Protection, Relays, Circuit Breakers and Earthing

What to remember

• A protection scheme must be selective, fast, sensitive, reliable and stable. The relay senses the fault; the circuit breaker clears it. CTs and PTs feed the relay.
• Overcurrent relays use plug setting (PSM) and time multiplier setting (TMS); differential relays protect transformers, generators and busbars; distance relays protect lines.
• Breaker choice depends on voltage: vacuum for medium voltage, SF6 for high and extra-high voltage. Neutral earthing type decides the size of the earth-fault current.

Basics of protection

Parts of a protection system: current transformer (CT) and potential transformer (PT) to measure, relay to decide, circuit breaker to trip, battery (DC supply) for tripping.

Requirements

• Selectivity: only the faulty part is removed.
• Speed: fault removed quickly, to limit damage and keep stability.
• Sensitivity: operates for the smallest fault current required.
• Reliability: operates when needed and not when not needed.
• Stability: does not trip for through-faults or heavy load.

Zones and backup. The system is divided into overlapping protection zones (generator, transformer, bus, line). Primary protection acts first. Backup protection acts if the primary fails, usually with a time delay and possibly at another location.

Instrument transformers

• CT secondary: 1 A or 5 A. Never open-circuit a CT secondary while the primary carries current; very high voltage appears.
• PT secondary: usually 110 V line-to-line. Never short-circuit a PT secondary.
• CT ratio 400/5 means 80:1.

Relays

By construction

• Electromagnetic attraction (plunger, hinged armature): used for instantaneous operation; works on AC and DC.
• Electromagnetic induction (disc and cup types): work only on AC; used for time-delay overcurrent relays.
• Static (solid state) and numerical (microprocessor) relays: fast, multi-function, with self-checking and event records.

Overcurrent relay

• Plug setting multiplier (PSM) = fault current in relay coil ÷ relay pickup current = (fault current ÷ CT ratio) ÷ (plug setting × relay rated current).
• Time of operation = (time from curve at that PSM) × TMS.
• Pickup setting is usually in steps of 25% from 50% to 200% (for earth fault relays, 10% to 40%).

Worked example. Fault current 2000 A, CT 400/5, relay setting 125% of 5 A.

• Relay current = 2000 ÷ 80 = 25 A.
• Pickup = 1.25 × 5 = 6.25 A.
• PSM = 25 ÷ 6.25 = 4.

If the curve gives 3 s at this PSM with TMS 1, then TMS 0.5 gives 1.5 s.

Characteristics

• IDMT stands for inverse definite minimum time.
• For the normal inverse curve (IEC), t = TMS × 0.14 / (PSM^0.02 − 1).
• Extremely inverse is useful for grading with fuses; very inverse gives large time spread for large fault current changes.

Other relay types

• Directional relay: responds to the direction of power flow; used in ring mains and parallel feeders.
• Distance (impedance) relay: measures the ratio V/I, which is proportional to the distance to the fault. Zone 1 usually covers 80–85% of the line instantaneously; Zone 2 covers the rest of the line plus part of the next line with delay; Zone 3 gives backup.
• Plain impedance relay: for medium lines.
• Reactance relay: for short lines, not affected by arc resistance.
• Mho relay: inherently directional; suitable for long lines.
• Differential relay (Merz-Price): compares currents entering and leaving the protected zone. Operates for internal faults only. Percentage (biased) differential relay avoids false tripping due to CT mismatch.
• Transformer differential protection: CTs on the star side of a star-delta transformer are connected in delta, and CTs on the delta side in star, to compensate the phase shift. The CT ratio compensates for the transformer ratio. Harmonic restraint prevents operation on magnetising inrush.
• Buchholz relay: gas-actuated device placed in the pipe between the transformer tank and the conservator. Slow gas formation gives an alarm (incipient fault); heavy oil surge trips the breaker. It works only with oil-filled transformers that have a conservator.
• Other transformer protections: oil and winding temperature, pressure relief device, restricted earth fault.
• Generator protection: stator differential, rotor earth fault, loss of excitation, reverse power (motoring), negative-sequence (unbalanced load), overspeed, overvoltage.
• Busbar protection: differential scheme with high-impedance or low-impedance relays, and bus-zone sections.
• Motor protection: thermal overload, single-phasing, short-circuit and earth fault.
• Under-frequency relay: used for load shedding.

