Junior Lineman / Energy Assistant — ITI Electrical Quick Book · Chapter 8

Substations and Distribution: DTR, Fuses, Breakers, Protection Basics

What to remember

A substation changes voltage and controls power. A distribution transformer (DTR) steps 11 kV down to 415 V (240 V per phase). Switchgear opens, closes and protects the circuit.
Protection has a fixed order of devices: fuse or relay senses the fault, the circuit breaker clears it, the isolator gives a visible gap for safe work, and the lightning arrester guards against surges.
Isolator is never operated on load. Open the breaker first, then the isolator. Close in the reverse order.

Substation basics

A substation is a place where voltage is changed, power is switched and the system is protected.

Type	Job
Step-up	At the power station, raises voltage for transmission
Grid / receiving	Steps down transmission voltage (for example 220 or 132 kV) to sub-transmission level
Primary distribution	Steps down to 33 kV or 11 kV, for example 33/11 kV
Secondary distribution (DTR)	Steps 11 kV down to 415 V for consumers
Switching	Only switching, no transformation

By construction a substation may be outdoor (open air), indoor, pole-mounted, plinth-mounted or gas-insulated (GIS, which uses SF6 gas and needs little space).

Main equipment in a substation

Busbar: a thick conductor or bar that collects and distributes power.
Isolator (disconnector): gives a visible open gap. No arc-quenching, so it is operated only when the circuit carries no current.
Circuit breaker: opens and closes the circuit on load and on fault.
Current transformer (CT) and potential (voltage) transformer (PT or VT): feed meters and relays.
Relay: the "brain" that senses a fault and trips the breaker.
Lightning (surge) arrester: sends surge current to earth. Modern ones use zinc oxide.
Power transformer, capacitor bank, earthing system, battery bank for tripping supply.
Earthing mat, fire fighting set and fencing.

Distribution system

Generation -> transmission (high voltage, low loss) -> sub-transmission -> primary distribution (33 kV and 11 kV feeders) -> DTR -> secondary distribution (LT lines) -> service connection to the consumer.
Feeder: a line from a substation that carries power to a distribution area. No tapping is done on it along its way, except at planned points.
Distributor: the line from which consumers' service lines are taken.
Service main: the wire from the distributor to the meter.

Scheme	Features
Radial	One path from substation to load. Cheap and simple. A fault interrupts all downstream customers.
Ring main	Feeders form a loop. Each load can be fed from two sides. A fault section is isolated and the rest keep supply.
Interconnected	Several sources feed a network. Best continuity and highest cost.

In Andhra Pradesh, transmission is looked after by APTRANSCO. Distribution is handled by three DISCOMs for the regions (southern, eastern and central). Feeders are often separated by load type, for example agriculture, domestic and industrial.

Distribution transformer (DTR)

Standard ratio: 11 kV / 433 V (commonly called 11 kV / 415 V, 3-phase 4-wire).
Common ratings are 25, 63, 100, 160 and 250 kVA. Small ratings are pole-mounted. Bigger ones sit on a plinth.
Winding connection: delta on the HV side and star with neutral on the LV side (Dyn11). The neutral gives 240 V between phase and neutral.
Cooling: oil natural, air natural (ONAN). Oil insulates and cools.

Parts of a DTR

Tank with radiators or cooling tubes.
Conservator: an expansion tank for oil. It also keeps the oil level steady.
Breather with silica gel: dries the air that enters. Blue silica gel turns pink when wet and must be dried or changed.
HV and LV bushings with terminals.
Oil level gauge, thermometer pocket, drain valve and filling plug.
Explosion vent (relief) on bigger units, and Buchholz relay on larger rated units (above about 500 kVA).
Tap changer (off-circuit) to adjust the voltage by a few percent.
Name plate: gives kVA, voltages, current, vector group, impedance and oil quantity.

