power factor presentation

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A.C POWERS AND POWER FACTOR

PRESENTATION FLOW

A.C POWERSActive Power:

Measured in watts (normally shown as kW). Provides the “working” part of the power system. Producing heat, light, motion etc.

Reactive Power: Measured in volt-ampere-reactive (normally shown as kVAR). It only

maintains the electromagnetic field and provides no “working” part of the power system.

Apparent Power: Measured in Volt-Ampere (normally shown as kVA). Provided both

working and nonworking parts of the power system.

A.C POWERS

Positive value of power waveform represents the real or working power which is delivered from the source to the load.Negative value of power waveform represents the reactive power which is returned from the load to the source.

From waveforms, 0 to t1, V and I are both positive; therefore, power is positive. At t=t1, V is 0V and thus P is 0W. From t1 to t2, I is positive and V is negative; therefore, P is negative. From t2 to t3, both V and I are negative; therefore power is positive, and so on.

A.C POWERS - Power Comparison -

A.C POWERS

POWER FACTORThe cosine angle between the voltage and current in an a.c circuit is known as power factor.Power factor involves the relationship between two types of power, working power and reactive power.

Poor P.F 0.60 P.F >0.90

POWER FACTOR Electrical energy is almost exclusively generated, transmitted and

distributed in the form of alternating current, therefore the question of power factor comes immediately into picture.

Most loads (80%) in electrical distribution systems are inductive in nature and hence they have low P.F which is highly undesirable.

Low P.F causes an increase in reactive current, resulting in additional losses of active power in all elements of power system.

It is important to have power factor as close to unity as possible. Note that leading power factor is also undesirable for power system.

POWER FACTORIf the circuit is inductive, the current lags behind the voltage by an angle Ѳ and power factor is referred to as lagging.

POWER FACTORIf the circuit is capacitive, the current leads the voltage by an angle Ѳ and power factor is referred to as leading.

POWER FACTORIf circuit is resistive, both current and voltage are in phase with each other and power factor is referred as unity.

POWER FACTOR -Power Triangle-

Apparent power in a.c circuit has two components, active or working power and reactive power.From trigonometric relation,

Thus the power factor of a circuit may also be defined as the ratio of active power to the apparent power.For leading currents, the power triangle becomes reversed.

POWER FACTOR -Power Triangle-Now the power factor can be defined in one of the following 3 ways:

1. Power factor = cosѲ = cosine angle between V and I.2. Power factor = cosѲ = Resistance/Impedance.3. Power factor = cosѲ = Active power/Apparent power.

Let us assume a circuit having current of 10 A at a voltage of 200 V and its power factor is 0.8 lagging.

S = VI = 200(10) = 2000VA = 2KVA P = VI cosѲ = 200(10)(0.8) = 1.6KW

Q = VI sinѲ = 200(10)(0.6) = 1.2kVARThe circuit receives an apparent power of 2KVA and is able to convert only 1.6KW into working or active power. Reactive power 1.2KVAR neither consumed nor it does any useful work, it flows back and forth in both direction periodically.

POWER FACTORDifferent types of electrical load have different Power Factors according to its nature.

Name of Equipment Name of Equipment Power Factor PercentPower Factor Percent

Lightly loaded induction motor 0.20 LaggingFull Loaded induction motor 0.80 Lagging

Neon-lighting equipment 0.30 - 0.70 Lagging Incandescent lamps 1.0 Unity

All types of resistance heating devices (e.g. toaster, heater)

1.0 Unity

CAUSES OF LOW POWER FACTOR MOST LOADS (80%) IN ELECTRICAL DISTRIBUTION SYSTEM ARE INDUCTIVE INMOST LOADS (80%) IN ELECTRICAL DISTRIBUTION SYSTEM ARE INDUCTIVE IN

NATURE. THE CAUSE OF LOW POWER FACTOR IS ONLY DUE TO INDUCTIVENATURE. THE CAUSE OF LOW POWER FACTOR IS ONLY DUE TO INDUCTIVELOADS. INDUCTIVE LOADS RESPONSIBLE FOR LOW P.F ARE AS FOLLOWS:LOADS. INDUCTIVE LOADS RESPONSIBLE FOR LOW P.F ARE AS FOLLOWS:

Most of ac motors are induction type motors (1Φ and 3 Φ). At light load these motors work at extremely low P.F of the order of 0.2 to 0.3 and rises to 0.8 or 0.9 at full load.

Arc lamps, electric discharge lamps, welding equipment and industrial heating furnaces operated at low lagging power factor.

Transformers draws a magnetizing current from the supply. At normal load this current does not affect the P.F, but they draws larger amount of magnetizing current at light load, causes low primary current P.F.

DE’MERITS OF LOW POWER FACTORLOW POWER FACTOR RESULTS IN THE FOLLOWING DISADVANTAGESLOW POWER FACTOR RESULTS IN THE FOLLOWING DISADVANTAGES

LARGER KVA RATING: LARGER KVA RATING: Electric machines (alternators, transformers, switchgears) are always rated in KVA. KVA rating of any equipment varies inversely with the P.F, therefore low P.F increases the KVA rating, as a result KVA rating making the equipment larger and expensive.

GREATER CONDUCTOR SIZE: GREATER CONDUCTOR SIZE: To transmit a fixed amount of power at constant voltage, the conductor will have to carry more current at low factor.

DE’MERITS OF POWER FACTORLARGE COPPER LOSSES: LARGE COPPER LOSSES: The larger amount of current at low power factor causes more I R losses in all the elements of the system.

POOR VOLTAGE REGULATION: POOR VOLTAGE REGULATION: Larger amount of current produces large voltage drops (IR drops) in alternators, transformers, transmission lines and distributors. This results in poor voltage regulation.

PAYED EXTRA UTILITY CHARGES: PAYED EXTRA UTILITY CHARGES: System operated at low P.F is uneconomical from supplier`s point of view. Suppliers usually takes extra charges from consumers who operated their load at low power factor.

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POWER FACTOR CORRECTIONThe process of supplying reactive power (lagging or leading) to bring the P.F closer to unity is known as power factor correction.

This can be done by following equipments, should be connected in parallel with the load.

POWER FACTOR CORRECTION -STATIC CAPACITORS-The capacitor draws a leading current and neutralizes the lagging reactive component of the load current, this raises the power factor.Correct amount of capacitance must be used, otherwise P.F goes from lagging to leading which is also undesired condition.

POWER FACTOR CORRECTION -STATIC CAPACITORS-

When inductor released the energy, capacitor stored that energy as shown.

Assume a circuit shown in fig: A in which pure resistive load of 120KW is connected with 600V generator, supplying 200A as shown.

POWER FACTOR CORRECTION -STATIC CAPACITORS-

Now lagging inductive load is added as shown in fig: B with the same 600V generator. In this case P.F drops from unity to 0.6 lagging.

Even though it is supplying same 120KW to the load, but current exceeds from 200A to 333A. This extra current is known as reactive current, causing decrease in P.F from unity to 0.6.

Now we are determining the value of capacitor, placed in parallel with the load to raise the P.F = 1

POWER FACTOR CORRECTION -STATIC CAPACITORS-

An over-excited synchronous motor running on no-load is known as Synchronous Condenser.

Increasing the field excitation of synchronous motor will improve the P.F from lagging to leading by operating in over excitation mode.

When synchronous machine is connected in parallel with the supply, it takes the leading current which partly neutralizes the lagging reactive component of the load.

POWER FACTOR CORRECTION –SYNCH CONDENSER-