POWER FACTOR CORRECTION

POWER FACTOR CORRECTION Components & Systems August 2002 Table of Contents What is Power Factor 1 ϕ mean What does Cosϕ 1 Disadvantages of Low...
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POWER FACTOR CORRECTION

Components & Systems

August 2002

Table of Contents What is Power Factor

1

ϕ mean What does Cosϕ

1

Disadvantages of Low Power Factor

1

Improving Power Factor

1

Power Factor Correction using Capacitors

2

Centralised Compensation

2

Substantiating Power Factor Costs

2

Calculating Capacitor Requirements

2

Power Factor Components

3

- Capacitors

4-7

- Contactors

4-5

- Fuse Protection

5

- Isolating

6

- Reactive Control Relay

7

Series 4000 Rack System

8-10

Rack System Components

8-10

- Capacitors

9

- Contactors

9

- Fuse Protection

9

- Busbars

9

Series 5000 Power Factor Systems

10

Harmonics

11

What are Harmonics?

11

Series 6000 Harmonic Racks

11

Series 7000 Harmonic Systems

12

Power Factor Correction What is power factor correction?

Disadvantages of Low Power Factor

Power factor is simply a name given to the ratio of “actual” power (active power) being used in a circuit, expressed in watts or more commonly kilowatts (kW), to the power which is “apparently” being drawn from the mains, expressed in volt-ampere or more commonly kilo volt-ampere (kVA).

1. Increased authorities cost since more current has to be transmitted, and this cost is directly billed to consumers on maximum demand kVA systems.

P.F.

2. Causes overloaded generators, transformers and distribution lines within a plant, resulting in greater voltage drops and power losses, all representing waste, inefficiency and needless wear and tear on industrial electrical equipment.

Active Power (kW) Apparent Power (kVA)

=

All modern industries utilise electrical energy in some form or other. Two basic categories of load are encountered in alternate current (AC) networks.

3. Reduces load handling capability of the plants electrical system. Most electrical supply authorities have changed to kVA demand systems from the inefficient kW demand system. Consumers are now billed and penalised for their inefficient systems according to the apparent power being used. In future, consumers will be penalised for plants with power factor below a pre-determined value.

1. Resistive Loads Devices containing only resistance e.g. incandescent lamps, heaters, soldering irons, ovens, etc. The current drawn from the supply is directly converted into heat or light. Since the voltage is assumed to be constant, the actual power (kW) being used is identical to the apparent power (kVA) being drawn from the line. The power factor is therefore unity or 1. In these purely resistive circuits, the current and voltage sinewave peaks occur simultaneously and are said to be “in phase”.

Improving Power Factor The most practical and economical power factor improvement device is the capacitor. As stated previously, all inductive loads produce inductive reactive power (lagging by a phase angle of 90°). Capacitors on the other hand produce capacitive reactive power, which is the exact opposite of inductive reactive power. In this instance, the current peak occurs before the voltage peak, leading by a phase angle of 90°. By careful selection of capacitance required, it is possible totally cancel out the inductive reactive power when placed in circuit together.

2. Inductive Loads All motors and transformers depend on magnetism as the basis of their operation. Magnetism is a force and in the physical sense is not consumed. In AC motors and transformers, magnetic forces are only required periodically. Consequently, a permanent magnet cannot be used and the necessary magnetism is produced by electrical means. The electrical current needed for this purpose is not fully utilised. Having produced the magnetic force, the current flows back to the power station again. This current is called the reactive current in contrast to the active current which performs work and is fully utilised in so doing. Although the reactive current is not utilised, it imposes a load on the electrical distribution system and supply authorities demand payment for this load according to specific tariffs.

kW ϕ1

KVA2

KV A1

The current drawn from the supply is made up of two separate kinds of current “power producing current” and “magnetising current”. Therefore the current flowing in an AC circuit (unless corrected) is generally larger than is necessary to supply the power being by the expended point.

To prevent the continual flow of reactive current back and forth between the load and power station, a capacitor, which is in effect a reactive current storage device, is connected in parallel with the load. The reactive current supplied by the power station and used for the magnetic force when the load is switched on does not now return to the power station but instead flows into the capacitor and merely circulates between the latter and the load. Consequently the distribution lines from the power station are relieved of the reactive current.

