Power System Protection 1

Power System Protection

Protection of power system deals with:

Protection against short-circuit faults

Causes of over-voltages

Insulation to withstand faults

Surge arresters and insulation coordination

Basic components of protection systems are:

Instrument transformers (voltage transformer and

current transformer)

Protective relays

Circuit breakers

Instrument Transformers

There are two basic types of instrument transformers: voltage transformers (VTs), formerly called potential transformers (PTs),

and current transformers (CTs). The transformer primary is connected to or into the power system and is insulated for the power

system voltage. The VT reduces the primary voltage and the CT reduces the primary current to much lower, standardized levels

suitable for operation of relays. Advantages associated with reduction of primary voltage and current values are:

Safety: Instrument transformers provide electrical isolation from the power system so that personnel working with relays

will work in a safer environment.

Economy: Lower-level relay inputs enable relays to be smaller, simpler, and less expensive.

Current Transformer (CT)

The primary winding of a current transformer usually consists of a single turn, obtained by

running the power systems primary conductor through the CT core. The secondary

generally has a large number of turns and produces a much smaller current, primary current

divided by the turn-ratio. Ideally, the CT secondary is connected to a current-sensing device

with zero impedance, such that the entire CT secondary current flows through the sensing

device.

Voltage Transformer (VT)

There are basically, two types of voltage transformers

used for protection equipment.

1. Electromagnetic type

2. Capacitor type (referred to as a CVT).

The electromagnetic transformers are used in voltage

circuits upto 110/132 kV. The capacitor VT is more

commonly used on extra high-voltage (EHV) networks.

Here the primary portion consists of capacitors

connected in series to split the primary voltage to

convenient values. Here the output voltage is isolated

through a transformer for safety purposes. Ideally, the

secondary of voltage transformers is connected to a voltage-sensing device with infinite impedance, such that the entire VT

secondary voltage is across the sensing device.

Fig. 0: Current transformer

Fig. 0: Equipment of protection system

Fig. 0: a) Electromagnetic type VT, b) Capacitor type VT (CVT)

Power System Protection 2

Protective Relays

Protective relays are intelligent electronic devices which receive measured signals from the secondary side of CTs and VTs and

detect whether the protected unit is in a stressed condition or not. A trip signal is sent by protective relays to the circuit breakers

to disconnect the faulty components from power system if necessary. It is important that relays operate, when they should, in order

to protect power system but it is equally important that they dont operate falsely in order to avoid causing unnecessary power

disturbances.

Differential Relays

Differential relays are commonly used to protect generators,

buses, and transformers against internal faults. As the name

implies, these relays compare currents entering and leaving the

protected zone and operate when the differential current between

these currents exceed a pre-determined level. The basic method

of differential relaying for generator protection is illustrated in

the figure. The protection of only one phase is shown. The

method is repeated for the other two phases. When the relay in

any one phase operates, all three phases of the main circuit

breaker will open, as well as the generator neutral and field

breakers (not shown).

For the case of no internal fault within the generator windings, 1 = 2, and assuming identical CTs, 1 = 2

. For this case the

current in the relay operating coil is zero, and the relay does not operate. On the other hand, for an internal fault such as a phase-

to-ground or phase-to-phase short within the generator winding, 1 2, and 1 2

. Therefore, a difference current 1 2

flows

in the relay operating coil, which may cause the relay to operate.

Overcurrent Relays

In these relays, if the current being measured exceeds a minimum value, the relay

determines that a fault has occurred, giving a trip command to the circuit breaker

to operate. Instantaneous overcurrent relays respond to the magnitude of their

input current, CT secondary current , as shown by the trip and block regions in the figure. If the current magnitude || exceeds a specified adjustable current magnitude , called the pickup current, then the relay contacts close

instantaneously to energize the circuit breaker trip coil. If is less than the pickup current , then the relay contacts remain open, blocking the trip coil.

Generally, such relays operate on a delay time basis where this delay time is a

nonlinear function of the magnitude of the fault current i.e. larger the current magnitude, shorter the delay time. If is a large multiple of the pickup current , then the relay operates (or trips) after a small time delay. For smaller multiples of pickup, the

relay trips after a longer time delay. And if < , the relay remains in the blocking position.

Directional Overcurrent Relays

The protection offered by this relay is for faults only in one direction. For

example, in the figure, if the fault occurs right of its CT location, the

current I sensed by this relay would be lagging with respect to the voltage

at this location, causing the relay to trip the circuit breaker. Whereas for

the fault left of its CT location, the current would be leading and the relay

would be blocked from tripping the circuit breaker.

Fig. 0: Differential relaying for one phase of generator protection

Fig. 0: Instantaneous overcurrent relay block and

trip regions

Fig. 0: Directional overcurrent relay

Power System Protection 3

Impedance (Distance) Relays

A distance relay, as its name implies, has the ability to detect a fault within a pre-set distance

along a transmission line or power cable from its location. Every power line has a resistance

and reactive per kilometer related to its design and construction so its total impedance will be

a function of its length or distance. A distance relay therefore looks at current and voltage and

responds to a voltage-to-current ratio. During a three-phase fault, current increases while bus

voltages close to the fault decrease. If, for example, current increases by a factor of 5 while

voltage decreases by a factor of 2, then the voltage-to-current ratio decreases by a factor of

10. That is, the voltage-to-current ratio is more sensitive to faults than current alone. A relay

that operates on the basis of voltage-to-current ratio is also called an impedance relay.

If the relays operating boundary is plotted, on an R/X diagram, its impedance characteristic

is a circle with its center at the origin of the coordinates and its radius will be the setting (reach) in ohms. The relay will operate

for all values less than its setting i.e. is for all points within the circle. This is known as a plain impedance relay and it will be

noted that it is non-directional, in that it can operate for faults behind the relaying point.

Pilot Relaying

Pilot relaying refers to a type of differential protection that compares the quantities at the terminals via a communication channel

rather than by a direct wire interconnection of the relays. Differential relaying of transmission lines requires pilot relaying because

the terminals are widely separated. Four types of communication channels are used for pilot relaying:

1. Pilot wires: Separate electrical circuits operating at dc, 50 to 60 Hz, or audio frequencies.

2. Power-line carrier: The transmission line itself is used as the communication circuit, with frequencies between 30 and

300 kHz being transmitted.

3. Microwave: A 2 to 12 GHz signal transmitted by line-of-sight paths between terminals using dish antennas.

4. Fiber optic cable: Signals transmitted by light modulation through electrically non-conducting cable.

Zones of Protection

To realize complete selectivity of protection, the power system is divided into discrete zones. Each zone encompasses one or more

power system equipment, and adjacent zones are overlapping so that no part of the power system is left unprotected.

In the figure, the first zone for the relay at A encompasses for

example 90% of the line. The remaining 10% is protected by

the relay at B. If the fault occurs in As first zone, the relay acts

instantly without any time delay. The second zone for the relay

at A encompasses for example 120% of the line, thus

overreaching in the next section. If the fault occurs in the

second zone assigned to it, it operates with a time delay of a

few hundred msec, thus allowing it to coordinate with the relay

that is its first zone. The adjacent zone consisting of line CD is

the third zone for the relay A for which it provides backup protection with a further time delay of 1 to 3 seconds.

Fig. 0: Impedance characteristic of a

distance relay

Fig. 0: Zones of protection