Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network.

Power system protection is the art and science of detecting problems with power system components and isolating these components

The main function of protection is to selectively and rapidly detect and disconnect a fault on the protected circuit to:

• Ensure optimal power quality to customers

• Minimize damage to the primary plant

• Prevent damage to healthy equipment that conducts fault current during faults

• Restore supply over the remaining healthy network

• Sustain stability and integrity of the power system

• Limit safety hazards to the power utility personnel and the public

The objective of a protection system is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation.

In order that the protective relay system perform this function satisfactorily, it should have the following qualities:

A. Dependability

The facet of reliability relates to the degree of certainty that a relay or relay system will operate correctly. Dependability is therefore the measure of the level of confidence or assurance that a protective system will operate accurately when the need arises and faster than designed.

Dependability becomes applicable when an abnormal current situation occurs within the protected zone. A relay system is hence said to be dependable if it actuates only when it is required to do so.

False actuation of protective relay or relay system does not only weaken the system process but could also cause unwarranted discomposure and nuisances for the power system and or outright loss of service.

The protection system must Operate When Required and that could be through

1. Proper system design

2. Backup

• To operate when the main system fails

• To cover any parts that may fall in-between protected zones

B. Security

This is defined as the facet of reliability that relates to the degree of certainty that a relay or relay system will not operate correctly.

Security is therefore the measure of the degree of confidence that a protection system will not actuate erroneously or above the designed speed. Therefore, security becomes relatable in an event of external faults in normal circuit operating conditions. A relay is therefore said to be secure if it does not actuate when it is not required to do so. Hence, the security of a relay is the level of sureness that it will not actuate erroneously.

Important Note:

In designing a protection system, balance must be struck between dependability and security as there is always a tradeoff between them

C. Selectivity "Discrimination"

The ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system. The amount of the power system that must be shut down in response to the event is kept to the absolute minimum

Selectivity can be achieved with devices that are “inherently selective.” That is, they operate only on faults within their “zone of protection” and do not ordinarily sense faults outside that zone.

D. Speed

The clearance of faults in the shortest time is a fundamental requirement but this must be seen in conjunction with the associated cost implications and the performance requirements for a specific application.

E. Sensitivity

The Relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.

The protective system generally shall have the following components

• Sensors [Instrument Transformers]

Current and voltage transformers to measure the parameters of the system

• Main Brain [Protective Relay]

Relays are analyzing the analogue signals from CT and VT as well as the digital signals to detect abnormalities

• Executer [Circuit Breaker]

Circuit Breakers or Contactors respond to the trip command received from the protective relay

• DC Supply

The DC supply is used to feed the control and protection circuits

Any protection relay could be considered as a programmable controller that consists of

a. Inputs

• Hardware Digital Inputs [Opto Coupler]:

Binary inputs are used for statuses, alarms and protection logic signals, the contacts marked with a plus (+) are polarity sensitive. Those contacts not marked with a plus (+) high-isolation control inputs are ground-isolated circuits and are not polarity sensitive. In other words, the relay will detect input changes with voltage applied at either polarity.

Opto-Isolated >> The threshold voltage is fixed depending on the Relay order code e.g., 24,48,110,125,250Vdc

Independent >> The threshold voltage can be configured through the relay software e.g. 15-265Vdc.

• Analogue Inputs: Analogue inputs as current and voltage transformer inputs that are used for different protection functions, also there is another type of analogue inputs that receive signals from a transducer in the form of 4 - 20mA or 0 – 10 Volts that could be used as measurements of RTD sensors and others.

b. Outputs

I/O control outputs from the relay include Standard outputs, Hybrid (high current-interrupting) outputs, and Fast Hybrid (fast high-current-interrupting) outputs either normally open or normally closed output contacts.

c. Interface Ports

Protection relays are equipped with different interface ports used for different purposes as

• User Interface Ports: where the users can apply settings and configuration

• Protection Interface Ports: where it is used for telecommunications between relays

• System Interface: where the relay could be connected to the Substation Automation System

d. Software Protection Functions

Different protection functions could be included in the relay based on the relay application and type. Each protection function consists of

• Virtual Inputs: It could be configured with relay hardware input or from a virtual output of another function

• Virtual Outputs: it could be routed to another function virtual input, LED, or relay hardware BO

• Settings: bases on each function as fault I, V, Z or any other parameters as well as the tripping time.

Zoning in Power system Protection is an important philosophy and must be done carefully so that no part of the system remains unprotected in any condition. To limit the extent of the power system that is disconnected when a fault occurs, protection is arranged in zones. Ideally, the zones of protection should overlap, so that no part of the power system is left unprotected.

CT starpoint shall be considered and standarized to avoid mal-operation for reverse faults and wrong measurements.

CT shorting and isolating facility is very important to allow maintenance of protection relays even if the feeder is energized. Shorting and isolating shall be executed from the terminal block [Disconnected type TB] or through the test switch [if applicable based on the test switch type].

CT Circuit Normalization

The 3-phase CT requires 3 terminal blocks for each phase if the CT was a single ratio, but if the CT is a multi-ratio it would require:

Terminal blocks number = number of CT tabs + 1

To normalize that CT circuit, close terminal blocks number 2,3,5,6,8,9,10 and 11

CT Shorting and Isolating through Terminal Block

Shorting is for the CT secondary winding and Isolating is for the protection relay under maintenance. To make shorting and isolating,

First close terminal blocks number 1,4 and 7 then open 2,5,8 and 10

Shorting Through Terminal Block

The below Terminal Blocks are used in the CT circuit. The disconnected type is used for shoring and isolating as illustrated is the previous point but the other fixed type terminal block is providing the shorting facility by fixing a metal bar as shown below and fix screws that will contact the terminals together through the metal bar.

a. ABB FT test switch

This type of test switch is using fingers, opening fingers would short the CT circuit. However, the isolation of the IED device (Protection Relay or Meter) is achieved when the Test plug handle is inserted in.

b. ABB COMBITEST test switch

The COMBITEST system for secondary injection testing of protection relays and protection IEDs is built up around

the test switches RTXP 8, RTXP 18 or RTXP 24.

The COMBITEST relay test system provides a safe, secure, and user-friendly method for secondary injection testing of relay protection systems, switchboards, and meters during commissioning and maintenance. It helps standardize panel designs across the install base.

When Test-plug handles (RTXH 8, RTXH 18, RTXH 24) is inserted:

• First, shorting the CT secondary circuit

• Then, completely isolates the secondary circuits of the measuring transformers from the IED device.

c. ALSTOM test switch

This type of test switches is providing only isolating facility when the test plug is inserted.

Warning:

It is important that the sockets in the test plug which correspond to current transformer secondary windings are shorted using external links before the test plug is inserted into the test switch.

d. SIEMENS test switch

The test switch serves for testing protection devices including CT circuits and command contacts. With the help of the switches located on the front side, the current and voltage inputs as well as the circuits of the protection device to be tested are interrupted and applied to the plug-in connector located on the front side. Via this plug-in connector currents and voltages can be fed by an injection test set and the different commands and indications can be tested.