L24 PQ Mitigation Asean School 2006.pdf

7/27/2019 L24 PQ Mitigation Asean School 2006.pdf

1/48

Asean Residential School in Electric Power Engineering (ARSEPE06),

13th

to 24th

November 2006, Universiti Tenaga Nasional

Power Quality Mitigations

MOHAMED FUAD FAISAL

SENIOR ENGINEER (POWER QUALITY)ENGINEERING DEPARTMENTDISTRIBUTION DIVISION TNB

November 2006

Page 1 of 48 MFF/2006


2/48


13th

to 24th


Preface

In line with the power quality survey conducted recently, voltage sags andharmonics remained to be the most concerned power quality problems

encountered by member utilities and their customers.

This course note starts with discussion on current dilemma besieging thepower quality mitigations efforts (Part 1), and followed by overview ofcommon power quality solutions (Part 2) for:

Voltage Sags, and Harmonics

Please enjoy the notes.

(Mohamed Fuad Faisal)Senior Engineer (Power Quality)Engineering DepartmentDistribution Division TNB

[email protected]

November 2006



3/48


13th

to 24th


Contents Page

Part 1 Power Quality Issues 4-16

Part 2 Power Quality Solutions

1.0 Harmonic Solutions 17-28

2.0 Voltage Sag Solutions 29-48



4/48


13th

to 24th


PART 1 POWER QUALITY ISSUES

1.0 Voltage Sag Issues

Problems encountered by utilities in handling voltage sag included thefollowings.

a) Lack of recognized standard for voltage sag immunity

There were a number of standards related to voltage sag. This standarddefined the immunity of equipment in terms of magnitude and durationof voltage sag encountered. ITIC, IEC and SEMI were threeorganizations who had issued guidelines governing voltage dip immunityon information technology and semiconductor manufacturing equipment

respectively.

To overcome this problem, utilities could work together with localregulatory bodies to work out a set of guideline or standard on voltagesags immunity with which susceptible equipment had to comply.

b) Equipment manufacturer

In todays competitive world, equipment manufacturer strived their best

to minimize the cost of production and hence voltage dip immunity wasusually not included if it was not explicitly requested by customers.

To overcome this problem, customer should be made aware of the factthat voltage sags were not uncommon in today power supply network andcould not be totally got rid of economically. For sensitive equipment thatmight be susceptible to voltage sags, the customer should request for

protection from voltage sags during the specification and procurementstage of these equipment.

Utilities could facilitate this solution through:

1) Improve the awareness of customer on voltage sag problem throughproactive communication programme;

2) Start dialogue with equipment manufacturers on improving voltagesag immunity of their product;



5/48


13th

to 24th


3) Provide incentive for the equipment manufacturers in improving theirproduct. This could be achieved by compiling equipment list, whichprovides a certain level of voltage sag immunity for reference bycustomer. A utility member was considering to award compliancelogo to equipment, which could pass the SEMI F42 or MS IEC61000-4-11 (Class 3) testing protocols.

c) Violation of equipment warranty

Sometimes even when technical solutions were available, the customermight not freely modify their equipment because this might violate thewarranty of the machine.

To overcome this problem, utilities together with the customer might

adopt the following approach:

1) Involved original equipment manufacture from the very beginning,especially during the simulation test of the proposed solution

d) Customer refusing services from utilities

Some utilities reported that some of their customers refused theinvestigation service offered. This might be due to the fact that

proprietary production process or equipment was in used by theconcerned customers, or irrational behavior of the concerned customer.

To overcome this problem, the following approach might be adopted byutility:1) Involved third party such as the regulatory, academics or professional

body as a moderator and made clear the position of the utility to thesethird parties. This could provide evidence on the due diligence paid bythe utility, in case the customer escalated the issue.

2) Utility could benchmark its performance with worldwide standard in

order to convince customers that their power quality was not inferioras compared with the norm in the world.



6/48


13th

to 24th


e) Liability issues related to testing and modification of equipment

Risks were associated with testing and modification of equipment.Therefore, the utility should pay special attention on this risk and took

precautionary measures.To overcome this problem, the following precautionary measure could betaken

1) Made clear to all parties involved on the responsibility of the utility,i.e., the utility was assisting the customer on testing instead of takingcharge of the test.

2) Involved original equipment manufacture to witness the test andapprove the subsequent modification made on the machine involved.

3) Included a disclaimer statement on the recommended solution

prepared for the customers.4) Discuss with insurance undertaker to ensure that provision were in

place to protect the utility from any liability during the testing.

f) Commitment from Top management of the customer

Provision of technical solution was not sufficient in some cases. Theutility might need to take on more steps in providing with the customerthe cost-benefit analysis for the customer to justify the investment

proposed. In some cases, the utility might need to adopt a top-downapproach, i.e., to sell the solution to the top management of the customerin order to convince them that voltage sag could not be totally eliminatedon utility side and solution on customer could solve the problem.

The following initiatives could be adopted:

1) Arrange power quality seminar targeting at management level ofcustomers. The cause, impact, and solution of power quality problemshould be presented at layman term so that managerial staff of

customers could really understand the issue.2) TNB had invested to build a Power Quality Lab, in which a number of

real demonstration were built so that customers could reallyexperience the impact and saw how mitigation device could be used toeliminate problems related to voltage sags.



7/48


13th

to 24th


g) Availability of solution alternatives on utility side

Voltage sag mitigation solutions on utility side were still economicallyunfavorable as compared with solution on customer side. Nevertheless,the following mitigation measures were adopted by some utilities, whichcould help to improve voltage sag performance on utility side:

1) Line Arrestor: Some utilities had installed line arrestor on selectedtower of the overhead transmission system which can minimize thevoltage dip magnitude even when the tower or overhead line werestruck by lightning. These devices were proved to be effective;

2) Vegetation Management Programme: Some utilities had adoptedvegetation management programme, i.e., directional pruningtechniques which were effective to minimize voltage sags caused by

vegetation.3) Penalty for 3rd party damage: Some utilities had worked together with

regulatory bodies to enact law which imposed heavy penalty tocontractor who damaged the electricity transmission or distributionequipment negligibly. This measure had been proved to be quiteeffective by monitoring the statistics of damage before and afterenactment of the law.

