ZESCO SPECIFICATION
=====
SPECIFICATION
33/11kV POWER TRANSFORMER SPECIFICATION
SCOPE
This specification covers the design, manufacture, supply, delivery installation, testing
and performance requirements of 33/11kV POWER TRANSFORMER
Normative References
The requirements of the following standards/ specifications shall be met where applicable
unless otherwise stated in this specification.
Specification / Standard
Number
Title
IEC 76
Power Transformers
IEC 137
Insulated bushings for alternating voltages above
1000V
IEC 214
On-Load Tap Changers
IEC 296
Specification for unused mineral insulating oils for
transformers and switchgear
IEC 529
Degrees of protection provided by enclosures
IEC 551
Determination of Transformer and reactor sound level
General
Type of Transformer
The transformer shall be double wound and be core type. The Transformer shall be
suitable for outdoor use and shall have protection classification IP54W (IEC 529).
IEC60076 recommendations shall be followed.
The transformer shall be fitted with bushings which satisfy the requirements for creepage
distance and for impulse levels for altitudes up to 1400 m, such that the transformer can
be interchanged one with another at any site.
Electrical Connections
Unless otherwise approved, the direction of winding of the higher and lower voltage
windings shall be such that they are of subtractive polarity.
As specified in the Technical Schedules, tappings shall be provided on a three-phase set
of transformer windings to provide an alternative ratio of transformation. The transformer
shall be capable of delivering the rated power when connected for the alternative ratio
and the unit must be capable of withstanding the specified test voltages when connected
for the most severe condition.
Duty Under Fault Condition
Except where modified below, it is to be assumed that the amount of generation
simultaneously connected is such that normal voltage will be maintained on one side of
any transformer when there is a short circuit between phases or to earth on the other side.
The transformer may be directly connected to an underground or overhead transmission
circuit and switched into and out of service together with its associated transmission
circuit.
Cores
The cores, framework, clamping arrangement and general structure of the transformer
shall be of robust design capable of withstanding any shocks to which the transformer
may be subjected during transport, installation and service.
The magnetic circuit shall be of cold rolled grain orientated steel and the flux density
shall be between 1.65...1.75 Tesla when the voltage applied to any tapping is not in
excess of that for which the tapping was designed.
All reasonable care shall be taken in the design and construction of the transformers to
reduce noise and vibration to a minimum. Each core shall be earthed to the tank and
securely fastened to prevent movement relative to the tank. Suitable hooks shall be
provided to allow for the lifting of the core from the tank.
The framework and core bolts shall be efficiently insulated from the core so as to reduce
circulating current to a minimum.
The core shall be earthed at one point only by means of a link or bolted connection which
must be located in an accessible position requiring the minimum amount of oil to be
drained from the tank. Preferably the link shall be brought to cover through a separate
bushing and earthed through removable link for inspection purpose.
Windings
The windings shall be fully insulated for continuous working at the specified service
voltage.
The insulation of windings and all connections shall be of approved materials which shall
not deteriorate under the action of hot oil or from other causes when the transformer is
operated continuously at the maximum permissible loads.
The windings shall be braced in an approved manner to withstand the effect of short
circuit and shocks which may occur during transport.
Tanks
The tank (including the tank lid) of the transformer shall be of mild steel plate of
sufficient thickness and it will have to stand static pressure up to 130 kPa
Inspection openings shall be provided in the tank cover to give access to changeover
links, bushing core earth links etc. and these inspection covers shall not weigh more than
25 kg each.
Each tank is to be designed to prevent the collection of moisture on any part and shall be
sufficient rigid to resist any permanent distortion when the transformer is lifted complete
with oil and subjected to the rigors of road and sea transport.
Tank covers shall be provided with lifting lugs or lifting eyebolts.
The steel used shall comply with BS 4360 or corresponding ISO standard and must be of
a quality suitable for deep drawing not prone to work hardness and accepting the welding
processes used in the tank manufacture. The manufacturing process used shall not
introduce any permanent elongation within the steel.
The complete tank shall be pressure tested for oil tightness and tank rigidity and strength.
Such testing shall be completed and proven satisfactory before the core and windings are
installed. The methods of construction testing and quality control of the tanks shall be
subject to the approval of the Engineer at all stages of construction.
The Bidder shall submit comprehensive details of the methods of construction and testing
procedures employed and shall be prepared to demonstrate to the Engineer the complete
operation.
Details shall be submitted of the standard method of packaging employed to protect the
transformer from mechanical damage during transportation to site. The Bidder shall
indicate in detail what additional packing will be provided to protect the transformer
during shipping by sea and road and handling on sites due consideration being given to
the limited resources available en route.
Accessories
Each transformer shall be fitted with the accessories specified in the Schedules.
Earthing Terminal
Stainless steel or mild steel earthing flag approximately 63 mm square with a thickness
not less than 6 mm and a centre hole of 14 mm diameter shall be welded externally to the
tank.
Oil
The transformer shall be supplied with a sufficient quantity of oil suitable in all respects
for continuous service under tropical conditions. The oil shall be of approved grade and
comply with IEC 296 Cl I or II.
31.5MVA TRANSFORMER
SCHEDULE A: ZESCO MINIMUM TECHNICAL REQUIREMENTS
S/N
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
TECHNICAL DETAILS
TYPE OF TRANSFORMER
UNITS
Rated power
Nominal Primary Voltage
Highest system voltage - primary
Nominal Secondary Voltage
Highest system voltage –
secondary
Vector Group
Phases
Type of cooling
Type of insulating oil
PCB content (attach specification
without fail)
Frequency
Winding Material
Impedance
Temperature rise
Winding
limits at rated
voltage and power
Top oil
Lightning impulse withstand
level at primary
Lightning impulse withstand
level at secondary
Power Frequency withstand
voltage – primary
Power Frequency withstand
voltage – secondary
Bushing Type
Specific Bushing creepage
distance
Parallel transformer operation
Equipped with Jacking pads
Equipped with lifting lugs
Buccholz for main transformer
Temperature Gauges (Windings)
Temperature Gauge (oil)
Pressure Release Valve (oil tank)
Skid under base
TAP CHANGER DETAILS
Tap changer Type
MVA
kV
kV
kV
kV
31 Tapping range
32 Auxiliary supply to Tap changer
controls
DATA
Free breathing oil immersed outdoor power
transformers to IEC 60076
10
33
36
11
12
Dyn1
03
ONAN
Mineral oil as per IEC 296
Nil
Hz
50
Copper
7
65
%
0
C
0
C
kV
60
200
kV
95
kV
70
kV
28
mm/kV
Silicon
31
















