The Load Flow Analysis is done to determine the flow of real and reactive between different buses in a power system.  It also helps in determining the voltage and current at different locations.

To conduct a Load Flow Analysis, the components in a power system need to be modelled.  The modelling is done by developing equivalent circuits of the components, such as the generator, transmission lines and line capacitances.


The Generator equivalent circuit is shown below.

The Thevenin equivalent circuit is as shown below.

This consists of a voltage source and a resistance and an inductance in series with the load.

E = V + IZ

where

Z is the steady stage impedance
V is the voltage and
I is the current

The Norton equivalent circuit

The Norton equivalent circuit consists of a power source and an admittance in parallel.


INorton = V/Z

INorton = YV





Load

The load is modelled as a resistance and inductance in a series circuit that is earth







Transmission lines

Transmission lines are modelled as

Short transmission lines (less than 80 km)


Short lines that are less than 80 kms long are modelled as a resistance and reactance in series with the load.  The line capacitances are neglected.



Medium (80 to 250 km)

Medium lines are modelled as a resistance and reactance in series.  The admittance is in parallel in two sections.











Long lines ( 250 km and above)

Long lines are also modelled as a resistance and reactance in series.  The admittance is in parallel in two sections.















Circuit breakers are used in a wide range of applications.  They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria.  Some of the classifications are below

Based on the interrupting medium
  1. Air circuit breaker
  2. Oil circuit breaker
  3. SF6 circuit breaker
Based on type of action
  1. Automatic circuit breakers
  2. Non-automatic circuit breakers
Based on the method of control
  1. Locally controlled circuit breakers
  2. Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)
Based on the type of mounting
  1. Panel mounted circuit breakers
  2. Remote from panel mounted circuit breakers
  3. Rear of panel mounted circuit breakers
Based on location
  1. Outdoor circuit breakers
  2. Indooor circuit breakers
Based on voltage
  1. Low voltage circuit breakers
  2. Medium voltage circuit breakers
  3. High voltage circuit breakers




The relay in a protection system should be sensitive enough to operate when a fault occurs.  A sensitive relay improves the reliability of the system.

When the parameter exceeds the set value, the relay should start operating.

The sensitivity of a relay is mentioned as a ratio of the minimum value of short circuit current to the minimum value of the quantity for the operation.

The sensitivity is indicated by a sensitivity factor Ks

Sensitivity of a Relay
where

Is is the minimum short circuit current in the zone and
Io is the minimum operating current for the relay.

The sensitivity of a relay is also related to the VA of the input to the relay.  Lesser the VA of the input, greater will be the sensitivity and vice versa. For instance, a relay which has 1 VA as its measuring input will be more sensitive than a relay, which has 5 VA as its measuring input.


A reliable and effective protection system is a crucial part of any power system.  The protection system protects the equipment in the power system, such as generators, motors, transformers, etc from damage to faults.

The protection system also ensures reliability by localizing and isolating the fault and minimizing its impact on the rest of the power system.

The requirements of a power system are as follows

1. To isolate the equipment or component in which the fault has occurred.  The isolation should be quick enough to prevent damage to the component itself.  For instance, a short circuit inside can severely damage a transformer.  The differential relay, in such a situation, should immediately act and trip the transformer.

2. To isolate the smallest possible section of the power system to minimize the interruption.

3. To prevent disturbance or disruption to other parts of the power system.  The fault, if not isolated in time, can cause the upstream breakers to trip.

A well-designed protection system will greatly increase the reliability and performance of a power system.


The faults in a power system can be caused by a wide range of causes.  Below is a list of some of the most probable causes for electrical faults.

Overvoltage is caused due to surges such as lightning or due to switching loads on and off.  In generators, it can be caused due to the failure of the field controller.

Heavy winds which can cause lines to snap or trees to fall on them. This can cause open circuits, earth faults and short circuits.

Ageing which can result in weakening of insulation and failure. This can result in short circuits and earth faults.

Chemical pollution and deposition on the insulators that can result in flash overs

Faults due to wildlife, such as birds, snakes, mice, etc which can cause short circuits and earth faults.

Collision of vehicles on towers and transmission poles can cause the towers to fall.

The table below will give an idea of the percentage of the types of faults in the different equipment in a power system

Overhead lines                      50%
Transformers                         12%
Switch gear                            15%
Cables                                   10%
Miscellaneous                         8%
Instrument Transformers        2%
Control Equipment                 3%


Silicon Controlled Rectifiers
Silicon Controlled Rectifier
A silicon controlled rectifier is a three terminal electronic component.  It consists of an anode, a cathode and a gate.

The device is similar to a diode, except that it needs to be switched on with an external voltage applied to the gate during the positive half cycle.  Once, the SCR has been trigerred, it "fires" and conducts as long as it is positively biased.

In AC circuits, during the negative half cycle, the SCR switches off automatically.

SCRS find application in applications where the current needs to be controlled.  They are used in many electronic equipments, such as inverters, converters, speed controller, etc.


There are different methods of trigerring the gate of a Silicon Controlled Rectifiers (SCRs).  They are

Using DC Voltage
By applying a positive voltage to the gate with respect to the cathode, the junction J2 can be forward biased.  This will switch on the SCR.  This process requires a constant dc voltage to be applied between the Gate and the Cathode, which is a disadvantage.  Besides, there is no isolation between this triggering dc voltage and the main dc supply

Using AC voltage
In AC applications, the trigger voltage can be obtained from the AC voltage suitable reduced.  The phase of the AC voltage is modified and applied to trigger the SCR at the desired instant.  The SCR will continue to conduct till the negative half cycle.

A separate transformer is required for the trigger circuit which increases the cost

Pulse triggering
This is the most widely used form of triggering.  In this method, a pulse of a small duration is applied to the gate to switch it ON.  Sometimes, a series of pulses are applied.  The pulse need not be continuous.  This reduces the losses in the gate.