Power Elect

8/3/2019 Power Elect

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AUTOMATIC POWER FACTOR CONTROLLING PANEL



1. Introduction

1.1 Defination

There are two type of electrical supplies viz. A.C. and D.C. But the mostly

electrical generated in the form of A.C.

Then by using transformer it is step up to high voltage for transmission

purpose.

At the end it is utilized by the various electrical equipments reduced voltage.

The power factor is defined as “It is cosine of angle between the voltage andcurrent in an A.C. circuit”.

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vvvV

In electrical circuit there are three main elements Resistor, Capacitor and

Inductor.

For purely resistive load circuit, the voltage and current are in phase with each

other, so power factor of circuit is unity.

For purely inductive load circuit, the current lags behind the voltage, so

power factor is lagging by 90’.

For purely capacitive load circuit, the current leads the voltage, so power

factor is leading by 90’.

I

Ø V

Power factor = cos ø



In other words,

It is also define as,

The ratio of resistance to impedance of any circuit

cos ø =

But in actual practice in industry we find the 80% load inductive in

nature( mostly as induction motor ) and remaining are in resistive in nature.

So power factors commonly very between 0’ to 90’ (lagging).



1.2 Disadvantage of low power factor

1. Increase in load current :

The power consumed by load is given by,

P = VL IL cosø (for single phase)

IL =

IL

From above equation load current is inversely proportionally to

the power factor for constant power and voltage

Hence for low power factor, load current increases.

2. Large copper loss :

As the power factor is low, current drawn by the load is more.

Here copper loss (I2

R) of the system as well as load increases with

causes in poor efficiency.

3. Poor voltage regulation :

For low power factor current magnitude increase, hence voltagedrop (IZ) in all the equipments connected in line such as

alternator, transformer, and transmission line etc, increases.

This results in decrease in receiving end and voltage, causing poor

voltage regulation.



4. Grater conductor size :

Current density is given as,

Where,

= current density ……… A/mm2

I = current through conductor……A

a = cross sectional area of Conductor ……….mm2

Hence cross sectional area

For same current density if current increases the cross sectional

area of conductor has to increase.

Increase in cross sectional area of conductor causes increase in

weight of conductor.

Hence cost of the supporting structure also increases.

This increases the overall transmission cost of the system.

5. Large kVA rating of equipment:

kVA rating of the alternator and transformer is

kVA =

i.e. kVA rating of equipments is inversely proportional to power

factor.

For low power factor, the kVA rating of the equipments has to be

made more.

This increase the size and cost of the equipment.



CAUSES OF LOW POWER FACTOR

I. All ac motors (expect over excited synchronous motors and

certain type of commutator motors) and a transformer operates

at lagging power factor. The power factor falls with the decrease

in load. For example, an induction motor has reasonable higher as

follow

Load Power factor

Full load 0.85

75% of full load 0.8

Half load 0.7

25% of full load 0.5

No load 0.2 to 0.3

II. Arc lamps and electric discharge lamp operate at low lagging

power factor.

III. Due to increase the supply mains voltage, which usually occurs

during low load periods such as lunch hours, night hours etc, the

magnetizing current of inductive reactance increase and power

factor of the electrical plant as a whole comes down.

IV. The power factor at which motors operate falls due to improper

maintenance and repairs of motors. In repaired motors, less wireis sometimes used then originally wound motor, therefore in such

motor leakage of magnetic flux increase and power factor of the

motor decrease.In case of heavily worn-out bearings, the rotor

may catch at the stator.

Some metal is sometimes removed for the rotor by turning

instead of replacing the defective bearings. In doing so, the length

of air gap between stator and rotor increase due to which greater



magnetizing current is required and therefore power factor

drops.

V. Industrial heating furnaces such as arc and induction furnaces

operate on very lagging power factor.

Hence it is very important to maintain a good power factor for a healthy

system.

The average power factor of some of the common appliances is give

below:

TYPE OF LOAD POWER FACTOR

Incandescent lamps 0.9 - 1.0

Fluorescent lamps 0.6 - 0.8

Neon lamps 0.4 - 0.5

Arc lamps 0.3 - 0.4

Fans 0.5 - 0.7Induction motors 0.4 - 0.8

Fractional kW motors 0.4 - 0.7

Induction heaters 0.85

Resistance furnaces 0.6 – 0.9

Arc furnaces 0.85

Arc welders 0.3 – 0.4

Induction furnaces 0.6

Resistance welders 0.4 – 0.7

Thyristorised APFC Panel



INTRODUCTION :

Power (KVA) Supplied by Electricity Boards consist of Real Power (KW), which

Produce energy & reactive power (KVAR) generated by Inductive Loads, whichdecreases the loading capacity & the efficiency of the Supply System. The PowerFactor (PF) is defined as a ratio between the Real Power & the Total Power.Electricity Boards charge heavy Penalty to H.T. Consumer, if the Power Factor of thesystem is below a required minimum average value. Power Factor of the System canbe improved by switching Capacitors as per KVA demand with the help of AutomaticPower Factor Controllers.

