Calculation of kVAr and Basics of Protection Systems

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3.2 CALCULATION OF KVAR REQUIRED

In electrical installations the operating load kW and its average power factor (PF) can be ascertained from the electricity bill. Alternatively, it can also be calculated from the formula,

Average PF = kWh/kVAhOperating load kW = kVA demand x Average PF

The average PF is considered as the initial PF and the final PF can be suitable assumed as required. In such cases required capacitor kVAr can be calculated as shown in the following example:

Calculate the required kVAr compensation for 500 kW installations to improve the PF from 0.75 to 0.96.kVAr= kW x multiplying factor from table .=500 x 0.59=295 kVAr

The following table on the next page is based on the following formula,

kVAr required = kW (tan 1 tan 2)Where, 1 = cos-1(PF1) and 2 = cos-1(PF2)PF1 and PF2 are initial and final PF respectively

Initial PFFinal PF

0.90.910.920.930.940.950.960.970.980.99

0.41.8071.8361.8651.8961.9281.96322.0412.0882.149

0.421.6761.7051.7351.7661.7981.8321.8691.911.9582.018

0.441.5571.5851.6151.6461.6781.7121.7491.791.8381.898

0.451.51.5291.5591.5891.6221.6561.6931.7341.7811.842

0.461.4461.4751.5041.5351.5671.6021.6391.681.7271.788

0.481.3431.3721.4021.4321.4651.4991.5361.5771.6251.685

0.51.2481.2761.3061.3371.3691.4031.441.4811.5291.59

0.521.1581.1871.2171.2471.281.3141.3511.3921.441.5

0.541.0741.1031.1131.1631.1961.231.2671.3081.3561.416

0.551.0341.0631.0921.1231.1561.191.2271.2681.3151.376

0.560.9951.0241.0531.0841.1161.1511.1881.2291.2761.337

0.580.920.9490.9791.0091.0421.0761.1131.1541.2011.262

0.60.8490.8780.9070.9380.971.0051.0421.0831.131.191

0.620.7810.810.8390.870.9030.9370.9741.0151.0621.123

0.640.7160.7450.7750.8050.8380.8720.9090.950.9981.058

0.650.6850.7140.7430.7740.8060.840.8770.9190.9661.027

0.660.6540.6830.7120.7430.7750.810.8470.8880.9350.996

0.680.5940.6230.6520.6830.7150.750.7870.8280.8750.936

0.70.5360.5650.5940.6250.6570.6920.7290.770.8170.878

0.720.480.5080.5380.5690.6010.6350.6720.7130.7610.821

0.740.4250.4530.4830.5140.5460.580.6170.6580.7060.766

0.750.3980.4260.4560.4870.5190.5530.590.6310.6790.739

0.760.3710.40.4290.460.4920.5260.5630.6050.6520.713

0.780.3180.3470.3760.4070.4390.4740.5110.5520.5990.66

0.80.2660.2940.3240.3550.3870.4210.4580.4990.5470.608

0.820.2140.2420.2720.3030.3350.3690.4060.4470.4950.556

0.840.1620.190.220.2510.2830.3170.3540.3950.4430.503

0.850.1350.1640.1940.2250.2570.2910.3280.3690.4170.477

Table 3.1: Multiplying factors for kVAr calculations

Notes:1. It is considered uneconomical in industrial applications to improve power factor by individual compensation for motor ratings below 15 HP.2. For motor ratings above 250 HP, the capacitor kVAr rating would be about 25% of the motor rating in HP.3. In all cases it should be ensured that the capacitor current at rated voltage is always less than 90% of the no load current of the motor. This is due to the fact that when capacitor current exceeds the no load magnetizing current of the motor, excessive voltage surges can occur due to self excitation in the event of an interruption in power supply which will prove harmful to both the motor as well as the capacitor.4. The capacitor kVAr values indicated in the above table are after taking into consideration the condition specified in the point 3 above and assuming motor loading of greater than 80%.5. If the motor is loaded less than 80% the capacitor kVAr required may be greater than the values indicated in the above table. In such cases the capacitor should be connected upstream in group or central compensation mode.

Capacitor kVAr for Transformer

Power and distribution transformers, which work on the principle of electromagnetic induction, consume reactive power for their own needs even when its secondary is not connected to any load. The power factor will be very low under such a situation.