4) Condition monitoring programme: Some utilities had extensiveequipment condition monitoring programme in place (e.g., onlineDGA, cable partial discharge measurement) which could help todetect equipment failure proactively and thereby reducing the numberof voltage sags.



8/48


13th

to 24th


2.0 Harmonics Issues

It is generally recognized that the problem of harmonics was still tolerable atthe present. But the problem might become more and more serious incoming years with proliferated use of power electronic devices andequipment. Issues related to handling of harmonics were summarized asfollows.

a) Lack of regional regulatory standard

There were some regional guidelines or standards such as G5/4 in UK,IEEE 519 in US, and Guideline for Customers Connected to MV or HVnetwork in Japan. Since these guidelines or standards were not legally

binding, it might not be easy for utilities to request their customers to

take measures against harmonics by themselves. Furthermore accordingto The AESIEAP Power Quality Survey 2003 Harmonics, 70% ofcountries or regions replied that they did not have any guidelines orregulations concerning harmonic emission, so the need for regional orlocal guidelines and standard governing harmonic emission became more

prominent. The international standard might be taken as reference andcustomized to suit local environment if necessary. Nevertheless, theseguideline or standard has no regulatory binding power.

To overcome this problem, the utilities might adopt the followingapproach:

1) Utilities should closely monitor harmonics level (e.g. THD) andharmonics troubles in order to know the actual harmonics condition.Utilities should also open a dialogue with local regulatory body toestablish local or regional guideline and standard governing harmonicemission. Based on these data, guides or regulations, utilities shouldexplain to customers that measure against harmonics was importantand measure against mitigation closer to harmonics source was the

most effective solution.

b) Equipment manufacturer

There were some regulations and standards for equipment manufacturesand customers concerning harmonic emission in some countries.However, no measurement was made to check that whether the



9/48


13th

to 24th


equipment were really complied with these standards. Sample check wererecommended to ensure that these standards or guidelines wereimplemented effectively

c) Location of harmonic source

Harmonic voltage could not be used as a mean to check against harmonicemission because the utility could not tell which customer was the sourceof harmonic. Even if the utility could measure the THD of harmoniccurrent, it was also difficult to tell whether the customer was a source ora sink of harmonics. This also posed a problem in handling of harmonicsemission.



10/48


13th

to 24th


3.0 Unbalance Issues

Problems encountered by utilities on unbalance load included thefollowings.

a) Mal-operation of protection relay

Unbalance load might cause mal-operation of grounding overcurrentprotection relay (LCO) due to different line/load characteristics between3 phases. It could happen under normal operation due to loadcharacteristics change, load transfer between 2 feeders, ormaking/breaking parallel operation for 2 lines. This would result inunnecessary interruption to customers.

Phase load flow characteristics would be changed duringmaking/breaking parallel operation or load transfer, and might producesevere current unbalance. The parallel flow was determined by the loadangle difference between the points making/breaking parallel and theimpedance between the paths.

To avoid the problem, the following precaution could be adopted:1) Choose an appropriate time when the load or parallel flow is minimal

to carry out the operation.2) Rearrange the single-phase load in three-phase four-wire distribution

system to keep the three-phase load balance.3) Desensitize the protection relay. However, careful consideration must

be made in order not to jeopardize the protection grading of thesystem upstream.

4) Reconfigure the system to adjust the impedances of the parallel pathsor of the 3 phases.

5) Load transfer to adjust the power angle between the points of parallel.6) Switch in/out reactive devices (e.g., reactor and capacitor bank) to

control the load angle.

7) Adopt load break switch (LBS) instead of disconnecting switch (DS)to minimize the time difference when the phase conductors are made.

b) Distribution/Feeder Automation

To reduce the chance of causing mal-operation of grounding overcurrentprotection relay (LCO) due to different line/load characteristics between



11/48


13th

to 24th


3 phases under normal operation, load transfer between 2 feeders, ormaking/breaking parallel operation for 2 lines, distribution or feederautomation system should be implement.



12/48


13th

to 24th


4.0 Grounding Issues

There are several reasons for the need of grounding. They are:

a) Personnel safety

Personnel safety was the primary reason that all equipment must have asafety equipment ground. This was designed to prevent the possibility ofhigh touch voltages when there was a fault in a piece of equipment. Thetouch voltage was the voltage between any two conducting surfaces thatcould be simultaneously touched by an individual. The earth might beone of these surfaces.

b) Equipment safety

A ground fault return path to the point where the power source neutralconductor was grounded was an essential safety feature to assureoperation of protective device. An insulation failure or other fault thatallowed a phase wire to make contact with an enclosure would find alow-impedance path back to the power source neutral. The resultingover-current would cause the circuit breaker or fuse to disconnect thefaulted circuit promptly.

c) Noise control

Noise control included transients from all sources. This was wheregrounding related to power quality. The primary objective of groundingfor noise control was to create an equip-potential ground system.Potential difference between different ground locations could stressinsulation, create circulating ground current in low-voltage cables, andinterference with sensitive equipment that might be grounded in multiplelocations.

However, the grounding system should be designed to accomplish theseminimum objectives:

1) There should never be load currents flowing in the grounding systemunder normal operating conditions. One could likely measure verysmall currents in the grounding system due to inductive coupling,capacitive coupling, and the connection of surge suppressors and the



13/48


13th

to 24th


like. In fact, if the ground current was exactly zero, there wasprobably an open ground connection. However, these currents shouldbe only a tiny fraction of the load current.

