Authorisation Test Questions

___________________________________________________________

Part 1 – Theory, Section A – Safety Rules

1. Define the term DEAD. What is meant by the word EARTHED? What is the procedure for making
DEAD?

S I C
DEAD (SIC = witched off, solated & onnected to Earth)
 Switched off and isolated from all live systems and is at or about zero volts with respect to
general mass of the earth and connected to earth.
• Rotating plant shall be regarded as dead when it is stationary or is being slowly rotated
by means of barring gear and is not excited.
• Pressurised systems shall be regarded as dead when they are completely isolated and
at atmospheric pressure.
C A
EARTHED (CAW = onnected to earth, t all times immediate discharge & Without Danger)
 Connected to general mass of the earth in such a manner as to ensure at all times an
immediate discharge of electric energy without danger.

Difference between DEAD and EARTHED

 To make DEAD is part of a Switching Instruction but EARTHED is not part of the switching
instruction.
 Only SAP can make DEAD an HV apparatus while HV apparatus are EARTHED at Installation.
 A DEAD apparatus cannot be LIVE but an EARTHED apparatus can be LIVE, e.g. an energized
transformer is LIVE but its body and neutral point of its Y windings connections are EARTHED.

In order to make “dead”, one must: SIPEEA (Switch OFF, Isolate, Prove absence of

voltage, Examine Earthing Leads, Establish Earthing point, Apply Circuit Main Earth)
• Switch off from all live and render inoperative and (verify that all poles have OPENED)
• Isolate from all live and render inoperative electrically and mechanically.
Rendering electrically inoperative: Switch Selector Switch (Local/Remote switch) to
OFF, Switch OFF MCBs, pull out fuses.
Rendering mechanically inoperative: remove castell keys, lock out.
Deposit the safety lock key, castell keys and fuses in the lockout box. Then and only
then is Isolation complete.
• Prove for absence of voltage even when you know that the poles of the breaker have
been OPENED and there is a physical gap (Isolation). With safety you cannot overdo.
Check the voltage rating and use the appropriate Voltage Tester. A voltage tester has
two buttons (Self Check Tester button and Test Button). Press the Self-check button
on the tester (if tester is OK, both the RED and GREEN LEDs will light up and buzz).
Press the TEST button, and test on a known nearby live point with same voltage level
as the isolated point (on live, it will buzz and flicker RED). Go now test on the isolated
points (it will flicker green if there is absence of voltage). Go back and test your voltage
tester on the known nearby live point to verify that your tester is OK. Where there is
no nearby known similar voltage LIVE point, self-test the voltage tester before and
after proving dead.
For LMT breakers, use phasing stick to prove absence of voltage. Go on known live
e.g. 11kV against ground will read 5.5kV. Isolated point will read Zero volts.
 • Examine earthing leads for loose nuts and bolts, broken strands and pilled insulation.
Earthing leads shall have a cross section area of not less than 80mm2 copper
equivalent.
• Establish Earthing. 5m, 10m (East, West, North and South) and get average value and
should be less than 2 ohms for Transmission substations and 5 ohms for Distribution
substations.
• Apply Earth. Deliberately connect to Earth at all points of isolation. Attach earth clamp
to earth system first, then phase clamps to all the three phase one at a time. All phases
shall be earthed even if work is to be carried out on one phase only.
• (everything above is the procedure to follow when applying Earth). The procedure for
removing an earth starts with removing clamps from the phases first (Isolated points),
thereafter remove from earth system.

2. What is the minimum number of people in an Earthing Party compared to Working Party? Tell
us the differences between Circuit-Main-Earth and Temporary Earth. Whose responsibility is it
to apply Temporary Earths at or near the site of work?

At least a minimum of two people in an Earthing Party while the working part can be one (To be
revised also to be like earthing party).

CME TE
Applied by SAP Applied by person in-charge of
work
Applied before PTW or SFT is Applied after PTW or SFT is issued
issued
Applied with consent of Shift Applied without consent of Shift
Charge Controllers Charge Controllers
Applied at or near point of isolation Applied at or near point of work
Counted as one unit at point of Counted per contact
isolation
Removed by SAP Removed by person in-charge of
work
Removed after cancellation of PTW Removed before cancellation of
& SFT PTW & SFT
Removed with consent of Shift Removed without consent of Shift
Charge Controllers Charge Controllers
Status indication Terms used: Status indication Terms used:
OPEN or CLOSE APPLY or REMOVE

3. Who is a Senior Authorised Person, Authorised Person or Competent Person? Who is a

Responsible Person? (Tip: What documents, issue, receive & cancel)

SAP : A Competent Person possessing technical knowledge, tested and appointed in writing by
ZESCO issued with certificate to issue, receive and cancel P.T.W, S.F.T and L.O.A within the terms
of his appointment.
- Certificate of appointment states apparatus or section of system to which it applies and
extent to which person is authorized to issue, receive & cancel P.T.W, S.F.T. & LOA
- Main job is switching operations.
- As already stated, SAP can issue all documents to himself.

