equipment that

result from different types of faults and

transformer connections

Math Bollen developed the concept of voltage sag types to describe the different voltage sagcharacteristics26 M.SURESH AP/EEE/KEC/PERUNDURAI


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Transformer Secondary Voltages with a SingleTransformer Secondary Voltages with a SingleTransformer Secondary Voltages with a SingleTransformer Secondary Voltages with a Single----LineLineLineLine----totototo----

Ground Fault on the PrimaryGround Fault on the PrimaryGround Fault on the PrimaryGround Fault on the Primary



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Utility distribution system sagUtility distribution system sagUtility distribution system sagUtility distribution system sag

performance evaluationperformance evaluationperformance evaluationperformance evaluation Customers supplied at distribution voltage levels are impacted by faults on both the

transmission system and the distribution system

The analysis at the distribution level must also include momentary interruptionscaused by the operation of protective devices to clear the faults. These interruptions will

most likely trip out sensitive equipment.

The critical information needed to compute voltage sag performance can be summarized

as follows: Number of feeders supplied from the substation.

Average feeder length.

Average feeder reactance.

Short-circuit equivalent reactance at the substation.

Feeder reactors, if any.

Average feeder fault performance which includes (3LG)

faults and (SLG) faults in faults per mile per month.28 M.SURESH AP/EEE/KEC/PERUNDURAI


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There are two possible locations for faults on the distribution

systems,

Faults on the same feeder

Faults on parallel feeders.

Utility distribution system sagUtility distribution system sagUtility distribution system sagUtility distribution system sag

performance evaluationperformance evaluationperformance evaluationperformance evaluation



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Faults on the same feederFaults on the same feederFaults on the same feederFaults on the same feeder

The voltage magnitude at the substation is impacted by the faultimpedance and location, the configuration of the power system, andthe system protection scheme

The voltage sag performance for a specific sensitive equipment having the

minimum ride-through voltage ofvs can be computedas follows:

where

N1 and N3 are the fault performance data for SLG and 3LG faults in faults per miles per month, and Ep1 andEp3 are the total circuit miles of exposure to SLG and 3LG faults on parallel feeders that result in voltage sagsbelow the minimum ride-through voltage vs at the end-user location.



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Voltage sag magnitude at an endVoltage sag magnitude at an endVoltage sag magnitude at an endVoltage sag magnitude at an end----user location asuser location asuser location asuser location as

a function of the fault location along a parallela function of the fault location along a parallela function of the fault location along a parallela function of the fault location along a parallelfeeder circuitfeeder circuitfeeder circuitfeeder circuit



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Faults on the same feederFaults on the same feederFaults on the same feederFaults on the same feeder

Here the expected voltage sag magnitude at the end-user

location is computed as a function of fault location on the

same feeder. Note that, however, the computation is

performed only for fault locations that will result in a sag but

will not result in a momentary interruption, which will becomputed separately.



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Protection of Equipments

Figure illustrates voltage sag solution alternatives and their relative costs.

As this chart indicates, it is generally less costly to tackle the problem at

its lowest level, close to the load.



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Avoid problems with Sag

Voltage Sag ride through capability curves should be given tocustomers by the manufacturers so that the initial evaluation of

the equipment can be performed

Before purchasing new equipments the company should rate theimportance of the equipment. If it is a critical equipment thecompany must make sure that adequate ride through capability is

included when the equipment is purchased. If the equipment is ofleast importance the voltage sag protection may not be justified

Equipment should at least be able to ride through voltage sags

with a minimum voltage of 70 % (ITI Curve).The relativeprobability of experiencing a voltage sag to 70% is much less than

experiencing a sag of 90%



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End User level SolutionsTo improve the reliability and performance of a process or facilityDifferent technologies are used to improve the overall sag performance

1) Protection for small loads (< 5KVA)

These are the single phase loads that need to be protected.

Protection for equipment control or small individual machine.

2) Protection for individual equipment or groups of equipment up to about 300 kVA

Power Conditioning technologies for protecting Critical equipment

3) Protection for large groups of loads or whole facilities at the low-voltage level. If large portion of the facility is critical or needs protection

4) Protection at the medium-voltage level or on the supply system.

If the whole facility needs protection or improved PQ solution this may be considered.



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Ferroresonant TransformerFerroresonant TransformerFerroresonant TransformerFerroresonant TransformerFerroresonance is a phenomenon associated with the behaviour of iron cores while operating near a point ofmagnetic saturation (where the core is so strongly magnetized that further increases in winding current results in

little or no increase in magnetic flux).

