i

TECHNICAL SPECIFICATION FOR THE

SUPPLY AND DELIVERY OF CURRENT

TRANSFORMERS

Volume 2

NamPower

15 Luther Street

PO Box 2864 Windhoek

Namibia

Tel : (+264 61) 205 4111

Fax : (+264 61) 205 2326

2

TABLE OF CONTENTS

1 Scope ................................................................................................................... 3

2 Obligation ............................................................................................................. 3

3 Normative References .......................................................................................... 3

4 Definitions ............................................................................................................ 4

5 Requirements ....................................................................................................... 4

6 Insulation Media ................................................................................................... 6

7 Design Requirements ........................................................................................... 8

8 Construction Requirements ................................................................................ 13

9 Tests .................................................................................................................. 17

10 Marking/labelling/documentation ........................................................................ 24

11 Delivery and Installation ..................................................................................... 26

12 Right of Purchase ............................................................................................... 27

13 Approval List ...................................................................................................... 27

14 Change of Equipment Design ............................................................................ 28

3

1 Scope

This specification covers the Purchaser’s requirements for high-voltage outdoor current

transformers for voltages above 3,6 kV up to and including 420 kV for use with electrical

measuring instruments, electrical protective devices, or both at a service frequency of 50 Hz.

This specification covers the design, manufacture, fabrication, testing, supply, delivery,

erection and guarantee of outdoor type current transformers operating on purely

electromagnetic principles.

This specification is based on the recommendations contained in the most recent revision

of NRS 029-1.

2 Obligation

Nothing in this specification shall lessen the Contractor’s obligations detailed in any other

document forming part of the contract.

The following referenced documents are indispensable for the application of this document.

For dated references, only the edition cited applies. For undated references, the latest

edition of the referenced document (including any amendments) applies. Information on

currently valid national and international standards can be obtained from the SABS

Standards Division.

3 Normative References

BS 1872:1984, Specification for electroplated coatings of tin.

SANS 60044-1, Instrument transformers - Part 1: Current transformers..

IEC 60028:1925, International standard of resistance for copper.

IEC 60044-6:2004, Instrument transformers — Part 6: Requirements for protective current

transformers for transient performance.

IEC 60050, International Electro technical Vocabulary.

IEC 60121:1960, Recommendation for commercial annealed aluminium electrical conductor

wires.

IEC 60815: 2008, Selection and dimensioning of high-voltage insulators intended for use in

polluted conditions

4

IEC 60270:2000, High-voltage test techniques — Partial discharge measurements

NOTE: Includes corrigendum of October 2001.

IEC 60296:1982, Specification for unused mineral insulating oils for transformers and

switchgear.

(Amendment No. 1:1986).

IEC 60376:1971, Specification and acceptance of new sulphur hexafluoride.

SABS 555:1985, Mineral insulating oil for transformers and switchgear

SANS 60060-2:1994, High-voltage test techniques — Part 2: Measuring systems.

SANS 60298:1990, A.C. metal-enclosed switchgear and controlgear for rated voltages

above 1 kV and up to and including 52 kV.

SANS 60529:1989, Degrees of protection provided by enclosures (IP code).

SANS 60694:1996, Common specifications for high-voltage switchgear and controlgear

standards.

SANS.60947-7-1:2009, Low voltage switchgear and controlgear – Part 7-1: Ancillary

equipment – Terminal blocks for copper conductors.

SABS ISO 1461:2000, Hot-dip (galvanized) zinc coatings (other than on continuously zinc-

coated sheet and wire).

SANS 61869-1, General requirements for instrument transformers

NRS 079-1:2004, Mineral insulating oils (unihibited) – Part 1: Purchase, management,

maintenance and testing

4 Definitions

For the purposes of this specification, the definitions and abbreviations given in

SANS 60044-1 and IEC 60050 apply.

5 Requirements

5.1 General

The life expectancy shall be 25 years under normal service conditions.

5

5.2 Service Conditions

Unless otherwise specified in schedule A, the following standard service conditions apply:

a) Ambient air temperature: -10 °C to 45 °C;

b) Daily average: 30 °C

c) Yearly average: 20 °C

d) Altitude: not exceeding 1 800 m;

e) Daily average humidity: not exceeding 95 %;

f) Wind pressure: not exceeding 700 Pa (equivalent to 34 m/s);

g) Level of atmospheric pollution: very heavy;

Notes:

For CT’s rated at ≤ 145kV, the insulation total creepage distance in mm shall be as per the

“very heavy” definition in IEC 60815

For CT’s rated at > 145kV, the insulation total creepage distance in mm shall be specified

in the schedules

5.3 Transport

Transportation of current transformers shall comply with the requirements of the relevant

transport authorities.

5.4 Erection

Erection shall include off-loading, the supply of all necessary tools, staging and lifting tackle

for handling and installation of the equipment. The Contractor shall remove all the above

mentioned items from site when erection has been completed.

5.5 Insulation

The insulation levels of the current transformers shall comply with the value as detailed in

Table 1. The current transformer shall be suitable for operation on a system that has

insulation levels as specified in schedule A.

