BUREAU OF INDIAN STANDARDS Manak Bhavan, 9 …bis.org.in/sf/etd/ETD16(10326)_26052016.pdfBUREAU OF...

BUREAU OF INDIAN STANDARDS

Manak Bhavan, 9 Bahadur Shah Zafar

Marg New Delhi 110002

Phones 2323 0131 TeleFax +91 11 2323 1192 Website :www.bis.org.in

2323 3375 Extn 4284 email : [email protected]

‍या प‍ चाला म‍ म‍ द‍ ‍‍‍‍‍‍‍‍

र ख‍रषण‍ ञा म

‍‍ ‍ दभ ददमाप

ईटीडी ‍16/टी- 44 ‍‍‍‍‍20-05-2016

तपमीपी‍ म तत‍:‍ईटीडी 16

.................................................................................................................................

रषती :

1. ईटीडी 16 क सभी सद य 2. वियत तकनीकी विभाग परिषद क सभी सद य ताा 3. चि िखन िाल अ य सभी िनकाय

मह दय,

क या िन‍ नलल खखत सश धन मस द की एक रित सल न ह :

ईटीडी 16( 10326 ) आई एस 1180 )भाग 1 ( : 2014 की सश धन स या – 1

क या इस सश धन मस द का अिल कन कि औि अपनी स‍ मितय यह बतात हए भज कक अतत: यदद य सश धन क प म रकालशत ह जाऍ त इस पि अमल किन म आपक यिसाय अािा काि बाि म ‍ या कदिनाइय आ सकती ह ।

स‍ मितय यदद क ई ह त क या अगल प‍ ि पि ददए पर म अध ह ताीिी क रपरिललखखत पत पि भज द ।

स‍ मितय भजन की अितम तािीख 26-06-2016

ध यिाद,

भिदीय,

( डी ग स िामी ) ि ञािनक एफ एि रमख (वियत तकनीकी)

सलग न : रपरिललखखत

सल न – रपिललखखत

BUREAU OF INDIAN STANDARDS

Manak Bhavan, 9 Bahadur Shah Zafar

Marg New Delhi 110002

Phones 2323 0131 TeleFax +91 11 2323 1192 Website :www.bis.org.in

2323 3375 Extn 4284 email : [email protected]

DRAFT AMENDMENT IN WIDE CIRCULATION

DOCUMENT DESPATCH ADVICE

Our Ref

Date

ETD 16/T-44 20-05-2016

TECHNICAL COMMITTEE : ETD 16

______________________________________________________________________________

ADDRESSED TO:

1. All Members of Transformers, Sectional Committee, ETD 16;

2. All Members of Electrotechnical Division Council; and

3. All Interested.

Dear Sir(s),

Please find enclosed copy of the following draft amendment:

Document no. Title

Doc ETD 16 (10326) Draft Amendment No.1, to IS 1180(Part 1) : 2014 Outdoor

type oil immersed distribution transformers upto

and including 2500 kVA, 33kV - Specification Part 1

Mineral Oil Immersed (Fourth Revision)

Kindly examine the draft amendment and forward your views stating any difficulties which you are

likely to experience in your business or profession, if these are finally adopted as AMENDMENT..

Comments, if any, may please be made in the format given overleaf and mailed to the undersigned.

Last date for comments:‍26-06-2016.

Thanking you,

Yours faithfully

(D Goswami)

Sc ‘F’ & Head (Electrotechnical)

Encl: As above

Date Document No.

20-05-2016 ETD 16( 10326)

Sl.

No.

Name of the

Organization

Clause/

Sub-

clause

Paragraph/

Figure/Table

Type of

Comment

(General/

Technical/

Editorial

Comments Proposed Change

Doc: ETD 16 (10326)

DRAFT AMENDMENT NO. 1 MAY 2016

TO

OUTDOOR TYPE OIL IMMERSED DISTRIBUTION

TRANSFORMERS UPTO AND INCLUDING

2 500 kVA, 33kV — SPECIFICATION

(Fourth Revision)

(Page 1, clause 1) — Add following as note after paragraph:

The following type of special transformers are not covered under the scope of this standard:

a. Inverter duty transformers

b. Traction transformers

c. Instrument transformers

d. Transformers for static converters

e. Starting transformers

f. Testing transformers

g. Welding transformers

h. Earthin g transformers

i. Mining transformers

j. Transformers for (Solar, wind power application)

k. Transformers for Railways( Locomotive and other applications)

l. Furnace transformers

m. Rectifier transformers

(, clause 7.7) — Add following as note under this clause:

Any other tapping is subject to agreement between supplier and the purchaser

(Page 6, clause 10.1.2) — Substitute the following for the existing:

‘If required by the user, a suitable cable-end box may be provided on the high voltage and or low

voltage side. Alternatively bus duct arrangement may be provided on low voltage side by

agreement between user and supplier’

(Page 6, clause 10.2, lines 3 and 9) — Substitute ‘1 kV for 1.1 kV’.

(Page 7, clause 11.1) — Add following note after paragraph:

“Alternatively epoxy cast terminals may also be used in the cable box subject to agreement

between user and supplier”

(Page 7, clause 13) — Add following note at the end of the clause:

‘Marking Plate Dimensions can be changed subject to Agreement between the User and the

Supplier’.

(Page 7, clause 14.2, line 2) — Substitute ‘may for shall’

{Page 12, clause 20.1(b), Note 2} — Substitute the following for the existing:

‘Only minimum position corresponds to the operating temperature of 30°C (for sealed type

transformers).

{Page 12, clause 20.1(f)} — Substitute the following for the existing:

Drain-cum-sampling valve (¾″ nominal size thread, IS 554) preferably steel with plug for three

phase transformers (for transformers above 100 kVA)

(Page 12, Fig. 7) — Replace the existing table given in Fig. 7 with the following table:

[Page 13, clause 20.1{(h), (q), (r)}] — Delete these fittings

{Page 13, clause 20.1(m)} — Substitute the following for the existing:

Oil/Nitrogen/air filling device/pipe with welded cover capable of reuse (for sealed type

transformer)

(Page 13, clause 20.2) — Add following as new optional fitting (q):

Unidirectional flat rollers (for transformers above 200 kVA);

Rating (kVA) X Y

2500 1200 800

2000 900 800

1600 900 800

1250 800 800

1000 800 750

630 700 800

500 700 450

400 800 450

315 600 450

250 500 415

(Page 13, clause 20.2) — Add following as new optional fitting (q):

Drain-cum-sampling valve (¾″ nominal size thread, IS 554) preferably steel with plug for three

phase transformers (for transformers upto 100 kVA)

(Page 13) — Add following as new clause 20.3:

The transformer may be fitted with Self Protection Devices subject to agreement between the

user and the supplier. Requirements of Self Protection Devices are given in Annex F.

{Page 13, clause 21.2(g)} — Add following note:

‘For single phase transformer with 11/√3 or 22 / √3 or 33/√3 volts, this test on HV side is not applicable’.

(Page 14, clause 21.5.1.1, para 1) — Substitute the following for the existing:

‘For non-sealed and sealed type transformers, the transformer tank shall be subjected to air

pressure of 80 kpa for 30 min (25 kpa for 30 minutes for corrugated tanks) and vacuum of 250

mm of mercury for 30 min’.

(Page 16, Annex A, second column, lines 11 and 12) — Substitute the following for the existing:

(Part 0/Sec 1):2012 Part 0 General requirements, Section 1 Enameled round copper wire (First

Revision)

(Part 0/Sec 2):2011 Part 0 General requirements, Section 2 Enameled rectangular copper wire

(First Revision)

(Part 0/Sec 3):2012 Part 0 General requirements, Section 2 Enamelled round aluminium wire

[First Revision of IS 13730 (Part 0/Sec 3):1994]

(Page 18) — Insert following as Annex E:

ANNEX E

ADDITIONAL INFORMATION ON LEAKAGE TEST

(Cl. 21.2 j, 21.5.1.3, 21.5.2.3 and 21.5.3.3)

E-1 Calculation of Gauge pressure during oil leakage test from Normal static head

Hydrostatic pressure in oil is given by: � = ��ℎ

Where, � = Pressure at a point (Pa)

