DISTRIBUTION PROTECTION

R.Panneer Selvam B,E.,M.I.E.,Former Superintending EngineerProtection and CommunicationCHENNAI

There are three aspects of a power system

These aspects are as follows:A. Normal operationB. Prevention of electrical failure.C. Mitigation of the effects of electrical failure.

Normal Operation

The term normal operation assumes no failures of equipment, no mistakes of personnel, nor Acts of God. It involves the minimum requirements for supplying the existing load and a certain amount of anticipated future load.

 

A. Choice between hydro, steam, or other sources of power.

B. Location of generating stations.

C. Transmission of power to the load.

D. Study of the load characteristics and planning for its future growth.

E. Metering

F. Voltage and frequency regulation.

G. System operation.

E. Normal maintenance

Outages

Electrical equipment failures would cause intolerable outages.

There must be additional provisions to minimize damage to equipment and interruptions to the service when failures occur.

  Two recourses are open:

(1) to incorporate features of design aimed at preventing failures, and

(2) to include provisions for mitigating the effects of failure when it occurs

Prevention of Electrical Failure

The type of electrical failure that causes greatest concern is the short circuit, or “fault” as it is usually called, but there are other abnormal operating conditions peculiar to certain elements of the system that also require attention. Some of the features of design and operation aimed at preventing electrical failure are: A. Provision of adequate insulation.

B. Coordination of insulation strength with the capabilities of lightning arresters.

C. Use of overhead ground wires and low tower-footing resistance.

D. Design for mechanical strength to reduce exposure, and to minimize the likelihood

of failure causable by animals, birds, insects, dirt, etc.

E. Proper operation and maintenance practices.

Design And Operation For Mitigating The Effects Of Failure

Some of the features of design and operation for mitigating the effects of failure are:

A. Features that mitigate the immediate effects of an electrical failure.

1. Design to limit the magnitude of short-circuit current.1

a. By avoiding too large concentrations of generating capacity.

b. By using current-limiting impedance.

2. Design to withstand mechanical stresses and heating owing to short-circuit currents.

3. Time-delay under-voltage devices on circuit breakers to prevent dropping loads

during momentary voltage dips.

4. Ground-fault neutralizers (Petersen coils).

B. Features for promptly disconnecting the faulty element.

1. Protective relaying.

2. Circuit breakers with sufficient interrupting capacity.

3. Fuses.

C. Features that mitigate the loss of the faulty element.

1. Alternate circuits.

2. Reserve generator and transformer capacity.

3. Automatic reclosing.

 

Protective Relaying

Thus, protective relaying is one of several features of system design concerned with minimizing damage to equipment and interruptions to service when electrical failures occur.

When we say that relays “protect,” we mean that, together with other equipment, the relays help to minimize damage and improve service.

Therefore, the capabilities and the application requirements of protective-relaying equipments should be considered concurrently with the other features.

Protection Technology

The last thirty years have seen enormous changes in relay technology.

The electromechanical relay in all of its different forms has been replaced successively by static, digital and numerical relays

ELECTROMECHANICAL RELAYS

The mechanical force is generated through current flow in one or more windings on a magnetic core or cores, hence the term electromechanical relay

The principle advantage of such relays is that – they provide galvanic isolation between the inputs and

outputs in a simple, cheap and reliable form – therefore for simple on/off switching functions where the

output contacts have to carry substantial currents, they are still used.

Philosophy of over Current Protection

Over current protection is directed entirely to the clearance of faults, although with the settings usually adopted some measure of overload protection may be obtained.

Attracted Armature Relays

These generally consist of an iron-cored electro- magnet that attracts a hinged armature when energized.

A restoring force is provided by means of a spring or gravity so that the armature will return to its original position when the electromagnet is de-energized.

Typical attracted armature relays

The contacts can be made quite robust and hence able to make, carry and break relatively large currents under quite onerous conditions (highly inductive circuits).

This is still a significant advantage of this type of relay that ensures its continued use.

The basic connections of a protective relay

Torque production in an induction relay.

Shaded-pole structure. Watthour-meter structure.