A capacitive tap from the second last screen, in addition to the last screen (which is

adequately earthed), is required for dielectric dissipation factor (DF = tangent-delta) testing

and should be brought to a separate terminal in the terminal box.

6

Table 1: Standard voltages and insulation levels of outdoor current transformers

Highest r.m.s.

voltage for

equipment

Um

kV

Nominal system r.m.s.

phase-to-phase

voltage

Un

kV

Rated short-time

power-frequency r.m.s.

withstand voltage

PFWL

kV

Rated switching impulse

peak withstand

voltage

SIWL

kV

Rated lightning impulse

peak withstand

voltage

LIWL

kV

Minimum

Insulation

Creepage

Distance

mm

3,6

7,2

12

24

36

52

72,5

100

145

245

300

362

420

800

3,3

6,6

11

22

33

44

66

88

132

220

275

330

400

765

16

22

28

50

70

95

140

185

275

395

460

510

630

1 150

-

-

-

-

-

-

-

-

-

690

850

950

1 050

1 425

45

75

95

150

200

250

350

450

650

950

1 050

1 175

1 425

2 100

115

225

375

750

1 125

1 625

2 250

3 100

4 500

7 600 (6 100)

9 300 (7 500)

11 300 (9 000)

13 100 (10 500)

25 000 (20 000)

Note 1: The creepage for units rated above 145kV (Um) and shall either be “very heavy” or

“heavy” as specified in the technical schedules. For voltage levels up to and including

145kV, a creepage of 31 mm/kV in relation to the highest r.m.s. phase-to- voltage (Um).

Note 2: The arcing distance of the insulator shall comply with the requirements for external

insulation specified in table 7 of SANS 60044-1.

6 Insulation Media

All materials used in construction of the CTs shall, based on environmental performance

tests, be declared to have the reasonable minimum negative impact on the environment

(and construction personnel) as a result of any releases into the environment and also when

eventually disposed of. Details of insulation materials shall be stated in schedule B.

Insulation media shall comply with international requirements and the Contractors detail

shall be submitted when tendering. This shall apply to all insulation materials utilised in the

construction of the complete unit.

Dry type CTs shall not be accepted.

7

6.1 Oil/Paper Insulated CTs

The oil used in CTs shall comply with NRS 079-1. Details of the Contractor and Contractor

type designation of oil shall be stated in schedule B. The rating plate shall be marked to

indicate the Contractor and type designation of the oil used to fill the voltage transformer.

Oil-insulated CTs shall be hermetically sealed. Details of the sealing arrangement shall be

submitted for approval if requested in schedule A. Where the Contractor’s design requires

specially designed gasketted joints to be above the oil level, machined surfaces and ‘O-

rings’ shall be used. Details of such joints shall be submitted for approval if requested in

schedule A.

The expansion of the insulating oil shall be accommodated by means of bellows or an inert

gas in the top housing of the current transformer. If bellows are used, they shall be of

stainless steel.

Facilities for oil filling and draining shall be provided. These facilities shall be suitably sealed

below the normal operating oil level and shall not leak oil when the transformer is tested in

accordance with SANS 61869-1.

If so specified in schedule A, oil sample valves shall be provided. Details of the oil sample

valves shall be submitted for approval before manufacture is undertaken. Details of sample

valves shall be submitted to NamPower for approval.

Oil level indicators shall be provided and arranged so as to be readable from ground level

with the transformer mounted on its structure as in service. Oil gauges shall be flush

mounted, and shall be securely attached throughout its perimeter to the tank or expansion

chamber. The sight glasses shall be ultra-violet light resistant and shall be protected from

accidental damage. Details of the oil level indicators shall be submitted to NamPower for

approval.

6.2 Gas Insulated CTs

The unit shall be labelled to clearly indicate that it is gas filled under pressure, the type of

gas (SF6, etc.) and the gas pressure. Facilities shall be provided at the base of the unit for

filling, draining and topping up of SF6 gas.

The method of sealing of the CTs and any need for gas replenishment shall be stated by

the supplier in schedule B. Refer to SANS 60298 for details of leakage rates and expected

life before replenishment.

8

Sulphur hexafluoride (SF6) gas shall comply with the requirements of IEC 60376. Details of

any alternative gases or gas mixtures offered, including applicable specifications, shall be

stated in schedule B.

Permanent gas-density monitoring devices shall be provided at the base of the CTs. This

device shall also incorporate first and second stage alarm signalling contacts, wired to the

secondary terminal strips, as well as a visible indication of the pressure available in the unit.

The readable gas-density monitoring devices shall be self-powered.

7 Design Requirements

7.1 Core and winding design

CTs shall be of the low leakage flux type in accordance with clause 3.18 of IEC 60044-6 on

all ratios, or of the low-reactance type as per NRS 029 for classes PX and TPS cores on all

ratios.

Each secondary winding shall be so designed that it will meet the following condition, with

the lowest specified ratio winding connected to its nominal burden. The maximum voltage

with respect to earth which will appear on any terminal, whether connected to the burden or

not, shall be less than the value specified for the inter-turn over-voltage test and shall be

stated in schedule B.

For post type current transformers, the secondary cores shall be positioned such that the

protection cores are on the P1 terminal side and the metering cores are on the P2 terminal

side or as otherwise specified in schedule A. This arrangement is shown in Figure 2 and

Figure 1 below.