Ferroresonance is a phenomenon associated with the behaviour of iron cores while operating near a point ofmagnetic saturation (where the core is so strongly magnetized that further increases in winding current results in

little or no increase in magnetic flux).

Its iron core is stuffed full of magnetic lines of flux for a large

portion of the AC cycle so that variations in supply voltage(primary winding current) have little effect on the core's

magnetic flux density, which means the secondary winding

outputs a nearly constant voltage despite significant variations

in supply (primary winding) voltage.

Normally, core saturation in a transformer results in distortionof the sine wave shape, and the ferroresonant transformer isno exception. To combat this side effect, ferroresonant

transformers have an auxiliary secondary winding paralleledwith one or more capacitors, forming a resonant circuit tuned

to the power supply frequency.This tank circuit serves as a filter to reject harmonics createdby the core saturation, and provides the added benefit of storing

energy in the form of AC oscillations, which is available for

sustaining output winding voltage for brief periods of input

voltage loss36 M.SURESH AP/EEE/KEC/PERUNDURAI


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Ferroresonant TransformerFerroresonant TransformerFerroresonant TransformerFerroresonant Transformer

Also called constant-voltage

transformers (CVTs), can handle most

voltage sag conditions ( Power

Conditioning)

Attractive for constant, low-power loads

Basically 1:1 transformers which are excited

high on their saturation curves, thereby

providing an output voltage which is not

significantly affected by input voltage

variations Harmonic Filtering

Reserve of energy in its resonant tank circuit.

Ferroresonance is a phenomenon associated with the behaviour of iron cores while operating near apoint of magnetic saturation (where the core is so strongly magnetized that further increases in

winding current results in little or no increase in magnetic flux).

Ferroresonance is a phenomenon associated with the behaviour of iron cores while operating near apoint of magnetic saturation (where the core is so strongly magnetized that further increases in

winding current results in little or no increase in magnetic flux).


Ch i i


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CharacteristicsThe graphic on the left shows a simplified version

of a magnetization curve to demonstrate this

concept. In the saturation region of the curve (red),

a large change in input voltage results in a small

change in output voltage. Operation in the

saturation region has the disadvantage of very

poor electrical efficiency. Standard powertransformers are designed to operate in the normal

range (blue) where electrical efficiency is muchhigher.

The graphic on the left shows a simplified version

of a magnetization curve to demonstrate this

concept. In the saturation region of the curve (red),

a large change in input voltage results in a small

change in output voltage. Operation in the

saturation region has the disadvantage of very

poor electrical efficiency. Standard powertransformers are designed to operate in the normal

range (blue) where electrical efficiency is muchhigher.

The graphic at the Left shows a typical range of

ferro efficiency versus percent load. It can be seen

that at 50% load, the ferro voltage regulator

efficiency ranges from 75 to 85%. As a comparison,a typical power transformer would have an

efficiency of 90% or more at 50% load.

The graphic at the Left shows a typical range of

ferro efficiency versus percent load. It can be seen

that at 50% load, the ferro voltage regulator

efficiency ranges from 75 to 85%. As a comparison,a typical power transformer would have an

efficiency of 90% or more at 50% load.


Di d t


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Disadvantage

They waste a lot of energy (due to hysteresis losses in thesaturated core)

Generates significant heat in the process intolerant offrequency variations, which means they don't work

very well when powered by small engine-driven generators having

poor speed regulation.

Voltages produced in the resonant winding/capacitor circuit tend

to be very high, necessitating expensive capacitors and presenting

the service technician with very dangerous working voltages


Voltage sag improvement withVoltage sag improvement withVoltage sag improvement withVoltage sag improvement with


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Voltage sag improvement withVoltage sag improvement withVoltage sag improvement withVoltage sag improvement with

ferroresonant transformerferroresonant transformerferroresonant transformerferroresonant transformer

Voltage sag ride-through improvement of a process controller

fed from a 120-VAferroresonant transformer

The Process controller can ride

through a voltage sag down to

30 percent of nominal, as opposed to 82 percent without

one.



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Voltage sag versus ferroresonantVoltage sag versus ferroresonantVoltage sag versus ferroresonantVoltage sag versus ferroresonant

transformer loading.transformer loading.transformer loading.transformer loading.

At 25 percent of loading, the allowable voltage

sag is 30 percent of nominal, which means thatthe CVT will output over 90 percent normal

voltage as long as the input voltage is above 30

percent.

At 25 percent of loading, the allowable voltage

sag is 30 percent of nominal, which means thatthe CVT will output over 90 percent normal

voltage as long as the input voltage is above 30

percent.