V ac
OLTC Type-MRI Germany (with automatic
and manual voltage regulation)
105% to 85% in steps of 1.25% ( 17 taps)
400
33 Visual position indicator of tap
position
34 Buccholz for tap changer
35 Transformer secondary neutral
earthing
ENVIRONMENTAL
CONDITIONS
36 Altitude above sea level
37 Relative humidity (maximum)
38 Operating ambient temperature
39 Applicable standards




Solidly earthed
m
%
0
C
IEC60076
1400
85
-1 to 40
IEC60076
NOTE: “” denotes that the item stated is required to be provided.
OTHER REQUIREMENTS
1.0 TRANSFORMER SHOULD COME WITH NEUTRAL CTs FOR STANDBY EARTH & REF
PROTECTION.
Ratio: 150/1; Burden: 20VA; Class: 5P20 for standby earth fault protection and a 300/150/1 CT; Burden
20VA; Class X; Vk=300V for REF protection.
2.0 MARKINGS
RATING AND DIAGRAM PLATES: To conform to IEC 60076
In addition, the following markings must appear on the rating plate


Purchase Order No.
Clearly indicate : “OIL contains NO PCB's”
3.0 Drawings, Test certificates/results and quality documents
 Tenderer shall provide drawings showing the dimensions of the transformers offered, the
arrangement of connections and rating plate details
 Successful bidder shall provide routine test certificate on delivery of transformer
SCHEDULE B: TO BE FULLY COMPLETED BY THE TENDERER
S/
N
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
TECHNICAL DETAILS
TYPE OF TRANSFORMER
Rated power
Nominal Primary Voltage
Highest system voltage - primary
Nominal Secondary Voltage
Highest system voltage – secondary
Vector Group
Phases
Type of cooling
Type of insulating oil
PCB content (attach specification without fail)
Frequency
Winding Material
Impedance
Temperature rise limits
at rated voltage and power
Lightning impulse withstand level at primary
Lightning impulse withstand level at secondary
Power Frequency withstand voltage – primary
Power Frequency withstand voltage – secondary
Bushing Type
Bushing creepage distance
Parallel transformer operation
Equipped with Jacking pads
Equipped with lifting lugs
Buccholz for main transformer
Temperature Gauges (Windings)
Temperature Gauge (oil)
Pressure Release Valve (oil tank)
Skid under base
TAP CHANGER DETAILS
Tap changer Type
Tapping range
Auxiliary supply to Tap changer
Auxiliary supply to Tap changer controls
Visual position indicator of tap position
Buccholz for tap changer
ENVIRONMENTAL CONDITIONS
Altitude above sea level
Relative humidity (maximum)
Operating ambient temperature
UNITS
MVA
kV
kV
kV
kV
Hz
Winding
Top oil
%
0
C
0
C
kV
kV
kV
kV
mm/kV










M
%
0
C
DATA
39
Applicable standards
IEC60076