02_Basic Protection Technology

8/9/2019 02_Basic Protection Technology

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Power Transmission and Distribution

No. 1 withEnergy Automation

Energy Automation Badiya Page 1

Power System Structure

Generation Transmission / Sub transmission Distribution

Extra High Voltage 765 kV

400 kV

220 kV

High Voltage 132 kV

110 kV

66 kV

Medium Voltage 33 kV

22 kV

11 kV

Medium 24 kV

Voltage 21 kV

15 kV

13.8 kV

The purpose of an electrical power system is to generate and supply electrical energy toconsumers. The system should be designed and managed to deliver this energy to the

utilisation points with both reliability and economy.

The purpose of an electrical power system is to generate and supply electrical energy toconsumers. The system should be designed and managed to deliver this energy to the

utilisation points with both reliability and economy.

Many items of equipment are very expensive, and so the complete power system represents avery large capital investment.

.

Many items of equipment are very expensive, and so the complete power system represents avery large capital investment.

.


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System Disturbances

Short Circuits in earthed systems

Symmetrical (3 phase)

Phase to Phase (and Earth)

Phase to Earth

Earth Faults in non effectively

earthed systems

Overload Conditions

Underfrequency/Undervoltage

Overvoltage


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Protective Relaying is the most important feature

of power system design aimed at minimising the

damage to equipment and interruption to service in

the event of faults. It is therefore a co-factor

among other factors resorted to improve reliability

of power system.

Protective Relaying

Role of Protection


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The Purpose of Protection

But it can:Limit the damage caused by shortcircuits

While:Protecting people and plant fromdamage

Selectively clearing faults inmiliseconds

Protecting plant from overloadconditions

The protection can not prevent system faults,

Power system must operate in a safe manner at all times.


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Causes and Probability of System Disturbances

Causes

Operator Mistakes

Pollution/Condensation

Equipment failures, e.g. P.T.'s, Isolators

Transient OvervoltagesProbability

System faults (220/400 kV): 3p.a. and 100 km

10-20 kV metal clad switchgear: 10-3 p.a. and feeder

GIS switchgear: 5-10-2 p.a. and bus

outdoor switchgear: 110/132 kV 7*10-2 p.a. and bus220/275 kV 10-1 p.a. and bus

400 kV 2*10-1 p.a. and bus


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Since protective relaying comes into action at the time of

equipment distress, a certain safeguard is necessary in

the unlikely event of its failure to act at the hour of need.

Hence, two groups of protective schemes are generally

employed -

a) Primary Protection

b) Back-up Protection

Primary Protection is the first line of defense, whereas

back-up relaying takes over the protection of equipment,

should the primary protection fail.

Principles of Relaying


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The Primary Protection has following characteristic features -

1. It has always a defined zone of operation.

2. It should operate before any back-up protection

could operate, therefore, it should be faster inoperation.

3. It should be able to completely isolate the fault

from all the current feeding sources.

4. It should be stable for all operating conditions.

Primary Protection


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1. Back-up protection should provide sufficient time

for the primary protection to perform its duty.

2. Back-up protection covers a wider zone of

protection. Therefore, there is always a possibility

of large scale disturbance, when back-up relays

operate.

3. Under primary protection failure, several back-up

relays may operate for complete isolation of fault.

Back-up Protection


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Primary protections failure could be due to any of the

following reasons -

1. Current or Potential Transformer failure

2. Loss of Auxiliary Control Voltage3. Defective Primary Relays

4. Open Circuits in Control & Trip Coil

5. Failure of Breaker

It is therefore logical that back-up relays should not

utilise any of the above items as common with primary

relays.

Reasons of Primary Protection Failure


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Protection Concept

The system is only as strong as the weakest link!

DISTANCE RELAY

Circuit BreakerCT / VT

Protection Battery

Cabling


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Basic Protection Requirements

Reliability dependability (availability)

high dependability = low risk of failure to trip

Security stable for all operating conditions ,

high security = low risk of over-trip

Speed high speed minimizes damage

high speed reduces stability problems

Selectivity trip the minimum number of circuit breakers

Sensitivity notice smallest fault value


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Zones of Protection

To limit the extent of the power system that is disconnected when a fault

occurs, protection is arranged in zones

Zones of protection should overlap, so that no part of the power system is left

unprotected

Location of the CT connection to the protection usually defines the zone Unit type protections have clear zones reach e.g Diff. Relay, REF relay

Zone reach depends on measurement of the system quantities e.g OC , EF,

distance relays . The start will be defined but the extent (or reach) is subject

to variation, owing to changes in system conditions and measurement errors.


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Protection - One Out of Two Principle

System1

TripCoil

1

Battery 1

System2

TripCoil

2

Battery 2


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Redundancy Concept of DC Circuits

Battery 1

Battery 2

Main Protection

87T

TR

TC 1

L-

Back-up Protection

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TR

TC 2

Busbar Protection

87BB

TR

BF

L-

Trip remote infeed


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Factors that influence fault current magnitude

G

Short circuit power of the infeed Voltage level Line impedance Fault resistance (arc) Treatment of star point

Infeed Consumer Line

Estimate of short circuit currents:

Medium Voltage (10 kV upto 30kV) ISCmin > ILmaxHigh Voltage (110 kV) I

SCmin>= I

LmaxExtra High Voltage (220kV + ) ISCmin = 0,25 ILmax


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Earth faults: Star-point configuration

R L

earthed system Peterson Coil isolated neutral

Earth fault = short circuitis recognised by normalover-current protection.

With low impedance earthingthe residual current detectionmust be more sensitive.

Earth faults = no short circuit Supply is not disrupted Earth fault must be alarmed and removedas fast as possible

Earth fault location is achieved withwattmetric earth fault detection


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Protection Criterion - Current

The overcurrent condition is evaluated I>

Suitable for:

Additional criterion - Time(to ensure selectivity)

Protection: Fuses

inverse time protection (IDMT)

definite time protection (DT)

I

ISCminILmax

I>


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Protection Criterion - Current Difference

Evaluation of node I1 + I2 + I3 + ... In = 0; if the equation is notsatisfied the fault is internal

Security is increased by stabilisation |I1|+|I2|+ ... |In| = Istab Characteristic:

definite distinction internal / external faults (no back-up)

Protection: Line differential protection Generator-, motor-, transformer differential protection Busbar protection

Trip

Istab

Idiff


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Protection Criterion - Impedance

From the voltage and current signals the

impedance is calculated

The impedance is proportional to the fault distance

Characteristic:

Additional criterion - Time

(Required for selectivity and back-up protection)Protection: Multiple stage distance protection

R

X

Z

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02_Basic Protection Technology