4.protective relays

we shall discuss on protection relays:

A special type of relay is one which monitors the current, voltage, frequency, or any other type of electric power measurement either from a generating source or to a load for the purpose of triggering a circuit breaker to open in the event of an abnormal condition. These relays are referred to in the electrical power industry as protective relays.

The circuit breakers which are used to switch large quantities of electric power on and off are actually electromechanical relays, themselves. Unlike the circuit breakers found in residential and commercial use which determine when to trip (open) by means of a bimetallic strip inside that bends when it gets too hot from overcurrent, large industrial circuit breakers must be "told" by an external device when to open. Such breakers have two electromagnetic coils inside: one to close the breaker contacts and one to open them. The "trip" coil can be energized by one or more protective relays, as well as by hand switches, connected to switch 125 Volt DC power. DC power is used because it allows for a battery bank to supply close/trip power to the breaker control circuits in the event of a complete (AC) power failure.

Protective relays can monitor large AC currents by means of current transformers (CT's), which encircle the current-carrying conductors exiting a large circuit breaker, transformer, generator, or other device. Current transformers step down the monitored current to a secondary (output) range of 0 to 5 amps AC to power the protective relay. The current relay uses this 0-5 amp signal to power its internal mechanism, closing a contact to switch 125 Volt DC power to the breaker's trip coil if the monitored current becomes excessive.

Likewise, (protective) voltage relays can monitor high AC voltages by means of voltage, or potential, transformers (PT's) which step down the monitored voltage to a secondary range of 0 to 120 Volts AC, typically. Like (protective) current relays, this voltage signal powers the internal mechanism of the relay, closing a contact to switch 125 Volt DC power to the breaker's trip coil is the monitored voltage becomes excessive.

There are many types of protective relays, some with highly specialized functions. Not all monitor voltage or current, either. They all, however, share the common feature of outputting a contact closure signal which can be used to switch power to a breaker trip coil, close coil, or operator alarm panel. Most protective relay functions have been categorized into an ANSI standard number code. Here are a few examples from that code list:

ANSI protective relay designation numbers

12 = Overspeed          
24 = Overexcitation          
25 = Syncrocheck
27 = Bus/Line undervoltage        
32 = Reverse power (anti-motoring)
38 = Stator overtemp (RTD)     
39 = Bearing vibration     
40 = Loss of excitation           
46 = Negative sequence undercurrent (phase current imbalance)
47 = Negative sequence undervoltage (phase voltage imbalance)
49 = Bearing overtemp (RTD)      
50 = Instantaneous overcurrent
51 = Time overcurrent      
51V = Time overcurrent -- voltage restrained
55 = Power factor          
59 = Bus overvoltage 
60FL = Voltage transformer fuse failure
67 = Phase/Ground directional current
79 = Autoreclose
81 = Bus over/underfrequency

• REVIEW:
• Large electric circuit breakers do not contain within themselves the necessary mechanisms to automatically trip (open) in the event of overcurrent conditions. They must be "told" to trip by external devices.
• Protective relays are devices built to automatically trigger the actuation coils of large electric circuit breakers under certain conditions.

Information on protective relays from a different site:

Introduction to Protective Relay

Protective relay works in the way of sensing and control devices to accomplish its function. Under normal power system operation, a protective relay remains idle and serves no active function.

But when fault or undesirable condition arrives Protective Relay must be operated and function correctly.

A Power System consists of various electrical components like Generator, transformers, transmission lines, isolators, circuit breakers, bus bars, cables, relays, instrument transformers, distribution feeders, and various types of loads.

Faults may occur in any part of power system as a short circuit and earth fault. Fault may be Single Line to Ground, Double Line to Ground, Line to Line,three phase short circuit etc. This results in flow of heavy fault current through the system.

Fault level also depends on the fault impedance which depends on the location of fault referred from the source side. To calculate fault level at various points in the power system, fault analysis is necessary.

The protection system operates and isolates the faulty section. The operation of the protection system should be fast and selective i.e. it should isolate only the faulty section in the shortest possible time causing minimum disturbance to the system. Also, if main protection fails to operate, there should be a backup protection for which proper relay co-ordination is necessary.

Failure of a protective relay can result in devastating equipment damage and prolonged downtime.

Working Principle of Protective Scheme

Protective relaying senses the abnormal condition in a part of power system and gives an alarm or isolates that part from healthy system. Protective relaying is a team work of CT, PT, protective relays, time delay relays, trip circuits, circuit breakers etc.

Protective relaying plays an important role in minimizing the faults and also in minimizing the damage in the event of faults.

Basic connections of circuit breaker control for the opening operation

Figure above shows basic connections of circuit breaker control for the opening operation. The protected circuit X is shown by dashed line. When a fault occurs in the protected circuit the relay connected to CT and PT actuates and closes its contacts.

