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.

Degrees of Protection

Equipment has to fulfil its functional requirement. But only that is not enough. The equipment has also to be protected against possible external influences and the equipment may not be harmful to the user or operator or to the environment. Several means or combination of means can be used to comply with the requirements:

•   Placing out of reach, for instance by means of an obstacle;

•   Total insulation, which is not easily applicable for moving parts;

•   Use of an enclosure

To support the easy use of understanding between user and manufacturer this requirement IEC60529 is made. This requirement can be used for electronic equipment as for telecommunication equipment as well as for high-voltage as for low-voltage equipment or rotating machines. The enclosure can be build as a part of the equipment or as a loose component, sold empty.

1.   The IEC protection code  (International Protection IP)

The IP codes are used to define protection against the ingress of solid foreign objects and water with harmful effects. Since the IP code is used for all equipment annex B of the IEC standard gives instructions to technical committees for applying IP codes. The protection against the ingress of solid foreign objects is also used as a means of specifying protection against accidental contact with live parts and therefore serves two different but related purposes.

2.    Interpreting the IP code

The IP code defined in the second edition of "IEC60529 Degrees of protection provided by enclosures (IP code)" consists of 2 numerals and 2 optional extra letters. The first numeral 0, 1,...6, or X defines the protection against ingress of solid foreign objets and against access to hazardous parts. The second numeral 0, 1,...8, or X defines the protection against the ingress of water.

3.   Elements of the IP Code and their meanings:

NEMA Insulation Classes

DC supply & Trip Indications of 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

• Supervision relay shall be provided for continuously monitoring the healthy conditions of breaker trip circuit and presence of DC supply on all applicable panels. The presence of healthy trip circuit shall be indicated by the continuous glow of a yellow lamp. The indicating lamps shall be of neon type with screwed base. The rating shall be 5 Watt at 150V or 260V DC depending on control and auxiliary supply voltage.
• One hooter and its acknowledge and cancelling arrangement shall be installed in the incoming panel to indicate failure of DC main supply, DC supply to any panel and failure of healthy trip circuit and tripping of any breaker. This arrangement should be self contained and securely fixed to the panel. The alarm cancelling arrangement shall be flush mounted on the front of the panel.
• For circuit breaker fitted with AC series trip mechanism, auxiliary contacts shall be provided in the protective relay for operation of a hooter, installed in the incoming panel, to indicate breaker trip. The contact shall be suitable for operation for the hooter. 

Differential Protection of 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

• All incoming panels shall be provided three additional CTs for differential protection.
• The differential protection zone of power transformers is required to trip the 11KV incoming breakers. For this purpose the incoming panels of 12.5KA rating shall be provided with an additional DC operated trip coil.
  

Over Current Protection of 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

• All panels shall have overcurrent & earth fault protection which shall be provided by means of inverse induction type/numeric type relays with two over current elements and one earth fault element. The relays shall have three instantaneous elements.
• For 12.5KA switchgear, the relays shall be arranged to operate on AC and without any external source of supply using saturable auxiliary current transformer.

• For 25KA switchgear, the over current relays shall operate on DC.
• For 25KA Incoming Panels, in addition to the DC protection scheme, a back up AC series protection scheme shall also be provide. The back up AC series protection shall be operated by a separate earth fault relay. The characteristics of earth fault relay for back up protection shall be same as for earth fault unit in the main over current  and earth fault relay. For AC back up protection the trip coils shall be energized directly from the CTs without  requiring any external source 

Short-Circuit Strength of 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

• The Switchgear, all apparatus and connections shall be so supported and proportioned as to be capable of safely withstanding stresses to which they may be subjected during service including those due to short circuit of 12.5KA or 25KA as required and under the cinematic conditions specified herein.
• Each panels shall be designed  to carry and withstand thermal and mechanical stresses caused by the following rated currents:

•   12.5 KA Panels:

1. Rated normal current  (for main-circuits)                                                400A & 1250A 
2. Rated short-time (1 sec) withstand current (for main & earthing circuits)        12.5KA
3. Rated peak withstand current (for main & earthing circuits)                                 32KA

•   25 KA Panels:

1. Rated normal current  (for main-circuits)                                                630A & 2500A 
2. Rated short-time (1 sec) withstand current (for main & earthing circuits)           25KA
3. Rated peak withstand current (for main & earthing circuits)                                 63KA

