High voltage transformer - 138kv power transformer
This article is about the one of the high voltage class transformer: 138kv power transformer. About the basic spec, the accessories, the test and ship.
There have different voltage classes, one is low voltage, second is medium voltage, third is high voltage.
As low voltage transformer, their voltage class can is 10 kv class, 20 kv class, they are distribution transformers.
As medium voltage transformer, their voltage class is 30 kv class, 35 kv class, they are substation transformer.
For the 66 kv voltage class and 69 kv voltage class, much higher class, include 115 kv voltage class, 138 kv voltage class, and 230 kv their should be high voltage transformer, we also call them power transformers.
Oil immersed transformer
Table of Content
What is a high voltage 138kv power transformer?
A high voltage transformer need to have the following spec tech.
1) . The Altitude of the high voltage transformer:
Maximum altitude can be – 100 meters (about 330ft.)
2) . Short Circuit Withstand of a high voltage transformer:
The high voltage transformer shall be designed to withstand the duration of 2 seconds in short circuit condition.
In order to allow for possible flash over of the neutral grounding resistor, the high voltage transformer shall be capable of withstanding the mechanical and thermal stresses for a single line-to-ground short circuit with a solidly grounded neutral on the secondary side.
3) . Temperature Rise for a high voltage transformer:
The average temperature rise of any winding above ambient temperature, measured by resistance, shall not exceed 65 °C.
4) .Forced Air Cooling of a high voltage transformer
The high voltage transformer shall include two stages of forced air-cooling to give ratings 133% of the selfcooled rating. The following minimum equipment shall be included:
a) Adequate capacity in all current carrying parts and bushings.
b) Cooling fans, to operate from 208/120V three phase, 60 Hz supplied by purchaser or from the station service power transformer.
c) Fan control contactors and hand-off-auto selector switch. Contactor to be complete with one extra normally open contact wired to terminal blocks for purchaser’s use.
d) Contacts on transformer winding temperature indicator for fan control.
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What is the accessories for the high voltage 138kv power transformers?
1). Surge Arresters
Station class, surge arresters metal oxide type shall be provided and mounted on brackets on the primary side of the transformer. Arresters shall be connected to a ground pad welded to the top of the transformer tank. Surge arrester shall be rated 120kV with MCOV of 98kV.
2) . Indicating-type shut-off valve in liquid line between conservator and main tank(conservator type units).
3) Non-dehydrating breather (conservator type units).
4) Lifting, moving and jacking facilities capable of handling the total filled weight of the transformer.
5) Manholes or hand-holes for tank access.
6) Pressure relief device, self-reset, with shroud to deflect hot oil away from the front of the high voltage transformer.
7) Liquid level gauges with preset alarm contact.
8) Liquid temperature gauges with drag hard for maximum indication and preset fan control, alarm and trip contacts. Drag hand with ability to reset from ground level.
9) Winding temperature gauges with drag hand for maximum indication and preset fan control, alarm and trip contacts. Drag hand has ability to be reset from the ground level.
10) Liquid drain valve and sampling valve.
11) Valves (2” NPT) for connections to liquid treatment facilities so that liquid may be filtered and re-circulated in the transformer.
12) Grounding pads, predrilled and tapped, and welded on the base or on the tank wall. One ground pad shall be located towards the extreme right and on the side of the low voltage bushings and the other diagonally opposite.
13) Anti-condensation heater in control cabinet.
14) Vacuum/pressure gauge (sealed tank type units) with facilities for isolating the gauge so that it may be removed for calibration without draining the tank or allowing moisture into the tank when the gauge is removed.
15) Gas sampling valve.
16) All gauges visible from ground level.
17) Off-circuit tap position indicator and all gauges clearly visible from ground level.
18) Diagram instruction nameplate, stainless steel.
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How do you make a high voltage transformer?
1）High Voltage Terminations of the high voltage transformer
The 138kV windings shall be brought out through cover-mounted bushings. The bushings shall be oil filled condenser type, rated 154kV, 650kV BIL, complete with suitable threaded stud connectors Bushings shall include a capacitor tap for testing.