Induction Cup Unit

• These two structures are shown in Fig

• They most closely resemble an induction motor, except that the rotor iron is stationary,

• Only the rotor-conductor portion being free to rotate.

Induction-Cup and Double-Induction-Loop Structures

Double-induction-loop structure. Single-induction-loop structure.

The data required for a relay The data required for a relay setting studysetting study

i. i. A single-line diagram of the power system involved, showing the A single-line diagram of the power system involved, showing the type and rating of the protection devices and their associated type and rating of the protection devices and their associated current transformers current transformers ii. Tii. The impedances in ohms, per cent or per unit, of all power he impedances in ohms, per cent or per unit, of all power transformers, rotating machine and feeder circuits transformers, rotating machine and feeder circuits iii. Tiii. The maximum and minimum values of short circuit currents he maximum and minimum values of short circuit currents that are expected to flow through each protection device that are expected to flow through each protection device iv. Tiv. The maximum load current through protection devices he maximum load current through protection devices v. Tv. The starting current requirements of motors and the starting and he starting current requirements of motors and the starting and locked rotor/stalling times of induction motorslocked rotor/stalling times of induction motorsvi. Tvi. The transformer inrush, thermal withstand and damage he transformer inrush, thermal withstand and damage characteristicscharacteristicsvii. Dvii. Decrement curves showing the rate of decay of the fault current ecrement curves showing the rate of decay of the fault current supplied by the generators supplied by the generators viii.Pviii.Performance curves of the current transformerserformance curves of the current transformers..

Over current ProtectionOver current Protectionfor Phase and Earth Faultsfor Phase and Earth Faults

PRINCIPLES OF PRINCIPLES OF TIME/CURRENT GRADINGTIME/CURRENT GRADING

Fault withstand levels

• The typical duration of external short circuits that a transformer can sustain without damage if the current is limited only by the self-reactance is shown in Table

• Maximum mechanical stress on windings occurs during the first cycle of the fault.

• Avoidance of damage is a matter of transformer design.

Over current and Earth Fault Relay

• Starting current103 - 110% of current setting.

• Closing current130% of current setting.

• Resetting currentThe maximum current up to whichthe disc will completely reset is 90%of current setting.

• Time settings0 - 3s at 10 times current setting.0 - 1.3s at 10 times current setting.

• Resetting timeWith the time multiplier set at 1.0 the resetting times of the above are 9s and 4s respectively.

• OvershootOn removal of a current equal to 20 times current setting the overshoot times of the above are 0.065s and 0.04s respectively.

Inverse-time curves.

Inverse-time curves.

Operating characteristic of over current relays

:-

characteristic operating time 1 RI curves t = x K

- 0.236/ I

0.14 Normal inverse t = x K 0.02

I – 1

13.5 very inverse t = x K I – 1

80 Extremely inverse t = x K I2 -1

120 Long time stand by earth fault t = x K I - 1

Logarithmic inverse t = 5.8 - 1.35 log n (I / I n)

where t = relay operating time K = scale constant or TMS according to curve I = multiple of set current Is or PSM = Fault current / CT ratio x PSM

Grading MarginGrading Margin

The time interval that must be allowed between the The time interval that must be allowed between the operation of two adjacent relays in order to achieve operation of two adjacent relays in order to achieve correct discrimination between them is called the correct discrimination between them is called the grading margingrading margin..

The grading margin depends on a number of factors:The grading margin depends on a number of factors:i. i. the fault current interrupting time of the circuitthe fault current interrupting time of the circuit

breakerbreakerii. ii. relay timing errorsrelay timing errorsiii. iii. the overshoot time of the relaythe overshoot time of the relayiv. iv. CT errorsCT errorsv. v. final margin on completion of operationfinal margin on completion of operation

Grading MarginGrading Margin

At one time 0.5s was a normal At one time 0.5s was a normal grading margin. With faster modern grading margin. With faster modern circuit breakers and a lower relay circuit breakers and a lower relay overshoot time, 0.4s isovershoot time, 0.4s isreasonable, while under the best reasonable, while under the best conditions even lower intervals may conditions even lower intervals may be practical.be practical.