Reactive power and active power flow through the motor or transformer. Geometrical calculation of these two powers yield the apparent power. The ratio of the active and apparent power is denoted by cosϕ and indicates what fraction of apparent power flowing is actually used by the motor. Active Power (kW)

Capacitors can therefore be utilised to reduce kVA and electrical costs. Improved power factor results in:

ϕ App

aren

Inductive Reactive Power (kVAr)

t Po

wer

Capacitor kVAr Required

Cosϕ1 is the kVA used before Power Factor Improvement equipment was added to the network. Cosϕ2 is the kVA used after Power Factor improvement equipment was added to the network.

ϕ mean? What does Cosϕ

Fig. 1

ϕ2

1. Reduced kVA charges

(kVA

)

2. Improved plant efficiency 3. Additional loads can be added to the system

As can be seen from Fig. 1, the apparent power is greater than the active power and hence the power factor is a value considerably less than unity.

4. Reduced overloading of cables, transformers, switchgear, etc. 5. Improved starting torque of motors

P.F.

=

Reactive Power (kW) Apparent Power (kVA)

=

ϕ Cosϕ

6. Reduce fuel requirements to generate power due to lower losses. 1 Contents are illustrative only - final details must be checked when placing orders

Power Factor Correction Centralised Compensation

Power Factor Correction using Capacitors

(Automatic Power Factor Correction) Two methods of improving power factor using capacitors are: In large industrial plants where many motors are generally in use or, when the main reason for power factor is to obtain lower electricity bills, then centralised compensation is far more practical and economical than individual motor compensation.

a) Individual motor compensation (static capacitors) b) Centralised compensation (automatic capacitor banks)

In this instance, large banks or racks of capacitors are installed at the main incoming distribution boards of the plant and are sub-divided into steps which are automatically switched in or out depending on specific load requirements by means of an automatic control system, improving the overall power factor of the network.

Individual Motor Compensation Most effective correction is obtained by connecting individual capacitors directly to the terminals of each motor. The motor and capacitor can be controlled jointly by the motor switchgear. The capacitor rating should be matched as closely as possible so that the power factor of the entire plant can be corrected to the optimum value, irrespective of the number of motors switched on.

Utilising Table 4 for calculating capacitor kVAr size requirements for power factor correction. The following information must be known beforehand.

The size of capacitor required may be determined from Table 3 by taking the motor kW and speed into consideration. Table 3 is a guide only and no guarantee of correct power factor. The correct method of maximum capacitor rating can be determined by using the following formula:

where

Qc

=

0.9Io V√3

Io Qc

= =

motor magnetising current capacitor power in VAr

a) The average plant power factor b) The maximum running load of the plant in kW To avoid ferro-resistance and dangerous voltage rises, the total kVAr required should never exceed 65% of incoming transformer kVA. In practice, to be absolutely safe, this limit should be set at approx. 50%. Generally an automatic power factor system consist of: a) b) c) d)

If the magnetising current is not known, 95% of the motor no-load current can be used as an approximate value. Care should be taken not to exceed the value calculated to avoid dangerous overvoltages and possible self excitation of motors at switch-off.

a main load-break isolator (or circuit breaker) an automatic reactive control relay power factor capacitors backed by suitable fuse protection suitably rated contactors for capacitor switching

The automatic reactive control relay monitors the total network and will switch-in the required capacitor banks at pre-determined intervals compensating for capacitor discharge times and load dependant requirements.

Over compensation can cause higher supply voltages which can cause consequent break down of motor insulation and flashover at motor terminals. To be safe, rather use standard capacitor sizes (as indicated below). For this reason, individual motor compensation is not recommended for motors which are rapidly reversed e.g. cranes, hoists, etc.

As capacitor switching subjects components to exceptionally high stresses it is imperative to correctly size and rate all components utilised in a system.

Table 3 Substantiating Power Factor Correction Costs

Individual Capacitor Rating in kVAr to improve Power Factor to 0.95 or better at all loads. Motor Rating kW

2 Pole 3000 rpm

4 Pole 1500 rpm

This question can best be answered by an example. Assuming a plant has a total load of 500 kW and a power factor (cosϕ) of say 0.75 lagging. Supply authorities kVA demand charge is approximately R40.00 per kVA (actually above R50.00 in most areas. Johannesburg is currently R53.10).