2) There should be, as near as possible, an equip-potential reference forall devices and locations in the system.

3) To avoid excessive touch potential safety risk, the housings of allequipment and enclosures should be connected to the equip-potentialgrounding system.



14/48


13th

to 24th


5.0 Voltage Regulation Issues

Problems encountered by utilities on voltage regulation included thefollowings.

a) Overloading of cables or transformers

Cables and transformers were generally capable of taking someoverloading current for a short-period of time without causing anynoticeable damage to them. However, some undervoltage situationsmight arise. These overloading might be caused by:

1) Load growth faster than expected due to the high electricity demand.2) Unbalancing of single-phase loads on three-phase transformers.

b) Sources of supply and loads were too far apart

The distance between sources and customer locations were outside thelimit of the designed voltage regulation.

c) Poor reactive power compensation

Highly inductive load were frequently the major causes of undervoltageproblems. This would also cause the overloading condition above.

d) Voltage regulating equipments were not properly adjusted or applied

Those voltage regulating equipments such as an off-load or on-load tapchanger on the transformer were not properly adjusted, controlled orapplied on the right situation or locations.

To avoid the mentioned undervoltage problems, the following measuresshould be taken.

1) Standardize within the utility an engineering standard as well as aconstruction standard for a proper voltage regulation on every voltagelevel.

2) Continuously monitor of the loading condition and take measuringactions. (Balance of load, construct a new circuit, change a new cablesizing, etc.)



15/48


13th

to 24th


3) Properly adjustment of all the voltage regulating equipment(Transformer tapping either on-load or off-load, AVR). A distributionline voltage regulator might also be applied in a proper location.

4) Apply some reactive power compensation equipment, mostly a shuntcapacitor either or both of fixed-type and automatic switched-type. Insome very special application on transmission system, a seriescapacitor or a SVC might be applied for the add-on stability situation.Also a D-Statcom could be applied on a very special load for a fastvarying voltage problem on the distribution circuit.



16/48


13th

to 24th


6.0 Flicker Issues

It was generally recognized that the problem of flicker was still tolerable atthe present. But the problem might become more serious in coming years.Under these circumstances, pending issues in handling flicker aresummarized as follows.

a) Regulatory standard

There were two indices that evaluate the level of flicker. One was Pst

(IEC) and another was V10. The former was applied in many countriesand the latter was applied only in Japan, Taiwan and South Korea.

Permissible levels of them were 1.0 for Pst and 45% for V10respectively. Since these levels were not binding, it might not be easy for

utilities to ask their customers to take measures against flicker bythemselves. To facilitate solution to this problem, the followingapproaches could be considered.

1) Roles of utilities: Utilities should monitor flicker level and surveyflicker troubles in order to know the actual flicker condition whenthere were possible flicker sources (e.g. arc furnace). And utilitiesshould also investigate the prospective effect of flicker when theyknew that customers newly introduced flicker sources or expanded

their facilities that included flicker sources. Based on these data andrelevant indices, utilities should explain to customers that measureagainst flicker was quite important and measure against flicker atflicker source was the most effective way.

2) Roles of customers: If there were some indices for customersconcerning flicker level depending on countries and regions, theyshould comply with these indices.



17/48


13th

to 24th


PART 2 POWER QUALITY SOLUTIONS

1.0 Solutions for Harmonics

1.1 Measures to mitigate harmonic problems

1.1.1 Current distortion

Whenever loads draw current in a non-linear manner, such as thatexperienced with rectifier based equipment, harmonic distortion iscommonly experienced. The amount of current distortion will depend on thesize of this non-linear load in relation to the capacity of the electrical powersource. Input circuit impedance helps to reduce the input current distortion.For this reason, many drive users add impedance to the drive in the form of a

line reactor. Typical levels of total harmonic current distortion (THID) forsix pulse rectifiers are listed in Table 6. The % THID is representative ofdrives, which do not have any means of filtering included in the inputcircuit.

Table 1 THID for common drives without any filter

kW % THD

15 kW > 100 %18-30 kW 80-100 %

37-112 kW 60-80%

> 150 kW 50-70%

Figure 1: Actual input current waveform for six pulse VFDwithout any filtering.



18/48


13th

to 24th


1.2 Use of Ac line reactors for VFD

The input harmonic current distortion can be reduced significantly by thesimple addition of input line reactance. The inductive reactance of an inputline reactor allows 50 Hz or 60 Hz current to pass easily but presentsconsiderably higher impedance to all of the harmonic frequencies. Harmoniccurrents are thus attenuated by the inductive reactance of the input reactor.Table 7 illustrates the expected harmonic current distortion for six pulseinput rectifier type drives (VFD) having various amounts of total inputreactance (inductive impedance).

Table 2 THID for common drives with filter

Inputimpedance

% THD

< 1 % > 75 %

2 % 52 %

3 % 45%

4 % 40%

5 % 35%

8 % 28%

Figure 2: Actual input current waveform for six pulse VFD with 3% linereactor

Input impedance may consist of source impedance (upstream transformer),line reactor and / or DC link choke. The 8% data reflects the performance ofa typical VFD when a combination of a 3% impedance DC link choke and a



19/48


13th

to 24th


5% impedance AC line reactor are used. It is easy to see how the simpleaddition of either a line reactor or equivalent DC link choke can have asignificant effect on the input harmonic current distortion of a six pulse VFD.Reactors are by far, the most economical means of reducing input currentdistortion on a drive system.The actual distortion at the main input (metering point) will vary dependingon the system impedance and the distribution of loads (linear vs. non-linear).Figure 5 is a chart that indicates the expected current distortion levels at thePCC for various combinations of linear and non-linear loads. Add up thetotal motor and other linear loads plus VFD loads to determine the total load.Divide the VFD load by the total load. Look up this number in the % VFDload column and read the distortion level (at PCC) for the appropriate linereactor ahead of each drive. This will result in a conservative number

because any additional source impedance will cause the actual distortion to

be even lower.