Standard Types of 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

1. Incoming Panel for connecting the 11KV switchboard to the Power Transformer.
2. Outgoing Panel for connecting supply to 11KV feeder. 
3. Bus Coupler Panel for connecting two 11KV sections in Grid stations where two Power transformers are installed and for Grid station auxiliary supply. 
4. Industrial Panel for giving 11KV supply to individual consumers.
5. Ring main Panel for giving 11KV supply to individual consumers fed from two different feeders.
6. Consumer Panels for giving connection/ 11KV supply to an individual consumer from the Grid stations through an express or dedicated 11KV feeder 

Standardization of Panels according to WAPDA Specification P-44:96

1. Panels manufactured by different manufacturers shall be interchangeable.
2. The following components shall be standardized:
1. CTs & PTs
2. Supporting Insulators
3. Bus Bars
4. Meters
3. At every Panel, the incoming AC/DC circuits shall be connected to the terminal block of the panel from where the supply shall further be distributed through miniature circuit breakers of appropriate rating & characteristics. 

Transformer Efficency:

Transformer is a static device which transfers the power from one circuit to other through electromagnetic induction. In electrical transformer as there are no moving parts,

 1. no friction and
 2. windage losses occur

 and other losses in transformer are less as magnetic path of a transformer is a closed loop (no air gap exist like induction motor between two circuits). Losses in the transformer are very less compared to any other rotating machine, hence efficiency of transformers will be very high. Efficiency will be of the order of 95% to 98%.

Transformer Taps:

Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Standard tap arrangements are at two-and-one-half and five percent of the rated primary voltage for both high and low voltage conditions.

Transformer & Its Working:

"A transformer is an electrical apparatus designed to convert alternating current from one voltage to another" 

 

• It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle.
• A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life. 
• Construction of transformer is very simple, two or more coils of insulated wire wound on a laminated steel or iron core. When voltage is introduced to one coil, called the primary winding, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary winding or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

General Requirements for 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

• The Switchgear shall be designed for indoor use only.
• each panel will be comprise of a draw-out type circuit breaker and shall be capable of being connected to a switch board on either side.
• All the associated equipment shall be self contained and self supporting.
• All live parts such as circuit breakers, bus bars, voltage transformers, current transformers etc. shall be enclosed within grounded metal compartments separated by grounded metal barriers. 
• Automatic shutters shall be provided to prevent exposure of live parts.
• Busbars & their connections shall be insulated.
• Mechanical interlocks shall be provided so as to ensure safe operation of the switchgear in the correct sequence.  
• All the secondary circuits, wiring & instruments etc., shall as far as possible be isolated by grounded metal barriers from all primary circuit elements with the exception of short lengths of wire such as at instrument transformer terminals.
• Switchgears shall be designed & manufactured to secure safety during operation, inspection, cleaning and maintenance & shall be so arranged as to minimize the risk of fire arising or spreading.

Service Conditions for 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

Ambient Temperature:
Maximum Temperature    50 deg. C
Maximum Mean over an hour    45 deg. C
Mean in any year   30 deg. C
Minimum Temperature - 10 deg. C

Relative Humidity:
Relative Humidity may range upto 100%.

Altitude:
Installation upto 1000m above sea level.

Supply System:
The Switchgear shall be suitable for installation in 11KV three phase AC supply system of the following characteristics:

Nominal Voltage 11KV
Rated Voltage 12KV 
Frequency  50Hz

Metalcald Switchgear WAPDA specification P-44:96

• This specification has been prepared by Design Department , Power Wing, WAPDA.
• These specifications was first introduced in August 1968.
• Its revised editions were issued in 1979,1985 & 1987.
• These specifications applies to 11KV A.C three phase metal clad switchgear for indoor application.
• the switchgear covered by this specification comprises of SF6 & Vacuum Type Circuit Breakers. 

• Reference Standards

The switchgear, including the operating devices and the auxiliary equipment which form an integral part of it,shall be designed, manufactured and tested in accordance with the following International Electro-technical Commission (IEC) Publications:

• IEC -    56                 HV AC Circuit Breakers 
• IEC -  298                 AC metal enclosed Switchgear & Control
• IEC -  694                 HV Switchgear & Control Gear COmmon Clauses
• IEC - 185                  Current Transformer
• IEC - 186                  Voltage Transformer
• IEC - 265-1               HV Switches
• IEC -   51-1               Electrical Measuring Instruments & their Accessories

• IEC -  51-2                 Same as above
• IEC -  51-3                 Same as above
• IEC -  51-5                 Same as above
• IEC -  51-8                 Same as above
• IEC -  468                  Measurement of Resistivity of Metallic materials