Transformer Construction

Most of the transformers installed in various area substations are supplied by Voltamp.

Some of the main fittings and accessories of  these transformers are :

TAP CHANGING ARRANGEMENT:

Off-Circuit Switch

The transformer is normally fitted with an off circuit tap changing switch to obtain required voltage ratio. It can be hand operated by a switch handle mounted either on tankl cover or on the tank side.The locking devices is fitted to the handle to lock in any lock position. The switch mechanism is such that it can be locked only when it is located in its proper position and not in any intermediate position.

THE TRANSFORMER MUST BE ISOLATED FROM ALL THE LIVE LINES, BEFORE OPERATING THE SWITCH.

Operating the switch when transformer is energized, will damage the switch contacts due to severe arcing between the contacts and may damage transformer winding.

When the switch handle is provided on the side wall, it is necessary that Switch handle assembly is dismantled before untanking.

Off-CircuitRatio Changing Links
Sometimes links are provided inside the transformer tank to obtain required voltage ratio. Links are required to be loosened and fixed in new required position as given in name plate. Links are accessible from the inspection cover. In case of conservator units, oil level has to be lowered below the inspection cover before unbolting inspection cover.

On-Loading Tap Changer

The on-load tap changer is an optional fitting. The on-load tap changers are provided with local manual control, local electrical control and remote electrical control. The automatic voltage regulation can also be provided as optional fittings.

The tappings are located on high voltage winding.

For further details, please refer to the attached manual of OLTC and schematics.

Earthing Terminals

The core laminations assembly is connected to core clamping frame which is in turn connected to the tank. Two earthing terminals are provided on the transformer tank. The earhing terminals should be connected to the earthing.

Lifting Lugs

Two/Four fittings lugs of adequate capacity are provided on tank  sides/top cover to lift fully assembled transformer filled with oil.

All lugs are designed for simultaneous use and must be used accordingly. Two/Four lifting lugs are provided for untaking the core and windings of larger capacity transformers.

All heavy fittings are also provided with individuals lifting lugs.

Valves

Every transformer is provided with drain cum filter valve at bottom of the tank, and filter valve at top of the tank. Valves are fitted with plugs/blanking plates to stop oil coming out.

Mainly two types of valves are provided:

1.      Wheel Valves

2.      Butterfly valves

The wheel valves are used either with female  screw threads or with flanges. These are of gun-metals/cast iron type’s    

Generally, one Isolating Valve also known as shut off valve is provided for transformer upto 2000 KVA between conservator and buchholz relay.

The Butterfly type cast-steel valves with the machined flanges are used at points of connection between tank and detachable radiators.

BUSHINGS

Oil Communicating Type

Transformers windings are connected to the external circuit through terminal bushings. The bushings are installed on the cover or, on side walls of the transformer tank. The lower end of the bushing protrudes into the tank and at both their ends are provided with suitable fasteners to connect the line leads inside the transformers  an external conductors outside it.

The shape and size of the bushings depend on the voltage class, type of current. Electrical performance of these bushings conform to I.S. 2099 & I.S. 7421 Dimensional details and associated parts generally conform to IS-3 upto 36 KV class. Bushings of 1000Volts are of two piece constructions without arcing horns, whereas all other bushings are of single piece porcelain type. Assembly and dismantling of single tank cover is required to be removed for necessary access to the inner (lower) end of the bushings. These bushings are not detached at the time of transportation.  

Condenser Bushings

Generally, condenser bushings are used for 72.5 KV and above. These bushings contain their own oil and are sealed to retain the same. Whenever these bushings are mounted on bushing pockets or raised truncated portions, air vent pipes are provided for carrying away air or gases from these pockets to Buchholz relay during service typical assembly is shown in flg.5.

These bushings are detached from the transformers and dispatched separately, they are packed as per manufacturers instructions. The draw through type lead is coiled and kept temporarily below the bushing blanking plate. The equipment required for mounting the bushings are (i) rope slings (ii) flexible steel wire approx 2mm in dia, of suitable length,

Typing procedure for mounting is as follows:

(i)                Open the bushing case carefully and check it for transit damage and clean the bushing thoroughly.

(ii)             Check whether the gasket on the pocket is in order. If not, clean the seat of the gasket and replace it by new one.

(iii)           Remove the locating pin (3).the cable bolt to be used on the unit is brazed to the end of the flexible draw through type lead mentioned above.