As the loading is increased, the corresponding ride-

through capability is reduced, and when theferroresonant transformer is overloaded (e.g., 150

percent loading), the voltage will collapse to zero.

As the loading is increased, the corresponding ride-

through capability is reduced, and when theferroresonant transformer is overloaded (e.g., 150

percent loading), the voltage will collapse to zero.

41M.SURESH AP/EEE/KEC/PERUNDURAI

M i h iM i h iM i h iM i h i


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Magnetic synthesizersMagnetic synthesizersMagnetic synthesizersMagnetic synthesizers Similar to CVT Provide improved voltage sag support and regulation for three-phase loads except

they are three phase devices

Size : 15 to 200 KVA

Used for process loads of larger computer systems where voltage sags or steady-state voltage variations are important issues

Magnetic synthesizers were used to protect against oscillatory transients and sagsand swells

When equipped with a lightning arrestor accessory, they protect against voltagesurges. They don't protect a critical load against a complete power outage, but via alimited stored energy capability using capacitors, they can provide ride-through,typically to one cycle. Battery backup capabilities permit additional operating time.

These devices use ferromagnetics (pulsed transformers and inductors) andcapacitors to synthesize a high-quality 3-phase output, using only the input as asource of energy. The output is fully isolated from imperfections in the inputpower.


Characteristics


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Characteristics

A magnetic synthesizer typically has the following operating characteristics: It can operate with an input voltage range as low as 40% or more of the nominal voltage.

Output power factorremains in the range of0.96or higher from half to full load.

Because it regenerates an output voltage waveform, output distortion, which is typically less than

4%, is independent of any input voltage distortion, including notching Efficiency at full loadis typically in the range of89% to 93%.

Minimum maintenance is required beyond annual replacement of failed capacitors. Redundantcapacitors built into the units allow several capacitors to fail between inspections without any

noticeable effect to the device's performance.

Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2-Hzchange in generator frequency, which is very large, results in an output voltage change of only 4%,

which has little effect for most loads.

It accepts 100% single-phase switch-mode power supply loading without any requirement for derating,

including all neutral components. Input current distortion remains less than 8% THD even when supplying nonlinear loads with more

than 100% currentTHD.


M g ti th iM g ti th iM g ti th iM g ti th i


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Magnetic synthesizersMagnetic synthesizersMagnetic synthesizersMagnetic synthesizers


Active series compensatorsActive series compensatorsActive series compensatorsActive series compensators


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Active series compensatorsActive series compensatorsActive series compensatorsActive series compensators

Boost the voltage by injecting a voltage in series with the remaining voltage during avoltage sag condition

Size : (1 to 5 KVA) for small single phase devices

( 2MVA and above) for very large devices

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 avoltage signal to the input voltage so that the output voltage remains within a specifiedtolerance during the disturbance.

The switch is very fast so that the disturbance seen by the load is less than a quartercycle 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 foralmost all voltage sag conditions.



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Uninterruptible Power SupplyUninterruptible Power SupplyUninterruptible Power SupplyUninterruptible Power Supply

An uninterruptible power supply (UPS), alsoknown as a battery back-up, provides emergency

power and, depending on the topology, lineregulation as well to connected equipment by

supplying power from a separate source when utility

power is not available. It differs from an auxiliary or

emergency power system or standby generator, which

does not provide instant protection from amomentary power interruption. A UPS, however, can

be used to provide uninterrupted power to equipment,

typically for 515 minutes until an auxiliary power

supply can be turned on or utility power is restored.

46

M.SURESH AP/EEE/KEC/PERUNDURAI

O li UPSO li UPSO li UPSO li UPS


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Online UPSOnline UPSOnline UPSOnline UPS

Fig: During Normal Power Supply

Fig: During Power interruption

Here Load is always

Fed through UPS

47


Offline / standbyOffline / standbyOffline / standbyOffline / standby


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Offline / standbyOffline / standbyOffline / standbyOffline / standby

The Offline / Standby UPS (SPS) offers only the most basic features, providing surge protection and batterybackup. Usually the Standby UPS offers no battery capacity monitoring or self-test capability, making it theleast reliable type of UPS since it could fail at any moment without warning. These are also the leastexpensive.When the incoming utility voltage falls below a predetermined level the UPS turns on its internal DC-AC

inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the

connected equipment on to its DC-AC inverter output. The switch over time is stated by most manufacturers

as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount oftime it takes the Standby UPS to detect the lost utility voltage.

