Protection System

Basic of Protection in Power System

Md Sajid AkhterAsstt. Manager (E&C)

Introduction• System Protection: the equipment

use to detect and isolate the faulty section from the system automatically.

Introduction• Short circuit occur when equipment

insulation fails due to system overvoltages caused by:– Lightning or switching surges

• Flashover line-line (caused by wind)• Flashover to tree

– Insulation contamination by dirt/salt– Mechanical failure

• Cable insulation failure– Natural causes

• Tower/pole or conductor falls• Objects fall on conductors

Introduction• Short circuit currents can be several

orders of magnitude larger than normal operating currents

• If it is allowed to persist, may cause:– Damage to the equipment due to heavy

currents, unbalanced current, or low voltage produces by the short circuit

– Fire and explosion effect equipment/people– interruption of service in the entire power

system area

Introduction• Careful design, operation and

maintenance of system protection can minimize the occurrence of short circuit but cannot eliminate them.

Fault Currents and Voltages

Function of System Protection

• Cause the prompt removal from service of any elements of power system when it suffers a short circuit, or when it start to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system.

• Provide indication of the location and type of failure so that the data can be used to assist in expediting repair and analyzing the effectiveness of fault-prevention and improvement features.

Function of System Protection

• Why do we need system protection:– Detect fault– Isolate faulted component– Restore faulted component

• Aims:– Continued supply for rest of system– Protect faulted part from damage

Types of ProtectionA – Fuses• For LV Systems, Distribution Feeders

and Transformers, VT’s, Auxiliary Supplies

B - Over current and earth fault• Widely used in All Power Systems

– Non-Directional – Directional

Types of ProtectionC - Differential• For Distribution Feeders, Busbars,

Transformers, Generators etc

High Impedance Restricted E/F Biased DifferentialPilot Wire

Types of ProtectionD - Distance• For Transmission and Sub-transmission

Lines and Distribution Feeders, • Also used as back-up protection for

transformers and generators without signaling with signaling to provide unit protection e.g.:– Time-stepped distance protection – Phase comparison for transmission lines – Directional comparison for transmission lines

Types of ProtectionE - Miscellaneous:• Under and over voltage • Under and over frequency • A special relay for generators,

transformers, motors etc. • Control relays: auto-reclose, tap change

control, etc. • Tripping and auxiliary relays

Design Criteria/Characteristics

Simplicity

Economy

Speed

SensitivitySelectivity

Reliability


• Reliability– Operate dependably and in healthy operating

condition when fault conditions occur, even after remaining idle for months or years.

• Selectivity– Clearly discriminate between normal and

abnormal system condition to avoid unnecessary, false trips.

• Sensitivity– Ability to distinguish the fault condition,

although the different between fault and normal condition is small.


• Speed– Fault at any point in the system must be

detected and isolated rapidly to minimize fault duration and equipment damage. Any intentional time delays should be precise.

• Economy– Provide maximum protection at minimum

cost• Simplicity

– Minimize protection equipment and circuitry

System Protection Components

• Transducer / Instrument Transformer• Relay• Circuit Breaker


Function:• Transducers/Instrument Transformers

– Provide low current and voltage, standardized levels suitable for the relays operation.

• Relays– Discriminate between normal operating and fault

conditions.– When current exceed a specified value relay will be

operated and cause the trip coil of CB to be energized/open their contact.

• Circuit Breakers– Open the line



System Protection Flow

Relay Transducer Fault Occur

Circuit Breaker

Fault Clear

voltage or current rise from normal condition

voltage/current is reduced to match with relay rating

activate circuit breaker

circuit isolation

Transducers• Also known as Instrument Transformer• Use to reduce abnormal current &

voltage levels and transmit input signals to the relays of a protection system.

• Why do we need transducer:– The lower level input to the relays ensures

that the physical hardware used to construct the relays will be small & cheap

– The personnel who work with the relays will be working in a safe environment.

Transducers• Current and Voltage Transformers

– Correct connection of CTs and VTs to the protection is important directional, distance, phase comparison and differential protections.

– Earth CT and VT circuits at one point only;

Voltage Transformers• VT is considered to be sufficiently

accurate. • It is generally modeled as an ideal

transformer.• VT secondary connected to

voltage-sensing device with infinite impedance.

Voltage Transformers• Types of VTs

– Electromagnetic VT – Capacitive VT

• Busbar VTs– Special consideration needed when used for line

protection• LV application(15 kV or lower)

– Industry standard – transformer with a primary winding at a system voltage and secondary winding at 66.4 V(line-to-neutral) and 115 V(line-to-line).

Voltage Transformers


Voltage/PotentialTransformer

(VT/PT)



Voltage TransformersHV and EHV• Capacitor-coupled VT (CVT)

– C1 & C2 are adjusted, so that a few kVs of voltage is obtains across C2

– Then, stepped down by T• VTs must be fused or protected by

MCB.