It is important to note that whenever one is considering the impedance of areactor, it is the effective impedance that does the work, not the ratedimpedance. Effective impedance is based on the actual fundamental current,which is flowing and the actual inductance of the reactor.

Effective impedance (percent) =

L x f x 2 x x 1.732 x Fundamental amps x 100 (Eq.1)

Volts (line to line)

Note:

L Inductance

Frequency (f) 50 Hz or 60 Hz

3.14

It is easy to predict the current distortion at the PCC when each VFDemploys the same filtering technique (such as line reactors). If 5%impedance line reactors are installed on the input of each VFD, then the



20/48


13th

to 24th


input current distortion would be < 35% depending on the amount of sourceimpedance (in addition to the line reactor).

If the electrical load was entirely made up of VFDs, each having a 5%impedance line reactor, then the distortion at the PCC would simply be 35%THID x 100%VFD / 100% total load = 35% THID at the PCC.

Now if the same VFDs were only 20% of the total load at the PCC, then35% THID x 20% VFD / 100% Total Load = 7% THID at PCC.

1.3 Harmonic Filters

In some cases, reactors alone will not be capable of reducing the harmoniccurrent distortion to the desired levels. In these cases, a more sophisticatedfilter will be required. The common choices include shunt connected, tuned

& detuned harmonic filters (harmonic traps) and series connected low passfilters (broad band suppressors).

Harmonic traps have been used for nearly thirty years. They consist of acapacitor and an inductor, which are tuned or detuned to a single harmonicfrequency. Since they offer very low impedances to that frequency, thespecific (tuned) harmonic current is supplied to the drive by the filter ratherthan from the power source. If tuned harmonic filters (traps) are selected asthe mitigation technique, then you may need multiple tuned filters to meetthe distortion limits, which are imposed.

Figure 3 Symbols for tuned and detuned harmonic filters

When employing tuned & detuned harmonic filters, you will also need totake special precautions to prevent interference between the filter and the

power system. A harmonic trap presents a low impedance path to a specificharmonic frequency regardless of its source. The trap cannot discernharmonics from one load versus another. Therefore, the trap tries to absorb



21/48


13th

to 24th


all harmonics, which may be present from all combined sources (non-linearloads) on the system. This can lead to premature filter failure.

Since harmonic trap type filters are connected in shunt with the powersystem, they cause a shift in the power system natural resonant frequency. Ifthe new frequency is near any harmonic frequencies, then it is possible toexperience an adverse resonant condition, which can result in amplificationof harmonics and capacitor or inductor failures. Whenever using harmonictrap type filters, one must always perform a complete system analysis. Youmust determine the total harmonics, which will be absorbed by the filter, the

present power system resonant frequency, and the expected system resonantfrequency after the filter (trap) is installed. Field tuning of this filter may berequired if adverse conditions are experienced.

1.4 Design of harmonic filters

When designing or applying a harmonic filter, the question that comes to themind of many engineers is; What harmonic or frequency should theharmonic filter bank be tuned too? i.e., 4.2 (de-tuned), 4.8 (partially de-tuned) or 5.0 (tuned). To answer this question, the engineer should knowwhy the filters are being installed in the first place.

Harmonic filters are generally installed to achieve one of the followingobjectives:

Capacitors are required to improve power factor, and possible systeminteraction may occur with the installation of a plain capacitor bank.

Permissible distortion limits of the local utility or IEEE-519 are exceeded,and filters are required to reduce them.

A combination of 1 and 2 above, whereby capacitors are required toimprove power factor and with the addition of the capacitors, permissible

distortion limits are exceeded.



22/48


13th

to 24th


Figure 4: Typical Industrial System & Harmonic filters

1.5 Tuning the Circuit

The most effective solution to this problem consists of series tuning thecapacitor bank to the lowest offending harmonic, usually the 5th. This isdone by introducing an inductor in series with the capacitor as shown inFigure 5.

Figure 5 Tuning circuit



23/48


13th

to 24th


The impedance versus frequency plot, as seen by the harmonic source, isshown in Figure 6; the original impedance response (untuned) is shown forcomparison.

Figure 6 Comparison between tuning and untuned capacitors

The minimum impedance occurs at the series resonant point, the 4.7thharmonic, while the peak represents a parallel resonance due to the capacitorand the two inductors. Harmonic currents generated at or near the seriesresonant frequency (such as the 5th) will flow to the trap harmlessly,

provided the capacitor and reactor are sized properly to withstand theadditional stresses. These currents are simply following the path of leastimpedance. The system will not resonate above this frequency since it isinductive.

This approach will accomplish two objectives. %in the line side of the

capacitor filter bank, system power factor is corrected and harmonic voltagedistortion is reduced, Harmonic voltage (Vh) is the result of a harmoniccurrent Oh) flowing through the system impedance (Zh), i.e. Ohm's Law (Vh= Ih Zh)- By reducing the system impedance (Zh) we can reduce theharmonic voltage (Vh) even though the harmonic current (Ih) remains thesame.



24/48


13th

to 24th


When the main objective is to reduce harmonic distortion, the engineer willconsider the use of more filter stages, each tuned to the next higher harmonic(7th, 1 1th, . . .). In some cases, where harmonic currents are excessive, theuse of capacitors rated at the next higher voltage may be required. [an erroroccurred while processing this directive]

1.6 Size of reactors for tuning and detuning filters

Capacitor along a reactor forms a series resonant circuit. This filter circuitcan be tuned to one of the harmonic frequencies occurring in the network.