6 Pole 1000 rpm

0.75

0.5 kVAr

0.5 kVAr

0.5 kVAr

1.1

0.5 kVAr

0.5 kVAr

1.0 kVAr

1.5

0.5 kVAr

1.0 kVAr

1.0 kVAr

2.2

1.0 kVAr

1.0 kVAr

1.5 kVAr

4.0

1.5 kVAr

1.5 kVAr

2.0 kVAr

5.5

2.0 kVAr

2.0 kVAr

3.0 kVAr

7.5

2.0 kVAr

2.0 kVAr

3.0 kVAr

11.0

3.0 kVAr

4.0 kVAr

5.0 kVAr

15

4.0 kVAr

5.0 kVAr

6.0 kVAr

18.5

5.0 kVAr

7.0 kVAr

8.0 kVAr

22

6.0 kVAr

8.0 kVAr

9.0 kVAr

30

8.0 kVAr

10.0 kVAr

12.0 kVAr

37

10.0 kVAr

12.0 kVAr

14.0 kVAr

45

12.0 kVAr

14.0 kVAr

16.0 kVAr

55

16.0 kVAr

22.0 kVAr

25.0 kVAr

75

18.0 kVAr

25.0 kVAr

30.0 kVAr

90

20.0 kVAr

30.0 kVAr

35.0 kVAr

110

25.0 kVAr

30.0 kVAr

40.0 kVAr

132

35.0 kVAr

40.0 kVAr

40.0 kVAr

160

40.0 kVAr

45.0 kVAr

50.0 kVAr

kW PF

=

kVA

500 kW 0.75 PF

=

666 kVA

Total costs @ R40.00/kVA = R26,640.00/month By installing capacitors to improve power factor (cosϕ) to 0.98 lagging new costs are; 500 kW 0.98 PF

=

510 kVA

Total costs @ R40.00/kVA = R20,400.00/month therefore savings monthly = R6,240.00 A complete system required to effect power factor from 0.75 to 0.98 (as in above example) would require a system of 360 kVAr which would currently cost approximately R43,000.00 Power factor correction usually pays for itself well within 12 months of the initial purchase (7 months in above example ) and continues saving indefinitely. It therefore stands to reason that more significant savings can be anticipated with the ever increasing escalation costs of electricity in the future. 2 Contents are illustrative only - final details must be checked when placing orders

Power Factor Correction Table 4

Calculating Capacitor Requirements It is imperative that correct capacitor sizes be selected when calculating capacitor requirements. In the case of centralised compensation, it is recommended that the first capacitor step be equal to half the value of the following steps, to allow a smooth overall linear correction system.

Existing PF Cosϕ

Target Power Factor Required Cosϕ

Before applying capacitors

0.80

0.85

0.90

0.92

0.95

0.98

1.0

0.40

1.54

1.67

1.81

1.87

1.96

2.09

2.29

Table 4 (right) will assist in calculating capacitor values in specific applications.

0.42

1.41

1.54

1.68

1.73

1.83

1.96

2.16

0.44

1.29

1.42

1.56

1.61

1.71

1.84

2.04

Prior knowledge of the following is required:

0.46

1.18

1.31

1.45

1.50

1.60

1.73

1.93

0.48

1.08

1.21

1.34

1.40

1.50

1.60

1.83

0.50

0.98

1.11

1.25

1.31

1.40

1.53

1.73

0.52

0.89

1.02

1.16

1.22

1.31

1.44

1.64

0.54

0.81

0.94

1.07

1.13

1.23

1.36

1.56

0.56

0.73

0.86

1.00

1.05

1.15

1.28

1.48

0.58

0.65

0.78

0.92

0.98

1.08

1.20

1.40

a) Power factor before applying capacitors (left vertical column) b) Required power factor (top horizontal row) c) Total consumption in kW The correct capacitor size can be calculated by multiplying the factor when crossing the horizontal and vertical columns in the table below by kW.