� =Density of oil (kg/m3) (as per IS 335, the density can be taken as 890 kg/m

3)

G = gravitational force (9.81 m/s2)

H = height of oil column at a particular point (m) (measured from top)

E-2 As per 21.5.1.3, 21.5.2.3 and 21.5.3.3, the amount of pressure application during the leakage

test on assembled transformer for non-sealed and sealed type transformers with all fittings

including bushing in position is summarized below:

a) Tank without corrugations:

i. Pressure equivalent to twice the normal head measured at base of tank for 8 h (for 3

phase transformer) and 6 h (for 1 phase transformers)

b) Tank without corrugations

i. 15 kPa measured at top of the tank for 6 h for both 3 phase and 1 phase transformer

E-3 Position of Pressure gauge is not specified. Based on facility available,the pressure gauge

can be mounted near the base of the tank or near the top cover(or on the cover)

a) The depth of static head at bottom gauge position shall be the height from highest oil

level to base of tank

b) The depth of static head at top of the tank shall be the height from highest oil level in

conservator up to tank top gauge location.

c) In case the conservator is not provided, (eg. Single phase transformers and small 3 phase

transformer below 63 kVA) and pressure gauge is mounted on cover of tank, a pressure

equivalent to one static head (tank height in this case) shall be applied since as per

requirement test pressure is equivalent to twice the static head.

E-4 Sample calculation of pressure for a transformer having oil level of 1000 mm:

Density of T/F oil, ρ = 0.89 gr/cc = 890 kg/m3

Oil level in the T/F, h = 1000 mm = 1 m

Hence, normal head pressure = (890 x 9.81 x 1) Pa = 8730.9 Pa ≈ 8.7 kPa

Twice the normal head pressure = 2 * 8.7 kPa = 17.4 kPa

Hence, Pressure to be measured in the gauge is,

a) 17.4 kPa, if gauge is fixed at base of tank and

b) 8.7 kPa, if gauge is fixed at top

(Page 20) — Insert following as Annex F:

ANNEX F

REQUIREMENTS OF SELF-PROTECTION DEVICES

F-1 SCOPE

This part of standard applies to high-voltage/low voltage self-protected liquid filled and naturally

cooled transformers for rated power 5kVA to 500kVA for indoor or outdoor use having a

a) Primary Winding (high-voltage) with highest voltage for equipment up to 24kV

b) Secondary Winding (Low-voltage) with highest voltage for equipment of 1100 V

These transformers are equipped with a self-protection and disconnection device to protect the

environment, property and people and prevent any disturbance of the high-voltage networks

from the consequences of an internal transformer fault.

Transformers covered by this standard must comply with the relevant requirements prescribed in

the IS: 1180 (Part-1).

F- 2 SELF-PROTECTED TRANSFORMERS

F- 2.1 Functional Requirements ― These transformers are equipped with an internal self-protection and disconnection device which, in the case of an internal fault, is designed to

a) Prevent external effects;

b) Keep the damage within the transformer tank. However, tank distortion as the result

of the fault is permitted;

c) Prevent ejection of dielectric fluid, material or gas from the inside to the

outside of the tank;

d) Prevent the propagation of an electric arc from the inside to the outside of the tank;

e) Eliminate the fault current in the transformer (disconnection);

f) Disconnect only the faulty transformer without tripping the high-voltage feeder.

F- 2.2 Coordination Principle ― The manufacturer shall provide information about the

characteristics for the high-voltage self-protection and disconnection device to enable the user

a) To verify that the characteristics of the high-voltage upstream network protection

have been coordinated to prevent false tripping of the feeder;

b) To verify that the characteristics of the low-voltage downstream network protection

have been coordinated to prevent false tripping of the transformer.

c) To verify that the disconnection device is coordinated with the self-protection device

such that disconnection device does not act for faults in low voltage network and

thermal overloading.

The user shall also ensure that the protection scheme of the low-voltage network is coordinated

in order to avoid unintended tripping of the self-protection and disconnection device inside the

transformer tank.

The self-protection and disconnection device inside the transformer tank is neither accessible nor

adjustable on site except the handle which enables to make, break and reset of self-protection

device.

