ELECTRICAL POWER DISTRIBUTION & UTILIZATION

The electrical power distribution system is that part of the electric utility system between the

bulk power source and the customers’ service switches. This definition of the distribution

system includes the following components:

1.  Subtransmission system2.  Distribution substations   [T/Fs, buses & switchgears]3.  Distribution or primary feeders [4.16 to 34.5kV(11kV Pak)]4.  Distribution transformers5.  Secondary circuits6.  Service drops/mains

Some distribution system engineers prefer to define the electrical distribution system as that part of the electric utility system between the distribution substations and the consumers’ service entrance.

A one line diagram of a typical distribution system.

The subtransmission circuits delivers energy from bulk power sources to the distribution substations. The distribution voltage ranges between 12.47 and 245kV.

The distribution substation reduces the subtransmission voltage for local distribution. The 3-phase primary feeders usually operate in between 4.16 to 34.5kV (11kV Pak)
Theses primary feeders distribute energy from the low – voltage bus of the substation to its load centers where it branches into 3-phase sub feeders and single phase laterals.Distribution transformers ranging from 10 to 500kVA, are usually connected to each primary feeder, sub feeders, and laterals. They reduce the distribution voltage to the utilization voltage.The secondary of the distribution transformers are connected to the consumer’s service mains/service
drops 

What is power factor?

Power Factor

Power factor definition

The power factor is equal to the real or true power P in watts (W) divided by the apparent power |S| in volt-ampere (VA):

PF = P_(W) / |S_(VA)|

PF - power factor.

P   - real power in watts (W).

S   - apparent power - the magnitude of the complex power in volt·amps (VA).

In AC circuits, the power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit.

The power factor can get values in the range from 0 to 1.

When all the power is reactive power with no real power (usually inductive load) - the power factor is 0.

When all the power is real power with no reactive power (resistive load) - the power factor is 1.

Power factor calculations

For sinusoidal current, the power factor PF is equal to the absolute value of the cosine of the apparent power phase angle φ (which is also is impedance phase angle):

PF = cos φ

PF is the power factor.

φ   is the apparent power phase angle.

The real power P in watts (W) is equal to the apparent power |S| in volt-ampere (VA) times the power factor PF:

P_(W) = S_(VA) × PF = S_(VA) × cos φ

Resistive load

 When the circuit has a resistive impedance load, the real power P is equal to the apparent power |S| and the power factor PF is equal to 1:

PF_{(resistive load)} = P / S = 1

  Reactive power

The reactive power Q in volt-amps reactive (VAR) is equal to the apparent power |S| in volt-ampere (VA) times the sine of the phase angle φ:

Q_(VAR) = S_(VA) × sin φ

Single phase circuit calculation from real power meter reading P in kilowatts (kW), voltage V in volts (V) and current I in amps (A):

PF = cos φ = 1000 × P_(kW) / (V × I)

Three phase circuit calculation from real power meter reading P in kilowatts (kW), line to line voltage V_L-L in volts (V) and current I in amps (A):

PF = cos φ= 1000 × P_(kW) / (√3 × V_L-L(V) × I_(A))

Three phase circuit calculation from real power meter reading P in kilowatts (kW), line to line neutralV_L-N in volts (V) and current I in amps (A):

PF = cos φ= 1000 × P_(kW) / (3 × V_L-N(V) × I_(A))

Power factor correction

Power factor correction is an adjustment of the electrical circuit in order to change the power factor near 1.

Power factor near 1 will reduce the reactive power in the circuit and most of the power in the circuit will be real power. This will also reduce power lines losses.

The power factor correction is usually done by adding capacitors to the load circuit, when the circuit has inductive components, like an electric motor.

what is ELECTRICAL POWER

Electric power is the rate at which electric energy  is transferred by an electric circuit.
Unit of power=watt
Denoted by=W

Electric power calculation

P = V · I

or

P = I ² · R

or

P = V ² / R

P is the electric power in watt (W).

V is the voltage in volts (V).

I is the current in amps (A).

R is the resistance in ohms (Ω).

Power of AC circuits

The formulas are for single phase AC power.

For 3 phase AC power:

 When line to line voltage (V_L-L) is used in the formula, multiply the single phase power by square root of 3 (√3=1.73).

When line to zero voltage (V_L-0) is used in the formula, multiply the single phase power by 3.

Real power

Real or true power is the power that is used to do the work on the load.

P = V_rms I_rms cos φ                                                                                        

P      is the real power in watts

V_rms  is the rms voltage = V_peak/√2 in Volts 

I_rms   is the rms current = I_peak/√2 in Amperes

φ      is the impedance phase angle = phase difference between voltage and current.

 Reactive power

Reactive power is the power that is wasted and not used to do work on the load.

Q = V_rms I_rms sin φ

Q      is the reactive power in volt-ampere-reactive [VAR]

V_rms  is the rms voltage = V_peak/√2 in Volts

I_rms   is the rms current = I_peak/√2 in Amperes

φ      is the impedance phase angle = phase difference between voltage and current.

 Apparent power

The apparent power is the power that is supplied to the circuit.