Current calculation

Full-load current I = kVA × 1000 / (√3 × V).
100 kVA at 433 V: I = 100000 / (1.732 × 433) = 100000 / 750 = about 133 A.
100 kVA at 11 kV: I = 100000 / (1.732 × 11000) = 100000 / 19052 = about 5.25 A.
Transformer turns ratio = primary voltage / secondary voltage. 11000 / 433 = about 25.4.
Efficiency = output / input. All-day efficiency matters for DTRs because their load varies in the day.

Earthing of a DTR

Body (frame) earth, neutral earth and lightning arrester earth should have separate earth pits. Pits are kept moist and joined by GI strip or wire.
Earth resistance must be low. It is tested with an earth tester and records are kept.

Common DTR faults and causes

Overloading and unbalanced load, which overheat the oil.
Loose connections or hot spots at bushings.
Oil leaks or low oil level, and moist silica gel.
LT side short circuit, and lightning surges.
Wrong or oversized fuse so that the fault is not cleared in time.

Routine maintenance: check oil level and leaks, check the breather, tighten terminals, check earth resistance, clean bushings, check fuse rating and note load current.

Fuses

A fuse is the simplest protective device. A thin element melts when current is too high and opens the circuit. It is a one-time device and must be replaced after use.

Fuse materials: tinned copper, lead-tin alloy, aluminium and silver. Silver and copper are used for small, accurate fuses.
Rated current: the current the fuse can carry continuously.
Fusing current: the minimum current at which the element melts.
Fusing factor = fusing current / rated current. It is always more than 1. Example: fusing current 15 A, rated current 10 A: factor = 1.5.
Fusing current relation: I = k × d^(3/2). A thicker wire (larger d) needs more current to melt.
Rewirable (kit-kat) fuse: used on LT circuits and in DTR LT boxes. Cheap, but the breaking capacity is low and the wire can be wrongly replaced.
HRC (high rupturing capacity) fuse: a sealed cartridge filled with sand or quartz that absorbs the arc. Safe, with high breaking capacity.
Drop-out (DO) fuse: used on 11 kV pole-mounted DTRs. On melting, the fuse tube drops open, giving a visible break.
Rule of thumb: the HT fuse for a DTR is chosen at about 125 to 150 percent of the full-load current. Do not oversize it.

Circuit breakers

A circuit breaker opens a circuit with load or fault current and can be closed again. The arc between the opening contacts must be quenched.

Type	Arc-quenching medium	Note
ACB (air circuit breaker)	Air	LT and some 11 kV use
OCB (oil circuit breaker)	Transformer oil	Older design. Fire risk, needs oil care
VCB (vacuum circuit breaker)	Vacuum	Very common at 11 kV and 33 kV. Compact, little maintenance
SF6 circuit breaker	SF6 gas	Used at higher voltages
MCB / MCCB	Air	Small and moulded-case breakers for LT circuits

Breaking capacity: highest fault current the breaker can safely interrupt.
Making capacity: highest current the breaker can close on.
Auto-recloser: a breaker that trips and recloses a few times to clear temporary faults on a feeder. Sectionaliser: opens the faulty section in the dead time.
Isolator vs breaker: breaker is for load and fault, isolator is for isolation only.
Operating sequence: to switch off, open the breaker, then the isolator. To switch on, close the isolator, then the breaker.

Protection basics

Fault types: line to ground (the most common), line to line, double line to ground and 3-phase (rare but severe). Overload is not a fault but a long-lasting heavy current.
Overcurrent relay: trips when current goes above a set value. IDMT (inverse definite minimum time) relays trip faster at higher current.
Earth fault relay: senses current to earth.
Buchholz relay: gas-operated relay on the pipe between tank and conservator. It senses gas from internal faults in an oil transformer.
Differential relay: compares current entering and leaving the transformer. Used on larger power transformers.
Lightning arrester: across line and earth. Normally open. At a surge it conducts, then it resets.
CT and PT rules: never open-circuit the secondary of a CT, because a dangerously high voltage appears. Never short-circuit the secondary of a PT. CT secondary rating is usually 1 A or 5 A. PT secondary is usually 110 V.
Capacitor bank: improves power factor and reduces losses.
Protection system aims: speed, selectivity (only the faulty part trips), sensitivity and reliability.