When the resonant frequency of the series resonant circuit is tuned to afrequency that is similar to the harmonic occurring in the system, the filter

circuit is termed as Tuned Filter

When the resonant frequency of the series resonant circuit is tuned to afrequency lower than the harmonic occurring in the system, the filter circuitis termed as Detuned Filter

Figure 7 Symbol for Harmonic filters

The reactor to capacitance ratio p (%) reflects the ratio of reactor reactanceto capacitor reactance.

The resonant freq (fR) of the series resonant filter circuit is indicatedindirectly by p (%).



25/48


13th

to 24th


Example for Tuned Filter

System frequency= 50 Hz

Harmonic content: 5th harmonics (THDV = 5.4 %)

Design of Tuned filter for 5th

harmonics

fR= f1. 1/(p)

The value of the 5th

harmonics = fR= 5x50 = 250 Hz

f1 = 50 Hz p = (f1/fR)2

= (50/250)2

= 4.00 %

Tuned at 5thHarmonics

Figure 8 Tuning at 5th

Harmonic



26/48


13th

to 24th


Table 3 Common sizes for Tuned reactors

Tunedindex at

harmonic

Tunedfrequency

Reactor valuein %

3 150 11.11%

5 250 4.00%

7 350 2.04%

9 450 1.23%

11 550 0.83%

13 650 0.59%



27/48


13th

to 24th


Example for Detuned Filter

System frequency= 50 Hz

Harmonic content: 7th harmonics (THDV = 6.9 %)

Design of Detuned filter for 4.06th

harmonics

fR= f1. 1/(p)

The value of the 4.06th

harmonics = fR= 4.06x50 = 203 Hz

f1 = 50 Hz

p = (f1/fR)2 = (50/203)2 = 6.00 %

Impedance-Frequency

0 50 100 150 200 250 300 350 400

0

0.05

0. 1

0

Frequency (Hz)

Z (ohm)

Tuned at 4.06thHarmonics

Figure 9 Detuning at 4.06th

Harmonic



28/48


13th

to 24th


Table 4 Common sizes for Detuned reactors

Tuned indexat harmonic

Detunedfrequency

Reactorvalue in %

2.77 138.5 13.03%

3.78 189 7.00%

4.06 203 6.07%

4.4 220 5.17%

4.7 235 4.53%

4.8 240 4.34%

4.2 210 5.67%



29/48


13th

to 24th


2.0 Solution for Voltage Sags

As mentioned earlier, solutions for power quality problems frequentlyinvolve some combination of wiring upgrades and mitigation equipment. Insome cases, it may be more cost effective for the customer to purchase andinstall mitigation equipment than to re-wire all or part of a facility. This is

particularly true when the facility includes numerous additions or hasundergone many remodeling.

Many manufacturing and process industries focus on ensuring sag protectionto maintain maximum competitiveness, productivity and quality. TheSemiconductor Equipment and Materials Institute (SEMI, www.semi.org)has gone so far as to establish a minimum standard with regard to sagimmunity performance for semiconductor tools and equipment. SEMI F47

introduced a well thought out voltage-to-time curve that most equipmentwill be exposed to during normal operation. In addition, a specific method oftest and reporting has been developed.

The first evaluation step is to identify which components are critical tomachine operation and would be adversely affected by voltage sag. Mostmotors, lighting and indicators can tolerate short-duration sag withnegligible detriment to production.

2.1 Mitigation measures for voltage sags

* Reference: Copper Development Association (CDA)

Figure 10 Possible mitigating methods



30/48


13th

to 24th


Modifications in the process equipment itself (Nos 1 and 2 in Figure 28)tend to be the cheapest to implement but are not always possible becausemanufacturers have not made suitable equipment readily available in themarket. Modifying the grid, (No 4 in Figure 28), although an interestingoption, is not always possible and is likely to be very expensive. The onlymethods that can generally be applied are protective measures installed

between the sensitive process and the grid (No 3 in Figure 28), and these arediscussed in this section.

In theory, installing an uninterruptible power supply (UPS) is the easiestway to protect sensitive processes against all sags. However, due to itsconsiderable purchase and maintenance costs, UPS equipment is installed ona structural basis only in places where the damage resulting from powersupply problems is very high, such as in hospitals, computer facilities and

financial institutions. In other cases, including most industrial processes, theinstallation of protective equipment must be subject to a cost-benefitanalysis, which often shows that installing a UPS is too expensive.

Stimulated by the high prevalence of voltage sag problems in industrialprocesses due to equipment being sensitive to voltage sags, solutions toprotect equipment against these sags have been made commerciallyavailable. Due to the wide variety and exotic vendor specific names of thesesystems, choosing the optimal techno-economic solution for a given problemis not straightforward.

This section analyses a number of systems that can be installed in existingfacilities containing processes susceptible to voltage sags. Taking intoaccount sag statistics from various countries, this Section provides guidanceon the effectiveness (in terms of the percentage of process outages avoided)that can be expected by installing these systems. Firstly, the equipment typesare described. Subsequently the sag immunization capability and othertechnical and economic aspects are evaluated. Taking into account the

performance of the described systems with regard to these aspects,

guidelines for practical situations are given.



31/48


13th

to 24th


2.2 Equipment procurement specifications

Generally, the least expensive approach is to purchase controls and otherelectronic equipment designed with minimum immunity requirement.

Improvement of equipment immunity is probably the most effective solutionagainst equipment trips due to voltage sags. But it is often not suitable as ashort time solution. A customer often only finds out about equipmentimmunity after the equipment has been installed. For customer electronics itis very hard for a customer to find out about immunity of the equipment, ashe is not in direct contact with the manufacturer. Even most adjustable-speeddrives have become off the shelf equipment where customer has noinfluence on the specifications. Only large industrial equipment is custom-made for a certain application, which enables the incorporation of voltage

tolerance requirements.