2) 13.8kV Voltage Terminations of the high voltage transformer
The 13.8kV windings shall be brought out through cover-mounted bushings located 180 degrees with respect to the high voltage bushings. The bushings shall be rated 15kV, 110kV BIL suitable for 2x1c -1000MCM/phase cable connections.
3) Neutral Termination of the high voltage transformer
The neutral point of the 13.8kV winding shall be brought out through a cover-mounted bushing. The neutral connection and bushing shall have minimum current rating of at least 50% of that of the self-cooled (ONAF) transformer rating.
4) Grounding Resistor of the high voltage transformer
The transformer shall be complete with a grounding resistor, stainless steel type, 8kV, 200A for 10 seconds, 40 Ohms, completely assembled in a screened outdoor enclosure and mounted on the side (or top) of the transformer, The resistor shall have a continuous current rating of at least 10% of the short time (10 second) rating. All connections from the resistor to the 13.8kV neutral bushing and to the tank ground shall be provided.
5) Off-Circuit Taps of the high voltage transformer
Four only 2.5% full capacity taps, 2 above rated voltage and 2 below, shall be provided in the13.8kV winding.
The tap position switch shall be externally operated, with provision for padlocking, and is to include a nameplate specifying that the high voltage transformer must be deenergized before operating the switch.
6) On-Load Tap Changer (OLTC) of the high voltage transformer
The high voltage transformer shall be complete with three-phase type on-load tap changing equipment 21 steps with 10 above and 10 below to automatically provide voltage compensation of +/-10% of the 138kV winding for variations in supply voltage to the 13.8kV winding and. All taps shall be full capacity. The OLTC oil-filled compartment shall withstand full vacuum.
All relays, selector switches, breakers, etc. located within the control cabinet shall be clearly identified with laminated plastic type nameplate and this identification shall agree with that on the schematic and wiring diagrams.
Which test for high voltage transformer need to do?
Daelim will do the routine tests in our factory before shipping, after the all the test passed, we will send the test report to the customer to confirm. All things have no problem, we will deliver them from our factory.
Factory routine tests are as follow:
• Resistance tests of all windings
• Ratio tests on the rated voltage and all tap connections
• Polarity and phase relation tests at rated voltage
• No load loss at rated voltage
• Exciting current at rated voltage
• Impedance and load losses
• Applied voltage test
• Induced voltage test
• Full wave impulse test
If the customer has special requirements on the test items, such as temperature rise test, lightning impulse test, they will have extra costs.
The customer should inform Daelim that they need these tests, and Daelim will add the cost to our quotation list. Or we will do the standard factory routine tests.
Some customer needs CSA certification for the transformer. CSA group can do CSA listed for transformer under 44kV, if the customer wants CSA group to inspect the transformer test and make the CSA listing for their transformer, Daelim will contact CSA group to confirm the cost and time, then add them into the quotation list. Customers can also do the CSA listing after they receive the transformer in their local.
Other customers may want a third-party inspection to their transformer, they will arrange third party companies such as SGS, BV, ANSI IBS to visit our factory and do the inspection work. Daelim also welcomes them to visit our factory.
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How to ship a high voltage power transformer?
Transportation- the transformer main tank, core, windings and any attached accessories shall be shipped filled with dry nitrogen gas from a suitably sized cylinder maintained at a positive pressure.
The moisture content of the Nitrogen gas shall not exceed 5ppm and a pressure monitoring gauge shall be installed. The above shall be the property of the client upon arrival to the site.
All other detached accessories that normally function under oil, shall be appropriately sealed to protect them against moisture or attack by weather.
All items shall be so packed as to protect them against weather and mechanical damage during the shipment.
The manufacturer shall supply and install disturbance / shock recorder to record any impact on the transformer during shipment. The shock absorber shall be installed at the factory and activated. The procedure for impact data retrieval shall be sent in soft, 2 weeks prior to shipment of the transformer. The impact recorder shall remain the property.