Importance of Power factor In the Electrical System & Some of the effects of the leading power factor.

In AC system we consider Three types of power

S - Apparent Power – Rating of Source - VA, KVA, MVA

P – Active Power – Average Power - W, KW, MW

Q – Reactive Power - Play Power – VAR, KVAR, MVAR

The relationship between those Powers brings the Power Factor, The power factor depends on the relative Phase of Voltage and Current.

There are different combination of loads are avaialble in industries i.e Resistive (Incandecent Lamps , Heating Elements..etc) Inductive ( induction Motors, fans, pumps,solenoids, relays..etc.,) & Capacitive (Synchronous Motors ,Capacitor Banks, Drives, UPS Systems ...etc.,).

Loads which obsorb the average power is called active power (P) & Play with source using some other power called reactive Power(Q).

Power Factor of a Transformer :

Since the Power factor of a transformer depends up on load, If it is a resistive load then the power factor is unity, if the load is inductive then the Power factor is lag, If the load is Capacitive then the power factor is Capacitive.

Transformer load magnitude and nature has a bearing on its power factor. It is lowest at no load, when current is mainly magnetizing in nature.

No Load Current of transformer:

Even on no load,a small amount of current is drawn from the primary side,to set up the required magnetic flux in the magnetic core. This current is known as the "No Load Current" The no load current is about 3-5% of the full load current and it accounts for the losses in a transformer.

When Transformer said to be on no load, if the secondary current is zero then the Primar current should be zero too. However when transformer on no load, excitation current flows in the primary because of core losses and Magnetic flux in core. This situation clearly indicates that there is no path available for the current to flow in the secondary side . And if there is no current flowing in the secondary side , there is no de-magnetising flux generated that means there is no need to draw more current from the source . So primary current would contain only the exciting current (i.e. 'NO LOAD CURRENT'). which is highly lagging due to inductance effect of core so the angle between the current and voltage increases thus the cosine value of the angle decreases and THAT MAKES POWER FACTOR VERY POOR AT NO LOAD CONDITION.

In case there is some loading in the transformer secondary winding, some more current will be drawn from the source to compensate the demagnetizing effect of secondary current . So now net primary current would contain 'NO LOAD CURRENT' + 'EXTRA COMPENSATING CURRENT'. That would shift the net current phasor more towards the voltage axis and would result in the increment of the power factor. We can have extra compensation at no load also by adding of fixed Capacitor bank and Variable Capacitors in Light load and full load condition to achieve unity power factor in all load conditions.

Effect of Leading Power Factor :

Current is generally lagged than voltage for phase at the larg load because most of the industry loads are combination of linear loads (Motors,Pumps, Solenoid Coils, Relays..etc,) and non linear loads ( Rectifiers, Variable Frequency Drives, SMPS,UPS Systems, Electronics cards…etc.,). But in case of light or no load, current is leaded than voltage for phase because of line capacitance and adding of extra capacitors for power factor Improvement.

1.Increasing of system voltage :

Leading reactive current which is on capacitor for Power factor Improvement and cables in light loads makes increase in system voltage.

Voltage drop is showed as V = I ( R cos θ + X sin θ ) is by increasing △ of leading capacitance, When R cos θ < X sin θ (Power factor angle is (-) value by leading power factor) is, voltage rise will be increased.

2.Increasing of Transformer Power Loss :

Transformer Power Loss will be increased with serious heating by current increasing the same as line power loss. Voltage rise by leading power factor makes eddy current loss and hysteresis loss of transformer to increase and at the same time makes capacitor for power factor improvement to get out of order and makes varieties equipments to give an electrical stress, More losses always lead to less efficiency and its decreasing the capacity of transformers.

3.Increase of Line Power Loss:

Line Power loss is square of line current x line resistor and if power factor make to lead with un necessary capacitor installation power loss will be increased by current increasing same as lagging power factor.

In case of non-linear loads, the term distortion power factor (DPF) is introduced because of the harmonics generated by these loads. Its the ratio of fundamental rms current to the total rms current. ie., DPF = Ifrms/Irms. It can also be termed in terms of THD as Total Harmonic Distortion.

Another term called displacement power factor (Pfd) which is the ratio of active power P to the apparent power S. True power factor PF = PFd * DPF.

Now that we know these things, it is actually dangerous to have low values of power factor in non-linear loads since it will cause some serious phase and neutral loading.

Harmonics are a distortion of the normal electrical current waveform, generally transmitted by nonlinear loads. (loads which have capacitor or inductive component). Power factor capacitors while correcting power factor also introduce harmonics that can cause stability and heating issues. Harmonics will distort the original waveform of the current signal like a sinusoidal wave will no longer remain sinusoidal. You can consider them as noise in waveform.

The even harmonics are not of much problem as they do not distort the basic waveform only the amplitude. But odd harmonics cause loss in the circuit due to heating. Odd harmonics alter he shape of the waveform.

In the presence of high harmonic currents, the Voltage waveform becomes distorted with the distorted voltage applied to the terminals of the capacitors. Harmonic voltages increase the currents flowing through the capacitors and can cause premature capacitor failure.

Detuned Capacitors :

The addition of detuning reactors to power factor correction capacitors causes the terminal voltage of the capacitors to rise in the presence of harmonics. It is important to increase the voltage rating of the capacitors condsiderably. Capacitors designed for use with detuning reactors are typically rated at 525V or 565 volts for a 380/400 volt system. The KVAR rating of the capacitors must be based on the actual supply voltage and the continuous applied voltage rating must be much higher.

Detuning reactors :

The detuning reactors are designed to be used with special high voltage capacitors and are rated in KVAR where the KVAR rating is the KVAR of the capacitor that they are designed to be used with. You use a 25KVAR detuning reactor to detune a 25KVAR capacitor. The series circuit of reactor and capacitor forms a resonant circuit. It is important that the resonant frequency of this tuned circuit is not near a harmonic frequency. This is why the detuning reactor must match the detuned capacitor.

Conclusions :

1. Entire power system should ideally operate at unity power factor. Continuous operation of power system either at lagging or leading power factor is not envisaged. Both leading and lagging power factors amounts to increased losses in the system, oversizing of cables and increased operational costs.
2. If by Knowingly /Unknowingly too many capacitors are placed in service on the distribution system during peak load / Light load /No load periods, not only rise the distribution voltage to an intolerable level, but the total apparent power flows through the transformer could exceed its rating, as the excess reactive power supplied by the capacitors but not drawn by the load will flow out of the transformer to the supply side.

Due to Transformers with on load tap changers and automatic voltage regulators only nominal voltages are maintaining in the systems now and if, while at bottom tap load is lost the voltage may rise unacceptably.

3. Voltage rise by leading power factor makes eddy current loss and hysteresis loss of transformer to increase and at the same time makes capacitor for power factor improvement to get out of order and makes varieties equipments to give an electrical stress, More losses always lead to less efficiency and its decreasing the capacity of transformers.

Loss in the electrical system always leads to draw more current than actual, Drawing more current leads to generation of heat in the equipment, Continuous heat in the equipment leads to damage/failure of equipment. This is only the reason for failure of sensitive electronic equipment like Electronic cards, PLC Power supplies, Input Output Modules, Drive & Drive Components..etc.,

4. In the presence of high harmonic currents, the Voltage waveform becomes distorted with the distorted voltage applied to the terminals of the capacitors. Harmonic voltages increase the currents flowing through the capacitors and can cause premature capacitor failure.