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CHAPTER 6
PROTECTIVE RELAY

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6.1 INTRODUCTION
-Power systems and their components need protection from natural hazards as well as human error.
-Lightning, wind, ice, switching surges, resonance, trees, animals and humans are some of the causes of faults.
-These faults produce overcurrents and/or overvoltages at various locations in a power system and must be cleared before they cause before they damage any machines, transformers, lines etc.
-This is generally accomplished by isolating the faulted portion (as small a portion as possible) of the system so that the remainder of the system can serve without interruption.

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In low-voltage distribution systems, lightning (surge) arresters are used for overvoltage protection and fuses and slow-acting circuit breakers are employed for overcurrent protection.
In high-voltage transmission systems reliable sensors, fast-acting relays and circuit breakers are needed to clear the fault quickly so that the stability of the remaining system is secured.
- In short, the protection, stability and security of a power system are affected by the ability of the protection devices to detect and respond to system abnormalities like overvoltages and overcurrents

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6.1 Protection Components
Protection systems have three basic components:
-Sensors (transducers, detectors) to detect system abnormalities
-Relays (activators) to provide signals to activate the protection devices.
-Circuit breakers (interrupters) to open (disconnect) the circuits

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Figure 6.0

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In order to perform its functions properly, the protection system must have the following characteristic:
Reliability: The reliability of the protection system is its ability to operate upon the occurrence of any fault for which it was designed to protect. In other words, the protection system should operate when it is supposed to and not operate when it is not required.
Selectivity: Selectivity is the ability of the protection system to detect a fault, identify the point at which the fault occurred and isolate the faulted circuit element by tripping the minimum number of circuit breakers. Selectivity of the protection system is obtained by proper coordination of the operating currents and time delays of the protective relays.

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6.2 CLASSIFICATIONS OF RELAYS
Relays may be classified according to the technology
used:
a. electromechanical
b. static
c. digital
d. numerical

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c) Speed: The speed of the protection system refers to the operating times of the protective relay. The potential damage to the faulted element depends on the length of time the short-circuit currents are allowed to flow. The speed of clearing or isolating the faulted system component also affects the stability of the whole system.
d) Sensitivity: Sensitivity refers to the characteristic of a protective relay that it operates reliably, when required, in response to a fault that produces the minimum short-circuit current flowing through the relay.

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6.2.1 ELECTROMECHANICAL RELAYS
Electromechanical relays can be classified into several
different types as follows:
attracted armature – comprises of iron cored electromagnet which attracts
an armature which is pivoted, hinged or supported.
b. moving coil- light coil which can be energised moves in a strong permanent
magnet field.
c. Induction – operated on the same principle as the induction motor
d. Thermal – operates based on the generated heats in a resistance winding
and temperature sensitive component
e. Timing relays- are used in conjunction with protection relays

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Static Relays
-The term ‘static’ implies that the relay has no moving parts.
-In a protection relay, the term ‘static’ refers to the absence of moving parts to create the relay characteristic.
-Their design is based on the use of analogue electronic devices instead of coils and magnets to create the relay characteristic.
-Early versions used discrete devices such as transistors and diodes in conjunction with resistors,capacitors, inductors,
- Advance versions enabled the use of linear and digital integrated circuits in later versions for signal processing and implementation of logic functions.

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Digital Relays
-Digital protection relays introduced a step change in technology.
Microprocessors and microcontrollers replaced analogue circuits used
in static relays to implement relay functions.
Compared to static relays, digital relays introduce A/D conversion of all measured analogue quantities and use a microprocessor to implement the protection algorithm.
-The microprocessor may use some kind of counting technique, or use the Discrete Fourier Transform (DFT) to implement the algorithm.

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6.2.4 Numerical Relays
Typically, they use a specialised digital signal processor (DSP) as the computational hardware, together with the associated software tools.
- The input analogue signals are converted into a digital representation and processed according to the appropriate mathematical algorithm.
- Processing is carried out using a specialised microprocessor that is optimised for signal processing applications, known as a digital signal
processor

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6.3 Zone of Protection
To limit the extent of the power system that is disconnected when a fault occurs, protection is arranged in zones.(Figure 6.1).
Ideally, the zones of protection should overlap, so that no part of the power system is left unprotected. (Figure 6.2).

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Figure 6.1

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Figure 6.2

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6.4 Protection principles
The best and common protection techniques can be classified into
2 categories
a) Overcurrent/earth fault-distance protection
b) Differential protection

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6.4.1 Overcurrent and Earth-fault Protection
Protection against excess current was naturally the earliest protection system to evolve.
The actuating quantity of an overcurrent relay is a current.
The relay is designed to operate when the actuating quantity equals or exceeds its pickup value.
An overcurrent relay can either be an instantaneous type or a time-delay type.

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IDMTL relay
Most commonly used type of relay.
Characteristics of relay is the higher the current, the shorter the
operating time.
The current/time tripping characteristics of IDMT relays may need to be varied according to the tripping time required and the characteristics of other protection devices used in the network.
IEC defines a number of standard characteristics as follows:
a) Standard Inverse (SI)
b) Very Inverse (VI)
c) Extremely Inverse (EI)
d) Definite Time (DT)

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The tripping characteristics for
different TMS settings using the SI curve

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Setting of IDMTL relays
In order to adjust the current setting, the relay coil is arranged to have
a tapped winding which is connected to a plug bridge.
(electromechanical relay)
In modern relays, it is known as PLUG SETTING MULTIPLIER.
Example: if Tap is set at 80% of 5A, then the current into the relay is
0.8 x 5A = 4A.
TIME MULTIPLIER SETTING (TMS) the time the relay disc to move through 180 degrees or relay operates to trip
- The setting is in term of percentage such as 10 %(0.1), 20%(0.2), 30% (0.3) etc.

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Tripping time of standard overcurrent relay:
Where: TMS = time multiplier setting

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Example:
Calculate tripping time of an overcurrent relay at 150% with the
The PSM setting is at 80%. CT ratio connected to the CT is 1000/5A
The TMS setting is 0.1
Solutions:
Time,
t = 1.72 s

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6.5 RELAY TESTING AND COMMISIONING

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Tripping time of standard overcurrent relay:
Where: TMS = time multiplier setting

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Tripping time of standard overcurrent relay:
Where: TMS = time multiplier setting