Modern Industries use Large Nos. of AC Motors, DC Drives, Rectifiers, Invertors,Compressors, Inductive Furnaces and Welding Machines, causing poor Power Factorof the System. The Reactive Power requirement in such Industries varies rapidlywithin every few cycles. The response speed of the conventional Power FactorController is not suitable to take care of rapid switching cycle. The ConventionalPower Factor Control System incorporates Electromechanical Switches to SwitchCapacitors and or not suitable for Fast switching where the load is rapidly changing.

Also fast switching of Capacitors and Contractors results in inherent stresses on theCapacitors and on the Switchgear components.

Uncompensated reactive power leads to poor Power Factor. Voltage instability,flickering, high current consumption, increased Maximum Demand, high losses, lowelectric supply, loading capacity and overrating of switchgears & cables results in high

wear and tear and high maintenance cost.SOLUTION :

Jaivic Thyristorised Automatic Power Factor Correction System is a modern PowerfulSystem constituting a Technological Breakthrough in the field of Power FactorControl.



This new Modern System utilisesElectronic Switching Element that switchPower Capacitor Banks into the newworth without creating switchingtransients. Connection to the network is

performed with the help of ‘ZERO-CROSSING Detector’, providing smoothconnection to the Capacitor Banks. Thereis no limit to the no. of switchingoperations, since the Electronic switchingelements do not wear out or deteriorateduring the switching process.

Technical Comparison :

Contractor Switching Thyristorised Switching

1. Network Compensation atextremely low voltage is notpassible.

2. Welding of the contactor contactsdue to high inrush currentsresulting in dangerous overcompensations.

3. Repeated and momentary ShortCircuit currents due to capacitorSwitching will have impact on theLife of the transformer.

4. Contactors and Capacitorsdeteriorate in performance,resulting in down time and nonavailibility of compensation.

1. Compensation at extreely lowvoltage is possible.

2. No over compensation, undercompensations and unbalancedcompensation.

3. No inrush current. Distributiontransformer never subjected toshort circuit current, life of thetransformer increases.

4. The Capacitor bank unit remains inuseful operational service with

maximum up time.

Direct On-line Contactor Switching Thyristorised Switching (with ZeroCrossing detect))

Ton Control Command

Thyristorised switching along with zerocrossing detect circuit brings the switching

surge currents to almost zero. Direct On-line Contactor Switching



Switching surge current is reduced and

controlled but not totally eliminated Advantages :

On-line Power Factor Correction. Fast Correction time Selectable

- up to 100 ms- Cycle to Cycle- 2 Sec. / 2 min. (Optional)

Zero Switch on Surge Current. Zero Differential Voltage Switching. Switching Devices Rating suitable for upto 50% THD. Increase in Life of Capacitors. Protection against System Resources. RS232 Serial Interface useful for Data Logging Application. Avoids Distribution System Deterioration. Transformers Self Inductive Load Compensation by pre-preprogramable

kVAr value. Suitable for Unbalanced Load. Loca/Remote Data Logging and Printer Interface. Programmable Settings for Tripping under Fault Conditions like –

- Capacitor Shorted- Capacitor Earth Fault

- Load Over Current- Load Imbalance etc.- Wrong Phase Sequence

100 % Depreciation.

Product Range

The product range in Power Factor Improvement Capacitors is as follows.

Low Voltage Metallised Polypropylene Film(MPP/Oil) Capacitors in single units from 1

kVAr to 30 kVAr with ISI Mark suitable forstandard Voltages of 415/440 V AC. HigherkVAr ratings can be supplied by banking ourstandard units with Aluminum - Copper bus-bars, with or without HRC fuses.

Low Voltage MPP Double Dielectric Capacitorsfor Heavy Duty Applications, with ISI Markfrom 1 to 30 kVAr up to 660 V AC.