The current density for short time thermal current shall not exceed the values specified in

schedule A.

9

Figure 1: 4-core without metering core(s)

P BZ BZ P P2 P1 Breaker

Core No. 1 2 3 4

P

BZ BZ P

P1 P2

Legend

P: Protection

BZ: Buszone

M: Metering

P1: Insulated primary terminal P2: Primary Terminal connected to the CT head

Hair Pin Design

Top Core Design

10

Figure 2: 6-core with metering core(s)

7.2 Ratios and Special Characteristics

Where multi-ratio (MR) CTs are specified, the various ratios shall be obtained by means of

tappings that can be obtained by changing the effective number of turns on the secondary

winding. Turns ratio compensation, where acceptable, shall not be by means of an additional

winding. The design shall be such as to minimise the voltage which can appear on a winding

terminal not connected to a burden.

Ratio selection on the primary side shall only be provided when it is called for under the

Special Requirement section.

The tap points and ratio selection (on primary or secondary windings) shall be those

specified in schedule A.

P BZ BZ P P2 P1 Breaker

Core No. 1 2 3 4 5 6

P BZ

BZ P

M M

P1

M M

P2

Legend

P: Protection

BZ: Buszone

M: Metering

P1: Insulated primary terminal P2: Primary Terminal connected to the CT head

Hair Pin Design

Top Core Design

11

7.3 MULTI RATIO (MR) METERING CURRENT TRANSFORMERS

Multi ratio metering current transformers having the nominal ratio specified in schedule A

shall provide the ratios, as per Table 2.

The secondary current rating Is shall be 1 A or as otherwise specified in schedule A.

Table 2: Ratios for Multi Ratio (Mr) Metering Current Transformers

Highest (nominal)

ratio

Ratios available Tap points required on secondary

winding

Accuracy class applicable to the ratio

Burden

VA

Class Ratio

90:1 90; 60; 30:1 1:30 10 0,5 90:1

150:1 150; 100; 50:1 1:50 10 0,5 100:1

10 0.2 150:1

200:1 200; 150; 125; 75; 50; 25:1 1:75; 50 10 0,5 125:1

10 0.2 150:1

400:1 (a) 400; 300; 200; 100:1 1:200; 100 10 0,5 100:1

10 0,2 200:1

400:1 (b) 400; 300; 250; 150; 100; 50:1 1:150; 100 10 0,5 100:1

10 0,2 150:1

600:1 600; 500; 400; 200; 100:1 1:200; 100 10 0,5 200:1

10 0,2 400:1

800:1 800; 600; 400; 200:1 1:600; 200 10 0,5 200:1

10 0,2 400:1

1 200:1 1200; 1000; 800; 600; 400; 200:1 1:800; 200 10 0,5 200:1

10 0,2 400:1

1 600:1 1600; 1200; 800; 400:1 1:800; 400 10 0,5 400:1

10 0,2 800:1

2 000:1 2000; 1600; 1200; 800; 400:1 1:800; 400 10 0.2 400:1

2 400:1 2400; 1600; 1400; 1000; 800; 600:1 1:1600; 1000 10 0.2 600:1

3 200:1 3200; 2400; 1800; 1400; 1000; 800:1 1:1800; 800 10 0.2 800:1

* For nominal ratio 400/1, option a) or b) is specified in schedule A.

7.4 MULTI RATIO (MR) PROTECTION CURRENT TRANSFORMERS

Multi ratio protection current transformers having the nominal ratio specified in schedule A

shall provide the ratios as per Table 3.

The required accuracy class is specified in schedule A and shall be either class 5P, 10P or

PX as per SANS 60044-1.

12

For bus zone cores, table 5 and table 6 apply for single ratio and multi-ratio cores

respectively.

Unless specified in schedule A, the secondary current rating Is shall be 1 A.

Table 3: Ratios for Multi Ratio (Mr) Protection Current Transformers

Highest (nominal)

ratio

Ratios available Tapping points on secondary

windings

Minimum knee point voltage, Ek, for class PX at the highest ratio

Exciting current

Ie

100:1 100; 80; 70; 60; 50; 40; 30; 20; 10:1 80; 40; 30 100 38

200:1 200; 175; 150; 125; 100; 75; 50; 25:1 150; 100; 25 200 38

400:1 400; 350; 300; 250; 200; 150; 100; 50:1 300; 200; 50 400 38

600:1 600; 500; 450; 400; 300; 250; 200; 150; 100; 50:1 400; 150; 100 600 38

800:1 800; 700; 600; 500; 400; 300; 200; 100:1 600; 400; 100 800 38

1 200:1 1200; 1000; 900; 800; 600; 500; 400; 300; 200; 100:1

800; 300; 200 1200 38

1 600:1 1600; 1400; 1200; 1000; 800; 600; 400; 200:1 1200; 800; 200 1600 38

2 000:1 2000; 1600; 1400; 1200; 1000; 800; 600; 400; 200:1 1600; 800; 600 2000 25

2 400:1 2400; 1600; 1400; 1000; 800; 600:1 1600; 1000; 2400 25

3 200:1 3200; 2800; 2400; 2000; 1600; 1200; 800; 400:1 2400; 1600; 400 3200 19

Note: Class PX definition – 10% increase in voltage for a 50% increase in current

7.5 Buszone Cores

Except where specified otherwise in schedule A, the buszone cores shall be class PX in

accordance with SANS 60044-1.

