Differential Relays MCQ Quiz in தமிழ் - Objective Question with Answer for Differential Relays - இலவச PDF ஐப் பதிவிறக்கவும்

Stator winding faults in alternator:

These faults mainly occur due to insulation failure in the stator windings.

The main faults that occur in the stator are

• Phase to ground faults
• Phase to phase faults

• Inter-turn fault involving turns of the same phase winding

Protection system:

• Differential protection system (Merz-price system) used against phase to phase or phase to earth faults
• The second most important protection scheme for stator winding is inter-turn fault protection.

Important points:

• Overcurrent relay: It is used for the protection of excessive currents due to short circuits, ground faults, etc.
• Thermocouple actuated alarm: It actuates an alarm when the heat or temperature is abnormal in the circuits.
• Reverse power relay: It is used to prevent power flow in the reverse direction.

Voltage (V) = 11 kV

VA rating = 10000 kVA

Full load current \(\left( {{I_L}} \right) = \frac{{10 \times {{10}^6}}}{{\sqrt 3 \times 11 \times {{10}^3}}} = 524.86\;A\)

Out of balance current = 20% of full load current

= 0.2 × 524.86

= 104.97 A

Let x% of the winding remain unprotected.

The fault current \(= \frac{{11 \times {{10}^3}}}{{\sqrt 3 }} \times \frac{1}{7} \times \frac{x}{{100}}\)

\(\Rightarrow \frac{{11 \times {{10}^3} \times x}}{{\sqrt 3 \times 7 \times 100}} > 104.97\) 

⇒ x > 11.57

The percentage of unprotected winding = 11.57%

The percentage of protected winding = 100 – 11.57

The correct answer is 'option 4'

Concept:

• Pilot relaying refers to any relaying scheme employing a communication network between relays, at different substations, to transmit a “trip or don’t trip” signal. The intent here is to trip the circuit breakers as fast as possible.
• Pilot protection schemes use communication channels to send information from the local relay terminal to the remote relay terminal, thereby allowing high-speed tripping for faults occurring within 100% of the protected line.

 Additional InformationWhy pilot relay is used?

• Faster fault clearing time
• When compared to no pilot channel, the protection scheme exhibits increased security and dependability
• Enables high speed reclosing
• Simplifies relay coordination

Differential Protection or Merz-Price protection of Alternators:

• The most common system used for the protection of stator winding faults is the circulating-current principle.
• In this method of protection, currents at the two ends of the protected section are compared.
• Under normal operating conditions, the currents are equal.
• Under abnormal or fault conditions, the current becomes unequal.
• The difference of the unequal currents under fault conditions is arranged to pass through the operating coil of the relay. The relay then closes its contacts to isolate the protected section from the system.
• The schematic arrangement of current differential protection for a 3-phase generator is shown.

• Two identical current transformer pairs CT1 and CT2 are placed on both sides of each phase of the stator windings.
• The secondaries of each set of current transformers are connected to the star.
• The two neutral points and the corresponding terminals of the two-star groups being connected together by means of a four-core pilot cable.
• This pilot cable provided an independent path for the current circulation in each pair of current transformers and the corresponding pilot P.
• The relay coils are connected in star, the neutral point is connected to the current-transformer common neutral, and the outer ends one to each of the other three pilots.
• The burden on each current transformer is the same as the burden of Relay.
• The relays are connected across equipotential points of the three pilot wires and these equipotential points would naturally be located at the middle of the pilot wires.
• The relays are used in this method of protection is instantaneous electromagnetic type.

The differential relay sends the trip signal to the circuit breaker to disconnect the power in the event of a fault, ensuring protection for the electrical equipment and system.