The Offline / Standby UPS (SPS) offers only the most basic features, providing surge protection and batterybackup. Usually the Standby UPS offers no battery capacity monitoring or self-test capability, making it theleast reliable type of UPS since it could fail at any moment without warning. These are also the leastexpensive.When the incoming utility voltage falls below a predetermined level the UPS turns on its internal DC-AC

inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the

connected equipment on to its DC-AC inverter output. The switch over time is stated by most manufacturers

as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount oftime it takes the Standby UPS to detect the lost utility voltage.

48


LineLineLineLine interactiveinteractiveinteractiveinteractive


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LineLineLineLine----interactiveinteractiveinteractiveinteractive

The Line-Interactive UPS is similar in operation to a Standby UPS, but with the addition of a multi-tapvariable-voltage autotransformer. This is a special type of electrical transformer that can add or subtract

powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of thetransformer.

This type of UPS is able to tolerate continuous under voltage brownouts and overvoltage surges withoutconsuming the limited reserve battery power. It instead compensates by auto-selecting different power tapson the autotransformer. Changing the autotransformer tap can cause a very brief output powerdisruption, so the UPS may chirp for a moment, as it briefly switches to battery before changing the selectedpower tap. Autotransformers can be engineered to cover a wide range of varying input voltages, but this also

increases the number of taps and the size, weight, complexity, and expense of the UPS.

.

The Line-Interactive UPS is similar in operation to a Standby UPS, but with the addition of a multi-tapvariable-voltage autotransformer. This is a special type of electrical transformer that can add or subtract

powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of thetransformer.

This type of UPS is able to tolerate continuous under voltage brownouts and overvoltage surges withoutconsuming the limited reserve battery power. It instead compensates by auto-selecting different power tapson the autotransformer. Changing the autotransformer tap can cause a very brief output powerdisruption, so the UPS may chirp for a moment, as it briefly switches to battery before changing the selectedpower tap. Autotransformers can be engineered to cover a wide range of varying input voltages, but this also

increases the number of taps and the size, weight, complexity, and expense of the UPS.

.

Line-Interactive UPS. Typical protection time: 5 - 30 minutes. Capacity expansion: Several hours

49


Hybrid UPSHybrid UPSHybrid UPSHybrid UPS


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Hybrid UPSHybrid UPSHybrid UPSHybrid UPS

It uses Voltage regulator in the standby

mode. It provides voltage regulation to the

load and momentary ride through when the

transfer from normal to UPS supply is made

50


MotorMotorMotorMotor----generator setsgenerator setsgenerator setsgenerator sets


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MotorMotorMotorMotor generator setsgenerator setsgenerator setsgenerator sets

A motor powered by the line drives a generator that powers 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 severalseconds.

51


Costs for the voltage sag andCosts for the voltage sag andCosts for the voltage sag andCosts for the voltage sag and


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Costs for the voltage sag andCosts for the voltage sag andCosts for the voltage sag andCosts for the voltage sag and

Interruption eventsInterruption eventsInterruption eventsInterruption eventsZero to several million Rupees per event

Cost varies for different industry and individualfacilities and also with market conditions

Higher costs are typically experienced if the endproduct is in short supply and there is limited

ability to make up for the lost production



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The cost of a power quality disturbance can be capturedprimarily through three major categories:

Product-related losses, such as loss of product and materials, lost

production capacity, disposal charges, and increased inventory

requirements

Labour-related losses, such as idled employees, overtime,cleanup, and repair.

Ancillary costs such as damaged equipment, lost opportunitycost, and penalties due to shipping delays.

Appendix A ofIEEE 1346-199818 for a more

detailed explanation of the factors to be considered

in determining the cost of power quality

disturbances.53 M.SURESH AP/EEE/KEC/PERUNDURAI

W i hi F tW i hi F tW i hi F tW i hi F t C t C l l tiC t C l l tiC t C l l tiC t C l l ti


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Weighing FactorWeighing FactorWeighing FactorWeighing Factor Cost CalculationsCost CalculationsCost CalculationsCost Calculations

Costs will typically vary with the severity (both magnitude andduration) of the power quality disturbance. This relationship can often bedefined by a matrix ofweighing factors.

The weighing factors are developed using the cost of a momentaryinterruption as the base

A momentary interruption will cause a disruption to any load orprocess that is not specifically protected with some type of energy storagetechnology

If a voltage sag to 40 percent causes 80 percent of the economic impact that a momentary interruptioncauses, then the weighting factor for a 40 percent sag would be 0.8.Similarly, if a sag to 75 percent onlyresults in 10 percent of the costs that an interruption causes,then the weighting factor is 0.1.

After the weighting factors are applied to an event, the costs of the event are expressed in per unit ofthe cost of a momentary interruption. The weighted events can then be summed and the total is thetotal cost of all the events expressed in the number of equivalent momentary interruptions.



