Troubleshooting # 01

Yesterday we  received a call from one of our clients, that whenever they  switch  on its VCB Trolley of 11KV Protection Panel tripping occurs in both the Metering Panel and also the Grid End Consumer Panel. 

When our team reached there site we did the following things.

• We checked the panels protection relay, all instantaneous tripping flags were down.
• We disconnected there INCOMING and OUTGOING cable.
• Removed the Trolley from the Panel.
• Checked The Trolley's Insulation of both the three Interrupters and also the resistance of three phases w.r.t ground, which was low as we expected.
• Then we checked the Insulation of all the panel circuit w.r.t ground, which was also low.

Note: Due to this low resistance Panel was not standing on 11KV.

         The resistance level for 11KV min. 22MΩ

Reasons:

When we inspected the Panel room and its  surroundings, we came across following causes of this fault

• High humidity/moisture level.
• Panel Room size less than recommended.
• No Proper Ventilation.
• 1window in the room without rain protection.
• An uncovered water trench in front of the room.

Three Phase Transformer connections

•  Wapda Transformers has a DY11 Connection.

Definition of Different Transformer Types and Parts according to IEC

Transformer Types
1. Power Transformer
A static piece of apparatus with two or more windings which, by electromagnetic induction,
transforms a system of alternating voltage and current into another system of voltage and
current usually of different values and at the same frequency for the purpose of transmitting
electrical power.
2. Auto-Transformer 
A transformer in which at least two windings have a common part.
3. Booster Transformer
A transformer of which one winding is intended to be connected in series with a circuit in order
to alter its voltage and/or shift its phase. The other winding is an energizing winding
4. Oil-Immersed Type Transformer
A transformer of which the magnetic circuit and windings are immersed in oil.
5. Dry-Type Transformer
A transformer of which the magnetic circuit and windings are not immersed in an insulating
liquid.
6. Oil Preservation System
The system in an oil-immersed transformer by which the thermal expansion of the oil is
accommodated. Contact between the oil and external air may sometimes be diminished or
prevented.

Terminals and neutral point
1. Terminal
A conducting element intended for connecting a winding to external conductors.
2. Line Terminal
A terminal intended for connection to a line conductor of a network.
3. Neutral Terminal
a) For three-phase transformers and three-phase banks of single-phase transformers:
The terminal or terminals connected to the common point (the neutral point) of a star-
connected or zigzag connected winding.
b) For single-phase transformers:
The terminal intended for connection to a neutral point of a network.
4. Neutral Point
The point of a symmetrical system of voltages which is normally at zero potential

Type Tests and Routine Tests of Transformer Oil and Their Purpose

Type Tests:

• Specific Gravity of Oil at 20 deg. C
• This test has special significance when the transformer is operated in very low temperature zone.
• The max. value of density is fixed at 27 deg. C ensures that water in the form of ice remains at the bottom and does not tend to float on the oil up to a temperature of about 10 deg. C.
• Oxidation Stability
• This test is the measure of  neutralization  value and sludge after oil is aged by simulating the actual service conditions of a transformer. 
• The oxidation stability test is very important for new oil but not for oil in service and shows presence of natural inhibitors which impart anti oxidation characteristics to oil.
• Sludge Value should be less than equal to 0.10
• Acidity after oxidation KOH/gm should be less than equal to 0.4
• Dielectric Dissipation Factor
• DDF is magnetically equal to sine of the loss angle and is good tool to indicate the quality of an insulation.
• A high value of DDF is an indication of the presence of contaminants of deterioration products such as water, oxidation products, metal soaps, soluble varnishes and resins.