Circuit breakers

A circuit breaker opens and closes a circuit under normal and fault conditions. An isolator only opens a circuit without current (no load, no arc quenching) and is operated after the breaker has opened.

Arc and its extinction

• Arc forms when contacts part; it carries current through ionised gas.
• In AC breakers the arc is extinguished at a current zero. After that, the voltage across the contacts rises (recovery voltage). If the dielectric strength recovers faster than the voltage rises, the arc does not restrike.
• Rate of rise of restriking voltage (RRRV) is a critical factor. Parallel resistors or capacitors reduce it.
• Current chopping: forcing the current to zero before its natural zero; it produces overvoltage in inductive circuits.

Types of circuit breakers

SF6 gas: colourless, odourless, non-toxic in pure form, non-flammable, strongly electronegative (it captures free electrons), with high dielectric strength (higher than air). Used in gas insulated substations (GIS). It is a strong greenhouse gas, so leaks must be avoided.

Ratings

• Breaking capacity (MVA) = √3 × rated line voltage (kV) × breaking current (kA). For 11 kV and 25 kA: 1.732 × 11 × 25 = 476 MVA.
• Making capacity = about 2.55 × symmetrical breaking current (peak value). For 25 kA, 63.75 kA.
• Short-time rating: current it can carry for a stated time (often 1 s or 3 s).
• Rated normal current and rated voltage.

Fuse. A fuse melts and breaks the circuit. HRC (high rupturing capacity) fuse has an element in sand or ceramic body and a high breaking capacity. Fusing factor = minimum fusing current ÷ rated current; it is always greater than 1 (about 1.5 for a rewirable fuse; closer to 1.1 to 1.2 for HRC).

Lightning arresters (surge arresters). Protect equipment against lightning and switching surges by diverting surge current to earth. Modern arresters use zinc oxide (metal oxide, gapless). The protection level must be lower than the insulation strength (BIL) of the equipment. Install them as close to the equipment as practical. Ground wires (shield wires) on top of towers shield lines from direct strokes.

Earthing

System (neutral) earthing

Equipment (protective) earthing connects metal frames to earth, so that a fault drives current through the earth path and trips a breaker, and so that touch voltage stays low.

Earth electrodes: plate earthing, pipe earthing, rod and strip. Salt and charcoal around a pipe electrode keep soil moist and low in resistance. Earth resistance should be as low as possible; it changes with soil type and moisture.

Safety terms. Touch potential is the voltage between a grounded structure and the ground where a person stands. Step potential is the voltage between two feet on the ground. Substation earth grids and gravel surface layers reduce both.

Exam traps

• Relay senses and decides; the circuit breaker interrupts.
• A CT secondary must not be open; a PT secondary must not be shorted.
• PSM uses relay current, not primary current.
• IDMT: time decreases as current increases.
• Mho relay is inherently directional; plain impedance relay is not.
• Buchholz relay is for oil-filled transformers with conservator, not for dry-type.
• Isolator is operated without load; circuit breaker can break fault current.
• SF6 is electronegative and heavier than air.
• Vacuum breakers suit medium voltage; SF6 suit high and EHV.
• Making capacity is higher than breaking capacity (about 2.55 times).
• Fusing factor is greater than 1.
• Peterson coil is used in a resonant earthed system, not a solidly earthed system.

One-liners

• 1. PSM = fault current in relay ÷ relay pickup current.
• 2. Operating time = curve time × TMS.
• 3. Overcurrent induction relay uses a disc; it works on AC only.
• 4. Differential protection compares current at the two ends of the zone.
• 5. Buchholz relay detects incipient faults in oil transformers.
• 6. Zone 1 of a distance relay covers about 80% of the line.
• 7. Mho relay characteristic is a circle through the origin.
• 8. CT secondary current is normally 1 A or 5 A.
• 9. AC arc is extinguished at a current zero.
• 10. Breaking MVA = √3 × kV × kA.
• 11. Making current = about 2.55 × symmetrical breaking current.
• 12. Peterson coil neutralises the capacitive earth-fault current.