The very best way to insure that a machine meets your requirements forvoltage sag ride through is to include the requirement in the purchasecontract language and require proof of compliance. The semi-conductormanufacturing industry trade group SEMI has prepared two standards in1999 to facilitate embedding the solution.

SEMI F47 provides for the definition and measurement of equipmentreliability and availability during voltage sags. SEMI F42 standard providesfor the compliance test protocol. See their website www.semi.org. The semi-conductor manufacturers have sought this solution because the cost of lost

product is extremely high. The bottom line is economics.

Therefore, the best solution is to keep problem equipment out of the plantthrough equipment procurement specifications. Equipment manufacturersshould design equipment with ride through capability curves available.

A ride through capability limit should be based according to the voltage

tolerance requirement of MS IEC 61000-4-11 (Class 3) to ensureminimum equipment mal-operation to voltage sags.



32/48


13th

to 24th


2.3 Lists of immediate equipment solutions

* Reference: Pacific Gas & Electric Company (USA)

2.3.1 Compliance Power Supply

The most critical and normally sag-sensitive component is the AC-DCpower supply used to power all DC control and logic circuits. A majority ofpower supplies currently on the market average 10 to 20 ms of hold-up timeat full load. These devices will not meet the sag immunity performanceneeded to work during common sag events without special considerationstaken by the system designer.

One option is to use a universal input power supply (85-264 VAC) and

power from the higher line voltage (208/240). This, of course, only meetsthe needed level of performance when powered from the higher line voltage.Another would be to de-rate the power supply to a lower output current inthe hopes that it will perform better when exposed to input sags.The preferred method is to use a power supply that meets the voltage sagcompatibility standards of SEMI F47 or IEC 61000-4-11, at full power andall voltage ranges. This gives the designer maximum flexibility withminimum design effort.

2.3.2 Change the trip setting

Another inexpensive and simple solution is to adjust the trip thresholds ofsensitive equipment. If you identify a relay that is inadvertently trippingduring a voltage sag, you can change its settingseither the voltagethreshold or the trip delay.

However, we can only do this if the trip settings were set too conservatively,so it is important to understand what they were designed to protect.

If you can identify an unbalance relay, an undervoltage relay, or an internalreset or protection circuit that is inadvertently tripping during a voltage sag,change its settings. Consider changing the threshold, and consider changingthe trip delay; either or both might make sense. Sometimes this can be assimple as twisting a knob; sometimes it may take a component change orfirmware adjustment. You can only use this solution when the trip settings



33/48


13th

to 24th


were set too conservatively to begin with; trips are useful and important, soyou don't want to eliminate them completely.

2.3.3 Slow down the relay

If the equipment is misoperating because a relay in the EMO circuit isoperating too quickly, consider slowing it down. You might use a relay withmore mechanical mass (such as a contactor), or you might use a relay hold-

in accessory. A reasonable delay time is 1.2 second.

2.3.4 Installing a coil hold-in device

Another option is to install a coil hold-in device. These devices are designed

to mitigate the effects of voltage sags on individual relays and contactors.Coil hold-in devices are installed between the relay or contactor coilconnection terminals and the incoming alternating current (AC) control line.They allow a relay or contactor to remain engaged until the voltage drops toabout 25 % of nominal, significantly improving its voltage sag tolerancewithout interfering with emergency shutoff functions. The best applicationfor this type of device is to support relays and contactors in an emergencyoff (EMO) circuit, master control relay, or motor control circuit.

During a voltage sag, this device maintains a current flow through the coil thatis sufficient to hold in the contacts closed. The circuit is designed to providecurrent to hold in the coil for sags down to 15-25% voltage. It is not designedto hold in the coil for cases where the voltage goes below 15%. This allowsemergency stop circuits to act correctly and will prevent any problems without-of-phase conditions following an interruption.

FACT: if the voltage drops to 60% of nominal, the available torque is

about 1/3. If the motor starts to lose synch with the supply power, then

you could have a major torque stress problem, which could result indamage to the drive system.



34/48


13th

to 24th


2.3.5 Built-in energy storage within the device

An example is a digital alarm clock with either a storage capacitor or abattery. A few VCRs have high-end models with ride-through capability. Thecost of modifying a PC switched-mode power supply could be less than $50.

In this competitive market, some manufacturers are reluctant to modifyingsince that may double the cost of the power supply.

2.3.6 Built-in software programming within the sensitive device.

Several small adjustable speed drive models have user programmableautomatic restart. Typically, they wait 30 to 60 seconds for the circuit tostabilize before trying to restart. The number of retries can be programmedtypically it is set at 5 retries. This strategy is commonly used in HVAC fansand pumps in an unattended environment. This applies to both sags and

momentary interruptions. There are several ASD manufacturers that have asan option, the ability of the ASD to restart before the load has come to acomplete stop and to apply the appropriate torque. Unfortunately, this conceptis only applicable to a single stand-alone drive as yet.

NEMA in the USAhas a working group to establish a new restart categorylabel for drives. The three categories of restart capabilities after a voltage sagare: 1) stop, 2) delay and try to restart but not in synch, and 3) try to restartimmediately in synch with prior speed. On better drives, these will all beavailable for the user to program as required. Some computer systems willsuspend operations until the voltage anomaly passes and then resumecomputing. It will issue a voltage out of tolerance error message, but thecomputing operation continues. For desktop computer users, most softwareshave an autosave feature to limit loss of data. This is a no-cost option. Forautomated process control software it would be very productive if thesoftware knew the status of the equipment when a sag shutdown is executed.This could be an extra cost option but very cost effective because it willreduce the restart time.