The excitation characteristic shall comply with table 4 or table 5, as appropriate, and as

further detailed in schedule A.

Table 4: Bus zone single ratios and excitation characteristics

Single ratio cores Minimum excitation voltage

V

Maximum excitation current

mA

Maximum winding resistance at 75 °C

1:500 1:1 000 1:1 200 1:1 600 1:2 400

550 550 550 550 550

50 50 50 50 50

2,0 4,0 4,8 6,4 8,0

13

Table 5: Buszone multi-ratios and excitation characteristics

Highest (nominal) ratio

Tapping points Excitation parameters at 1 000:1 ratio

Minimum excitation voltage

V

Maximum excitation

current mA

Maximum total winding resistance

at 75 °C

1:1 600

1 000-1 200 550 50 6,4

8 Construction Requirements

8.1 Primary Terminals

Primary terminals shall be of the type and orientation specified in schedule A. Terminals

shall be of either electrotinned copper or aluminium. Where electrotinned copper terminals

is offered this shall be without subsequent heat treatment or machining.

The primary terminals shall be marked P1 and P2, in accordance with clause 10 of

SANS 60044-1, with the additional requirement that:

P1 is the terminal which is insulated from the CTs head; and

P2 is the terminal connected to the CTs head.

The connection between terminal P2 and the CTs head shall be compatible with all materials

involved and shall not be of braided construction. It shall carry the rated short-time current

specified in schedule A without exceeding the current density specified in schedule A.

The insulated terminal P1 shall withstand the impulse voltage level specified in schedule A,

the test of which shall proceed as described in SANS 60044-1.

8.2 Secondary Terminals

The bushings used for bringing the secondary connections through the tank into the

secondary terminal box shall not be used as the secondary terminals for service

connections, unless approved.

The secondary terminals shall be rail-mounted, and shall be the spring-loaded screw clamp

type of 10 mm width in accordance with SANS.60947-7-1. The terminals shall accept two

back-to-back hook blade lugs and no more than two conductors shall be connected to any

side of a terminal. The manufacture and type of terminals required are specified in schedule

A.

14

8.3 Secondary Terminal Boxes

Each CT shall be fitted with one secondary terminal box that shall be located in an accessible

position and shall be provided with an easily removable slip-on weather-proof cover. When

in place, the cover shall be secured to the corresponding terminal box by means of

preferably not more than one M8 stainless steel set screw. Any other proposed method

shall be subject to approval.

The type of terminal box shall be stated in schedule B and, subject to approval, shall be

either:

Integrally cast with the CT case or

A steel box welded to the main tank.

The terminal box with the cover fixed in place shall have a degree of protection of at least

IP56 in accordance with SANS 60529.

Each terminal box shall be provided with an aperture, located at the bottom of the box to

permit vertical entry of the secondary control cables. The aperture shall be covered

externally by an un-drilled removable brass plate (minimum thickness of 2mm), aluminium

alloy (minimum thickness of 3mm) or stainless steel (minimum thickness of 2mm) for a steel

or aluminium box. Unless otherwise specified in schedule A, this gland plate, as well as the

opening, shall have an effective area of at least 75 mm by 50 mm. This area shall be stated

in schedule B. Access to the gland-plate opening shall unobstructed for cables entering the

terminal box vertically from below.

The space between the bottom terminals and the gland plate shall be at least 75 mm.

All terminal boxes shall be fitted with a breathing vent not less than 10 mm in diameter. This

vent shall be situated in the bottom of each box and shall be covered with a gauze of non-

corroding material to prevent entry of insects.

The beginning and end of each secondary winding with all secondary taps, if any, shall be

wired to suitable terminals accommodated in the terminal box.

Secondary terminal boards shall be made of non-tracking insulation material. If the

minimum clearance either between terminals or between walls of the terminal box and

nearest terminals is less than 25 mm, barriers interposed at these positions shall be

provided, but a minimum clearance of 13 mm shall be maintained between terminals.

15

An earth stud shall be provided for earthing of the secondary windings inside the terminal

box. The earth stud shall be of diameter at least 6 mm, and shall have an external connection

to the main earthing system.

The capacitive tap connection shall be brought out from the second last screen and shall be

connected to an insulated terminal inside the terminal box. This terminal shall be clearly

labelled and display a warning that it shall be solidly earthed during service.

8.4 Tanks

Corrugated tanks used to accommodate internal volume changes are not acceptable.

Tanks and fittings shall be of such a shape that water is not trapped on the outside surfaces.

An unpainted earthing flag of 5 x 50 x 100mm (minimum), with two 14mm holes at 50mm

centres, arranged vertically. The flag shall be situated in close proximity to a tank mounting

bolt hole on the same side as the terminal box.

8.5 Rating and Diagram Plates

Rating and diagram plates shall be engraved or stamped on stainless steel and externally

mounted. The plate(s) shall be mechanically affixed e.g. screwed or riveted, to the

equipment. Mounting by means of adhesives is not acceptable Anodised aluminium is not

acceptable for externally fitted diagram plates except for diagram plates located inside the

terminal box.