• Differential Relay Function: The primary function of a differential relay is to detect differences in current between two or more points in a power system, such as between the incoming and outgoing currents of a transformer or generator. If a significant difference (indicative of a fault) is detected, the relay initiates a protective action.
• Trip Signal: Upon detecting a fault condition, the differential relay sends a trip signal specifically to the circuit breaker. This action causes the circuit breaker to open, disconnecting the affected section of the electrical system to prevent damage or further faults.
• Importance of the Circuit Breaker: The circuit breaker acts as a switching device that interrupts the flow of current in response to the trip signal from the relay, protecting the equipment and maintaining system stability.
  
  Additional Information 
   

Differential relay:

• A differential relay is one that operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined value.
• Thus a current differential relay is one that compares the current entering a section of the system with the current leaving the section.
• Under normal operating conditions, the two currents are equal but as soon as a fault occurs, this condition no longer applies.
• The difference between the incoming and outgoing currents is arranged to flow through the operating coil of the relay.
• If this differential current is equal to or greater than the pickup value, the relay will operate and open the circuit breaker to isolate the faulty section.

Applications of differential relay:

• Generally, differential relays are used to protect the equipment against internal faults.
• Merz price protection scheme is one form of the differential relay which is used to protect the stator winding of Alternator from the internal faults.
• Differential relay is also used for the protection of transformer winding.
• We can make use of differential relay in the protection of Transmission line and Bus bar protection also.

Difficulties of differential protection:

(1) Saturation of CT magnetic circuit during short circuit condition: Due to these causes the relay may operate even for external faults, the relay may lose its stability for through faults.

To overcome this difficulty, the percentage differential relay or biased differential relay is used.

(2) The difference in pilot wire lengths: The current transformer and machine to be protected are located at different sites and normally it is not possible to connect the relay coil to the equipotential points.

This difficulty is overcome by connecting an adjustable resistor in service with the pilot wires.

(3) CT Ratio error during short circuit:

The current transformer may have an almost equal ratio at normal currents, but during short circuit conditions, the primary currents are unduly large. The ratio error of CT’s on either side differ during this condition due to.

• The inherent difference in CT characteristics arising out of the difference in the magnetic circuit.
• Unequal dc component in short circuit current

(4) Magnetic current inrush in transform while switching in: When a transformer is connected to the supply a large (6 to 10 times full load) current inrush takes place this certainly causes operation of differential relay though is no fault in the transformer.

To avoid this difficulty, harmonic restraint is provided for the differential relay. This relay filters the harmonic component from the inrush current and feeds it to the restraining coil. The coil magnetizing current contains a large content of several harmonic this harmonic content is used for obtaining restraining torque during switching in of transformer.

(5) Tap changing causes a change in the transformation ratio of the transformer:

Because of this, the C.T Ratio does not match with the new tap setting resulting in current in a pilot wire even during health condition, this aspect is taken care of by biased differential relay.

Merz-Price protection is used to protect transformers and generators/alternator.

Note:

Merz-Price protection is used to protect both transformer and alternator but if we have to choose any one option among these two, then the alternator will be the best answer.

Merz price protection:

• Merz price protection is also called differential protection.
• For alternator, the most severe outcome of fault is the damage of stator. For this, the differential protection of alternator or Merz price circulating current protection system is used which protects alternators stator from burning through at least 80% if it.
• If somehow the stator coil is burnt, welded together then there no such solution that to replace it causing longer downtime.
• Merz Price protection scheme or differential circulating current protection scheme functions on the concept of comparing two currents in and out of stator coil.
• In normal condition, the two currents will be the same, and if a fault occurs there will be the same difference, and merz price protection scheme work by detecting this difference or differential currents.

Fig: Merz price protection schemes.

• Under the normal working condition, there will be no current different in two sides of the protected zone, hence two CT’s in a phase will conduct same current (I1 = I2) or (I­1 – I­2 = 0)
• Thus the current is balanced and vectorially assumed up as zero.
• When the fault occurs, there will be a difference between the two current if two CT’s of a  phase (I1 ≠ I2) or (I­1 – I2 ≠ 0)
• Merz price protection detects this difference and operates the relay.