2.3.7 Change motor starter contactors from AC to DC

DC contactors and relays are readily available. A DC contactor or relay isstronger because the flux is constant. A rectifier is required to change AC toDC. A small storage capacitor could be added to DC contactor to extendride-through energy. Disadvantage of DC is increased arcing due to lack ofzero crossing to break the arc.



35/48


13th

to 24th


According to NEMA standard, an AC motor contactor should ride through asag to 85% of nominal and a DC contactor should ride through a sag downto 80% of nominal. Some industrial facilities use all DC control relays.Relays are much faster to open than motor contactors because they aresmaller, less mass, act quicker. They will open in 5 to 15 milliseconds; thisis less than one cycle. There are very important application considerationswhen considering extending the ride through of motor contactors. In makingmodifications, consider the system approach and consult your OEM. Onemethod to extend the ridethrough is to add a 70va ferroresonant transformer.This is cheaper and easier than changing the coils.



36/48


13th

to 24th


2.4 Other solutions for voltage sags

* Reference: www.powerstandards.com

2.4.1 Switch power supply settings.

Many power supplies can be set to accommodate different voltage ranges,and these ranges often overlap. Choose a range where your nominal voltageis near the top of the range, and you'll have more room for voltage sags. Forexample, if your power supply has Range #1, 95V-250V (accommodatingJapan and Europe), and Range #2, 110V-270V (accommodating NorthAmerica and Australia), and you have a 240V nominal voltage, you willhave greater sag immunity on Range #1.

2.4.2 Connect your single-phase power supply phase-to-phase.

If you can stay within your power supply's acceptable voltage range, and ifyou have three-phase power available, you can get a quick 70% boost inavailable voltage by connecting phase-to-phase. For example, if your powersupply is rated as 90V-250V, and you are using it on a 120V circuit, you canonly tolerate a voltage sag to 75%. But if you connect it phase-to-phase, thenominal voltage will be 208V and you will be able to tolerate a voltage sagto 45%.

2.4.3 Reduce the load on your power supply.

Lightly loaded power supplies always tolerate voltage sags better thanheavily loaded power supplies. If you can determine that a particular powersupply is causing your equipment to mal-operate during a voltage sag event,consider moving some of its loads to another power supply. Tradeoffs: May

be inconvenient to install; carefully consider effects of a shutdown on one ofthe power supplies.

2.4.4 Increase the rating of your power supply.

If you can't move the loads, use a bigger supply for the same load -- relative

to its rating, it will be more lightly loaded.2.4.5 Use a three-phase power supply instead of a single-phase

supply.

A properly designed (and lightly loaded) three-phase power supply willeffectively tolerate voltage sags on one or two phases that would shut downa single-phase power supply.



37/48


13th

to 24th


2.4.6 Run your power supply from a DC bus.

Sometimes you can substitute a DC-operated power supply for an AC-sourced supply. If it does nothing else, this will narrow down your problemsto supporting a DC bus, which can often be done with simple capacitors or

batteries. (This is the approach that high-reliability telecommunicationssystems take, using a 48 Vdc supply as their power distribution system.)

2.4.7 Get rid of the voltage sag itself.

As a last resort, consider installing a quick-operating voltage regulator onyour AC supply. There are a variety of technologies: ferroresonanttransformers, solid-state power conditioners, etc. But make sure that youaren't making the problem worse; if the original cause of the voltage sag isdownstream from your voltage sag regulator, the voltage sags will actually

get deeper and longer when you install the fix.



38/48


13th

to 24th


2.5 Understanding power conditioners

To ride through a voltage sag event, the load will need some kind of systemthat can react within about cycle and provide near-normal power for a fewseconds until the voltage is fully restored. This requires either a source ofstored energy at the site or an alternate source of energy. These devices musteither be capable of being switched very quickly or be always on-line.

To achieve this condition, one needs to install some form of a power-conditioning device. These solutions increase in cost with the size and scopeof the equipment or circuits being protected.

2.5.1 Location to install Power Conditioners

Figure 11 Locations for power conditioners



39/48


13th

to 24th


2.6 Types of power conditioners

2.6.1 Understanding UPS

Installing an uninterruptible power supply, on a PC, PLC or controls toswitch to battery during a voltage sag or an interruption will minimize

process interruption. The down side to this approach is the battery. Batterieshave the following disadvantages: a) generates hydrogen gas, must beventilated, b) battery lead is a hazardous waste, and c) battery life is limitedand decreases rapidly when cycled often. An advantage is that the UPS willride through not only sags, but also momentaries and extended interruptionsup to the limit of the battery capacity, maybe 5 to 10 minutes. For a full on-line UPS the cost doubles but you add the benefit of filtering out alldisturbances.

A UPS can be installed off-line, which is cheaper, or on-line, which doublesthe cost but adds the ability to filter out all types of voltage disturbances,including spikes and harmonic distortions.

2.6.2 Understanding Constant Voltage Transformer

Installing a Ferroresonant (constant voltage transformer- CVT) transformer onPC, PLC or controls will provide sag ride through capability. They also

provide filtering of transients. CVT will not ride through a momentary orsustained interruption. They have no moving parts, no battery and are veryreliable. Oversizing will extend ridethrough. This is probably the mostcommon mitigation measure used at present for this problem. CVTs are alsocommonly installed to protect large loads up to 15 kVA single phase.

CVTs provide voltage sag ride-through of 25 % for 1 second and also filterspikes, but they are not able to protect against interruptions, eithermomentary or sustained. CVTs are often used for relatively constant, low-

power loads, and have the advantage of lower operating and maintenance

costs than UPSs, because CVTs dont require batteries.



40/48


13th

to 24th


2.6.3 Understanding Dip Proofing Inverters

For an individual computer, process control circuits or a group of machines,another simple solution is to install a Dip Proofing Inverter (DPI), which canride through a voltage sag event down to 0 % of nominal voltage for up to3.1 seconds.