The information to be displayed shall be as specified in SANS 60044-1:

Special tests, when performed;

The customer order number, and shall be provided with a blank space for the

customer serial number;

The applicable version of NRS 029;

The volume of oil in litres for oil filled CTs or the gas type and the pressure for gas

filled CTs;

The terminal markings and the relative physical arrangement of the CTs secondary

windings with respect to the primary terminals. The P1 terminal shall be indicated as

being isolated from the CT head;

The dielectric dissipation factor (tangent delta) expressed as an absolute value and

capacitance (C12) expressed in picofarads, as measured at 10 kV, with the ambient

16

temperature (T amb ) at the time of measurement, shall be recorded either on the main

rating plate or a separate plate mounted under the main rating plate. This plate, if

separate, shall be manufactured from the same material, and mounted by the same

method as the main rating plate.

8.6 Mounting Arrangement

The base mounting arrangement for the CTs shall be such that it can be bolted to a support

structure, with mounting holes arranged on the corners of a square of dimensions not

exceeding those specified in schedule A.

Each current transformer shall be supplied complete with holding-down bolts, hot dip

galvanised.

All nuts and bolts of the assembly and clamping structure shall be effectively locked by

means of locking-plates or standard machined lock-nuts so that they will not be loosened by

vibration.

Peening of bolt-ends and/or threads alone or the use of tempered, pressed steel nuts will

not be accepted.

8.7 Finish and Surface Treatment

All materials shall be inherently corrosion-resistant or treated against corrosion for the

design lifetime of the equipment. All ferrous surfaces exposed to the elements shall be hot

dip galvanised or alternatively metal sprayed.

Unless otherwise approved, all ferrous parts associated with CTs shall be either:

Hot-dip galvanized in accordance with SANS 121, ISO 1461, shall not be less than

90 m. The Contractor shall ensure that the workmanship of all welded seams and

joints, are suitable for hot dip galvanising. Where additional coats of paints are

specified over the galvanising, the first coat of paint shall be an etching primer.

Zinc metal sprayed, in accordance with SANS 2063, shall not be less than 80 m. All

exterior surfaces shall be given a priming coat of zinc chromate based paint before

applying the finishing coats. If not hot dip galvanised the interior metal surfaces shall

be finished with a coat of oil resistant paint or varnish on a suitably prepared surface.

Electroplating on ferrous material is not acceptable.

17

Paintwork or galvanising damaged during transport and/or erection shall be made good by

the contractor to NamPower’s satisfaction.

8.8 Hollow Core Insulators

Insulators shall comply with IEC 61462 and IEC 60815.

The name of the Contractor and country of origin of the HV insulators shall be stated in

schedule B and detailed drawings of the insulator shall be supplied with the tender.

Permission must be obtained from the purchaser prior to a change of insulator supplier

during the course of a contract.

9 Tests

For CTs that are intended for service at altitudes that exceed the altitudes at which the type

tests and routine tests apply, corrections as set out in SANS 60044-1 and SANS 61869-1.

Tests are classified as type tests, routine tests and special tests as follows:

9.1 Type tests

Unless valid and approved in writing by the purchaser type test certificates specified in

SANS 60044-1 and in schedule A are available, type tests shall be carried out on one fully

assembled current transformer of each type and rating at an approved test facility stated in

schedule B. Type test certificates shall be a certified hardcopy, or protected softcopy, of the

original test certificate of a test conducted by a testing authority accepted by the purchaser.

The certificates of the tests shall be included in the test reports. Type tests shall be followed

by the routine tests.

Short-time current (SANS 60044-1 see 7.1)

Temperature rise (SANS 60044-1 see 7.2)

Lightning impulse test (SANS 60044-1 see 7.3.2)

Switching impulse test (SANS 60044-1 see 7.3.3)

Wet test for outdoor type transformers (SANS 60044-1 see 7.4)

Determination of errors (SANS 60044-1.see 11.4 and/or 12.4, 11.6,

12.5 and 14.3)

Radio interference voltage measurement (RIV) (SANS 60044-1 see 7.5)

18

9.2 Routine tests

Each fully assembled current transformer shall be subjected to the following routine tests at

the Contractor’s works, to prove compliance with this specification.

Verification of terminal markings (SANS 60044-1 see 8.1)

Power-frequency withstand test on primary winding (SANS 60044-1 see 8.2.1)

Partial discharge measurement (SANS 60044-1 see 8.2.2)

Power-frequency withstand test on secondary winding (SANS 60044-1 see 8.3 or

14.4.4)

Power-frequency withstand test, between sections (SANS 60044-1 see 8.3 or 14.4.4)

Inter-turn overvoltage (SANS 60044-1 see 8.4 or 14.4.5

Determination of errors (SANS 60044-1.see 11.4 and/or 12.4, 11.6, 12.5 and 14.3)

Polarity (SANS 60044-1 see 16 and 22)

Test for effectiveness of sealing (9.2.1)

Capacitance and dielectric dissipation factor (tangent delta) (9.2.3) and (SANS61869-

1)

9.2.1 Test for Effectiveness of Sealing

9.2.1.1 Oil-insulated current transformers

Each current transformer, when filled with oil to at least the normal level and with all sealed

fittings in place, but, where necessary, with pressure limiting devices blocked or removed,

shall be subjected for a period of 12 hours to a test pressure. The pressure measured at

the top of the unit shall not be less than twice the maximum operating pressure at ambient

air temperature of 40 °C or 70 kPa, whichever is the greater;

The current transformer shall not leak oil, including the facilities for oil filling and draining.