Practice questions

1. A fault current of 2000 A flows in a line with a CT of 400/5. The relay is set at 125% of 5 A. The plug setting multiplier is:

   1. 2.5
   2. 5
   3. 4
   4. 8
   Answer

   C. 4

   Relay current = 2000/80 = 25 A; pickup = 6.25 A; PSM = 25/6.25 = 4.

2. A CT of ratio 400/5 carries a primary fault current of 2000 A. The relay coil current is:

   1. 50 A
   2. 25 A
   3. 5 A
   4. 160 A
   Answer

   B. 25 A

   CT ratio = 80, so the secondary current is 2000/80 = 25 A.

3. An overcurrent relay gives an operating time of 3 s at a given PSM with TMS = 1. With TMS = 0.5 the time becomes:

   1. 1.5 s
   2. 3 s
   3. 6 s
   4. 0.5 s
   Answer

   A. 1.5 s

   Operating time = curve time × TMS = 3 × 0.5 = 1.5 s.

4. A circuit breaker on an 11 kV system has a breaking current of 25 kA. Its breaking capacity is about:

   1. 43 MVA
   2. 476 MVA
   3. 275 MVA
   4. 825 MVA
   Answer

   B. 476 MVA

   MVA = √3 × 11 × 25 = 476.

5. A 33 kV circuit breaker has a breaking current of 20 kA. Its breaking capacity is about:

   1. 572 MVA
   2. 1980 MVA
   3. 660 MVA
   4. 1143 MVA
   Answer

   D. 1143 MVA

   MVA = 1.732 × 33 × 20 = 1143.

6. A breaker has a symmetrical breaking current of 25 kA. Its making current is about:

   1. 63.75 kA
   2. 35.4 kA
   3. 25 kA
   4. 12.5 kA
   Answer

   A. 63.75 kA

   Making current = 2.55 × 25 = 63.75 kA.

7. A fuse of 10 A rating has a fusing factor of 1.5. The minimum fusing current is:

   1. 15 A
   2. 25 A
   3. 6.7 A
   4. 10 A
   Answer

   A. 15 A

   Fusing current = fusing factor × rating = 15 A.

8. A PT of ratio 11000/110 V has 5500 V on the primary. The secondary voltage is:

   1. 550 V
   2. 110 V
   3. 55 V
   4. 5.5 V
   Answer

   C. 55 V

   The ratio is 100, so 5500/100 = 55 V.

9. A relay has a plug setting of 150% with a 5 A rated coil and a CT of 100/5. The primary pickup current is:

   1. 300 A
   2. 7.5 A
   3. 100 A
   4. 150 A
   Answer

   D. 150 A

   Relay pickup = 1.5 × 5 = 7.5 A; primary = 7.5 × 20 = 150 A.

10. A line has an impedance of 10 ohm. Zone 1 of a distance relay is set at 80% of this. The Zone 1 setting is:

    1. 2 ohm
    2. 8 ohm
    3. 12 ohm
    4. 10 ohm
    Answer

    B. 8 ohm

    80% of 10 = 8 ohm.

11. In a resonant earthed system, the Peterson coil inductance for a per-phase capacitance C at angular frequency ω satisfies:

    1. L = ωC / 3
    2. L = 3 / (ω²C)
    3. L = 1 / (ω²C)
    4. L = 1 / (3ω²C)
    Answer

    D. L = 1 / (3ω²C)

    The coil must cancel the capacitive earth fault current: ωL = 1/(3ωC).

12. If the secondary of a current transformer is open-circuited while the primary carries current:

    1. The secondary current doubles
    2. The primary current falls to zero
    3. A dangerously high voltage develops
    4. Nothing happens
    Answer

    C. A dangerously high voltage develops

    No secondary ampere-turns oppose the flux, so the core saturates and a very high voltage appears.

13. The secondary of a potential transformer must never be:

    1. Short-circuited
    2. Connected to a relay
    3. Open-circuited
    4. Grounded at one point
    Answer

    A. Short-circuited

    A shorted PT secondary draws excessive current and burns out.

14. The Buchholz relay is located:

    1. Inside the winding
    2. In the pipe between the transformer tank and the conservator
    3. On the bushing
    4. In the radiator fins only
    Answer

    B. In the pipe between the transformer tank and the conservator

    Gas rising from the tank passes through this pipe.