Exam traps

Isolator vs circuit breaker: the isolator has no arc-quenching and is never opened on load.
Fuse vs MCB: a fuse is single-use. An MCB can be reset.
CT secondary must not be open. PT secondary must not be shorted.
Conservator holds the extra oil. The breather holds silica gel.
Buchholz relay detects gas. It does not detect over-voltage.
Fusing factor is more than 1. It is not less than 1.
Delta side is HV and star side is LV in a DTR, so that the LV neutral is available.
Line to ground fault is the most common. A 3-phase fault is the most severe but rare.

One-liners

1. DTR ratio is 11 kV / 433 V.
2. The LV side of a DTR is star with neutral.
3. A conservator takes up oil expansion.
4. Blue silica gel turns pink when it is damp.
5. A Buchholz relay detects gas in oil.
6. A VCB is common at 11 kV and 33 kV.
7. SF6 is used in gas-insulated substations and breakers.
8. Isolators are operated only with no load.
9. HRC fuse has a sealed cartridge filled with sand or quartz.
10. DO fuse gives a visible break at 11 kV poles.
11. Full-load current = kVA × 1000 / (√3 × V).
12. A ring main supply gives better continuity than a radial one.

Practice questions

The standard ratio of a pole-mounted distribution transformer in the LT network is
1. 33 kV / 11 kV
2. 11 kV / 3.3 kV
3. 11 kV / 433 V
4. 132 kV / 33 kV
Answer
C. 11 kV / 433 V
A DTR steps 11 kV down to about 433 V (415 V line voltage at load).
What is the main job of the conservator of a transformer?
1. To hold the tap changer
2. To cool the winding with air
3. To measure current
4. To take up expansion of the oil
Answer
D. To take up expansion of the oil
The conservator is an oil tank above the main tank.
Silica gel in a transformer breather is used to
1. reduce noise
2. absorb moisture from the air
3. cool the oil
4. detect gas
Answer
B. absorb moisture from the air
The breather dries the air drawn in when the oil cools.
Which relay is operated by gas formed in a transformer due to an internal fault?
1. Earth fault relay
2. Buchholz relay
3. Distance relay
4. Overcurrent relay
Answer
B. Buchholz relay
The Buchholz relay is in the pipe between the tank and the conservator.
An isolator should be operated only
1. during a fault
2. with the breaker closed
3. while carrying full load
4. when no current is flowing
Answer
D. when no current is flowing
An isolator has no arc-quenching, so it is opened only on no load.
What is the arc-quenching medium in a vacuum circuit breaker?
1. Transformer oil
2. Air at high pressure
3. Vacuum
4. SF6 gas
Answer
C. Vacuum
The arc is quenched inside a sealed vacuum bottle.
An HRC fuse cartridge is filled with
1. transformer oil
2. sand or quartz powder
3. silica gel
4. SF6 gas
Answer
B. sand or quartz powder
The filler absorbs the arc energy.
The LV winding of a distribution transformer is usually connected as
1. delta
2. star with a neutral
3. open delta
4. zig-zag only
Answer
B. star with a neutral
The star point gives the neutral for 240 V single-phase supply.
The secondary of a current transformer must never be
1. connected to a relay
2. earthed at one point
3. open-circuited
4. connected to an ammeter
Answer
C. open-circuited
An open CT secondary develops a dangerously high voltage.
Which device protects substation equipment from lightning surges?
1. Capacitor bank
2. Bus coupler
3. Isolator
4. Lightning arrester
Answer
D. Lightning arrester
The arrester diverts surge current to earth.
In a healthy silica gel breather, dry gel is blue. Damp gel turns
1. pink
2. black
3. yellow
4. green
Answer
A. pink
Pink gel shows that it has absorbed moisture and needs changing.
Which is the most common fault on a distribution line?
1. Double line fault
2. Three-phase fault
3. Open circuit in the neutral
4. Line to ground fault
Answer
D. Line to ground fault
Single line to ground faults are far more common than other faults.
A drop-out (DO) fuse is commonly fitted on the
1. 11 kV side of a pole-mounted DTR
2. bushing of a capacitor
3. LT meter board