6 Terminal Oil-Filled Capacitors from 3 kVAr to 25 kVAr, 440 V AC

Low Voltage MPP - Dry type Capacitors in single units from 1 kVAR to 9kVAr 415/440 V AC for Light Duty Applications, With ISI Mark.

Low Voltage Polypropylene Film Capacitors in Mixed Dielectric(M.D) and Allpolypropylene designs(APP) in Single units of 5 kVAr to 30 kVAr suitable forstandard voltages of 415/440V AC. with ISI Mark.

2 mfd to 30 mfd, 36 mfd, 72 mfd, 108mfd Dry and Oil Filled Condensers forVoltage upto 240 V & 440 V.

We also undertake Manufacture of Capacitors of single phase, 6 Terminalsand for special Voltage Application.

How Does Your Capacitor Work? APFC Panels

A load & therefore its KVAR are in a dynamic state - generally. A matching KVAR

output of a capacitor bank must also be dynamic i.e. must adjust itself-instantly to itsrequirement, if one is to obtain a uniform &'set' p.f. all along. This is best achieved byan automatic control that switches in & out, segments of a designed capacitor bank.A control panel serving this purpose is called on APFC panel or Automatic PowerFactor Controlling panel. It controls the load power factor by sensing variousavailable parameters. A) Sensing Parameters :-

1. Current - Sensing based APFC :- The current magnitude through a feeder or busis sensed and fed to a relay. As this magnitude crosses a set band-width, the relay

operates a power controlling a section of a capacitor bank. This is the simplest and

http://www.madhavcapacitors.com/APFCPanel/apfc.htm





possibly the cheapest relay. It has a disadvantage of functioning with no reference tothe actual load power factor - but assuming it. 2. Power Factor Sensing based APFC :- This relay senses the start of the voltagecurrent wave forms on a given feeder & measures the time difference between them.

It then converts this into a p.f. & compares this with a set value. Upon finding adifference, it operates the power contactor. This type of relay is most widely used. Ithas an advantage of being able to show the laod p.f. on an indicating meter. It'sdisadvantage :- It has no relation to the load magnitude & it's KVAR requirement. Itcan lead to severe hunting. 3. KVAR Sensing based APFC :- This relay senses the magnitudes of both thevoltage & current wave forms & also the time or phase difference between them. Itthen calculates the load KVAR & compares these with a possible combination ofsections within a capacitor bank and operates their controlling contractors to add therequired capacitor KVAR to the electrical system. This is the most sensitive relay -

capable of obtaining maximum benefit out of a given capacitor bank. It's disadvantages :- It is rather hard on the contractors and its related surgesuppression attachments. B) Sizing of Capacitor Switching Blocks :- 1) Power Factors for the purposes of levying penalties are based on the monthlyconsumption’s of KVA-Hrs, KW-Hrs & KVAR-Hrs as recorded on a tri-vector meter. Ifthe basic purpose of installing capacitors is to stay safely above the penalty limit,then average power factor correction based on a 24 hour basis is sufficient and notan elaborate "instant to instant" p.f. correction. This helps one in setting as wide aband-width as possible before changing a step. It prevents switching - too often.

It must be noted that KVA-Hrs and KVAR-Hrs do not subtract if excessive capacitorKVAR are dumped into the system by over corrections into a leading zone - say partof the time. It records this also as a low p.f. & subject to penalty. Besides, leadingp.f.'s are unhealthy for capacitors & the system itself.

a) A simple straight forward method of sizing the capacitor blocks would be to dividethem equally into targeted number of steps. Besides simplicity it has an advantage of

standard sizes for replacement of work out contractors, blown fuses etc. Many adesigner favour this. b) In ambitious method of sizing the blocks, they are designed in a binary sequenceso that a large number of combinations is available for a given set of contactors etc.If the accessories are chosen properly, this can be an ideal method though slightlycostlier than method (a) above. c) Each controls in an APFC Panel adds considerably to overall costs. It is advisable to keep as much capacitor KVAR out of the APFC control as possible,

for example, the first step i.e. load portion which is constant on a 24 hour basis,Continuous working industries offer this.



In the second step - divide the remainder in a number of steps. Keep this number ofstep as small as possible, by studing the load pattern. The portion that is likely to beoperated often, should be at the fag end. Large size contactors should at the startingend so that they operate as few times as possible.