(iv)           Removing the blanking plate and uncoil the draw through type lead. Tie the flexible steel wire to the head of the bolt fixing temporarily on the top of the cable bolt(2) to facilitate the threading of the main lead through the bushing tube and at the same time preventing the lead   

form falling back into the tank.

(v)              Suitably raise the bushing and remove the protective cover at the oil end of the bushing.

(vi)           Check that Oil level in the bushing is appearing in the Oil Level Indicating glass. In case it is not visible, do not mount that bushing. Also do not attempt to oil fill at site as such bushing is most likely to failure in service.

(vii)          During the lowering operation, thread the flexible steel wire through the inside tube of the bushing and pull it. Continue the operation till the bushing rests on its flange pocket. Bolt the two flanges and revove the slings.

(viii)      Hold the cable bolt in position, remove the flexible wire and insert the locking pin (3) to hold the cable bolt remove the slings.

(ix)           Fix the terminal cap (1) in position by screwing it on the terminal bolt.

(x)              For removing air from the central tube after oil filling, unscrew the terminal cap and press the terminal bolt downward.

(xi)           Fix upper and lower arcing horns and adjust the gap setting as per the required Insulation Co-ordination.

Cable Boxes

Cable boxes are designed for receiving and protecting cable ends.  Insulating paper is most hygroscopic and all paper insulated cable ends must be protected by suitable insulating Compound .These cable boxes are provided with brass wiping glands and are designed with Clearance inside the box suitable for compound filling. The cable box in such case must be Filled with compound as marked as indicated in the drawing.

Cable boxes for PVC or XLPE cables are designed with air clearance and hence these boxes are not required to be filled with compound.

Cable boxes of 3.6 KV and above are provided with detachable gland plates. Earthing terminals are also provided on these cable boxes for earthing the armouring of individual cable.

When cable boxes are provided with disconnecting chambers they permit removal of Transformers for servicing without disturbing cable terminations.

Bus-Duct/Trunkings

Some users prefer connections to load by means of Bus-Duct. Bus duct is supplied by some other Agency. However, we provide suitable flanges.trunkings around transformer bushings for receiving the busduct.

The level of the busduct flanges from groud/rail level is indicated in the General Arrangement Drawings of the transformer. The complete  details busduct flange is furnished by us giving complete dimensional details for the matching flanges, bolt spacing, bushing terminal details etc.

Marshalling Box

 The transformer is provided with certain fittings directly mounted on the transformer at various locations. These fittings are having electrical contacts or terminals which are required to be connected to the protection schemes to give alarm/annunciation  under abnormal conditions and if further required to disconnect the transformer form mains. In order to facilitate connections of all such devices to the protective scheme, the cable form all such contacts are wired upto a weather proof  terminal box. This box called Marshalling Box, is also used for housing Oil Temperature Indicator (OTI) and Winding

Temperature Indicator (WTI). 

The Marshalling box is made of sheet Metal and is provided with glass window for observing OTI & WTI.

It has hinged door with locking facility to prevent unauthorized access. The capillaries from OTI &WTI come out from the bottom of the Marshalling box through suitably recessed gland plate thus preventing ingress of dust.

It has detachable gland plate with glands through which cables enter and leave. It has a rain shed provided on top to prevent rain falling directly over it. All these provisions make Marshalling Box a Weather-proof enclosure.

Buchholz Relay

Buchholz relay is a very sensitive, gas and oil operated apparatus which detects formation of gas or development of sudden pressure inside the oil filled transformer. It is connected to protection circuits to give an early audible alarm of gas collection and to disconnect the transformer from supply in case of sever fault inside the transformer.

The basic function of the relay, is to initiate an electrical signal in the protection circuit when:

(a)  Gas is accumulated in the relay, as result of incipient fault.

(b)  Surge of oil is developed on account of sudden increase in pressure inside the transformer due to sever fault.

(c)  Oil level in the relay is reduced below the minimum level.

Buchholz relay operates in the following manner:

The relay comprises a housing containing two pivoted buckets/Floats counter balanced by dead weights. Each bucket assembly carries a mercury switch. The relay is fitted  in the oil connection between conservator and tank.

Due to gas collection, the oil level inside the relay drops and the upper bucket moves down. This tilts the mercury switch brining fluid mercury in such a position that it bridge the normally open (NO) contacts. Other probable reasons for droping of level in the relay are:

(a)  Leakage of oil from main tank, conservator or relay itself.

(b)  Collection of air in the relay which is trapped earlier in the tank and in the winding.