The curves or data necessary to define the protection system shall be given. Reference of the

model numbers of self-protection device and the disconnection device shall be reported on the

rating plate as below:

(1) (2) (3)

Self-Protection Device Disconnection Device

Type Thermal/ Thermal

Magnetic Breaker

Expulsion Fuse

Make/Model

Rating

Interruption Capacity

(kA)

F- 2.3 Overload Capability ― The self-protection and disconnection device shall not operate

under the permitted overload conditions defined in IS: 6600.

F- 3 CONSTRUCTIONAL FEATURES OF SELF-PROTECTED TRANSFORMERS ―

The following protective equipment must be installed at least in each self - protected

transformer:

a) Lightning Arrester on Primary Side

b) Expulsion fuse as Disconnection Device on primary side.

c) Thermal or thermal - magnetic circuit breaker as self-protection device.

d) Overload indicator.

Equipment and accessories mentioned above, must meet relevant IS Standards in this standard

reference and they are the responsibility of the manufacturer to be an integral part of the

transformer.

F- 3.1 Lightening Arrestor ― The Lightning Arrestor selection, testing, and operation must comply with IS-3070(PART-III). They must be installed directly on the transformer tank (on a

support base) and connected directly to the terminal of the primary terminal. The bridge between

the arrester and primary terminal must be firmly connected by a flexible copper braid with a

minimum length of 250mm.

F- 3.2 Expulsion Fuse ― The expulsion fuse in integrated into the transformer for the protection of the distribution system against internal faults in transformers. The fuse is installed

inside the transformer and connected in series between the internal terminal of the bushing and

the output of the primary winding. The fuse must be immersed in oil and / or inside the bushing.

If the transformer is fitted with a switch, the fuse must be coordinated so that only operate

as a backup switch or due to an internal fault in the transformer.

The fuse must comply with IS: 9385 (PART-2)-1980.

F- 3.3 Thermal Switch or Thermomagnetic Circuit Breaker ― To protect against secondary failures and overloads, you must integrate an oil immersed transformer circuit breaker,

coordinated with the expulsion fuse and meet the all the type test requirements of IS/IEC 60947-

2.

Table !: Magnetizing current

Time

(1)

Magnetising current

(2)

0.01s 25xIn

0.10s 12xIn

1.00s 6xIn

10.00s 3xIn

In = Rated current of the transformer

Type

(1)

Ratings (kVA)

(2)

Symmetrical Interrupting

Capacity (A rms)

(3)

Three

Phase

6.3, 10, 16, 20, 25, 40

4000 A

63, 100

6000 A

160, 200

10000 A

250, 315

15000 A

400, 500

25000 A

Single

Phase

5, 10, 16

4000 A

25

6000 A

The operating handle must be designed and constructed to enable operation from the ground. The

breaker must be mechanically restored from an accessible handle when it operates due to a fault

in the secondary network.

F- 3.4 Overload Indicator ― The signal device must be mounted on the wall of the tank near

the handle switch operation. The lamp must glow under the following conditions:

Primary (HV)

(1) Self-Protection Device

(2) LV Terminal

(3) Signal Light

(4)

Live Close Live Off

Live Open Off On

Live Close (Overload) Live On

Live Open (Trip) Off On

The indicator lamp also operates in overloaded state indicating to the outsider that the

transformer is overloaded and glows until it is reset by the operating handle of the breaker. It

must have an average life of 1000 h and in red colour.

F- 4 TESTS

F- 4.1 List and classification of tests (routine, type and special tests) ― The

tests shall be carried out in accordance with the relevant part of IS1180 (part-1)-2014,

except for the modified and additional tests stated below.

F- 4.2 Routine Tests ― The tests shall be as prescribed in 21.2 of IS 1180 (PART-1)-

2014

F- 4.3 Type Tests ― The tests shall be as prescribed in 21.3 of IS 1180 (part-1)-2014.

The following type tests shall be performed to demonstrate compliance with the standard:

a) Partial discharges (under consideration);

b) Low-voltage lightning impulse voltage;

c) overload temperature rise;

d) Switching and inrush current;

e) Low-voltage single phase overload;

f) Low-voltage three-phase impedance short circuit;

g) Dielectric liquid leak;

h) Low-voltage winding short-circuited turns;

i) High-voltage winding short-circuited winding leads;

j) Cover-tank electrical earth continuity;

k) Tank overpressure.