Routine Tests

• Viscosity
• It is the measure of oil resistance to  continuous  flow without the effect of external forces.
• The oil must be mobile, as heat transfer in transformers occurs mainly by convection currents.
• Since viscosity increase with decrease in temperature.
• It is necessary that viscosity be as low as possible at low temperature.
• Viscosity @40 deg. C should be less than equals to 16.5
• Viscosity @-15 deg. C should be less than equals to 800
• Flash Point
• It is the temperature at which oil gives so much vapour that this vapour, when mixed with the air, forms an ignitable mixture and gives a momentary flash on application of flame under prescribed conditions
• Flash point of oil should be greater than equal to 140 deg. C.
•  Acidity Value
• It is the measure of free organic and inorganic acids present in the oil and is expressed in terms of milligrams of KOH required to neutralize the total free acids in one gram of oil.
• Total Acid KOH/gm should  be less than equal to 0.03.
• Di-electric strength
• Breakdown Voltage is the voltage at which breakdown occurs between two electrodes when oil is subjected to an electric field under prescribed conditions.
• Electric strength is the basic parameter for insulation system design of transformer.
• It serves to indicate the presence of contaminating agents like moisture, fibrous materials, carbon particles and sediments.
• Di-electric strength should be greater than equal to 30kV.
• Pour Point
• The temperature at which oil will just flow under the prescribed conditions  is known as the pour point.
• If the oil becomes two  viscous  or solidifies, it will hinder the formation of convection currents and thus cooling of equipment will be  severely  affected. 
• Pour point should be less than equal to -30 deg. C .
• Moisture Content
• The amount of free and dissolve water present in the oil is known as moisture content and is expressed in ppm.
• Presence of moisture is harmful since it adversely affects the electrical characteristics of oil and accelerates deterioration of insulation paper.
• Corrosive Sulphur
• This test is designed to detect any traces of free corrosive sulphur that may be present in oil.
• Presence of corrosive sulphur in oil results in pitting and black deposit on the surface of bare copper used in transformer, which will adversely affects the dissipation of heat and consequently the performance of the equipment.

Routine Tests of Transformer and their Purpose

• Visual Inspection
• To check the cracks, rust or chipping.
• To check the Serial Number.
• To check the Level of Oil.
• To check the leakage/seepage of Oil.
• To check the Markings on transformers.
• To check the colour coding of LT/HT connections.
• To check the colour of Oil.
• To check the locking Washers at the end of HT/LT eye bolts.
• Power Frequency High Voltage Test
• This test is intended to check the adequacy of main insulation to earth and between windings.
• Induced Over Voltage withstand Test
• To check the insulation of HT & LT winding and the insulation between different turns of winding.
• Turn Ratio Test
• This test is carried out to ensure the correctness of Voltage Ratio of Transformer between HT & LT windings on each tapping.
• The voltage ratio of transformer is equal to the turn ratio therefore to verify the voltage ratio we check the turn ratio.
• Measurement of No Load Losses
• This test is carried out to check the quality of the silicon steel sheet (core) used in the transformer.
• No Load losses are very important in this respect that these are permanent and are independent of load.
• These losses should be as minimum as possible and this can be ensured by using good  quality  silicon steel sheet.
• Measurement of Load Losses & Impedance
• In order to ensure the quality of copper, copper losses are measured. 
• Copper losses comprises the sum of I2R losses in the winding and the stray losses due to eddy current in conductor, clamps and tank.

• Good quality copper not only increases the life of transformer but also reduces the system losses.
• As the stray losses vary with the frequency therefore it is important to give the supply to transformer at rated frequency.
• In distribution transformers stray losses are usually negligible.
• Bird Protection Test
• To prevent flashover caused by large birds, transformer cover shall be effectively coated with a weather proof permanent insulation material having a dry dielectric withstand voltage of not less than 8kV.
• Verification of Vector Group
• There are different types of connections according to which, limbs of HT/LT windings are connected. WAPDA has adopted DYn-11 and to verify this, vector group test is carried out.
• Tank Pressure Test
• Lot of welding is involved in the fabrication & manufacturing of transformer due to which there are welding joints at top, cooling tubes & base which can cause leakage/seepage of Oil.
• In order to ensure no leakage/seepage of Oil during normal operation of transformer, pressure test is conducted.

Transformer Oil Tests

The Oil shall conform to IEC-296 Class-1 Oil.

Type Tests

• Oxidation Stability.
• Dissipation Factor.

Routine Tests

• Specific Gravity of the Oil.
• Viscosity.
• Flash Point.
• Acidity Value.
• Dielectric Strength.
• Pour Point.

Routine Tests & Type Tests for Transformers

Routine Tests

• Visual Inspection
• Power Frequency Voltage withstand test at 36kV for 1min on H.T side.
• Induced Voltage withstand test at 830V and 100Hz for 1min on L.T side.
• Measurement of Voltage Ratio (Turn Ratio Test).
• Measurement of no load losses.
• Measurement of load losses and impedance voltage.
• Bird Protection test at 8kV for 1min.
• Verification of vector group.
• Tank Pressure Test at 15PSI.
• Paint Thickness Test (0.120mm Min.).
• Dimensional Check.

Type Tests

• Temperature Rise Test
• Impulse Voltage withstand Test.
• Short Circuit Test.