To improve the power factor it is required to connect a fixed capacitor or a capacitor bank at the LT side of the transformer. The approximate kVAr of capacitors required are given in the table below:-

S. No.kVA rating of the transformerkVAr required for compensation

1.Upto 315 kVA5% of kVA rating

2.315 kVA 1000 kVA6% of kVA rating

3.Above 1000 kVA8% of kVA rating

Table 3.2: Approximate kVAr of capacitors required

3.2.1 Objective of Power System Protection

The objective ofpower system protectionis to isolate a faulty section ofelectrical power systemfrom rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.

Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system. For satisfying this quick disconnection the protection relays should have following functional requirements.

3.2.2 Protection System in Power System

Lets have a discussion on basic concept ofProtection system in power systemand coordination ofprotection relays.

Fig 3.13: Basic Connection Diagram of Protection RelayIn the picture the basic connection ofprotection relay has been shown. It is quite simple. The secondary of current transformer is connected to the current coil of relay. And secondary of voltage transformer is connected to the voltage coil of the relay. Whenever any fault occurs in the feeder circuit, proportionate secondary current of the CT will flow through the current coil of the relay due to which mmf of that coil is increased. This increased mmf is sufficient to mechanically close the normally open contact of the relay. This relay contact actually closes and completes the DC trip coil circuit and hence the trip coil is energized. The mmf of the trip coil initiates the mechanical movement of the tripping mechanism of the circuit breaker and ultimately the circuit breaker is tripped to isolate the fault.

3.2.3 The Functional Requirements of Protection Relay

ReliabilityThe most important requisite of protective relay is reliability.They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.

SelectivityThe relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.

SensitivityThe relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.

SpeedThe protective relays must operate at the required speed. There must be a correct coordination provided in various power system protection relays in such a way that for fault at one portion of the system should not disturb other healthy portion. Fault current may flow through a part of healthy portion since they are electrically connected but relays associated with that healthy portion should not be operatedfaster than the relays of faulty portion otherwise undesired interruption of healthy system may occur. Again if relay associated with faulty portion is not operated in proper time due to any defect in it or other reason, then only the next relay associated with the healthy portion of the system must be operated to isolate the fault. Hence it should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation.

3.2.4 Important Elements for Power System Protection

Switch gearIt consists of mainly Bulk oil Circuit breaker, Minimum oil Circuit breaker, SF6 Circuit breaker, Air Blast Circuit breaker and Vacuum Circuit breaker etc. Different operating mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are employed in Circuit Breaker. Circuit Breaker is the main part of protection system in power system it automatically isolate the faulty portion of the system by opening its contacts.

Protective gearConsists of mainly power system protection relays like current relays, voltage relays, impedance relays, power relays, frequency relays, etc. based on operating parameter, definite time relays, inverse time relays, stepped relays etc. as per operating characteristic, logic wise such as differential relays, over fluxing relays etc. During fault the protection relay gives trip signal to the associated circuit breaker for opening its contacts.

Station BatteryAll the circuit breakers of electrical power system are DC (Direct Current) operated. Because DC power can be stored in battery and if situation comes when total failure of incoming power occurs, still the circuit breakers can be operated for restoring the situation by the power of storage battery. Hence the battery is another essential item of the power system. Some time it is referred as the heart of the electrical substation. A Substation battery or simply a Station battery containing a number of cells accumulate energy during the period of availability of A.C supply and discharge at the time when relays operate so that relevant circuit breaker is tripped.

3.2.5 Definition of Switchgear Aswitchgearorelectrical switchgear is a generic term which includes all the switching devices associated with mainly power system protection. It also includes all devices associated with control, metering and regulating ofelectrical power system. Assembly of such devices in a logical manner forms a switchgear. This is very basic definition of switchgear.

3.2.6 Switchgear and Protection

Fig. 3.14: Switchgears at Substations

We all familiar with low voltage switches and re-wirable fuses at our home. The switch is used to manually open and close the electrical circuit in our home and electrical fuseis used to protect our household electrical circuit from over current and short circuit faults. In same way every electrical circuit including high voltageelectrical power systemneeds switching and protective devices. But in high voltage and extra high voltage system, these switching and protective schemes becomes complicated one for high fault current interruption in safe and secure way. In addition to that from commercial point of view everyelectrical power systemneeds measuring, control and regulating arrangement. Collectively the whole system is calledSwitchgear and Protectionof power system. Theelectrical switchgearshave been developing in various forms.