Voltage Transformers• VT ratios:

– ratio of the high voltage/secondary voltage

1:1 2:1 2.5:14:15:1 20:1 40:1 60:180:1 100:1 200:1 300:1400:1 600:1 800:1 1000:12000:1 3000:1 4500:1

Current Transformers• CT is an instrument transformer that is

used to supply a reduced value of current to meters, protective relays, and other instruments.

• The primary winding consist of a single turn which is the power conductor itself.

• CT secondary is connected to a current-sensing device with ideally zero impedance.

Current Transformers• Various CTs ratio(secondary current

rating is 5A)50:5 100:5 150:5 200:5250:5 300:5 400:5 450:5500:5 600:5 800:5 900:51000:5 1200:5

• CTs also available with the secondary rating of 1A

Current Transformers



Exciting Current (IE)

Knee Point Voltage (Vk)

Initial region

Unsaturation region

Saturation region

Current Transformers• Knee Point Voltage: The 'knee-point' of the

excitation curve is defined as 'that point at which a further increase of 10% of secondary e.m.f. would require an increment of exciting current of 50%’

• Metering CT: Instruments and Meters are required to work accurately upto full load, so it advantageous to saturate and protect the instrument under fault condition and hence to have metering CTs with very sharp knee point voltage.

Current Transformers• Protection CT: It should govern with a wide range of

current from acceptable fault setting to maximum fault currents many times normal rating, hence it is important that saturation is avoided wherever possible to ensure positive operation of the relays mainly the currents are many times the normal current.

• Specification Example:Protection Metering

Rated Burdon : 15VA 15VAAccuracy Class : 5P 0.5Accuracy Limit Factor : 20 Class 1.0

ALF is 20 times of rated current

VT and CT Schematic

Zones of Protection• For fault anyway within the zone,

the protection system responsible to isolate everything within the zone from the rest of the system.

• Isolation done by CB• Must isolate only the faulty

equipment or section

Zones of Protection• Zones are defined for:

– Generators– Transformers– Buses– Transmission and distribution lines

Zones of Protection

Transformer Protection

Generator Protection• Following problems require consideration from the

point of view of applying Generator Protection:– Stator Earth Fault– Unbalancing Loading– Rotor Earth Fault– Loss of Synchronism (Out of Step)– Loss of Excitation (Field Failure)– Reverse Power– Over Voltage– Over/Under Frequency– Overspeeding– Exceesive Vibration– Bearing Temperature ..etc……

Stator Earth Fault Protection

• Failure of Stator Windings or connection insulation can result in severe damage to the winding and stator core.

• The most probable mode of insulation failure is phase to earth.

• An earth fault involving the stator core results in burning of the iron at the point of fault and welds laminations together.

• The flashover is more likely to occur in the end winding region, where electrical stresses are highest.

Stator Earth Fault Protection

• Phase to Phase fault are less common, they may occur in the slots if the winding invloves two coil sides in the same slot.

• Inturturn faults are rare but significant fault –loop current can arise where such a fault does occur.

• For machine greater than 1MVA high sensitive, high speed differential Protection is applied for protect heavy damage due to Stator Earth Fault.

• Following Protection are used for Stator Earth Fault Protection………

Differential Protection (87)

PLANT TO BE PROTECTED

I1 I2

I1 = I2 Healthy; no trip| I1 - I2 | > Limit Fault; trip

Differential Protection

• When a short circuit develop anywhere between the two CTs.

• If current flows to the short circuit from both sides as shown, the sum of the CT secondary currents will flow through the differential relay.

• It is not necessary that short-circuit current flow to the fault from both sides to cause secondary current to flow through the differential relay.

Biased Differential Protection

K1

K2

RESTRAIN

OPERATE

Operating Characteristic

Ibias

Idiff

Is2

Is1

In a biased differential relay, the through current is used to increase the setting of the differential element. For heavy through faults, it is unlikely that the CT outputs at each zone end will be identical, due to the effects of CT saturation. In this case a differential current can be produced. However, the biasing will increase the relay setting, such that the differential spill current is insufficient to operate the relay. The differential current setting, “Gen Diff Ιs1”, should be set to a low setting to protect as much of the machine winding as possible. A setting of 5% of rated current of the machine is generally considered to be adequate. “Gen Diff Ιs2”, the threshold above which the second bias setting is applied, should be set to 120% of the machine rated current.The initial bias slope setting, “Gen Diff k1”, should be set to 0% to provide optimum sensitivity for internal faults. The second bias slope may typically be set to 150% to provide adequate stability for external faults.

High Impedance Differential Protection

Due to limitation of percentage biased diff. scheme when assymetric growth in the system current can cause the CT (nearby fault) approach to saturation level.The relay can be made more stable for the maximum applied voltage by increasing the overall impedance of the relay circuit. As the impedance of the relay input is relatively small hence a series connected external resistor is require to increase the overall impedance. An additional Non Linear Resistor (Like Metrosil) shunted connected may be deployed to limit the peak secondary voltage during internal fault condition.

To be cont……

Thanks

Documents

Protection System