9.2.2 Gas-insulated current transformers

Determine the rate of gas leakage in accordance with the method described by the current

transformer Contractor and approved by the Client. Ensure that each individual SF6 gas-

filled compartment is leak-tested after sealing and that the rate of gas leakage is such as to

confirm compliance with SANS 60298.

19

9.2.3 Capacitance and dielectric dissipation factor (tangent delta)

This test shall be conducted after the power frequency withstand test, and applies to all oil-

immersed paper-insulated current transformers with a Um 24 kV. The tangent delta and

the capacitance (C12) readings at 10kV, shall both be recorded on the information plate

mounted on the tank for the purposes of condition monitoring. The measurements of

tangent delta shall be performed on the completed units.

9.2.4 For current transformers with 24kV≤Um < 52 kV

The voltage applied between primary terminals bonded together and the earth screen

terminal is to raised to 120 % of Um./√3, While the voltage is being raised, the tangent delta

measurements at voltages of 10kV, 100 %, and 120 % of Um./√3 are to be recorded.

Thereafter, the measurements should be taken in the reverse order back to 10kV i.e. 100%

and 10kV.

The tangent delta (absolute value) for a particular type must be stated by the Contractor in

schedule B.

The current transformer is deemed to have passed the test when:

The absolute value of tangent delta readings at each step, during both excursions is not

more than the value stated in schedule B.

The difference in percentage value, between the reading at the maximum test voltage (120%

Um./√3) and that at the reading at the minimum test voltage (10kV) is not more than 20%.

9.2.5 For current transformers with Um 52 kV

The voltage applied between primary terminals bonded together and the earth screen

terminal is to raised to 120 % of Um./√3..While the voltage is being raised, the tangent delta

measurements at voltages of 10kV, 63,5%, 100 %, and 120 % of Um./√3 are to be recorded.

Thereafter, the measurements should be taken in the reverse order back to 10kV i.e. 100%,

63,5% and 10kV.

The current transformer is deemed to have passed the test when:

a) The absolute value of tangent delta readings at each step, during both excursions is

not more than 0,0050.

b) The difference in absolute value, between the reading at the maximum test voltage

(120% Um./√3) and that at the minimum test voltage (10kV) is not more than 0,0010.

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9.3 Special tests

Where so specified in schedule A, the special tests shall be performed and may be specified

as type tests or routine tests.

The special tests can be required during the tender adjudication during the manufacturing

process or before the installation of the current transformers.

Where valid test certificates are available for identical current transformers, these may be

accepted in lieu the special tests.

The following tests are to be performed upon agreement between Contractor and purchaser:

Transmitted overvoltage test (SANS 61869-1 see 7.4.4);

Chopped impulse voltage withstand test on primary (SANS 61869-1 see 7.4.1);

Multiple chopped impulse test on primary winding (SANS 61869-1 see 7.4.2);

Mechanical tests (SANS 61869-1 see 7.4.5);

Impulse withstand (9.3.1);

Mechanical strength and sealing tests on primary terminals (9.2.1);

Capacitance and dielectric dissipation factor (tangent delta) at ambient temperature

and at 90°C (9.3.2);

Long duration voltage (9.3.3);

Internal arc fault protection requirements (SANS 61869-1 see 7.4.6) (9.3.4);

Enclosure tightness tests at low and high temperatures (SANS 61869-1 see 7.4.7);

Gas dew point test (SANS 61869-1 see 7.4.8);

Corrosion test (SANS 61869-1 see 7.4.9);

Fire hazard test (SANS 61869-1 see 7.4.10);

9.3.1 IMPULSE VOLTAGE TEST WITH THE CURRENT TRANSFORMER CONNECTED

AS IN SERVICE

This test shall be applied to all outdoor current transformers to check the capability of the

insulation, between the transformer cap (or conservator tank) and the terminal P1, of

withstanding the impulse voltage.

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Figure 3: Impulse voltage test circuit for current transformer connected as in service

The performance of the test shall be as follows:

A lightning impulse voltage, whose peak value is given in Table 1, is applied to P2

terminal while P1 is connected to earth via a low inductance resistance of 400 ;

The nominal burden, specified in schedule A shall be inserted across the full

secondary winding (i.e. maximum ratio) and connected to earth at one point. A

burden of 12 VA shall be used unless otherwise specified;

The insulation between P1 and P2 shall withstand the voltage appearing across them;

this voltage shall be measured and recorded in the type test report. The insulation

between terminal P1 and the current transformer head shall withstand the test without

any degradation.