Switchgear protectionplays a vital role in modern power system network, right from generation through transmission to distribution end. The current interruption device or switching device is called circuit breaker in Switchgear protectionsystem. The circuit breaker can be operated manually as when required and it is also operated during over current and short circuit or any other faults in the system by sensing the abnormality of system. The circuit breaker senses the faulty condition of system through protection relay and this relay is again actuated by faulty signal normally comes fromcurrent transformerorvoltage transformer.

Fig. 3.15: 123kV Switchgear Bay

A switchgear has to perform the function of carrying, making and breaking the normal load current like a switch and it has to perform the function of clearing the fault in addition to that it also has provision of metering and regulating the various parameters of electrical power system. Thus the circuit breaker includes circuit breaker,current transformer,voltage transformer, protection relay, measuring instrument, electrical switch, electrical, miniature circuit breaker, lightening arrestor or surge arrestor, isolator and other associated equipment.

Electric switchgear is necessary at every switching point in theelectrical power system. There are various voltage levels and hence various fault levels between the generating stations and load centres. Therefore various types of switchgear assembly are required depending upon different voltage levels of the system. Besides the power system network, electrical switchgear is also required in industrial works, industrial projects, domestic and commercial buildings.

3.2.7 What is a Circuit Breaker?

Definition of Circuit BreakerElectrical Circuit Breakeris a switching device which can be operated manually as well as automatically for controlling and protection ofelectrical power systemrespectively. As the modern power system deals with huge currents, the special attention should be given during designing ofcircuit breakerto safe interruption of arc produced during theoperation of circuit breaker. This was the basicdefinition of circuit breaker.

3.2.7.1 Introduction to Circuit Breaker

The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of fault current in them which may damage the equipment and networks permanently. For saving these equipments and the power networks the fault current should be cleared from the system as quickly as possible. Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under huge current carrying condition.

During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench these arcs in safe manner. Thecircuit breakeris the special device which does all the required switching operations during current carrying condition. This was the basicintroduction to circuit breaker.

3.2.7.2 Requirements of a Circuit Breaker

The power associated with circuit breaker is large and it forms a link between the consumers and the suppliers.

The necessary requirements of a circuit breaker is as follows,

1. The normal working current and the short circuit current must be safely interrupted by the breaker.2. The faulty section of the system must be isolated by the circuit breaker as quickly as possible and with the minimum delay.3. It should not operate with the flow of overcurrents during healthy conditions.4. The faulty current should only be isolated without affecting the healthy one.

3.2.7.3 Working Principle of Circuit Breaker

The circuit breaker mainly consists of fixed contacts and moving contacts. In normal on condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement stored potential energy in theoperating mechanism of circuit breakerwhich is realized if switching signal given to the breaker. The potential energy can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of potential energy, it must be released during operation. Release of potential energy makes sliding of the moving contact at extremely fast manner. All circuit breaker have operating coils (tripping coils and close coil). Whenever these coils are energized by switching pulse, the plunger inside them displaced. This operating coil plunger is typically attached to theoperating mechanism of circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic energy, which makes the moving contact to move as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism. After a cycle ofoperation of circuit breakerthe total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor or by any other means. Till now we have discussed about mechanicalworking principle of circuit breaker. But there are electrical characteristics of a circuit breaker which also should be considered in this discussion ofoperation of circuit breaker.

Lets have a discussion on electricalprinciple of circuit breaker.

The circuit breaker has to carry large rated or fault power. Due to this large power there is always dangerously high arcing between moving contacts and fixed contact during operation of circuit breaker. Again as we discussed earlier thearc in circuit breakercan be quenching safely if the dielectric strength between the current carrying contacts of circuit breaker increases rapidly during every current zero crossing of the alternating current. The dielectric strength of the media in between contacts can be increased in numbers of ways, like by compressing the ionized arcing media since compressing accelerates the deionization process of the media, by cooling the arcing media since cooling increase the resistance of arcing path or by replacing the ionized arcing media by fresh gasses. Hence a numbers of arc quenching processes should be involved.