units 3&7 relays.ppt

Protection and Relay Schemes

PRESENTED BY GADDALA JAYARAJU

NOVEMBER 4, 2015

topics

• Introduction of Protective Relays

• Electrical System Protection with Protective Relays

• Conclusion

What are Relays?• Relays are electrical switches that open or close another

circuit under certain conditions.

Relay Purpose

Isolate controlling circuit from controlled circuit.

Control high voltage system with low voltage.

Control high current system with low current.

Logic Functions

Relay Types• Electromagnetic Relays (EMRs)

– EMRs consist of an input coil that's wound to accept a particular voltage signal, plus a

set of one or more contacts that rely on an armature (or lever) activated by the

energized coil to open or close an electrical circuit.

• Solid-state Relays (SSRs)

– SSRs use semiconductor output instead of mechanical contacts to switch the circuit. The

output device is optically-coupled to an LED light source inside the relay. The relay is turned

on by energizing this LED, usually with low-voltage DC power.

• Microprocessor Based Relays

– Use microprocessor for switching mechanism. Commonly used in power system

monitoring and protection.

How a Relay Works

Sold-State Relay

Advantages/Disadvantages• Electromagnetic Relays (EMRs)

– Simplicity

– Not expensive

– Mechanical Wear

• Solid-state Relays (SSRs)

– No Mechanical movements

– Faster than EMR

– No sparking between contacts

• Microprocessor-based Relay

– Much higher precision and more reliable and durable.

– Improve the reliability and power quality of electrical power systems before, during and

after faults occur.

– Capable of both digital and analog I/O.

– Higher cost

Why A System Needs Protection?

• There is no ‘fault free’ system.

• It is neither practical nor economical to build a ‘fault free’ system.

• Electrical system shall tolerate certain degree of faults.

• Usually faults are caused by breakdown of insulation due to various

reasons: system aging, lighting, etc.

Electrical Faults

• majority are phase-to-ground faults• phase-to-phase • phase-phase-phase • double-phase-to-ground

Advantages for Using Protective Relays

• Detect system failures when they occur and isolate the

faulted section from the remaining of the system.

• Mitigating the effects of failures after they occur. Minimize

risk of fire, danger to personal and other high voltage

systems.

Protective Devices Comparison

Relays Circuit Breakers Fuses

Acquisition Detection

Activation Actuation

Protective Devices Comparison

Circuit Breakers V.S. Relays

• Relays are like human brain; circuit breakers are like human

muscle.

• Relays ‘make decisions’ based on settings.

• Relays send signals to circuit breakers. Based the sending

signals circuit breakers will open/close.

Protective Devices Comparison

Fuses V.S. Relays

• Relays have different settings and can be set based on protection

requirements.

• Relays can be reset.

• Fuses only have one specific characteristic for a individual type.

• Fuses cannot be reset but replaced if they blow.

Protection and Relay Schemes

• Motor Protection

• Transformer Protection

• Generator Protection

Essential Qualities of Protective Relaying

A protective relaying scheme should have certain important qualities.

Such an essential qualities of protective relaying are,

1. Reliability

2. Selectivity and Discrimination

3. Speed and Time

4. Sensitivity

5. Stability

6. Adequateness

7. Simplicity and Economy

All the relays consists of one or more elements which get energized and

actuated by the electrical quantities of the circuit.

Most of the relays used now a days are electro-mechanical type which work

on the principles of electromagnetic attraction and electromagnetic

induction.

Classification of Protective Relays

Electromagnetic Attraction Type Relays

The electromagnetic attraction type relays operate on the principle of attraction of

an armature by the magnetic force produced by undesirable current or movement of

plunger in a solenoid.

These can be actuated by a.c. or d.c. quantities. The various types of these relays

are,

1. Solenoid Type : In this relay, the plunger or iron core moves into a solenoid

and the operation of the relays depends on the movement of the plunger.

2. Attached Armature Type : This relay operates on the current setting. When

current in the circuit exceeds beyond the limit, the armature gets attracted

by the magnetic force produced by the undesirable current. The current

rating of the circuit in which relay is connected plays an important role in

the operation of the relay.

3. Balanced Beam Type : In this relay, the armature is fastened to a balanced

beam. For normal current, the beam remains horizontal but when current

exceeds, the armature gets attracted and beam gets tilled causing the

required operation.

Induction Type RelaysThese relays works on the principle of an electromagnetic

induction. The use of these relays is limited to a.c. quantities.

The various types of these relays are,

1.Induction Disc Type : In this relay, a metal disc is allowed to rotate

between the two electromagnets. The electromagnets are energized by

alternating currents. The two types of constructions used for this type

are shaded pole type and watt-hour meter type.

2. Induction Cup Type : In this relay, electromagnets act as a

stator and energised by relay coils. The rotor is metallic

cylindrical cup type.

Directional Type Relays

These relays work on the direction of current or power flow in the circuit. The

various types of these relays are,

1.Reverse Current Type : The relay is actuated when the direction

of the current is reversed or the phase of the current becomes

more than the predetermined value.

2. Reverse Power Type : The relay is actuated when the phase

displacement between applied voltage and current attains a

specified value.

Relays Based On TimingIn relays the time between instant of relay operation and instant at which

tripping of contacts takes place, can be controlled. This time is called operation

time. Based on this, the time relays are classified as,

1.Instantaneous Type : In this type no time is lost between operation of relay and

tripping of contacts. No intentional time relay delay is provided.

2. Definite Time Lag Type : In this type intentionally a definite time lag is provided

between operation of relay and tripping of contact.

3. Inverse Time Lag Type : In this type, the operating time is approximately inversely

proportional to the magnitude of the actuating quantity.