AP: A Competent Person possessing technical knowledge, tested and appointed in writing by
ZESCO with certificate issued to carry out specific work on its system or apparatus.
Certificate of appointment states class of work the person is authorized to carry out and the
apparatus or section of system to which it applies and may include authority to receive S.F.T.
- Can receive P.T.W or L.O.A
- may include authority to receive S.F.T
- Can issue P.T.W part A & L.O.A (depending on competence level)
- Can erect barricade, attach Danger live and Danger approach notices
- Can apply TEs

CP: A person who has sufficient technical knowledge or experience to enable him avoid danger
and is listed in Competent Persons Register kept by Control Engineer.
- Tested, appointed in writing
- Can receive part A of P.T.W only and L.O.A

RP : A person appointed by his employer to carry out switching, isolation and earthing on a
consumer’s own apparatus, from which ZESCO apparatus can be made live.
- Signs declaration (part 2) on P.T.W or S.F.T.
- On cancellation he signs again on part 8

4. Name the notices that you use in ZESCO. When do you use a Caution Notice and when do you
use a Danger Notice? Can a Danger Notice be used in place of a Caution Notice?
(Hit: Who applies, where and for whose information?)
Caution Notice is applied by SAP at the point of isolation when rendering an apparatus
inoperative, for the information of other SAPs not to interfere with the circuit.

Danger Notice (i) (Danger Approach) is applied by AP affixed on barricade facing inside
barricaded area for the information to working party members letting the workmen in
barricaded area know that they are approaching limit of safe working area; that the area
beyond the barricade is not a safe working area.

(ii) (Danger Live) is applied by AP affixed on adjacent similar LIVE apparatus for the
information of working party members making them know that they should keep away from
the LIVE apparatus as the apparatus may shock, burn or electrocute.

Warning Notice is applied on the perimeter fence of HV installations for the information of
the general public not to inter the substation or switching station as there are hazardous HV
voltages that may shock, burn or electrocute.

Circuit Identity Wristlet are applied by person in-charge of works won on arm wrists of
working party members that may be required to work on towers of a transmission line for the
information of working party to identify the designated transmission line. Examples:
H1: Kabwe – Luano 330kV Line 1
H4: Kabwe – Luano 330kV Line 2
H2: Kabwe – Kitwe 330kV Line 2
H3: Kabwe – Kitwe 330kV Line 3
T1: Kabwe – Pensulo 330kV Line
J2: Kitwe – Chambeshi East 330kV Line
J3: Chambeshi East – Luano 330kV Line

(iii) A Danger Notice cannot be used in place of a Caution Notice. Caution Notice conveys
warning against interfering with apparatus or control equipment.
Danger Notice calls attention to the risk of loss of life, bodily injury or damage to
health from shock as a result of approaching or interfering with apparatus that is LIVE.
5. At what point would you consider an isolation to be complete? Can a circuit breaker be used as
an Isolation Point? If so when?
Switch off from all live, Isolate from all live, render inoperative electrically and mechanically,
attach caution notice and deposit the safety lock key in the lockout box.
LMT 11kV breakers have integral earth and can be used for CME earthing.

6. What does Rendering Inoperative mean? What is a Safety Lock, Standard Lock and Non-standard
Lock?

- Render inoperative electrically (Selector Switch in OFF position, Switch OFF MCBs, Remove
fuses) and inoperative mechanically (remove keys, lock off with safety lock), attach caution
notice and deposit the safety lock key, fuses & castell keys in the lockout box.
- Safety Lock: used to lockout point of isolation, unique key different from locality, deposit key in
a lockout box.
- Non-standard Lock: used to lockout apparatus in its normal operative position, whose key is
kept in a key cabinet.
- Standard lock: used for locking out gates of HV installations, feeder pillar, have duplicate keys
or master keys.
7. What is Sanction-for-Test? In a Sanction-for-Test, What is meant by the term Exceptions? What
is the purpose of a Workman’s Declaration and how is this declaration effected?
- Sanction-for-Test an approved document issued by SAP to SAP/(AP Authorized for that task i.e
person in charge of testing HV apparatus) for carrying out tests on the HV apparatus.
- Exceptions, changes made on the original state of the circuit where tests are to be carried out
e.g. removed CMEs after SFT is issued.
- Workman’s Declaration, is for making known to the working party by the person in-change of
works, exactly which circuit is made dead, points of isolations, CMEs and TEs that have been
applied, dangers/risks associated. It’s effected by signing on the Workman’s declaration having
understood the above by all members of the working party.
- Indirect issue is when the person in-charge of works is not at the same site as SAP. SAP reads
to the person in-charge of works to record exactly the same information (accurate copy) on a
PTW and he reads back and then SAP signs on Part 9 of PTW. On indirect clearance, person in-
charge of work signs on part 10 of the PTW.
-
8. In Section 2 of the Permit-to-Work, when is this declaration used? Explain the portion that says
“OTHER RELEVANT PERMIT Nos.” In Part A of the Permit-to-Work under Section 1, explain how
you would obtain the proximity to a live point, and when do you use this? What is a Safe Distance
- The declaration is used when the person in-charge of works has explained to the working party
by the person in-change of works, the exactly which circuit is made dead, points of isolations,
CMEs and TEs that have been applied, barricade, Danger approach, Danger Live, mini risk
assessment and associated mitigation measures.
- “OTHER RELEVANT PERMIT Nos.” refer to other permit to work numbers that are associated on
the same circuit made dead. Breaker 105 PTW is written on the right top corner while someone’s
PTW will appear on “OTHER RELEVANT PERMIT Nos”. The qualifier is that “OTHER RELEVANT
PERMIT Nos” should have the same common isolation point as the “PERMIT No” on PTW.
- Part A is used when carrying out works above ground but within a safe working distance from
a live exposed points.
- Use a live line voltage tester stick/earth stick rated for that specific voltage, lay it on the ground
measure 5.2m for 330kV and mark on the stick. Use this template to locate the safe working
distance. Don’t use the measuring tape directly
- 330kV = 5.2m
- 220kV = 4.6m
- 132kV = 4.2m
- 88kV = 3.4m
- 66kV = 3.2m
- 33kV = 2.9m
- 17.5 = 2.8m
- 11kV = 2.6m