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The cost is 16.9 times the cost of an interruption. If an interruption costs $40,000, the total costs

associated with voltage sags and interruptions would be $676,000 per year



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Costs for Different Types of Power QualityCosts for Different Types of Power QualityCosts for Different Types of Power QualityCosts for Different Types of Power Quality

Improvement TechnologiesImprovement TechnologiesImprovement TechnologiesImprovement Technologies


Comparing solution alternatives with the baseComparing solution alternatives with the baseComparing solution alternatives with the baseComparing solution alternatives with the base


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Comparing solution alternatives with the baseComparing solution alternatives with the baseComparing solution alternatives with the baseComparing solution alternatives with the base

case using total annualized costs.case using total annualized costs.case using total annualized costs.case using total annualized costs.


THE POWER PATHTHE POWER PATHTHE POWER PATHTHE POWER PATH


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THE POWER PATHTHE POWER PATHTHE POWER PATHTHE POWER PATH

How Electricity Reaches Your HomeHow Electricity Reaches Your HomeHow Electricity Reaches Your HomeHow Electricity Reaches Your Home


InterruptionsInterruptionsInterruptionsInterruptions


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pppp

An interruption is defined as the complete loss of supplyvoltage or load current. Depending on itsduration, an interruption is categorized as instantaneous, momentary, temporary, or

sustained. Duration range for interruption types are as follows:

Instantaneous - 0.5 to 30 cycles

Momentary - 30 cycles to 2 seconds

Temporary - 2 seconds to 2 minutes

Sustained greater than 2 minutes

The causes of interruptions can vary, but are usually the result of some type of electrical supply grid

damage, such as lightning strikes, animals, trees, vehicle accidents, destructive weather (highwinds, heavy snow or ice on lines, etc.), equipment failure, or a basic circuit breaker tripping.This interruptions can be observed in the above fig.

An interruption, whether it is instantaneous, momentary, temporary, or sustained, can causedisruption,damage,and downtime, from the home user up to the industrial user. A home, or smallbusiness computer user, could lose valuable data when information is corrupted from loss of powerto their equipment.

Fig - Momentary interruption



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Momentary InterruptionsMomentary InterruptionsMomentary InterruptionsMomentary Interruptions



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OutageOutageOutageOutage


Short interruptions (< 1min)Short interruptions (< 1min)Short interruptions (< 1min)Short interruptions (< 1min)


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Short interruptions (< 1min)Short interruptions (< 1min)Short interruptions (< 1min)Short interruptions (< 1min)

Values depend on standard.

Short interruptions:

IEEE = - Un < 90% duration from 20 ms to 1 minute

EN= - Un < 99% duration from 20 ms to 3 minutes

Long interruptions (> 1min)

IEEE = - Un < 90% duration > 1 minute

EN= - Un < 99% duration > 3 minutes


Common causes of powerCommon causes of powerCommon causes of powerCommon causes of power


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interruptionsinterruptionsinterruptionsinterruptions Car accidents

Storms, high winds and lightning strikes

Trees

Debris and vegetation blown into powerlines

Wildlife (birds, possums etc)

Wilful damage and vandalism

Bush fires

Pole top fires

Overloads

Faulty wiring or meter box

Planned Interruptions63 M.SURESH AP/EEE/KEC/PERUNDURAI

Planned InterruptionPlanned InterruptionPlanned InterruptionPlanned Interruption


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Switch off and unplug sensitive appliances (such as computers, TVs, VCRs, DVDs,fax machines, clock radios, microwave ovens and sound systems) to reduce the risk of

damage from power surges or voltage sags that can occur when power is restored.

Be sure to disconnect electric heaters from the power supply so they wont present a

hazard when power is restored.

Turn offall but one light so that you will know when power is restored.

Keep fridge doors closed. Food in fridges and freezers will keep for at least 12 hours ifseals are in good condition and doors are kept closed.

If you have a generator, only use it to provide power directly to appliances. Never

connect it to the house switchboardor wiring as your equipment could be damagedwhen power is restored.


Planned Interruptions


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Only run generators outdoors where the fumes can't causeillness.

When power is restored please wait a few minutes before

switching on appliances to avoid overloading the localdistribution system.

Have a 'standard' telephone available that does not require

electricity to operate, or a mobile phone, so you can contactemergency services.

If you have automatic garage doors, know how to operate

the doors manually. Check your meter box to ensure that safety devices,

including residual current devices, have not operated.65 M.SURESH AP/EEE/KEC/PERUNDURAI


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Table 1.


Voltage sags and Interruptions

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