4. tank of a power transformer
Answer
A. 11 kV side of a pole-mounted DTR
It gives a visible break when the fuse blows and drops.
A substation that steps down 33 kV to 11 kV is called
1. a switching station
2. a step-up substation
3. a primary distribution substation
4. a secondary distribution substation
Answer
C. a primary distribution substation
Step-down from sub-transmission to feeder voltage is primary distribution.
Which gas is used in gas-insulated substations?
1. Nitrogen only
2. Hydrogen
3. Carbon dioxide
4. SF6
Answer
D. SF6
SF6 has high insulation strength and a compact design.
What is the main advantage of a ring main distribution?
1. It needs no switches
2. Supply can reach a load from two sides
3. It has no voltage drop
4. It is the cheapest layout
Answer
B. Supply can reach a load from two sides
A faulty section can be isolated and the rest stays on supply.
Which device senses a fault and sends the trip signal to the circuit breaker?
1. Isolator
2. Earth pit
3. Relay
4. Busbar
Answer
C. Relay
The relay compares measured values with its setting.
Transformer oil is used in a DTR for
1. insulation and cooling
2. increasing the voltage
3. reducing load current
4. lubrication of the core
Answer
A. insulation and cooling
Oil insulates the winding and carries heat to the tank.
The full-load current of a 100 kVA, 433 V, 3-phase transformer on its LV side is about
1. 231 A
2. 77 A
3. 133 A
4. 300 A
Answer
C. 133 A
I = 100000 / (1.732 × 433) = 100000 / 750 = about 133 A.
The full-load current of a 63 kVA, 433 V, 3-phase transformer on its LV side is about
1. 145 A
2. 210 A
3. 48 A
4. 84 A
Answer
D. 84 A
I = 63000 / (1.732 × 433) = 63000 / 750 = 84 A.
The full-load current of a 100 kVA, 11 kV, 3-phase transformer on its HV side is about
1. 5.25 A
2. 3.03 A
3. 52.5 A
4. 9.1 A
Answer
A. 5.25 A
I = 100000 / (1.732 × 11000) = 100000 / 19052 = 5.25 A.
The full-load current of a 250 kVA, 433 V, 3-phase transformer on its LV side is about
1. 433 A
2. 333 A
3. 192 A
4. 577 A
Answer
B. 333 A
I = 250000 / 750 = 333 A.
A transformer has a voltage ratio of 11000 V / 440 V and 2000 primary turns. The number of secondary turns is
1. 50
2. 25
3. 80
4. 160
Answer
C. 80
N2 = 2000 × 440 / 11000 = 80.
A fuse has a rated current of 10 A and a fusing current of 15 A. Its fusing factor is
1. 0.67
2. 1.5
3. 5
4. 25
Answer
B. 1.5
Fusing factor = fusing current / rated current = 15 / 10 = 1.5.
A fuse has a fusing factor of 1.5 and a fusing current of 18 A. Its rated current is
1. 9 A
2. 27 A
3. 16.5 A
4. 12 A
Answer
D. 12 A
Rated current = 18 / 1.5 = 12 A.
A fuse has a rated current of 20 A and a fusing factor of 1.25. The current at which it melts is
1. 25 A
2. 16 A
3. 21.25 A
4. 30 A
Answer
A. 25 A
Fusing current = 20 × 1.25 = 25 A.
If the fusing current of a wire follows I = k × d^(3/2), how many times does the fusing current rise when the diameter is doubled?
1. About 2.83 times
2. 1.41 times
3. 2 times
4. 4 times
Answer
A. About 2.83 times
2^(3/2) = 2 × 1.414 = 2.83.
A 3-phase load draws 100 A at 415 V. The apparent power is about
1. 124 kVA
2. 41.5 kVA
3. 72 kVA
4. 100 kVA
Answer
C. 72 kVA
kVA = 1.732 × 415 × 100 / 1000 = about 72 kVA.
A 63 kVA DTR carries 42 A on its 433 V side. Its percentage loading is about
1. 33 percent
2. 50 percent
3. 67 percent
4. 80 percent
Answer
B. 50 percent
Full-load current is 84 A, so 42 / 84 = 50 percent.
A transformer delivers 80 kW at the output and has total losses of 2 kW. Its efficiency is about
1. 96.0 percent
2. 99.0 percent
3. 98.0 percent
4. 97.6 percent
Answer
D. 97.6 percent
Efficiency = 80 / (80 + 2) = 97.56 percent.
Consider these statements. 1. An isolator can safely break load current. 2. A circuit breaker can interrupt fault current.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Isolators have no arc-quenching. Breakers are built to clear faults.
Consider these statements. 1. The secondary of a CT must not be open-circuited. 2. The secondary of a PT must not be short-circuited.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both rules prevent damage and danger.