15. A Buchholz relay is used to detect:

    1. Overload in a dry-type transformer
    2. Earth fault in a motor
    3. Internal faults in an oil-filled transformer
    4. Faults on a transmission line
    Answer

    C. Internal faults in an oil-filled transformer

    It senses gas and oil surge due to incipient and severe internal faults.

16. The principle of differential protection is to compare:

    1. Temperature and pressure
    2. Phase sequence
    3. Voltage and frequency
    4. The currents entering and leaving the protected zone
    Answer

    D. The currents entering and leaving the protected zone

    A difference in current means an internal fault.

17. A directional overcurrent relay responds to:

    1. Voltage only
    2. Frequency only
    3. Current and its direction of power flow
    4. Temperature
    Answer

    C. Current and its direction of power flow

    It operates for faults in one direction only.

18. Which distance relay is inherently directional?

    1. Reactance relay
    2. Mho relay
    3. Plain impedance relay
    4. Overcurrent relay
    Answer

    B. Mho relay

    The mho characteristic is a circle passing through the origin, so it only responds in one direction.

19. Which distance relay is best suited to short lines because it is not affected by arc resistance?

    1. Reactance relay
    2. Mho relay
    3. Directional relay
    4. Plain impedance relay
    Answer

    A. Reactance relay

    A reactance relay measures only the reactive component of impedance.

20. Induction-type relays (disc or cup) work on:

    1. DC only
    2. Both AC and DC
    3. High-frequency supply only
    4. AC only
    Answer

    D. AC only

    They need a changing flux to induce eddy currents.

21. In an IDMT relay, the operating time:

    1. Decreases as the fault current increases
    2. Is the same for all currents
    3. Is zero at all currents
    4. Increases as the current increases
    Answer

    A. Decreases as the fault current increases

    IDMT means inverse definite minimum time.

22. SF6 gas is used in circuit breakers mainly because it is:

    1. Light in weight
    2. Strongly electronegative, with high dielectric strength
    3. A good conductor
    4. Flammable
    Answer

    B. Strongly electronegative, with high dielectric strength

    It captures free electrons, which gives fast arc extinction.

23. Vacuum circuit breakers are commonly used in:

    1. Medium-voltage systems such as 11 kV to 33 kV
    2. 800 kV transmission only
    3. Low-voltage fuses
    4. Only in generators of 100 MW
    Answer

    A. Medium-voltage systems such as 11 kV to 33 kV

    They suit frequent switching and low maintenance at medium voltage.

24. An isolator:

    1. Can break fault current
    2. Has a tripping coil
    3. Extinguishes the arc by oil
    4. Is operated only when no current flows
    Answer

    D. Is operated only when no current flows

    Isolators open a circuit with no load; breakers handle load and fault current.

25. An AC arc in a circuit breaker is extinguished mainly at:

    1. Peak current
    2. Any instant by magnetic force
    3. A natural current zero
    4. Voltage maximum only
    Answer

    C. A natural current zero

    At current zero the arc can be deionised and prevented from restriking.

26. The rate of rise of restriking voltage can be reduced by:

    1. Increasing the arc length only
    2. Connecting a resistor in parallel with the breaker contacts
    3. Removing arc chutes
    4. Using a smaller contact gap
    Answer

    B. Connecting a resistor in parallel with the breaker contacts

    A parallel resistor damps the transient and lowers RRRV.

27. Current chopping in a circuit breaker causes:

    1. Increased insulation
    2. Lower arc voltage
    3. Overvoltage when inductive current is interrupted
    4. Improved fault clearing in all cases
    Answer

    C. Overvoltage when inductive current is interrupted

    Interrupting current before its natural zero stores energy in inductance, which produces a voltage surge.

28. Modern surge arresters generally use:

    1. Silicon carbide with an air gap only
    2. Copper strips
    3. Mica
    4. Zinc oxide (metal oxide) blocks
    Answer

    D. Zinc oxide (metal oxide) blocks

    Zinc oxide has a highly nonlinear voltage-current curve.

29. The purpose of overhead ground (shield) wires is to:

    1. Reduce line reactance
    2. Shield the line from direct lightning strokes
    3. Raise system voltage
    4. Carry load current
    Answer

    B. Shield the line from direct lightning strokes

    They intercept the stroke and send it to earth through the tower.

