p9 materials for discussion and conclusions

8/16/2019 P9 Materials for Discussion and Conclusions

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1. Fixed type capacitor banks

The reactive power supplied by the fixed capacitor bank is constant irrespective of any variations in the power factor and the load of the receivers. These capacitor banks are switched on either manually (circuit breaker / switch) or semi automatically by a remote-controlled contactor.This arrangement uses one or more capacitor to provide a constant level of compensation.

These capacitors are applied at the terminals of inductive loads (mainly motors), at bus bars.Disadvantages:

Manual ON/OFF operation. Not meet the require kvar under varying loads. Penalty by electricity authority. Power factor also varies as a function of the load requirements so it is difficult to maintain a consistent power factor by use of Fixed Compensation i.e. fixed capacitors. Fixed Capacitor may provide leading power factor under light load conditions, Due to this result in overvoltages, saturation of transformers, mal-operationof diesel generating sets, penalties by electric supply authorities.

Application:

Where the load factor is reasonably constant. Electrical installations with constant load operating 24 hours a day Reactive compensation of transformers. Individual compensation of motors. Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate. Size of Fixed Capacitor bank Qc = 15% kVA transformer

Method Advantages DisadvantagesIndividual capacitors Most technically efficient, most flexible Higher installation & maintenance costFixed bank Most economical, fewer installations Less flexible, requiresswitches and/or circuit breakers

The size of the inductive load is large enough to select the minimum size of capacitors that is practical.

For HT capacitors the minimum ratings that are practical are as follows:System Voltage Minimum rating of capacitor bank3.3 KV , 6.6KV 75 Kvar

11 KV 200 Kvar22 KV 400 Kvar33 KV 600 Kvar

Unit sizes lower than above is not practical and economical to manufacture.

When capacitors are connected directly across motors it must be ensured that the rated current of the capacitor bank should not exceed 90% of the no-load current of the motor to avoid self-excitation of the motor and also over compensation.Precaution must be taken to ensure the live parts of the equipment to be compens


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ated should not be handled for 10 minutes (in case of HT equipment) after disconnection of supply.

Crane motors or like, where the motors can be rotated by mechanical load and motors with electrical braking systems, should never be compensated by capacitors directly across motor terminals.

For direct compensation across transformers the capacitor rating should not exceed 90 % of the no-load KVA of the motor.

C. Size of Conductor for Capacitor Connections

Size of capacitor circuit conductors should be at least 135% of the rated capacitor current in accordance with NEC Article 460.8 (2005 Edition).

Placement of capacitors in Distribution system

The location of low voltage capacitors in Distribution System effect on the mode of compensation, which may be global (one location for the entire installation), by sectors (section-by-section), at load level, or some combination of the last two.

In principle, the ideal compensation is applied at a point of consumption and at the level required at any instant.

Compensation by sectorPrinciple

Capacitor banks are connected to bus bars of each local distribution Panel.

Most part of the installation System can benefits from this arrangement, mostlythe feeder cables from the main distribution Panel to each of the local distribution panel.Advantages

Reduces the tariff penalties for excessive consumption of kvar. Reduces the apparent power Kva demand, on which standing charges are usually

based. The size of the cables supplying the local distribution boards may be reduced, or will have additional capacity for possible load increases. Losses in the same cables will be reduced. No billing of reactive energy. Makes less demands on the supply Feeders and reduces the heat losses in these Feeders. Incorporates the expansion of each sector. Makes less demands on the transformer. Remains economical

Limitations

Reactive current still flows in all cables downstream of the local distribution Boards. For the above reason, the sizing of these cables, and the power losses in them, are not improved by compensation by sector Where large changes in loads occur, there is always a risk of overcompensation and consequent overvoltage problems.

Application

Compensation by sector is recommended when the installation is extensive, and wh


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ere the load/time patterns differ from one part of the installation to another.

This configuration is convenient for a very widespread factory Area, with workshops having different load factors

Load flow studies are performed in order to investigate:-

i) Flow of current, kW and kVar in the various points of the networkii) Bus bar voltages.iii) Effect of rearranging circuits and incorporating new circuits on system loadingiv) Optimum system running conditions and loads distributionv) Optimum system lossesvi) Optimum rating and tap range of transformersvii) Improvement from change of conductor size and system voltage

Load flow studies are essential not only for analyzing the existing system, butalso useful for theplanning of future development of the system in order to know the effect of newloads and newcables/lines before they are installed.

In a power system, voltage at various buses tends to increase or decrease during it dailyoperation. When the voltage is below the required level, reactive power produced by inductanceneeds to be offset by capacitance. There are several techniques can be used to increase thevoltage to its acceptable level. A well known technique is to use shunt capacitor in parallel to thetransmission lines.The capacitor is used to provide reactive power compensation in order to achieve power and

energy loss reduction, system capacity release and acceptable voltage profiles.The extent ofthese benefits depends on the location, size, type, and number of shunt capacitors and also ontheir control settings. Hence, an optimal solution for placement and sizing of shunt capacitors ina distribution system is a very important aspect of power system analysis.In this experiment, capacitor banks are required to be connected at the transmission network tohelp improving voltage profile for certain busbars. The PSS/Adept simulation software iscapable to optimise the size and the location of capacitors in the designated network.