9. How do you mark your work area near a 330 live necked conductor?
- Use a link stick rated for 330 kV. Lay it stretched out on the ground. With a measuring
tape, mark 5.2m on it from the tip using a yellow insulation tape. Transpose the mark on
the link stick in such a manner that the tip almost touches the 330 naked conductor and
mark on the surface of the tower member the safe working distance. That is the place
where you erect the barracked and affix the Danger Approach Notice facing towards the
working party.

10. What is a Limitation-of-Access? When or where is it used?

- Document approved by MD or Engineer deputising, issued by SAP and AP to CP on works to be
carried out on ground in proximity to HV apparatus.

11. What locks are used in ZESCO? (HIT: Where applied and where keys kept) Where can you find
a Lockout Box and how can it be used to enhance safety? What is a Circuit Wristlet?
- Non Standard Lock: a lock having a unique key different from keys in the locality used to lock
off switch gear in its normal operating position. The keys are kept in a key cabinet.
- Safety Lock: a lock having a unique key different from keys in the locality used to lock off
switch gear at points of isolation during switching to avoid energising the circuit which is being
worked on. The keys are kept in a lock out box. If there is no lock out box, the SAP keeps keys
out of reach by others.
- Standard Lock: a lock having common master keys or duplicate keys used to lock off switch
gear HV substations, switchyards and Feeder.
- A Key Cabinet is a box found in HV switching and Substations used to keep keys to Non
standard locks and Circuit Identity Wristlets.
- A lockout box is a box found in HV switching and Substations having a lockable door with 3
unique none-interchangeable keys. This box is used for holding isolation point keys, fuses and
castell keys which have been made inoperable under PTW and SFT
- A Circuit Wrist is bracelet-like attire which is worn on a wrist in such a manner that it is readily
visible on the wrist of every working party member who may have to climb a tower. It is marked
with a circuit designation of that particular line on which work has to be carried out.
- Example:
At Kitwe Substation, 330kV Kabwe Line 2 is designated H2
At Kitwe Substation, 330kV Kabwe Line 3 is designated H3
At Luano Substation, 330kV Kabwe Line 1 is designated H1
At Luano Substation, 330kV Kabwe Line 4 is designated H4
At Pensulo Substation, 330kV Kabwe Line is designated T1
At Chambishi East Substation, 330kV Kitwe Line is designated J2
At Chambishi East Substation, 330kV Luano Line is designated J3

Part 1 – Theory, Section B – Knowledge of the Network

1. Draw the single line diagram of your station reticulation system and label it, showing clearly the
position of isolators, circuit-breakers, voltage transformers, current transformers and their ratios,
transformer ratings and cable/conductor sizes and current-carrying capacities.
 Aluminium area Stranding & Wire Current Carrying
Weight mass
(sq mm) Diameter Capacity
Resistanc
Total
Overall e Ultimate
Sectiona
Cod Conducto Conducto Dia AT 20 deg Breakin 65 75 90
l
e r r (mm) C g Deg. Deg. Deg.
area
wor (Al) (Steel) (Approx Total Al. Steel. (Ohms/Km load C C C
(Sq.
d Nomina Sectiona ) Kg/K Kg/K Kg/K ) (Kn)
mm)
l l m m m (Max)
Nn Dia No Dia Amp Amp Amp
. (mm) . (mm) s s s

Mole 10 10.60 12.37 6 1.50 1 1.50 4.50 43 29 14 2.780 3.97 58 70 NA

Squirrel 20 20.98 24.48 6 2.11 1 2.11 6.33 85 58 27 1.394 7.61 89 107 NA

Weasel 30 31.61 36.88 6 2.59 1 2.59 7.77 128 87 41 0.9289 11.12 114 138 NA

Rabbit 50 52.88 61.7 6 3.35 1 3.35 10.05 214 145 69 0.5524 18.25 157 190 NA

Racoon 80 78.83 91.97 6 4.09 1 4.09 12.27 318 215 103 0.3712 26.91 200 244 NA

Dog 100 105.0 118.5 6 4.72 7 1.57 14.15 394 288.3 105.7 0.2792 32.41 239 291 NA

Wolf 150 158.1 194.9 30 2.59 7 2.59 18.13 727 438 289 0.1871 67.34 329 405 NA

Panther 200 212.1 261.5 30 3.00 7 3.00 21.00 976 588.5 387.5 0.1390 89.67 395 487 NA