30. Compared with other methods, solid neutral earthing gives:

    1. The highest earth-fault current and the lowest overvoltage
    2. The smallest fault current
    3. Very high overvoltages
    4. No fault current
    Answer

    A. The highest earth-fault current and the lowest overvoltage

    A direct earth connection has low impedance, so the fault current is high.

31. In a resonant earthed system, the Peterson coil:

    1. Blocks load current
    2. Increases the fault current
    3. Reduces insulation
    4. Cancels the capacitive earth-fault current
    Answer

    D. Cancels the capacitive earth-fault current

    It tunes with line capacitance so an arcing earth fault is extinguished.

32. In an ungrounded system, during a single line-to-ground fault, the voltage of the healthy phases to earth rises to:

    1. Line-to-line voltage
    2. Phase voltage
    3. Zero
    4. Half the phase voltage
    Answer

    A. Line-to-line voltage

    The neutral shifts, so healthy phase voltage becomes √3 times the phase voltage.

33. Touch potential is the voltage between:

    1. Neutral and a phase
    2. A grounded structure and the ground at the point where a person stands
    3. Two feet of a person
    4. Two phases
    Answer

    B. A grounded structure and the ground at the point where a person stands

    Step potential is the voltage between two feet.

34. In a star-delta power transformer differential scheme, the CTs on the star side are connected in:

    1. Star
    2. Delta
    3. Open delta only
    4. Series with the relay coil only
    Answer

    B. Delta

    This compensates the 30° phase shift of the power transformer.

35. Statements: 1. A relay senses a fault and sends the trip signal. 2. An isolator is used to interrupt fault current.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Isolators cannot break current; circuit breakers do.

36. Statements on SF6: 1. It has high dielectric strength. 2. It is non-electronegative.

    1. 2 only
    2. Both 1 and 2
    3. Neither 1 nor 2
    4. 1 only
    Answer

    D. 1 only

    SF6 is strongly electronegative, which makes it a good arc-quenching gas.

37. Statements on IDMT relay: 1. Operating time decreases as current increases. 2. Time of operation = time from curve × TMS.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

38. Statements on neutral earthing: 1. Resistance earthing limits earth-fault current. 2. Solid earthing gives a larger earth-fault current than resistance earthing.

    1. 1 only
    2. Both 1 and 2
    3. 2 only
    4. Neither 1 nor 2
    Answer

    B. Both 1 and 2

    A lower neutral impedance gives higher fault current.

39. Statements on instrument transformers: 1. The CT secondary must not be open-circuited. 2. The PT secondary must not be short-circuited.

    1. Both 1 and 2
    2. 1 only
    3. 2 only
    4. Neither 1 nor 2
    Answer

    A. Both 1 and 2

    Both are standard safety rules.

40. Statements on differential protection: 1. It operates for faults inside the protected zone. 2. It is the preferred protection for a transformer winding.

    1. 1 only
    2. 2 only
    3. Neither 1 nor 2
    4. Both 1 and 2
    Answer

    D. Both 1 and 2

    Differential relays are zone-selective and widely used for transformers, generators and buses.

41. Statements on breaker ratings: 1. Making current is about 2.55 times the symmetrical breaking current. 2. Breaking capacity in MVA = √3 × kV × kA.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are standard rating relationships.

42. Which pair is correctly matched?

    1. Mho relay – non-directional
    2. Peterson coil – solidly earthed neutral
    3. Buchholz relay – oil-filled transformer with conservator
    4. Vacuum breaker – 800 kV lines
    Answer

    C. Buchholz relay – oil-filled transformer with conservator

    A mho relay is directional; vacuum breakers suit medium voltage; the Peterson coil is for resonant earthing.

43. Statements on lightning protection: 1. The protective level of an arrester should be below the insulation strength of the equipment. 2. Arresters should be placed as close to the protected equipment as practical.

    1. 1 only
    2. 2 only
    3. Neither 1 nor 2
    4. Both 1 and 2
    Answer

    D. Both 1 and 2

    Both are correct for good insulation coordination.

44. The fusing factor of a fuse is always:

    1. Equal to the rated current
    2. Greater than 1
    3. Less than 1
    4. Equal to 0
    Answer

    B. Greater than 1

    Minimum fusing current is higher than the rated current.