Since the tests referring to the self-protection and disconnection device are

destructive, it should be useful to test up to five transformers. Table 1 gives the order

for tests carried out on five prototypes (A, B, C, D, E).

Table 1 - Order for tests carried out on prototypes A, B, C, D, E

Transformer tested A B C D E

W inding resistance X X

Short-circuit impedance and load loss X X X

No-load loss and no-load current X X X

Voltage ratio and vector group X X X

Short-circuit withstand (11.4) X

Temperature rise at rated power X X

Overload temperature rise (12.4.7) X X

Separate source a.c. withstand voltage (high voltage) X X

Separate source a.c. withstand voltage (low voltage) X X

Induced a.c. withstand voltage X X

High-voltage lightning impulse voltage X X

Low-voltage lightning impulse voltage if applicable

(12.4.8)

X X

Partial discharge (12.1) X X

Sound level X

Switching and inrush current (12.3) X

Low-voltage single-phase overload (12.4.4) X

Low-voltage three-phase impedance short circuit (12.4.5) X

Dielectric liquid leak (12.4.3) X

Low-voltage winding short-circuited turns (12.4.2 ) X

High-voltage winding short-circuited winding lead

(12.4.6)

X

Cover-tank electrical earth continuity (12.4.9) X

Tank overpressure (12.2) X

Untanking active part and analysis of the disconnecting

development

X X X X X

F- 4.4 Short-Circuit Test with the Self-Protection and Disconnection Device

Disconnected or Bypassed

The components of the self-protection and disconnection device are sensitive to high over

currents; they shall be inhibited during the performance of the test according

to IS 2026 (part-5)-2011 intended to demonstrate the ability to withstand the dynamic

effects of short circuit.

F- 5 TEST PROCEDURE

F- 5.1 Measurement of Partial Discharges (Under Consideration)

F- 5.2 Transformer Pressure Test

F- 5.2.1 Purpose of the Test

The purpose of the test is to verify that the transformer is able to withstand an

increase in internal pressure without external effects until and after the self-protection

and disconnection function has operated. The manufacturer shall state the maximum rated

internal pressure withstand for the tank (ΔP value).

The lowest admissible value of the internal withstand pressure is subject to

agreement between the manufacturer and the purchaser. If the transformer is fitted with

a pressure detecting device, its tripping value shall be set up accordingly.

F- 5.2.2 Test Methods

The test consists in submitting the de-energized transformer to an overpressure growing

progressively in about 1 min from 0 to ΔP value and maintaining this value for 10 min.

Leak detection is carried out by visual inspection while maintaining for 10 min the

initial pressure on the manometer and whitewashing the tank with a tracer if necessary.

If the transformer is fitted with a pressure-detecting device, the tripping threshold

of the system reacting with the overpressure shall be established.

F- 5.2.3 Test Results

No oil leak shall be detected. A permanent deformation of tank and cooling fins is

allowed.

F- 5.3 Switching Cycle Tests

F- 5.3.1 Purpose of the Test

The purpose of the test is to verify that the self-protection and disconnection device

does not operate due to the inrush current when the transformer is switched on at the

rated voltage. The short-circuit power should be at least 50 times the rated power of

the transformer under the test.

F- 5.3.2 Test Methods

The transformer is energized at a test voltage equal to the rated voltage (tolerance +5%)

for 1 s ± 20 ms. The cycle is repeated 20 times. Each cycle of the energizing period is

spaced out by at least 10 s. For convenience of testing, by the testing laboratory it can spaced

out by up to 180 seconds (3minutes).

Other cycles and tolerances for test voltage and duration are subject to agreement

between the manufacturer and the purchaser.

F- 5.3.3 Acceptance Criterion

Disconnection shall not occur.

F- 5.4 Behavior Test of Self-Protection and Disconnection Device

F- 5.4.1 Common Clauses Applicable For These Tests

F- 5.4.1.1 Test-circuit characteristics

The value of the short-circuit power shall be in accordance with IS 2026-5.