0.60

0.58

0.71

0.85

0.91

1.00

1.13

1.33

Example:

0.61

0.55

0.68

0.81

0.87

0.97

1.10

1.30

1. Convert the plant load to kW (kVA x PF = kW) 666 kVA x 0.75 Pf = 500 kW (useful power)

0.62

0.52

0.65

0.78

0.84

0.94

1.06

1.27

0.63

0.48

0.61

0.75

0.81

0.90

1.03

1.23

2. To correct a load of 500 kW at 0.75 PF to 0.98 PF. Follow the 0.75 value (in left vertical column) horizontally until below the 0.98 value (in top horizontal row). The factor value is 0.68.

0.64

0.45

0.58

0.72

0.77

0.87

1.00

1.20

0.65

0.42

0.55

0.68

0.74

0.84

0.97

1.17

0.66

0.39

0.52

0.65

0.71

0.81

0.94

1.14

0.67

0.36

0.49

0.63

0.68

0.78

0.90

1.11

0.68

0.33

0.46

0.59

0.65

0.75

0.88

1.08

0.69

0.30

0.43

0.56

0.62

0.72

0.85

1.05

0.70

0.27

0.40

0.54

0.59

0.69

0.82

1.02

0.71

0.24

0.37

0.51

0.57

0.66

0.79

0.99

0.72

0.21

0.34

0.48

0.54

0.64

0.76

0.96

0.73

0.19

0.32

0.45

0.51

0.61

0.73

0.94

0.74

0.16

0.29

0.42

0.48

0.58

0.71

0.91

0.75

0.13

0.26

0.40

0.46

0.55

0.68

0.88

0.76

0.11

0.24

0.37

0.43

0.53

0.65

0.86

0.77

0.08

0.21

0.34

0.40

0.50

0.63

0.83

0.78

0.05

0.18

0.32

0.38

0.47

0.60

0.80

0.79

0.03

0.16

0.29

0.35

0.45

0.57

0.78

0.80

0.13

0.27

0.32

0.42

0.55

0.75

0.81

0.10

0.24

0.30

0.40

0.52

0.72

0.82

0.08

0.21

0.27

0.37

0.49

0.70

0.83

0.05

0.19

0.25

0.34

0.47

0.67

0.84

0.03

3. Capacitor required to correct from 0.75 to 0.98 (power x capacitor from the table value) 500 kW x 0.68 = 340 kVAr savings:

500 kW @ 0.75 PF 500 kW @ 0.98 Pf Reduction of

= =

666 kVA 510 kVA 156 kVA

(23.4% less of transformer load)

0.16

0.22

0.32

0.44

0.65

0.14

0.19

0.29

0.42

0.62

0.86

0.11

0.17

0.26

0.39

0.59

0.87

0.08

0.14

0.24

0.36

0.57

0.88

0.06

0.11

0.21

0.34

0.54

0.89

0.03

0.09

0.18

0.31

0.51

0.90

0.06

0.16

0.28

0.48

0.91

0.03

0.85

0.13

0.25

0.46

0.92

0.10

0.22

0.43

0.93

0.07

0.19

0.40

0.94

0.03

0.16

0.36

0.13

0.33

0.95 3

Contents are illustrative only - final details must be checked when placing orders

Power Factor Components Power Factor Specifications

Components,

Systems

3 Phase Monobloc Capacitors 440V “H” Type 440V, 50 Hz, 3 Phase (Maximum Voltage 520V - 50Hz)

and

Electromechanica have been involved in all aspects of power factor correction in our specific environment for some 20 years now, and it is with this experience gained, that we can confidently recommend, specific components required in power factor correction, and complete ready to connect automatic power factor systems.

Alpivar heavy duty capacitors are totally dry units with no impregnation or insulation liquid.

Electromechanica offers a complete range of components and equipment for power factor correction viz. a) b) c) d) e)

Power factor capacitors 400-550 volt Contactors specifically for capacitor switching Fuse protection components for capacitor banks Automatic reactive control relays Special rack systems incorporating capacitors, contactors, fuse protection and busbars f) Complete power factor systems 30-1500 kVAr

Alpivar monoblock capacitors utilise windings insulated by a selfextinguishing casing, immersed in thermosetting polyurethane resin applied under vacuum with excellent heat dissipation qualities (measured internal temperature is