Kundah 400 404.1 425.2 42 3.50 7 1.96 26.88 1282 1119 163 0.07311 88.79 566 705 NA

Zebra 420 428.9 484.5 54 3.18 7 3.18 28.62 1621 1182 439 0.06868 130.32 590 737 NA

Moose 520 528.5 597.0 54 3.53 7 3.53 31.77 1998 1463 535 0.05595 159.60 667 836 NA

Morculla 560 562.7 591.7 42 4.13 7 2.30 31.68 1781 1553 228 0.05231 120.16 688 862 NA

BASIC DATA FOR ALUMINIUM CONDUCTORS STEEL REINFORCED (ACSR) AS PER IS 398 (PART - II) : 1976
 Stranding & Wire Diameter
Approx weight Kg./Km. Current Carrying Capacity
No. & (mm)

Nominal Calculated Ultimate

Nominal
Equivalent Conduc- tor Conductor Conductor Cond- uctor Resistance Breaking 65 75 90
Aluminium
Code Word Copper Area (Sqmm) (Al) (Steel) Diam- eter (mm) AT 200C Load Deg. C Deg. C Deg. C
Area (Sqmm)
Area (Sqmm) Total Al. Steel. Ohm/Km (Kn)

Kg/Km Kg/Km Kg/Km

Dia Dia
No. No. Amps Amps Amps
(mm) (mm)

Gopher 16 25.90 30.62 6 2.36 1 2.36 7.08 106.00 72.00 34 1.098 952 88 109 145

Ferret 25 41.87 49.48 6 3.00 1 3.00 9.00 171 116 55 0.6795 1503 125 155 195

Mink 40 63.32 73.65 6 3.66 1 3.66 10.98 255 173 82 0.4565 2207 167 208 250

Horse 42 71.58 116.20 12 2.79 7 2.79 13.95 542 204 338 0.3977 6108 189 235 280

Beaver 45 74.07 87.53 6 3.99 1 3.99 11.97 303 205 98 0.3841 2613 189 235 280

Otter 50 82.85 97.91 6 4.22 1 4.22 12.66 339 230 109 0.3434 2923 207 257 305

Cat 55 94.21 111.30 6 4.50 1 4.50 13.50 385 261 124 0.3020 3324 229 285 325

Leopard 80 129.70 148.40 6 5.28 7 1.76 15.48 493 360 133 0.2193 4137 315 378 440

Coyote 80 128.50 151.60 26 2.54 7 1.90 15.86 521 365 156 0.2214 4638 286 367 430

Tiger 80 128.10 161.80 30 2.36 7 2.36 16.52 604 363 241 0.2221 5758 312 373 435

Nynx 110 179.00 226.20 30 2.79 7 2.79 19.53 844 506 338 0.1589 7950 385 475 540

Lion 140 232.50 293.90 30 3.18 7 3.18 22.26 1097 659 438 0.1223 10210 465 574 640

Bear 160 258.10 326.10 30 3.35 7 3.35 23.45 1219 734 485 0.1102 11310 506 623 685
 Goat 185 316.50 400.00 30 3.71 7 3.71 25.97 1492 896 596 0.0898 13780 586 724 780

Sheep 225 366.1 462.60 30 3.99 7 3.99 27.93 1726 1036 690 0.0777 15910 655 808 860

Deer 260 419.30 529.80 30 4.27 7 4.27 29.89 1977 1188 789 0.0678 18230 726 896 940

Elk 300 465.70 588.40 30 4.50 7 4.50 31.50 2196 1320 876 0.0611 20240 787 970 1015

Camel 300 464.50 537.70 54 3.35 7 3.35 30.15 1804 1318 486 0.0612 14750 787 970 1015

Sparrow 20 33.16 39.22 6 2.67 1 2.67 8.01 135 92 43 0.8578 1208 110 140 174

Fox 22 36.21 42.92 6 2.79 1 2.79 8.37 149 101 48 0.7857 1313 116 144 180

Guinea 49 78.56 127.20 12 2.92 7 2.92 14.60 590 224 366 0.3620 6664 208 244 296

Lark 125 196.10 247.80 30 2.92 7 2.92 20.44 922 556 366 0.1451 8559 378 410 490

2. What protection features are found on the main transformers, incomer and feeders at your
station?

Transformer Diff. Prot.

The differential relay actually compares between primary current and secondary current of power
transformer bearing in mind the transformation ratio, if any unbalance found in between primary
and secondary currents the relay will actuate and inter trip both the primary and secondary
circuit breaker of the transformer.