5.1 Capacitor Placement Optimization (CAPO) analysis Capacitor placement optimization finds the suitable size of the capacitors to be located incertain bus of a network. The result of the optimization process will indicate the set of location(s)where capacitors should be placed; the size of the capacitors.


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From the results of 5 and 6, evaluate theperformance of the capacitors in terms of voltage stability and total power saving.

Discuss the result with and without capacitor placement to the power transmissionsystem.

CAPO places capacitors on the network as long as they are economic (i.e., as long as the value ofthe monetary savings from the placement is greater than the cost of the capacitor itself). CAPOselects the node for the nth capacitor that results in the largest monetary savings. Load snapshotsare implemented in PSS/ADEPT to provide modeling of the load variations, which occur with time,temperature, or other factors. When switched capacitors are placed by CAPO, thecapacitor

switching increment for each snapshot is also calculated.

The following paragraphs provide a complete description of CAPO, considering fixed and switchedcapacitors and multiple snapshots. First, for each snapshot a load flow is doneto let transformertaps and existing switched capacitors adjust. These transformer tap and capacitor increment settingsare then savedwitheachsnapshot.There

will be no further adjustmentsof thesetransformer/capacitorsettingsasCAPOprogresses.

CAPO first considers fixed capacitors, which, by definition, are on during all load snapshots. All theeligible nodes in the network are then examined to see at which one the capacitor placement offersthe greatest monetary savings. Since there are multiple snapshots, this reductio

n is calculated asthe weighted sum from each snapshot, where the weighting factor is the snapshotduration. Thefollowing conditions can then stop the capacitor from actually being placed on the selected node: The present worth of the savings does not offset the present worth of the costs. Withmultiple snapshots the savings are evaluated as in the simple example consideredabove, except now a weighted sum over all the profiles is calculated. There are no more fixed capacitors available to be placed (actually, this can be


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checkedbefore all the nodes are searched, but is listed here for completeness). An upper voltage limit is violated in one of the profiles (the network upper voltage limitis set from the Analysis Options Property sheet under the General tab).Fixed capacitors continue to be placed until one of the above three conditions are encountered; atthat point the fixed capacitor placement ends and the switched placement begins. This procedure

is a bit more complicated, and before we begin this is probably a good point tomake a comment. Ifonly one load snapshot is used, you might expect that after the fixed capacitors are placed therewill be no placement of switched capacitors. There are at least four conditionswhere this is not true: You had only a few fixed capacitors available, and there was still considerableopportunityforsavingswhenthesefixedunits

weredepleted. The eligible nodes for switched capacitors are different than those eligible for fixedcapacitor placement. You make the cost of switched capacitors less than that of fixed capacitors, and afterthe fixed capacitors are placed it will still be cost effective to place switched capacitors. You make the size of the switched capacitor bank smaller than that of the fixedbank.The eligible nodes (for switched capacitors) in the network are reviewed to find the node, which pro-

duces the greatest savings summed over all the snapshots. There are a couple ofsubtleties in thisevaluation. First, if placing the switched capacitor causes a voltage violationin any snapshot, thecapacitor is turned off during that period. Second, if the capacitor causes a cost penalty for a snap-shot, it is also turned off for that snapshot. The calculation of the present worth of the savings is thencalculated considering only the snapshots during which the capacitor is turned on. This processcontinues until a point is reached where: The savings do not offset the cost of the switched capacitor. CAPO runs out of switched capacitors to place.

In summary, CAPO places fixed capacitors on the network until one of the stop conditions areencountered. Then switched capacitors are placed until one of the switched capacitor stop conditionsoccurs.Thetotalcost ofthe


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optimizationis then the installationandmaintenancecostofallthecapacitorsplaced;thetotalsavingsisthesumof thesavingsfromeachcapacitor.

PSS/ADEPT 5.2U

SERS MANUALJune 2005

Siemens Power Transmission & Distribution, Inc.Power Technologies International1482 Erie Boulevard P.O. Box 1058Schenectady, NY 12301-1058 USPhone 518-395-5000www.pti-us.com

1) D. P. Kothari and I J Nagarath, Modern Power System Analysis, Tata Mc Graw Hill, Third Edition,

2005.2) Chapman,S.J, Electric Machinery and Power System Fundamentals, McGrawHill 2002

3) "PSS/ADEPT 5.2 USERS MANUAL June 2005",Siemens Power Transmission & Distribution, Inc. Power Technologies International, 1482 Erie Boulevard P.O. Box 1058, Schenectady, NY 12301-1058 US, Phone 518-395-5000, www.pti-us.com

4) Mohan, W.; Scott, M. "Modeling Power Electronics in Power System Using EMTP". University of Minnesota - BPA. 1993

5) Saadat, H, "Power system Analysis Paperback", PSA Publishing; THIRD EDITION

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