Distance Type RelaysThese relays work on the principle of measurement of voltage to current ratio. In this type, there are

two coils. One coil is energized by current while other by voltage. The torque produced is proportional

to the ratio of the two quantities. When the ratio reduces below a set value, the operates. The various

types of these relays are,

1.Impedance Type : In this type, the ratio of voltage to current is nothing but an

impedance which is proportional to the distance of the relay from the fault point.

2. Reactance Type : The operating time is proportional to the reactance which is

proportional to the distance of the relay from the fault point.

3. Admittance Type : This is also called mho type. In this type, the operating time is

proportional to the admittance.

Differential Type RelaysA differential relay operates when the vector difference of two or more electrical

quantities in the circuit in which relay is connected, exceeds a set value. These are

classified as,

1.Current Differential Type : In this type, the relay compares the current entering a

section of the system and the current leaving the section. Under fault condition, these

currents are different.

2. Voltage Differential Type : In this type, two transformer are used. The secondaries of

the transformers are connected in series with the relay in such a way that the induced

e.m.f.s are in opposition under normal conditions. Under fault condition, primaries carry

different currents due to which induced e.m.f.s no longer remain in opposition and the

relay operates.

Other Types of RelayVarious other types of relays which are used in practice are,

1. Under voltage, current power relay : This relay operates when the voltage, current or

power in a circuit falls below a set value.

2. Over voltage, current, power relay : This relay actuates when the voltage, current or

power in a circuit rises above a set value.

3. Thermal Relay : This relay actuates due to the heat produced by the current in the relay

coil.

4. Rectifier Relay : In this relay, the quantities to be sensed are rectified and then given to

the moving coil unit of the relay.

5. Permanent Magnet Moving Coil Relay : In this relay, the coil carrying current is free to

rotate in the magnetic field of a permanent magnet. This is used for d.c. only.

6. Static Relay : This relay uses some electronic method for sensing the actuating quantity. It

uses a stationary circuit.

7. Gas Operated Relay : The gas pressure is adjusted according to the variations in the actuating quantity.

This gas pressure is used to actuate the relay. Buchholz relay is an example of such type of relay.

Differential Protection

"A differential relay responds to vector difference between two or more

similar electrical quantities “

From the definition the following aspects are known ; -

1- The differential relay has at least two actuating quantities say I1, I2

2- The two or more quantities should be similar i.e. current/current.

3- The relay responds to the vector difference between the twoi.e. to I1-

I2,  which includes magnitude and/or phase angle difference.

       Differential protection is generally unit protection. The protected

zone is exactly determined by location of CT's and VT's. The vector

difference is achieved by suitable connections of current transformer or

voltage transformer secondary's.

Application of differential protection :-Most differential relays are current differential relays in which vector

difference between the current entering the winding and current

leaving the winding is used for sensing and relay operation.      Differential protection principle is used in the following applications.

Protection of generator, protection of generator transformer unit.

Protection of transformer 

Protection of feeder (transmission line) by pilot wire differential

protection.

Protection of transmission line by phase comparison carrier current

protection.

Protection of large motor.

Bus-zone protection.

Principle of circulating current differential (MERZ-PRIZE) protection

Difficulties of differential protection : -

Difference in pilot wire lengths. The current transformer and machine to be protected are

located at different sites and normally it is not possible to connect the relay coil to the equipotential points. The

difficulty is overcome by connecting adjustable resistor in series with the pilot wires. These are adjusted on site to

obtain the equipotential points

CT's ratio error during short circuits. Due to these causes the relay may operate even for external faults. The relay may loose its stability for through

faults. To overcome these difficulty, the percentage differential relay, or 'Based Differential Relay' is used. It is

essentially a circulating current differential relay which additional restraining coil. The current flowing in restraining

coil proportional to (I1+I2)/2 and this restraining current prevents the operation during external faults. Because, with

the rise in current, the restraining torque increases and I1-I2 arising out of difference in CT ratio is not enough to

cause the relay operation.

Saturation of CT magnetic circuit during short circuit condition.

Magnetizing current inrush in transformer while switching in.

Tap changing. The tap changing causes change in

transformation ratio of a transformer.

Differential protection of 3-phase circuit

Balanced voltage differential protection

Setting of differential relay :The circulating current differential relay has two principle settings namely,       - Setting of operating coil circuit.       - Setting of restraining coil circuit.

Setting of restraining coil circuit (pick up value). It is defined as the ratio :

While determining this setting the factors be considered include       - Ct errors                              -Tap-changing       - Resistance of pilot wires        - Stability of through faults        In case of power transformers, percentage basic setting is of the order of 20 % and percentage pick-up value of the order of 25%.

Distance Type RelaysThese relays work on the principle of measurement of voltage to current ratio.

In this type, there are two coils. One coil is energized by current while other by voltage.

The torque produced is proportional to the ratio of the two quantities. When the ratio reduces below a set value, the operates. The various types of these relays are,

1.Impedance Type : In this type, the ratio of voltage to current is nothing but an impedance which is proportional to the distance of the relay from the fault point.

2. Reactance Type : The operating time is proportional to the reactance which is proportional to the distance of the relay from the fault point.

3. Admittance Type : This is also called mho type. In this type, the operating time is proportional to the admittance.

Impedance TypeIn this type, the ratio of voltage to current is nothing but an impedance which is proportional to the distance of the relay from the fault point.

Reactance Type Distance Relay

Mho Type Distance RelayMho relay is also known as admittance relay and measures a component of admittance Y<θ .

It is also called as angle impedance relay.      The characteristic of Mho relay on admittance diagram is a straight line.

The Mho characteristic on R-X diagram is a circle passing through origin. This characteristic is obtained by polarizing the impedance relay and directional relay