Restricted Earth Fault

 An external fault in the star side will result in current flowing in the line current transformer of
the affected phase and at the same time a balancing current flows in the neutral current
transformer, hence the resultant current in the relay is therefore zero. So this REF relay will not
be actuated for external earth fault.

But during internal fault the neutral current transformer only carries the unbalance fault current
and operation of Restricted Earth Fault Relay takes place. This scheme of restricted earth fault
protection is very sensitive for internal earth fault of electrical power transformer.

Over Current Protection:

The overcurrent protection devices include:

 Fusible switches.
 Mains Circuit Breakers MCB.
 Over Current Relays.

FUSEs: In a fuse, the overcurrent protection function is accomplished by

a fuse fusing/melting open its current-responsive element when an overcurrent or short circuit
passes through the element.

Mains Circuit Breakers MCB: MCBs are designed to close a circuit by non-automatic means and
to open the circuit automatically on a predetermined overcurrent without injury to itself when
properly applied within their specific rating.

Over Current Relays: In an over current relay, normal current flows through a coil, the magnetic
effect generated by the coil is not sufficient to move the moving element of the relay, as in this
condition the restraining force (spring) is greater than deflecting force (magnetic pull). But when
the current through the coil increases, the magnetic effect increases, and after certain level of
current, the deflecting force generated by the magnetic effect of the coil, crosses the restraining
force, as a result, the moving element starts moving to change makes the contact in the relay
output.

OC can be instantaneous or time delayed.

Instantaneous Over Current Relay:

Construction and working principle of instantaneous over current relay quite simple i.e. generally a
magnetic core is wound by current coil. A piece of iron is so fitted by hinge support and restraining
spring in the relay, that when there is not sufficient current in the coil, the Normally Open (NO)
contacts remain open. When current in the coil crosses a pre-sent value, the attractive force
becomes sufficient to pull the iron piece towards the magnetic core and consequently the no
contacts are closed.

The pre-set value of current in the relay coil is referred as pick up setting current. This relay is
referred as instantaneous over current relay, as ideally, the relay operates as soon as the current
in the coil gets higher than pick up setting current. There is no intentional time delay applied. But
there is always an inherent time delay which cannot be avoided practically. In practice the operating
time of an instantaneous relay is of the order of a few milliseconds.

EXAMPLE OF pre-set LV Overcurrent on a 120 MVA 330/66/11kV Transformer

Apparent power = Root of three x Current x voltage

Q = 1.73*I*V

Maximum load current = Q/(1.73*V ) = 120000/(1.73*66) =1050.97 Amps

LV

Normally protection pre-set value at 10% above overload and therefore we have

Pre-set Over Current = 1.1 * 1050 = 1.156 Amps

Definite Time Over Current Relay:

This relay is created by applying intentional time delay after crossing pick up value of the current.
A definite time over current relay can be adjusted to issue a trip output at definite amount of time
after it picks up. Thus, it has a time setting adjustment and pick up adjustment.
“Time delay” is an intentional delay in trip operation after the device has sensed that the current
has exceeded the pickup value. Usually, time delays are used to provide time for another overcurrent
device to operate and clear the fault. It may also be used to allow for a normal temporary
overcurrent condition to pass or to allow the condition to clear itself. An example of normally
occurring temporary overcurrents are motor starting currents and transformer inrush currents. In
these instances, the inrush currents exceed the pickup of overload devices protecting the
transformer or motor, but the time delay of the overload should allow sufficient time for the inrush
current to flow and prevents an unnecessary trip from occurring

Inverse Time Over Current Relay:

Inverse time is a natural character of any induction type rotating device. This means the speed of
rotation of rotating art of the device is faster if input current is increased. In other words, time of
operation inversely varies with input current. This natural characteristic of electromechanical
induction disc relay in very suitable for over current protection. This is because, in this relay, if fault
is more severe, it would be cleared more faster. Although time inverse characteristic is inherent to
electromechanical induction disc relay, but the same characteristic can be achieved in
microprocessor based relay also by proper programming.
Earth-fault Protection with Core Balance Current Transformers. (Zero Sequence CT)

In this type of protection (Fig. 6) a single ring shaped core of magnetic material, encircles the
conductors of all the three phases. A secondary coil is connected to a relay unit. The cross-section
of ring-core is

(Fig.6) Principle of core-balance CT for earth fault protection

Earth Fault Protection:

Sometimes saturation (overcurrent) is not a problem. It is convenient to incorporate phase-fault

relays and earth-fault relay in a combined phase-fault and earth-fault protection. During no-earth-
fault condition, the components of fluxes due to the fields of three conductors are balanced and the
secondary current is negligible. During earth faults, such a balance is disturbed and current is
induced in the secondary. Core-balance protection can be conveniently used for protection of low-
voltage and medium voltage systems. The burden of relays and exciting current are deciding
factors. Very large cross-section of core is necessary for sensitivity less than 10 A. This form of
protection is likely to be more popular with static relays due to the fewer burdens of the latter.
Instantaneous relay unit is generally used with core balance schemes.