The purchaser shall define the characteristics given below.

a) The power factor of the test source: less than 0.15.

b) The high-voltage neutral terminal of the supply network shall be earthed

through an impedance that limits the current to the earth to certain values.

These values depend on the kind of system neutral earthing and the preferred

values are

1) For solid earthing the three-phase short-circuit value;

2) For isolated earthing: up to 100 A;

3) For resistance earthing: 100 A to 3 000 A

4) For impedance earthing: 10 A to 300 A. NOTE: In case of difficulties in setting the performance of the test laboratory, an agreement amon g the purchaser, the

manufacturer and the laboratory should be found to adjust the current.

F- 5.4.1.2 Transformer preparation

The tap changer, if fitted, is positioned on the tap that corresponds to the rated voltage.

The low-voltage neutral and the transformer earthing terminal are earthed.

The transformer is placed on an isolating support, so that the fault current between the

tank and the earth can be measured.

F- 5.4.1.3 Test duration and measurement

The supply voltage shall remain connected for 15 minutes after the self-protection a n d

disconnection device has operated.

Measurements to be taken during each test:

a) r.m.s. primary line currents;

b) r.m.s. primary phase-to-ground voltages;

c) r.m.s. tank earthing current;

d) r.m.s. high-voltage network earthing current

e) r.m.s. secondary line current;

f) r.m.s. neutral current;

g) r.m.s. secondary phase-to-neutral voltage;

h) transformer tank pressure (for tests with possible pressure increasing);

i) r.m.s. values can be defined from an analogue oscillogram or digital

recordings.

NOTE: A video camera may be used during the tests to aid subsequent analysis.

For convenience of testing the test duration may be reduced to 30 seconds.

F- 5.4.1.4 Acceptance criteria

The high-voltage line currents shall not exceed the value given by manufacturer and

shall be cleared by the self-protection and disconnection device.

The operation of the protection-disconnection function shall not cause overvoltages

on the high-voltage network or on the low-voltage network exceeding the maximum

values for the insulation level of the network.

During the performance time of the test including the 15-min period that follows

the elimination of the three high-voltage line currents, no event, such as fire, material

projections, dielectric liquid or gas leak, or propagation of an electric arc from inside

to outside the tank, shall take place.

After untanking the active part, it shall be checked that the self-protection and

disconnection function has operated properly.

If the disconnection of the three phases is required, it shall be checked phase by phase

by measurement.

These criteria are applicable to all following tests. In addition, special criteria are given

in the relevant clauses.

F- 5.4.2 Transformer With Short Circuit Between The Low-Voltage Turns

F- 5.4.2.1 Purpose of the test-(under consideration)

This test is performed to check the behaviour of the transformer in case of short

circuit between low-voltage turns.

F- 5.4.2.2 Transformer preparation

A transformer comprising one turn in short circuit in one low-voltage winding shall be

manufactured specially to carry out this test. The flowing section of the short-circuit

current shall be sufficient not to insulate the short circuit during the test.

In case of difficulties with the above conditions, the maximum number of short-circuit

turns should be increased to achieve the test. The maximum number of turns shall,

however, be limited to five.

F- 5.4.2.3 Particular test methods

During the test, the transformer shall supply an inductive three-phase load

corresponding to its rated power.


Sub clause 5.4.1.4 is applicable.

F- 5.4.3 Transformer with Dielectric Liquid Leak


The purpose of the test is to verify that a leak, in an energized transformer, shall not

lead to an explosion or a fire.


The transformer is connected to a 3-phase load at the rated power and the rated voltage. A

drain valve is provided and sized to decrease the volume of fluid by 50 % in about 1 h

in case the disconnection does not operate before.


Sub clause 5.4.1.4 is applicable.

F- 5.4.4 Transformers With Low-Voltage Single-Phase Overload


The purpose of the test is to create a temperature rise of the windings until their

deterioration with initial circulation currents insufficient to immediately trigger the self-

protection and disconnection device.


An inductive load is placed on the low-voltage side between one phase and the neutral.

This load is such that the initial low-voltage current is between 3 and 4 times the rated

low-voltage current of the transformer.