9.3.2 DIELECTRIC LOSS ANGLE (TANGENT DELTA) TEST AT AMBIENT

TEMPERATURE AND AT 90 °C

The test applies only to on oil/paper insulated units having Um 72,5 kV. The measurements

of the dielectric loss angle (tangent delta) on the primary insulation, as a special test, shall

be made after the impulse test, switching impulse test and power-frequency test, but before

the long-duration voltage test. The type test measurements of tangent delta on the

completed unit consist of two performances.

At ambient temperature, and the voltage applied between primary winding terminals shall

be raised to 120 % of the highest system voltage Um. While the voltage is being raised,

tangent delta measurements shall be taken at 5 %, 10 %, 15 %, 20 %, 30 %, 60 %, 75 %,

100 %, 120 % of Um. This voltage shall then be reduced to zero and tangent delta

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measurements taken, as the voltage is being reduced, at 100 %, 75 %, 60 %, 30 %, 20 %,

15 %, 10 %, 5 % of Um;

At a temperature of approximately 90 °C, tangent delta measurements shall be taken at 60

%, 75 %, 100 % and 120 % of Um.

Ambient temperature shall be recorded. All measurements at both temperatures and at

each step shall be recorded. The transformer has passed the test if the temperature

coefficient ‘’ for each step is less than or equal to 0,01/°C.

a

90n

a tan

tan

_

90

1

t

It

where

ta is the ambient temperature in degrees centigrade;

tan 90 is the tangent delta at 90 °C;

tan ta is the tangent delta at ambient temperature; and

nI is the natural logarithm.

Values of of 0,02/°C or larger are unacceptable. For intermediate values of (between

0,01/°C and 0,02/°C), units may be accepted if they pass the long-duration test.

9.3.3 LONG DURATION VOLTAGE TEST

This test shall be performed to determine the thermal stability on oil-paper insulated current

transformers having Um 72 kV.

A test voltage of 1,3 Um/3 shall be applied to the current transformer’s primary insulation

for 48 h at the ambient temperature of 40 C 2 C.

The value of tangent delta measured at the test voltage during the last 10 h, at regular

intervals of 1 h, shall not exceed the value at 75 % of Um during the tangent delta in 5.3.3,

and it shall be constant within a tolerance of 0,02 %. If this condition is not achieved, the

test shall be continued up to tangent delta stabilisation or to insulation failure.

During the specified time interval and the applied test voltage, the apparent charge of the

partial discharge shall be recorded and shall be well below the value at Us during the partial

discharge test (see table 8) and the apparent charge shall be constant (with insignificant

variations).

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Table 6: Partial Discharge Test Levels for Current Transformers

METHOD OF SYSTEM EARTHING

PRE-STRESS VOLTAGE UP

MIN DURATION OF UP SECONDS

MEASU-RING VOLTAGE US

MIN DURATION OF US SECONDS

MAX PERMISSIBLE PARTIAL DISCHARGE “Q” ABOVE THE BACKGROUND LEVEL (PC)

Effective Ut (as in Table 1)

60 1,2 Um 60 10

9.3.4 INTERNAL ARC WITHSTAND CAPABILITY

The internal arc fault protection requirement generally applies to current transformers with

Um ≥ 72 kV. The internal arc test shall be performed as prescribed in IEC 61869-1. In addition

to the above, a high speed video recording must be taken during the test which must be

made available to the purchaser on request.

For units rated at Um ≥ 245 kV, the internal arc fault protection requirement is mandatory. In

the range, 72 kV ≤ Um < 245 kV, the requirement shall be specified in schedule A if required.

9.4 Test Certificates

All tests shall be fully documented in English and shall be signed and stamped by the

purchaser’s inspector(s), and copies shall be forwarded to the purchaser.

The copies shall include test certificates, bound together in one volume, recording the results

of type tests, routine tests and special tests (if applicable) carried out on one fully assembled

current transformer of each type of rating.

Each CT shall be delivered with one copy of all routine test certificates together with a copy

of the excitation curve showing clearly where the knee-point occurs for each protective core

and, in the case of multi-ratio windings, stating to which ratio the curve applies. These

certificates and curves shall be packed in a waterproof container and housed inside the

terminal box of the respective current transformer.

One copy shall be retained by the Contractor for the service life of the equipment.

9.5 Inspection during manufacturing and witnessing of tests

NamPower reserves the right to appoint a representative to inspect the current transformers

at any stage of manufacture or to be present at any of the specified tests. Such inspection

shall not relieve the contractor of his responsibility for meeting all the requirements of the

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specification, nor shall it prevent subsequent rejection if such material or equipment is later

found to be defective.

The Contractor shall ascertain whether inspection or witnessing of tests, or both, are

required, and shall then give NamPower not less than seven days notice, in the case of local

manufacture, of the firm date when the equipment will be ready for the inspection or

witnessing testing.

The NamPower representative or the test witness, or both, shall not inspect any current

transformer and/or sign any release certificate unless relevant drawings approved by

NamPower are produced for viewing by the authorised inspection or witness authority.

Permission to proceed with the despatch of manufactured units from the Contractors works,

shall be subject to:

All test performed satisfactory;

All requirements of the specification accomplished according to the NamPower or

appointed representative.