Theory of Core Balance CT

Let Ia, Ib and Ic, be the three line currents and Φa, Φb and Φc be corresponding components
of magnetic flux in the core. Assuming linearity, we get resultant flux Φ as,

Φ=k (Ia + Ib + Ic)

where k is a constant Φ = K * Ia.

Referring to theory of symmetrical components

(Ia + Ib + Ic)=3Ic=In

Where, Io is zero sequence current and In, is current in neutral to ground circuit. During normal
condition, when earth fault is absent,
(Ia + Ib + Ic) =0

Hence Φr = 0 and relay does not operate

During earth fault the earth fault current flows through return neutral path.

For example for single line ground fault,

If = 3Iao = In

Hence the zero-sequence component of Io produces the resultant flux Φr in the core. Hence core
balance current transformer is also called as zero sequence current transformers (ZSCT).

Earth-fault protection can be achieved by following methods:

1. Residually connected relay.
2. Relay connected in neutral-to-ground circuit.
3. Core-balance-scheme.
4. Frame leakage method.
5. Distance relays arranged for detecting earth faults on lines.
6. Circulating current differential protection.

Buchholz Protection

Buchholz relay in transformer is an oil container housed in the connecting pipe from main tank to
conservator tank. It has mainly two elements. The upper element consists of a float. The float is
attached to a hinge in such a way that it can move up and down depending upon the oil level in the
Buchholz relay Container. One mercury switch is fixed on the float. The alignment of mercury switch
hence depends upon the position of the float.
The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just
in front of the inlet (main tank side) of Buchholz relay in transformer in such a way that when oil
enters in the relay from that inlet in high pressure the alignment of the baffle plate along with the
mercury switch attached to it, will change. In addition to these main elements a Buchholz relay has
gas release pockets on top. The electrical leads from both mercury switches are taken out through
a molded terminal block.

Buchholz relay function is based on very simple mechanical phenomenon. It is mechanically

actuated. Whenever there will be a minor internal fault in the transformer such as an insulation
faults between turns, break down of core of transformer, core heating, the transformer insulating
oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to
decomposition of transformer insulating oil will accumulate in the upper part the Buchholz container
which causes fall of oil level in it.

Fall of oil level means lowering the position of float and thereby tilting the mercury switch. The
contacts of this mercury switch are closed and an alarm circuit energized. Sometime due to oil
leakage on the main tank air bubbles may be accumulated in the upper part the Buchholz container
which may also cause fall of oil level in it and alarm circuit will be energized. By collecting the
accumulated gases from the gas release pockets on the top of the relay and by analyzing them one
can predict the type of fault in the transformer.

More severe types of faults, such as short circuit between phases or to earth and faults in the tap
changing equipment, are accompanied by a surge of oil which strikes the baffle plate and causes
the mercury switch of the lower element to close. This switch energized the trip circuit of the circuit
breakers associated with the transformer and immediately isolate the faulty transformer from the
rest of the electrical power system by inter tripping the circuit breakers associated with both LV and
HV sides of the transformer. This is how Buchholz relay functions.

Buchholz Relay Operation Certain Precaution

The Buchholz relay operation may be actuated without any fault in the transformer. For instance,
when oil is added to a transformer, air may get in together with oil, accumulated under the relay
cover and thus cause a false Buchholz relay operation. That is why mechanical lock is provided in
that relay so that one can lock the movement of mercury switches when oil is topping up in the
transformer. This mechanical locking also helps to prevent unnecessary movement of breakable
glass bulb of mercury switches during transportation of the Buchholz relays.
The lower float may also falsely operate if the oil velocity in the connection pipe through, not due
to internal fault, is sufficient to trip over the float. This can occurs in the event of external short
circuit when over currents flowing through the winding cause overheated the copper and the oil and
cause the oil to expand.

Difference between PRV and Buchholz

Buchholz relay is located between the conservator tank and the Main Tank of transformer oil. The
Pressure Relief Valve (PRV) is located on top of the Main Tank and on top of the Conservator Tank
of transformer oil. When it is required to limit the pressure rise inside a tank, in order to prevent an
excessive mechanical stress of the walls, it is necessary to use a safety valve set at a precise
overpressure value. The tank of oil-immersed transformers is usually fit with this kind of protecting
device; as matter of fact, in case of short-circuit due to an insulation failure, the dielectric arc
between alive parts vaporises the surrounding insulating fluid which generates a quick rise of the
pressure inside the tank, with the risk of permanent deformations, or, even, of the failure of the
tank walls with the consequent flow-out of hot oil.

Line Differential Protection:

Communications is an integral piece of a line differential relay, as the currents from one line terminal
must be sent to relays at other line terminals to perform the differential calculation. This requires
the use of a digital communications channel, which is commonly a multiplexed channel where
channel switching may occur.

Line Distance (Impedance) Protection:

The basic principle of distance protection involves the division of the voltage at the relaying point
by the measured current. The apparent impedance so
calculated is compared with the reach point impedance. If the measured impedance is less than the
reach point impedance, it is assumed that a fault exists on the line between the relay and the reach
point.
Since the impedance of a transmission line is proportional to its length, for distance measurement
it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined
point (the reach point).