Current flows from battery in the trip circuit. As the trip coil of circuit breaker is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation.

Thus the fault is sensed and the trip circuit is actuated by the relay and the faulty part is isolated.

What is Relay?

A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts.

These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.

Functions of Protective Relay

These are the main functions of protective relay:

1. To sound an alarm or to close the trip circuit of a circuit breaker so as to disconnect Faulty Section.
2. To disconnect the abnormally operating part so as to prevent subsequent faults. For e.g. Overload protection of a machine not only protects the machine but also prevents Insulation failure.
3. To isolate or disconnect faulted circuits or equipment quickly from the remainder of the system so the system can continue to function and to minimize the damage to the faulty part. For example – If machine is disconnected immediately after a winding fault, only a few coils may need replacement. But if the fault is sustained, the entire winding may get damaged and machine may be beyond repairs.
4. To localize the effect of fault by disconnecting the faulty part from healthy part, causing   least disturbance to the healthy system.
5. To disconnect the faulty part quickly so as to improve system stability, service continuity and system performance. Transient stability can be improved by means of improved   protective relaying.
6. To minimize hazards to personnel.

Desirable Qualities of Protective Relaying

1. Selectivity,
2. Discrimination
3. Stability
4. Sensitivity,
5. Power consumption
6. System Security
7. Reliability
8. Adequateness
9. Speed & Time

Terminology of protective relay

Pickup level of actuating signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated. If the value of actuating quantity is increased, the electromagnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.

Reset level: The value of current or voltage below which a relay opens its contacts and comes in original position.

Operating Time of Relay: Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.

Reset time of Relay: The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.

Reach of Relay: A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.

History of Protective Relay

The evolution of protective relays begins with the electromechanical relays. Over the past decade it upgraded from electromechanical to solid state technologies to predominate use of microprocessors and microcontrollers.

The timeline of the development of protective relays is shown below:

1900 to 1963	1963 to 1972	1972 to 1980	1980 to 1990
Electromechanical Relay	Static Relay	Digital Relay	Numerical Relay
1925=Single Disc Type Relay (Single Input)	1963=Static Relay  (All Purpose)	1980=Digital Type Relay (All Purpose)	1990=Numerical Type Relay (All Purpose)
1961=Single Cup Type Relay (Impedance Relay)	1972=Static Relay with self checking           (All Purpose)

Types of Relays

Types of protection relays are mainly:

A. Based on Characteristic:

1. Definite time Relays.
2. Inverse definite minimum time Relays (IDMT)
3. Instantaneous Relays
4. IDMT with Instantaneous.
5. Stepped Characteristic
6. Programmed Switches
7. Voltage restraint over current relay

B. Based on logic:

1. Differential
2. Unbalance
3. Neutral Displacement
4. Directional
5. Restricted Earth Fault
6. Over Fluxing
7. Distance Schemes
8. Bus bar Protection
9. Reverse Power Relays
10. Loss of excitation
11. Negative Phase Sequence Relays etc.

C. Based on Actuating parameter:

1. Current Relays
2. Voltage Relays
3. Frequency Relays
4. Power Relays etc.

D. Based on Operation Mechanism:

1. Electro Magnetic Relay
2. Static Relay……• Analog Relay
……• Digital Relay
……• Numerical /Microprocessor Relay
3. Mechanical relay

• Thermal• OT Trip (Oil Temperature Trip)
  • WT Trip (Winding Temperature Trip)
  • Bearing Temp Trip etc.
• Float Type• Buchholz
  • OSR
  • PRV
  • Water level Controls etc.
• Pressure Switches
• Mechanical Interlocks
• Pole discrepancy Relay

E. Based on Applications

1. Primary Relays
2. Backup Relays

Types of Relay based on Relay Operation Mechanism

1. Electromagnetic Relay

Electromagnetic relays are further categorized under two following categories.

1.1 Electromagnetic Attraction RelayThis Relay works on Electromagnetic Attraction Principle

1.2 Electromagnetic Induction Relay
This Relay works on Electromagnetic Induction Principle

2. Solid State (Static) Relay

Solid-state (and static) relays are further categorized under following designations:

2.1 Analog Relay
In Analog relays are measured quantities are converted into lower voltage but similar signals, which are then combined or compared directly to reference values in level detectors to produce the desired output.

2.2 Digital RelayIn Digital relays measured ac quantities are manipulated in analogue form and subsequently converted into square-wave (binary) voltages. Logic circuits or microprocessors compare the phase relationships of the square waves to make a trip decision.

2.3 Numerical Relay
In Numerical relays measured ac quantities are sequentially sampled and converted into numeric data form. A microprocessor performs mathematical and/or logical operations on the data to make trip decisions.