2.6.4 Understanding Voltage Dip Compensator

A Voltage Dip Compensator (VDC) can ride through a sag down to 37 % ofnominal voltage for up to 3.1 seconds can also be used to protect single

phase equipment and control circuits.

2.6.5 Understanding Dynamic Compensator (Dynacom)

DynaCom is a low voltage dynamic voltage compensator designed tomitigate voltage sags by injecting a compensating voltage directly into the

power supply.

Under normal system operating conditions, Dynacom allows system voltageto pass through with high efficiency. In the event of a voltage sag, Dynacom

produces a compensating voltage of an appropriate magnitude and durationto fill in the sag, thus reproducing the original voltage wave form. Thedirect injection technique used in Dynacom provides accurate and efficientvoltage compensation.

The Dynacom can correct input voltage to as low as 40 % of nominalvoltage up to 1 second.

2.6.6 Understanding Dynamic Sag Corrector

The DySC (pronounced disk), rated at 250VA to over 3,000 kVAspecifically protects sensitive equipment and manufacturing processes from

deep voltage sags and momentary interruptions, the most common powerquality events.

TheDySCcan correct input voltage to as low as 0 % of nominal voltage for50 ms and 50 % voltage for 2 seconds.



41/48


13th

to 24th


2.6.7 Understanding Active Voltage Conditioner

The Vectek Active Voltage Conditioner (AVC) is an inverter based systemthat protects sensitive industrial and commercial loads from voltagedisturbances. It provides fast, accurate voltage sag correction as well ascontinuous voltage regulation and load voltage compensation.

It has been optimally designed to provide the required equipment immunityfrom the level of voltage sags expected on the ac supply network. The AVCis available in load capacities of 20kVA - 10MVA and has an operatingefficiency exceeding 98%. It offers extremely fast response to three-phasesags down to 50%, and single-phase sags down to 25% on the ac supplynetwork. Standard models are optimized for sag correction and for enhancedregulation allowing correction of voltage sags and surges the AVC-R is

available. All AVC models provide continuous regulation within 10% ofthe nominal mains voltage and can remove voltage unbalance from thesupply. Optionally models can be configured to remove flicker and harmonicvoltages from the supply.



42/48


13th

to 24th


2.6.8 Understanding Datawave

The Datawave is a Magnetic Synthesizer that generates a stable outputwaveform to distribute to the sensitive electronic equipment. The self-contained system can be used to condition utility power, distribute it tosensitive electronics, and monitor power parameters. Systems are availablefrom 15-200 kVA.

Total power conditioning under the worst power conditions maintainingconsistent output quality even during 40% undervoltages and +40%overvoltages for 1 second. Power conditioning, monitoring and flexibleoutput distribution from a single factory tested unit. Handles non-linearloads and high neutral current without oversizing.

General specifications are:

Voltage Regulation: For input voltages of 40%, output voltage is within+5% for any load condition up to full load.

Single Phase Protection: For loss of one input phase, output voltages remainwithin 6% and 4% up to 60%load.

2.6.9 Understanding Flywheel

A flywheel is a simple form of mechanical (kinetic) energy storage. Energyis stored by causing a disk or rotor to spin on its axis. Stored energy is

proportional to the flywheels mass and the square of its rotational speed.Advances in power electronics, magnetic bearings, and flywheel materialscoupled with innovative integration of components have resulted in directcurrent (DC) flywheel and increasing battery life.

A flywheel could also be used alone for applications where longer-termbackup capability is not required or economically justified. Variations In

general, flywheels can be classified as low speed or high speed. The formeroperate at revolutions per minute (rpm) measured in thousands, while thelatter operate at rpm measured in the tens of thousands. As noted above,doubling the rpm quadruples the stored energy, all else equal, so increasingrpm significantly increases the energy density of a flywheel. Operating athigher rpm necessitates fundamental differences in design approach.



43/48


13th

to 24th


2.6.10 Dynamic Voltage Restorer (DVR)

The PureWave DVR is designed for series connection in a distribution line.It maintains the voltage applied to the load during sags and swells byinjecting a voltage of compensating amplitude and phase angle into the line.

The PureWave DVR is a means to satisfy the stringent power qualitydemands of industrial and commercial customers. It also provides a meansfor energy users to isolate themselves from voltage sags, swells, andunexpected load changes originating on the transmission or distributionsystem.

The PureWave DVR offers immediate benefits to power users. It

Reduces losses from process shutdowns and product degradation

Protects expensive and sensitive production equipment from voltageanomalies

Minimizes the potential for production shutdowns due to faultselsewhere on the distribution and transmission systems, including thosecaused by lightning strikes

Enables increased production for processes that previously ran at less-than-full output, due to voltage sag sensitivity.Power quality problem

Power quality solution

Figure



44/48


13th

to 24th


Table 5 Summary of available power conditioners

Item Coil lock CVT Dip-Proof

Inverter

Voltage Dip

Compensator

UPS Dynacom Dynamic Sag

Corrector

Flyw heel AVC

Ride-through

capability, %

Voltage sag

25 % for 3

second

50 % for 1

second

0 % for 3.1

second

37% for 3.1

second

0 % for 10

minutes

40 % for 1

second (1&3-

phase)

0 % for 50 ms,

50 % for 2

second

0 % for 8 second

& switch t o

standby

generator

25 % Single

faults for

second, 50

phase fault

30 cycle

Battery less Yes Yes Yes Yes No Yes Yes Yes Yes

Maintenance free Yes Yes Yes Yes No Yes Yes No Yes

Response time On-line On-lineOff-line (

Date post:	14-Apr-2018
Category:	Documents
Upload:	hizbi7
View:	223 times
Download:	0 times

L24 PQ Mitigation Asean School 2006.pdf

Documents