NOTE: The load current above the normal overload range has been selected in order to damage the winding within a reasonable

time.


Subclause 5.4.1.4 is applicable.

F- 5.4.5 Transformer With A Three-Phase Impedance Short Circuit On The Low-Voltage

Network


This test will be carried out only if the self-protection and disconnection device

comprises current-limiting fuses (back-up or general-purpose types) in accordance with

lS 9385-1.

The purpose of the test is to check the critical point of self-protection and

disconnection device when these critical points exist (see Note).

When the self-protection and disconnection device comprises other protection

components, an equivalent test should be defined between the manufacturer and the

purchaser.

NOTE: For back-up and general-purposes fuse types, currents slightly lower than I3 are considered to be critical.

F- 5.4.5.2 Particular test method

An inductive load is connected to the low-voltage terminals. This load is such that

high- voltage currents are between 0,75 and 0,8 times the value of the minimum

breaking current (I3 according to IS 9385-1).

lf the short-circuit impedance of the transformer does not allow the above current values

to be reached, the test is performed with a short-circuit link on the low-voltage terminals.

F- 5.4.5.3 Acceptance criteri


F- 5.4.6 Transformer with A Three-Phase Short Circuit At The High-Voltage Winding

Leads

F- 5.4.6.1 Purpose of the test

This test has two purposes ― to check the good mechanical and thermal behaviour of

the links between the high-voltage bushings and the high-voltage side of the self-

protection and disconnection device for the maximum fault current;

- to check the proper disconnection for the maximum fault current.


A short circuit is made between the self-protection and disconnection device and the

high- voltage winding leads at a location agreed between the manufacturer and the

purchaser.

The test can be performed on a model partially reproducing the arrangements of

the transformer from high-voltage bushing to the high-voltage winding leads

including the grounded parts.

The transient recovery voltage of the power test circuit shall be in accordance

with IS: 9385-1 concerning test series 1, even if no fuses are fitted in the transformer.



F- 5.4.7 Temperature Rise With A Three-Phase Overload


The purpose of the test is to check that an overload does not cause improper operation of

the self-protection and disconnection device.

The following test is carried out to demonstrate the behaviour of the self-protected

transformer under a specific overload cycle. The test shall be such that the final

temperature value is close to the maximum allowable values according to IS: 2026 (part-

7)-2009 (maximum winding hot-spot temperature of 140 °C and maximum top oil

temperature of 115 °C).


The transformer is first supplied at reduced voltage with the low-voltage phase

bushings short-circuited.

A current corresponding to the total losses is applied until thermal stabilization is

reached.

The test shall be achieved in accordance with IS: 2026 (part-7)-2009 with an overload of

1.4 In and the time of the overload shall be calculated by a formula taking into

account the ambient temperature of the laboratory. The formula is given in IS: 2026

(part-7)-2009


Disconnection of the self-protection and disconnection device shall not occur.

F- 5.4.8 Low-Voltage Lightning Impulse Voltage


The purpose of the test is to verify the low-voltage insulation on distribution

transformers for suburban or rural installation which are, in some countries, severely

exposed to overvoltages.


The lightning impulse is applied between the low-voltage bushings including the low-

voltage neutral bushing (if any) connected together and the tank; the high-voltage

bushings are linked together and connected to the earth.

Three impulses with negative polarity are applied: peak value and shape 30 kV;1.2/50

µs.

F- 5.4.8.3 Acceptance criterion

No flashover or failure shall occur.

F- 5.4.9 Electrical Connection Between Cover And Tank

F- 5.4.9.1 Purpose of the test (under consideration):

The purpose of the test is to verify that the electrical connection between cover and

tank is able to carry a 6 kA current for 1 s with an increasing potential lower than 50 V.


The test is performed with a sinusoidal single-phase supply voltage, the transformer is

connected to the test circuit by means of an earth terminal on the cover and an earth

terminal on the tank and a link to the ground terminal.

Throughout the test, the current is measured and the value shall be contained

within 6 kA ± 10 %; the voltage shall never exceed 50 V.


No failure or flashover, such as electrical arc spreading or material spattering, shall

occur.

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