Such a relay is described as a distance relay and is designed to operate only for faults occurring
between the relay location and the selected reach point,
thus giving discrimination for faults that may occur in different line sections.

Line Overcurrent Protection

A piece of iron is so fitted by hinge support and restraining spring in the relay, that when there is
not sufficient current in the coil, the Normally Open (NO) contacts remain open. When current in
the coil crosses a pre-sent value, the attractive force becomes sufficient to pull the iron piece
towards the magnetic core and consequently the no contacts are closed. The pre-set value of current
in the relay coil is referred as pick up setting current.

Advantages of Digital/Numerical Distance Relays

- Keep more information about the fault

- Have more consistent operating times.

Disadvantage of Digital/Numerical distance relays

- They are usually slightly slower than some of the older relay designs when operating under
the best conditions.
- Note very robust e.g. their maximum operating times are also less under adverse waveform
conditions or for boundary fault conditions.

Terminology

Dependability: the certainty that a protection system will operate

when it is supposed to.
Security: the certainty that a protection system will not operate
when it is not supposed to.

Reliability = Dependability + Security

Selectivity means that the protective devices will minimize effect of a short circuit or other
undesirable event on the power system. The amount of the power system that must be shut down
in response to the event is kept to the absolute minimum. Selectivity can be achieved with devices
that are “inherently selective.” That is, they operate only on faults within their “zone of protection”
and do not ordinarily sense faults outside that zone.

“Time delay” is an intentional delay in trip operation after the device has sensed that the current
has exceeded the pickup value. Usually, time delays are used to provide time for another overcurrent
device to operate and clear the fault. It may also be used to allow for a normal temporary
overcurrent condition to pass or to allow the condition to clear itself. An example of normally
occurring temporary overcurrents are motor starting currents and transformer inrush currents. In
these instances, the inrush currents exceed the pickup of overload devices protecting the
transformer or motor, but the time delay of the overload should allow sufficient time for the inrush
current to flow and prevents an unnecessary trip from occurring.

3. What do you do when you find the main transformer at your station has tripped on restricted
earth fault?

- Inform NCC and get instruction to carry out switchings in order to make transformer dead
- issue necessary document (PTW to remove conductors and SFT for investigations)
- after investigation, take the test results to NCC and Senior manger operations

4. List three main causes of a transformer tripping on Buchholz.

- Low oil level

- Winding faults
- Insulation failure
- Moisture
- The Buchholz relay operation may be actuated without any fault in the transformer. For
instance, when oil is added to a transformer, air may get in together with oil, accumulated
under the relay cover and thus cause a false Buchholz relay operation. That is why mechanical
lock is provided in that relay so that one can lock the movement of mercury switches when
oil is topping up in the transformer. This mechanical locking also helps to prevent
unnecessary movement of breakable glass bulb of mercury switches during transportation of
the Buchholz relays.

- The lower float may also falsely operate if the oil velocity in the connection pipe through, not
due to internal fault, is sufficient to trip over the float. This can occurs in the event of external
short circuit when over currents flowing through the winding cause overheated the copper
and the oil and cause the oil to expand

5. What is the current-carrying capacity of a 120x3 core PILC 11kv cable?

- 351A
SIZE NUMBER OF CURRENT
(CSA) INSULATION CORES CAPACITY(A)
70mm PILC 3 220
70mm XLPE 3 260
 95mm PILC 3 270
95mm XLPE 3 320
120mm XLPE 3 351
180mm XLPE 3 440
240mm XLPE (Cu) 3 502
240mm XLPE 1 656
300mm XLPE 1 766
300mm XLPE 3 560

CONVENTIONAL CURRENT MVA MVA MVA

SIZE (CSA) NAME CAPACITY(A) (33KV) (11KV) (132KV)
100mm Dog 370 21.15 7.05 84.59
150mm Wolf 480 27.43 9.14 109.74
200mm Panther 550 31.44 10.48 125.74
300mm Bison 760 43.44 14.48 173.75
350mm Goat 800 45.72 15.24 182.90

6. (a) What do you do when a radial feeder has tripped on both over-current and earth-fault?

- Inform NCC control engineer

- Isolate and
- Investigate through line patrol
- sectionalize

(b) State the Black start for your town.