C) Methods of Switching In & Switching Out :-

1) When the bank is controlled in equal steps, as in B(a) above, some designersprefer a first - in, first - out or FIFO method so that all contactors and steps haveuniform period of operation & can together last longer. 2) If method B(c) above is followed, then the switching control should be on the basisof 'First-in, Last-out' or FILO. 3) Method B.(b) above, calls for random switching which requires careful selection ofpower contactors or better still, opting out for thyristor switching - which has yet to

prove it's mettle in India. D) Structural Design of an APFC Panel :-

1) Capacitor bank step (section) 2) Discharge resistance on individual capacitor unit - external. 3) Incoming switch fuse for the bank.

4) Capacitor bank bus bar.

5) Capacitor bank CT's. 6) Ammeter selector switch. 7) Ammeter for bank current.

8) Thermal overload relay or sectional fuses. 9) Automatic control relay & p.f. meter. Time delay relays. 10) Power contactor. 11) Push Button sets. 12) Indicating lamps.

13) Cabinet (capacitor bus bars) 14) Earthing bus bars. 15) Isolating transformer for contactor coils.



16) Heater. 17) Lamp, extra piano type switches & sockets. 18) Cooling fan. 19) Auto-manual change over switch. E) Rating of Components :-

1) Should an APFC panel develop a 'short' from the main bus to body or betweenphases, a heavy current will flow till the back-up protection - like an HRC fuse,isolates this short.

The system voltage divided by the system impedance up to the point of a short,gives the short circuit current. This impedance consists main by of the step down

transformer impedance - generally 4% to 6%. Increasing this value by 10% takescare of impedances of intervening items like a switch, a bus bar, a C.T. etc. Theshort circuit current divided by 165 Amps gives an acceptable conductor crosssection, which can safely hold for a s.c. duration of one second. Generally, thesesections are not unduly large and fall within a current density of 2.5 to 3 Amps/mm2of the full capacitor bank current rating.

Should the length of this panel be large - then the bus bars must be laterally & rigidlysupported to prevent flexuring under s.c. forces. 2) A capacitor shorted to it's body restricts the s.c. current severely. Depending onthe inside construction and the wall thickness of the capacitor tank, this unit canwithstand the bursting forces till it's protective system takes over. Thus the doubleearthing of a panel can be safely standardised on G.I. strip of 50 x 6 mm2 size. 3) A capacitor at the instance of being switched on, is a dead short circuit. Theinrush current is limited in its peak value by system inductance’s upto that point,except that the circuit now goes into a natural resonance. A power contactor, bynature of its construction and contact material, can withstand a peak current of agiven magnitude - beyond which, the contactor points will weld on to themselves -leading to capacitor failure. If a capacitor is being switched on against other steps which are already on, then theother steps will discharge into this new - comer. The intervening bus bars have verylow inductance’s & these peak currents are very high - reaching 160 times the ratedcapacitor current or more. The capacitor should be able to handle this- withoutwelding.

There are three methods to deal with this :- a) Use a liberal & proven rating for a known contactor.



b) Use surge suppression choke coils on each capacitor, to introduceextra inductance & thus limit the peak current. For panels with 4 stepsor more & also for panels using MPP capacitors, this is essential. c) Use a special contactor with auxilliary contacts which introduce a

starting resistance at the begining, then short it.

4) A discharge resistor on a capacitor reduces the residual voltage on it - after beingswitched off to a safe value of 50 volts within less than a minute and readies it for re-switching should this be required. If this resistance were to burn out, the re-switchingwill take place against a charged unit. This will burn it out. It is highly essential toperiodically check the condition of these externally mounted discharge resistance’s. 5) Other Items : Main switch fuse is substituted by air-breakers for large banks. Draw- out type, electrically operated breakers increase cost of a panel tremoundously. 6) Time Delay Relays : Time Delay Relays with an adjustable one minute delayshould be incorporated - both in APFC or Manual mode to prevent re-switching of a

contactor within less than one minute of switching it off. What can go wrong in an APFC Panel. 1) Wrong connections to the Automatic Relay : The C.T. feeding this relay is the mains CT & not the CT within the panel itself. The

voltage connection to the relay should be from the same phase from which thecurrent is measured. These relays are single phase relays. 2) Too narrow a band-width, per step : The band-width can be set manually. A narrow band width leads to hunting betweensteps. 3) Contactor points welding together. 4) Discharge Resistor &/or choke coils burning out.

5) Time Delay Relays being bipassed or not working. 6) Failure of electronic components under the combined on slought of higherambient temperature and voltage surges - particularly for outdoor pole- mountedtype of panels.

7) Improper ventilation, Loose cable joints & similar causes commonly found. 8) Unattended leaks on capacitors.

Power Elect

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