The lower bucket operates similar to upper bucket when level in the relay drops further. However important function of this relay is to disconnect transformer from circuit under sudden development of pressure inside the transformer doe to severe internal fault In such cases, gas generation is rapid and displaced oil surges through the relay impinge on the baffle plates causing lower bucket to tilt and close the Normally Open (NO) contact of the mercury switch carried by it.

Upper bucket contacts are connected to audible alarm-‘A’- circuit and lower bucket contacts-‘T’- are connected to trip circuit.

The relay is mounted in position with associates piping and Isolating valves at works. In larger transformers, the buchholz relay assembly is dismantled and sent separately. When Test Lever is provided, it is sent in ‘Test’ position to prevent damage to window glasses and mercury switches.

Mount the Buchholz relay between conservator and transformer tank cover along with piping and valves without straining the pipe line. Isolating valve must be fully opened after mounting is completed

To ensure successful operation of the relay the pipe work on either side of the relay is set inclined to horizontally by 3-5⁰.

MOUNT RELAY SUCH THAT ARROW DIRECTION POINTS TOWARDS CONSERVATIOR

After filling oil in the transformer, air trapped inside the relay is to be released through the valve/petcock provided on top of buchholz collection, it must be ensured that this valve is kept open and the pipe is full of oil. When gas is collected, oil from pipe will flow out first and then gas will come out.

Radiators

The function of radiators is to limit the temperature of oil and winding by dissipating heat that is generate due to the losses within the transformer while in the service. The number of sections per radiator and the number of radiators per transformer will depend upon the losses and permissible temperature rise.

Distribution Transformers are normally provided with radiators welded to tank. Owing to transport limitation and possible transit damages, power transformers are provided with detachable radiator with radiator valves in position on tank flanges.

-         Each radiator consists of number of “Section” made from pressed CRCA Sheets forming channels for oil flow. These “Sections” are welded to Header Pipes at Top and Bottom. Detachable radiators are provided with  M.S. Flanges at Top and Bottom. Flanged radiators are fitted with Air Release Plug, Drain Plug and Lifting Lug.

-         Bracing Straps, made from M.S. Flat are provided on radiator to prevent vibration of section.

Radiators are cleaned internally to remove scales and a coat of varnish is applied. External surface cleaned off all rust and one coat of Red-Oxide primer is applied which is followed by final painting.   

Silica Gel Breather       

Whenever there is a change in the ambient temperature or in the load of an oil immersed transformer there is a change in oil pump, & hence in the volume of oil. Increase in oil volume, casuses the air above the oil level in the conservator to be pushed out and decrease causes air to be drawn in. Thus the transformer “Breathes”. When air is breathed in , there is a possibility of moisture and dust from atmosphere to be sucked in. These contaminants deteriorate the insulation properties of oil. Hence, Silica Gel Breather is provided which arrests moisture and dust from the air drawn in.

A Typical- Silica Gel Breather has following main components:

1. A Casing

2. Silica Gel Crystal

3. An oil Seal at the lower end of casing

The casing has a window at the upper part for observation of the color of the Gel crystals. It has flange connection at the top for the connecting the breather to the breather pipe. The lower part of the casing has at its lower end an oil seal arrangement, a window for observation of oil level and an oil filling hole with gasket and plug.

Due to the chemical affinity possessed by Silica Gel Crystals, they absorb moisture from the air drawn in. The color of silica gel is blue when dry and turns pink when it is saturated with moisture. The color of crystal can be observed from the outside of the casing.

Oil seal assembly at the lower end of the casing consists of little quantity of oil with an inverted cup

Partly dipped in the oil and a tube fixed at the center of the cup.  

The oil acts as  a coarse filter and removes the dust from the outside air when it is passes through oil.

Silica gel breather is depatched in a separate case.

Magnetic Oil Level Gauge (MOG)

This is a dial type oil level indicating device provided on larger transformers with conservator at relatively high levels from the ground. In large transformers conventional glass oil level indicators are difficult to observe due to their heights and color change/dust accumulation on the glass. Further, the low oil level in the conservator falls to a low level. This protection feature and clear visibility justify the cost of MOG on a bigger transformer.

It consists of two compartments:

1.      The oil side compartment which fixed on the opening in the conservator.

2.      The pointer side compartment.

These compartments are sealed against leakage of oil by a metallic diaphragm. On the oil side compartment, there is a bevel gear wheel and it is positioned near the diaphragm. Movement of the float due to rise and fall of oil level in the conservator results into circular motion of the driving magnet. A follower magnet is positioned in the pointer side compartment near the diaphragm. This magnet has its poles face to face poles of driving magnet from the oil side compartment coupling them magnetically. The movement of float is, therefore, transferred through the diaphragm, eliminating direct oil light mechanical coupling.