S = V_rms I_rms

S      is the apparent power in Volt-amper [VA]

V_rms  is the rms voltage = V_peak/√2 in Volts 

I_rms   is the rms current = I_peak/√2 in Amperes 

 Real / reactive / apparent powers relation

The real power P and reactive power Q give together the apparent power S:

P² + Q² = S²

P      is the real power in watts [W]

Q      is the reactive power in volt-ampere-reactive [VAR]

S      is the apparent power in Volt-amper [VA]

What is Resistance?

Resistance definition:

Resistance is property of any materiel to reduce the flow of current. 

Its unit is ohm (Ω). 

Resistance calculation:

The resistance of a conductor is resistivity of the conductor's material times the conductor's length divided by the conductor's cross sectional area.

R is the resistance in ohms (Ω).

ρ is the resistivity in ohms-meter (Ω×m)

l is the length of the conductor in meter (m)

A is the cross sectional area of the conductor in square meters (m²).

•  The length is bigger and the resistance will increase.
•  The cross sectional area is bigger and the resistance will decrease

Resistance calculation with ohm's law:

R is the resistance of the resistor in ohms (Ω).

V is the voltage drop on the resistor in volts (V).

I is the current of the resistor in amperes (A).

Resistance in series circuit:

The total equivalent resistance of resistors in series is the sum of the resistance values:

R_T= R₁+ R₂+ R₃+...

Resistance in parallel circuit:

The total equivalent resistance of resistors in parallel is given by:

Measuring electrical resistance:

Electrical resistance is measured with ohmmeter instrument.

In order to measure the resistance of a resistor or a circuit, the circuit should have the power supply turned off.

The ohmmeter should be connected to the two ends of the circuit so the resistance can be read.

What is electrical current?

Electrical current

Electrical current is the flow rate of electrical charge or flow of electrons  in electric field, usually in electrical circuit.

The electrical current is measured in ampere (amp) unit.

Amp Denotes As=A

what is ampere?

An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (6.24 x 1018charge carriers) moving past a specific point in one second.

Current in series circuits:

Current that flows through resistors in series is equal in all resistors - just like water flow through a single pipe.

I_T = I₁ = I₂ = I₃ =...

I_T - the equivalent current in amps (A).

I₁ - current of load #1 in amps (A).

I₂ - current of load #2 in amps (A).

I₃ - current of load #3 in amps (A).

Current in parallel circuits:

Current that flows through loads in parallel - just like water flow through parallel pipes.

The total current I_Total is the sum of the parallel currents of each load:

I_T = I₁ + I₂ + I₃ +...

I_T - the equivalent current in amps (A).

I₁ - current of load #1 in amps (A).

I₂ - current of load #2 in amps (A).

I₃ - current of load #3 in amps (A).

Current measurement:

Current measurement is done by connecting the ammeter in series to the measured object, so all the measured current will flow through the ammeter.

The ammeter has very low resistance, so it almost does not affect the measured circuit.

Electrical Voltage

Electrical voltage

Electrical voltage is defined as electric potential difference between two points of an electric field.

Using water pipe analogy, we can visualize the voltage as height difference that makes the water flow down.

In an electrical circuit, the electrical voltage V in volts (V) is equal to the energy consumption E in joules (J)

divided by the electrical charge  Q in coulombs (C).

V is the voltage measured in volts (V)

E is the energy measured in joules (J)

Q is the electric charge measured in coulombs (C)

Voltages in series

The total voltage of several voltage sources or voltage drops in series is their sum.

V_T = V₁ + V₂ + V₃ +...

V_T - the equivalent voltage source or voltage drop in volts (V).

V₁ - voltage source or voltage drop in volts (V).

V₂ - voltage source or voltage drop in volts (V).

V₃ - voltage source or voltage drop in volts (V).

Voltage in parallel

Voltage sources or voltage drops in parallel have equal voltage.

V_T = V₁ = V₂ = V₃ =...

V_T - the equivalent voltage source or voltage drop in volts (V).

V₁ - voltage source or voltage drop in volts (V).

V₂ - voltage source or voltage drop in volts (V).

V₃ - voltage source or voltage drop in volts (V).

Voltage drop

Voltage drop is the drop of electrical potential or potential difference on the load in an electrical circuit.

Voltage Measurement

Electrical voltage is measured with Voltmeter. The Voltmeter is connected in parallel to the measured component or circuit.

The voltmeter has very high resistance, so it almost does not affect the measured circuit.

Voltage standards at different countries

Australia	230V	50Hz
Brazil	110V	60Hz
Canada	120V	60Hz
China	220V	50Hz
France	230V	50Hz
Germany	230V	50Hz
Pakistan	220V	50Hz
Ireland	230V	50Hz
Israel	230V	50Hz
Italy	230V	50Hz
Japan	100V	50/60Hz
New Zealand	230V	50Hz
Philippines	220V	60Hz
Russia	220V	50Hz
South Africa	220V	50Hz
Thailand	220V	50Hz
UK	230V	50Hz
USA	120V	60Hz