Consider these statements about fuses. 1. The fusing factor is less than 1. 2. A fuse is a single-use device.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Fusing factor is more than 1. A blown element must be replaced.
Consider these statements. 1. A Buchholz relay detects over-voltage. 2. The conservator holds the silica gel.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
Buchholz relay senses gas. The breather holds silica gel.
Consider these statements about switching. 1. To switch off, open the breaker first and then the isolator. 2. To switch on, close the breaker first and then the isolator.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
For switching on, the isolator is closed first, then the breaker.
Consider these statements. 1. A rewirable fuse has a very high breaking capacity. 2. An HRC fuse has a sealed cartridge.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Rewirable fuses have low breaking capacity.
Consider these statements. 1. In a radial system, a fault interrupts the downstream customers. 2. In a ring main system, a load can be fed from two sides.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both describe the two layouts correctly.
Consider these statements about circuit breakers. 1. SF6 breakers use SF6 gas to quench the arc. 2. Oil circuit breakers use vacuum to quench the arc.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
OCBs use oil.
Consider these statements about a lightning arrester. 1. It conducts continuously at normal voltage. 2. It diverts surge current to earth.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
It is normally non-conducting and conducts only on a surge.
Consider these statements about DTR earthing. 1. Body, neutral and arrester earths should have separate pits. 2. A high earth resistance is better.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Earth resistance must be low.
Which pair is correctly matched?
1. Isolator - quenches the arc
2. Capacitor bank - raises the voltage of the line
3. Conservator - holds silica gel
4. Buchholz relay - gas from an internal transformer fault
Answer
D. Buchholz relay - gas from an internal transformer fault
The Buchholz relay is operated by gas or oil surge in the transformer.
Consider these statements about faults. 1. A line to ground fault is the most common. 2. A 3-phase fault is the most common.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Three-phase faults are rare but severe.
Which device gives a visible open gap in a substation for safe maintenance?
1. Current transformer
2. Capacitor bank
3. Lightning arrester
4. Isolator
Answer
D. Isolator
Isolators show a visible break, and earthing is applied after the gap is made.
Circuit breakers are rated for breaking capacity. What does it mean?
1. The voltage of its coil
2. The number of poles
3. The highest fault current it can safely interrupt
4. The weight it can carry
Answer
C. The highest fault current it can safely interrupt
Breaking capacity is the maximum fault current a breaker can clear.

Page 1 of 1

Chapter quiz · 7 questions

0/7 answered

On what you just read, from the live question bank. 🔒 Answers and solutions are shown to registered students (free).
अध्याय पढ़ने के बाद अभ्यास करें।

1.The distributions of electrons in the K, L, and M shell of the chlorine atom are:
2.The electron distribution in an aluminum atom is-
3.The electron distribution in a magnesium atom is-
4.Match the elements in Column I with their correct electron distribution in Column II according to the Bohr–Bury rules.
5.Which of the following is/are controlled by the use of insecticides in crop protection management?
6.Which of the following statements is NOT correct with reference to the distribution of elements in the periodic table?
7.Animals often blend in with their surroundings for protection. Which of the following do that?

7 left. You can check any time.