At the other end of the axis of the driven magnet an indicating pointer is fitted. The dial is calibrated to show the oil level in the conservator. The dial and the pointer area housed behind the front glass. The dial has three position marked. The follower magnet has also a cam fitted on it which operates a mercury switch. When this magnet is at a position corresponding to low oil level the mercury switch closes the Normal Open (NO) contacts. These contacts  are normally wired to give audible alarm. The contacts are brought to terminal box at the lower end of the dial, for external connection.

Oil Temperature Indicator (OTI)        

-         Oil Temperature Indicator (OTI) is generally provided on all transformers except for very small ratings. The direct reading pointer arrangement in this instrument greatly facilitates observation of working temperature of oil. It also helps, if need be, in deciding the permissible overloads in accordance with I.S. 6600-1972. Guide for loading of oil immersed transformers.

-         A Typical-Oil temperature indicator consists of a

-         Bourdon tube with a pointer arrangement mounted in a case comprising of a reading dial and a glass cover. There is a temperature sensing bulb which communicates to the Bourdon tube through the armoured capillary.

-         The oil temperature indicator is provided with two pointers and associated contacts for protection of transformers. Both the pointers are independently adjustable and can be set to desired temperature. Setting of these pointers at required temperatures can be done from outside through the knob by using special keys.

-         The OTI is generally housed and wired upto terminal strip in the Marshalling box having a glass window on the door for observation. The length of capillary does not influence the accuracy of measurement and extra length of capillary tubing must not be cut, as it would be break communication between bulb and Bourdon tube.

If the oil temperature increases beyond set limit due to overload or inadvertent closure of radiator valves or insufficient air draft, the indicating pointer touches the present alarm pointer and actuates the alarm contact. The alarm contacts, when duly wired give an alarm. If the alarm is not attended and there is a further increase of temperature, the trip contacts which are wired to the trip circuit will operate and isolate the transformer form mains.

Winding Temperature Indicator (WTI)    

A winding temperature indicator (WTI) is an optional fitting and is provided when ordered. It is set to read Hot Spot Temperature (HST).

A typical WTI arrangement comprises of the following:

(a) WTI instrument having a temperature sensing bulb and a capillary similar to OTI. In addition it is provided with a heater coil around its operating bellow.

(b) A current transformer mounted on one of the transformer leads, sensing load current.

The bulb of the instrument is placed in an oil filled pocket located on the top cover of the transformer similar to OTI pocket. The Heater Coil is fed by the W.T.I.C.T.

Thus the Temperature indicated by WTI accounts for

(i)                Temperature of Top Oil and

(ii)             Winding Gradient ( Temperature Rise of winding over surrounding oil) which is dependent on load Current.

And  is adjusted to read HST = [Top oil Temp.] + 1.1 x Gradient

The Heater Coil is provided with an adjustable resistive shunt. (Ref fig. 24) in parallel which allows the instrument to be adapted for a range of winding Gradients

The adjustable shunt by-passes certain amount of current (I_SH) from the C.T. Secondary Current (I_S).        Thus Heater Coil current

I_H = I_{S ­-}  I_SH

For the particular transformer, the shunt is adjusted  at factory for the applicable Winding Gradient. WTI also has Alarm & Trip Contacts. Foe Fan Cooled Transformers auxiliary contacts of WTI are used for Fan switching.

Repeater OTI & WTI Instruments

Repeater OTI & WTI instruments are Optional Fittings and are provided when ordered. They enable readings being taken in control room also. They are generally provided in the RTCC Panel, when it is a part of contact.

They are available in one of the following types:

(a)  Simple Analog  Repeater

(b)   4-20mA. d.c. Analog  Repeater

(c)   4-20mA. d.c. Digital  Repeater

(d)   4-20mA. d.c. Digital  Repeater plus signal for SCADA

They all require the Local Instrument to be provided with a precision potentiometer.

In cases (b), (c), & (d), they also require Auxiliary Devices like Power Supply Unit, Resistance to

Current Converter (RCXT) and/or Current to Current Converter (CCXT) etc. They are housed in a UNIT BOX mount on the transformer.

Wiring Diagrams and Interconnection Diagrams necessary for the wiring of Marshalling Box, UNIT BOX and RTCC Panel are supplied along with transformer.