- drive to site
- call NCC that you are at site
- Record all alarms (indications) in the relays
- reset all alarms
- inform NCC in case the fault is coming from your station
- move to prepare the station as follows
- Take auxiliaries onto the Gen set
- open all the capacitor breakers (close reactor breakers)
- open all the 66kV breakers
- open LV transformer breakers
- Open HV T2 breaker
- Make sure HV T1 breaker is closed
- Open HV outgoing breaker (Luano)
- make sure Kitwe Line breaker in closed

Black Start Procedure For Chambishi East 330/66/11 kV Substation

3.5 Chambishi East 330/66/11 kV Substation

i) Check the status of the station and record all alarms and indications on all
330kV and 66 kV Feeders as well as all the Power Transformers Compensation Equipment
(Capacitor Banks). Reset all alarms.
ii) If the cause of the fault is due to faulty equipment located in the substation, notify NCC and
proceed to fully isolate the faulty equipment.
iii) If the substation equipment is okay and available, proceed to prepare the substation for
restoration of supply as follows.
3.5.1 Changeover of Auxiliary Supply from Normal to Emergency
Diesel Generator Supply
Normal station auxiliary Supply is from 330/66/11kV transformers 1TX or 2TX.
On the auxiliary Services Panel.
a) Open all 400V auxiliary switches for feeders.
b) Open and render inoperative 400V incomer Circuit Breaker No.1FQ, 2FQ, 3FQ and 4FQ.
c) Open and render inoperative 11kV Isolator 1TX2 and 2TX2.
d) Offload unnecessary 400V Loads (Leaving relay and battery rooms, HMI, building and
perimeter supplies.
e) Start the station standby Generator
f) Close 400V Circuit Breakers from the Generator (Gen 1 and Gen 2).

3.5.2 Open the All 66 kV Circuit Breaker:-

Open all Transformer LV Breakers
a) On 330/66/11 kV transformer T1, CB 1TO
b) On 330/66/11 kV transformer T2, CB 2TO
Open all Capacitor Bank Breakers
c) On 66 kV Capacitor 1, CB 1CO
d) On 66 kV Capacitor 2, CB 2CO
e) On 66 kV Capacitor 3, CB 3CO
f) On 66 kV Capacitor 4, CB 4CO
Open all 66kV Feeder Breakers
g) On 66 kV South Feeder, CB 1LO
h) On 66 kV North Feeder, CB 2LO
i) On 66 kV Mwambeshi Line Feeder, CB 4LO
(ZCCZ1 and ZCCZ2 are spares and breakers are always in OPEN status)
j) On 66 kV Ndeke Line Feeder, CB 7LO

3.5.3 Switch Off T2:-

a) On 330/66 kV transformer T2 CB 210
b) On 330/66/11kV Transformer T2, CB 2TXO

3.5.4 Switch OFF 330kV Luano Line:-

a) On 330 kV Line, OPEN CB 205

3.5.5 Switch ON Kitwe 330kV Line, T1 and 330kV Bus Coupler:-

a) On 330kV Kitwe line, CB 105
b) On 330/66/11 kV transformer T1, CB 110
c) On 330kV Bus Coupler, CB 130

3.5.6 Switch ON 66kV Bus Bars

a) On 66 kV Bus Section Isolator 1S4
b) On 66 kV Bus Coupler CB 1WO

Procedure for Restoring Auxiliaries Supply on T1 or T2

Switch off the Standby Generator

a) Open 400V Standby Main Circuit Breakers (MCB'S) i.e. Gen 1 and Gen 2
b) Render operative and Close 11kV Isolator 1TX2 and 2TX2.
b) Close 11kV Circuit Breakers 1TX0 and 2TX0.
c) 400V CLOSE 1FQ and 3FQ or CLOSE 400V CB 2FQ and 4FQ depending on which leg
auxiliaries are selected.
3.5.7 Key Performance Indicators:-
a) 100% Correct Switching
b) Able to prepare the Substation for restoration of supply within 30minutes

3.5.8 Records :-
In the course of implementing this procedure the following would have
generated and maintained in hard copy form
NO. RECORD NAME WHERE KEPT RETENTION TIME
1 Substation Control Room 5 years
Occurrence Book
2 Substation Control Room 5 Years
Occurrence Book

7. What is the purpose of reducing the current setting when carrying out sectionalising?
- To trip out faster so as not to disturb the larger part of the healthy system

8. What do you do after discovering an HV cable fault, before restoring supply?

- Isolate and sectionalise.
- Procedure

9. What is the purpose of having good earthing in your substation or switchyard and how do you
improve earthing?
- Helps reduce the step and touch potential
- For safety of personnel and equipment
- Sink more Earth pits and mats
- Use earthing cement
- Run the earth mat into the water in a river

10. Explain why it is necessary to rack out the VT when isolating transformers for maintenance.
- Avoiding feedback from the auxiliaries to the secondary windings
 -

Part 1 – Theory, Section C – Procedures and Schedules

1. Procedure to follow when issuing a SFT

2. Procedure to follow when issuing a PTW
3. Procedure to follow when issuing a LOA
4. Procedure to follow when physically moving a damaged 5MVA transformer from the
switchyard at Waterworks Substation to the Transformer workshop.
5. Procedure to follow when testing the transformer winding insulation
6. Procedure to follow when reducing the current setting on sectionalising
7. Write a full switching schedule to take out TX1 for annual maintenance

PRACTICALS:

1. Chambishi East 330/66/11kV Substation

2. Fully Labelled SLD
3. Outage Request For Kitwe 330kV Line @ Chambishi East 330/66/11kV Substation
4. Do all this within Two Weeks (dead line 29/09/2017)
5. Practical evidence of Switching Instructions 3 times under the supervision of the SAP
6. Issue various documents (PTW & SFT)