Overcurrent protection:

• Overcurrent protection is protection against excessive currents beyond the acceptable current rating of the equipment. It generally operates instantly magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection be the line.

Earth fault protection:

• Earth fault protection is intended to protect equipment when an insulation fault occurs, for instance, direct contact between a live conductor and earth.
• When earth fault occurs, the electrical system gets short-circuited and the short-circuited current flows through the system.
• Earth fault relay, earth leakage circuit breaker, and ground fault circuit interrupter are used for earth fault protection.

Distance protection:

• Distance protection is a non-unit system of protection, which measures the impedance between the relay location and the point where the fault is incident and compares it with a predefined set value.
• If the measured impedance is less than the set value, the relay operates and isolates the faulty section.
• Two types of distance relay

a) Definite distance Relay – it operates instantly for fault up to a pre-determined distance from the relay.

b) Time distance relay – In this type of relay the time of operation is prepositional to the distance of fault from the relay point.

• Distance relay is employed as a primary as well as backup protection. It is used for the protection of transmission lines
• Distance relay can be used for both phase fault and ground fault protection.

Note:

The connections of the CTs on both the sides of the transformers are as shown in figure.

As the given transformer is connected in Y/Δ, the secondaries of CT should be connected in Δ/Y.

Let the line currents on the primary and secondary sides of the transformers be I_L1 and I_L2 respectively.

\(\sqrt 3 {{\rm{V}}_{{\rm{L}}1}}{{\rm{I}}_{{\rm{L}}1}} = \sqrt 3 {{\rm{V}}_{{\rm{L}}2}}{{\rm{I}}_{{\rm{L}}2}}\)

\(\sqrt 3 \times 66 \times {{\rm{I}}_{{\rm{L}}1}} = \sqrt 3 \times 11 \times {{\rm{I}}_{{\rm{L}}2}}\)

\(\Rightarrow {{\rm{I}}_{{\rm{L}}1}} = \frac{{{{\rm{I}}_{{\rm{L}}2}}}}{6}\)

Given CT ratio = 250/5

\(\Rightarrow {{\rm{I}}_{{\rm{L}}2}} = 250{\rm{\;A}},{\rm{\;}}{{\rm{I}}_{{\rm{L}}1}} = \frac{{256}}{6} = 41.67{\rm{\;A}}\)

Secondary of CT at low voltage side have current of 5 A

Secondary of CT at high voltage side have current of \(\frac{5}{{\sqrt 3 }}{\rm{A}}\)

Let x% be the percentage of winding which remains unprotected

The voltage of unprotected winding \(= \frac{{6.6 \times {{10}^3}}}{{\sqrt 3 }} \times \frac{x}{{100}}\)

The fault current \(\left( {{I_f}} \right) = \frac{{6.6 \times {{10}^3}}}{{\sqrt 3 }} \times \frac{x}{{100}} \times \frac{1}{{10}} = \frac{{6.6x}}{{\sqrt 3 }}\)

The current in pilot wires with of \(\frac{{800}}{5}\)

\(= \frac{{6.6x}}{{\sqrt 3 }} \times \frac{5}{{800}}\)

for the operation of relay \(= \frac{{6.6x}}{{\sqrt 3 }} \times \frac{5}{{800}} = 1\)

Stator Earth Fault Protection

• Stator earth fault protection in alternators involves preventing damage to the stator windings when an insulation failure causes a ground fault. This is typically achieved by limiting the ground fault current and detecting and tripping the circuit during a fault. 
• Stator windings can fail due to insulation breakdown, leading to a short circuit between a winding and the machine's frame (ground). 
• Stator earth fault protection in generators limits ground fault current to minimize damage to the stator core and windings.
• It's achieved by grounding the generator's neutral point with a limited impedance (e.g., a resistor) and using protective relays to detect and respond to earth faults.
• Different methods, like 20Hz injection or third harmonic monitoring, are employed to detect and protect various percentages of the stator winding.