OTI & WTI Instruments Operated By Resistance Temperature Detectors (RTDs)  

Temperature sensing can alternatively be done by Resistance Temperatures Detectors (RTDs). Thus OTI & WTI can be operated by RTDs. When, so ordered, they are provided accordingly. In this case also Power Supply Unit and RCXT Unit will be required and they are provided in the Unit Box.

Here again Repeater Instruments are provided when ordered.

Thermesyphon Filter: (Ref. Fig. 26) Thermosyphons

Thermosyphon Filter is devices which continuously improve the quality of oil in the transformer tank. It is an optional fitting and is provided when ordered.

It is a container having perforated trays filled with Activated Alumina. It is fitted to the transformer tank similar to a radiator with provision of isolating valves, air release plug, drain valve etc. Due to the convection current set up in oil, oil flows continuously over the exposed surface of Activated Alumina (i.e. the adsorbent material) and in the process, contaminants like moisture, organic acids etc. generated due to  ageing of insulation, get collected in the adsorbent material and thus improve the quality of oil.

Periodic recharging of adsorbent material is necessary.

Flexible Separator (Ref. fig. 27)   

Also known as ‘Diaphragm Conservator’ or ‘Rubber Diaphragm’. It is an optional fitting and is provided when ordered.

The ‘Flexible Separator’ is sealed, nonporous flexible bag of a highly resistant fabric, coated externally to resist transformer oil and internally to resist external atmosphere.

It is fitted inside the conservator such that variations in oil volume due to variations of temperature are taken up by the Flexible Bag. Oil in conservator is sealed from the outside atmosphere by the mounting flanged to which the Flexible Bag is attached. Thus the atmospheric moisture and gases cannot contaminate the oil inside the conservator.

Fig. (a) Shows the separator in inflated condition corresponding to low oil volume. When oil expand due to increase in temperature, atmosphere inside the separator is exhaled and the separator bag deflates as shown in Fig. (b).

Oil filling in the conservator is to be done as per the procedure given in 2.20

Oil Level Transmitter: (fig. 28)

When oil in conservator is to be remotely monitored Oil level transmitter is to be provided. It is an optional fitting and is provided when ordered.

Transmitter System comprises of:

(i)                Stainless Steel Rod which goes inside the conservator tank.

(ii)             Transducer which gives electrical signaldepedent on the oil level.

(iii)           Power Supply/V-I Module which provides 4-20mA signal from the transducer output.

On Load Tap Changer (OLTC): (Ref. Flag. 22&23)

OLTC is an optional device and is provided  when ordered.

Tapings are located on High Voltage Winding of the transformer.

One type of OLTC is fitted external to transformer main tank . In the other type, contacts mounted on insulated cylinder are immersed in the transformer main tank. In both the types, Drive Mechanism (DM) contained in a separate box.

Different arrangements used to effect tap changing . They are:

(a)  Linear: Commonly used arrangement when no of tap positions is moderate.

(b)  Coarse/Fine: By means of a changer over contacts, the Coarse tap winding is included in  or excluded from the circuit. Thus two voltages (i.e. tap positions) are obtained for one position of moveable arm.

(c)   Reverse/Forward: By Means of a changer-over contact, the taping winding is connected in Forward (Adding) or Reverse (i.e. Subtracting) direction w.r.t. main winding. Thus two voltages (i.e. tap positions) are obtained for one position of moveable arm.  

Arrangement (b) & (c) used when no. of tap positions is large.

OLTC is generally operated electrically through a Remote Tap Control Cubicle (RTCC) located in Control Room. A electrical control is also provided in the DM, along with a selector switch for Local or Remote Control. For emergency manual operation, an operating Handle is provided.

When Automatic Voltage Regulating  (AVR) Relay is ordered  and a Voltage Transformer (V.T.) is wired on the transformer output side, the output  voltage is compared in the AVR Relay with the suitable reference voltage and difference used to give command to OLTC so as to reduce the difference. Thus the output voltage is automatically controlled to the preset value.

A Line Drop Compensator (LDC) is included in the AVR when specially ordered R&X  of the cable, connecting transformer to load, are  to be set by means of adjustable knobs  In such a case; voltage at the end of of the cable  and not a transformer terminals, is controlled.

Direction of Power Flow is an important consideration in OLTC. Only some types are suitable for Full Power Flow in –terms of current and no. of operations in the reverse direction. These details are given in the plate fitted to OLTC.