10 Marking/labelling/documentation

10.1 Drawings and Instruction books

10.1.1 CONTRACT DRAWINGS

The following drawings shall be supplied for post type current transformers:

Outline dimension, mounting, main terminal and clamp drawing. This drawing shall

show the main terminal markings so that the physical arrangement can be correlated

with the desired electrical schematic arrangement

Drawing showing assembly drawing of the complete unit in cross-section, (not

necessarily the design drawing)

Insulator detail drawing;

Secondary terminal boxes, covers, cover screws and gland plate drawing showing

physical arrangement of secondary terminals and secondary earthing terminals,

barriers, etc. The secondary terminal markings shall be shown on this drawing

Connection diagram drawing showing the terminal markings and the relative polarity

and physical arrangement of the cores with respect to the primary terminals. This

drawing shall preferably take the form of the final diagram plate

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Rating and diagram plate drawing showing the technical performance and connection

data which will actually appear on these plates.

10.1.2 DESCRIPTIVE PAMPHLETS AND INSTRUCTION BOOKS

Unless otherwise specified in schedule A/B (annex B), three sets of descriptive pamphlets

and instruction books covering the equipment offered shall be forwarded to NamPower as

soon as possible but not later than the delivery date. A complete set of test certificates as

specified in clause 5.7, shall be included in each instruction book.

The instructions shall, inter alia, include details of the recommended oil sampling procedure,

together with the necessary sectional and other sketches of oil sampling devices provided

and recommended maintenance/inspections for the service life of the equipment.

Guidance with regard to the following is also required:

Long term storage of spare units;

Handling/preparation for transport indicating support positions;

Correct handling and slinging methods.

10.1.3 DRAWINGS AND LITERATURE

The following drawings and literature shall be submitted at the tendering stage where

applicable:

FOR EACH CORE OF THE CURRENT TRANSFORMER

Magnetisation curve (i.e. B/H not V/Io) for the core material as used (i.e. annealed or

not annealed), from “B” values not greater than 0,4 Tesla to not less than 23 Tesla;

Full details of type tests performed on an identical unit, if any;

Secondary windings resistance at 75 °C for each ratio.

FOR EACH POST TYPE CURRENT TRANSFORMER

Typical outline and dimension drawing which should include the following:

Mounting details;

Primary terminal dimensions and markings;

Overall dimensions;

Position of earthing terminal;

The total creepage and the arcing distance of the hollow core insulator

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Height of the gland-plate in secondary terminal box above base and distance of its

centre line from the centre line of the current transformer;

Dimensions of primary connections;

Mass of complete unit and volume of the oil and/or the mass of insulating gas.

Sectional arrangement drawing which should show the following details:

The position of the cores and windings;

Hollow core insulator;

Oil or gas seal arrangements;

The method, for oil filled units, used to accommodate expansion and contraction of

the insulation oil;

Pressure relief devices (if any).

A Sectional drawing of the hollow core insulator, depicting all pertinent dimensions for the

evaluation of SANS 60815 and SANS 61462 compliance shall be provided.

The following details of the secondary terminal box which may either be shown on the

general arrangement drawing or on a separate drawing:

Cover and method of fixing for box cover and gland plates;

Position and dimensions of the gland plate;

Secondary terminals, their arrangement and clearance, creepage distance, barriers

and markings;

Breathing arrangement;

Section drawing of the bushings;

Complete list of recommended spares;

Descriptive pamphlet and instruction book.

Failure to supply tender drawings and literature with the tender may lead to rejection of the

tender. Tender drawings do not authorise the Contractor to commence manufacture of the

units before the submission and approval of the contract drawings.

11 Delivery and Installation

11.1 General

Transportation of current transformers shall comply with the requirements of the relevant

transport authorities.

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The current transformers shall be delivered to the destinations required by NamPower.

The equipment must be protectively packed in such a way that it can be safely transported,

handled and stored at site, as it will not necessarily be possible for erection to commence

immediately upon delivery.

Attention is drawn to the fact that NamPower will accept delivery at the specified destination

only and that the Contractor is to make all necessary arrangements for acceptance, off-

loading and transhipping at all intermediate points as well as ultimate off-loading at the

specified destination.

11.2 Acceptance of equipment

NamPower will accept equipment at site provided that the site representative is satisfied that

the equipment has been delivered in good order. If any equipment is found to be damaged

or deficient, such equipment shall be replaced by the Contractor.

If agreed to by NamPower, all repairs within the guarantee period shall automatically

introduce a further one year guarantee from date of return to service.

11.3 Maintenance of equipment in storage

Equipment shall preferably not require any special attention or maintenance whilst it is being

stored at site prior to being erected. If any equipment requires maintenance during storage

this shall be clearly stated in the tender and the attention of NamPower shall be drawn to

this fact at the time of delivery of each item of equipment requiring maintenance.

Failure to comply with the above shall render the Contractor liable for any damage that may

occur owing to maintenance not having been carried out during storage.

12 Right of Purchase

The Owner reserves the right to purchase any or more of the recommended spares listed

by the Contractor or the current transformers only.

13 Approval List

All details requiring approval shall be submitted within 30 days from the date of contract

award.

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14 Change of Equipment Design

During the period covered by the contract no design changes shall be made to equipment

without the prior approval of NamPower.