THREE-PHASE UNIT VERSUS SINGLE-PHASE UNITS

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Three-phase unit versus single-phase units:

Since the transmission system is 3-phase, transformers may be built as 3-phase single units or as three single-phase units into delta and star combinations or groups.

Advantages of 3 phase units

• They occupy less space
• No extra support equipment is required to form a 3-phase Delta or Star

connection.

• They are cheaper
• They can be transported from factory as a compact unit, erected and

commissioned at site quickly

• Compact on-load tap changing (OLTC) gear can be provided as a built in unit.

Disadvantages of 3 phase units

·         Problem of transportation in case of large capacity units weighing more than 100 tons.

·         Takes time in assembling, erecting and commissioning if parts are dismantled and sent to site.

·         The cost of one spare 3-phase transformer is more.

·         Change of connections from star to delta or vice-versa cannot be done.

·         If reconditioning is undertaken then the complete unit has to be taken out of service and this becomes a problem if no spare capacity is available.

          Advantages of Single-Phase Units

• The cost of a spare transformer is the cost of a single-phase unit, which is comparatively very much less than the cost of a complete spare 3-phase unit.
• They can be transported to site as completely assembled units and commissioned quickly.
• Reconditioning can be undertaken on individual units with a minimum outage time.
• It is possible to obtain different possible pairs of connections between the primary and secondary.

          Disadvantages of Single-Phase Units

• They occupy more space
• They require additional support structure to form 3-phase connections.
• Expenditure on civil engineering works is more
• The problem of providing on-load tap changing gear and even if provided the cost of providing tap changing gear on each unit works out costlier by at least 50% when compared to a compact unit in a 3-phase transformer.

          Considering all the above, there is little argument in favour of the adoption of single-phase units as compared to 3-phase units.  Single-phase units are the only choice where 3-phase units cannot be transported because of their weight and dimensions and also if there are no facilities at site for the assembly, preparation and commissioning of the 3-phase unit

          Power Transformers

These are transformers of high rating of generally not less than 5MVA and 33KV and the rating also increases with the voltage rating. They may be of the step-up type installed at generating stations or of the step-down type installed at substations. They have a high utilisation factor, which means that they are arranged to work at a constant load equal to their rating.  Hence their maximum efficiency is designed to be at or near full load.  Such power transformers installed in substations are provided with OLTC gear to regulate the voltage to be within permissible limits during peak load and off peak load hours.

However, generator step-up power transformers are provided with only off circuit taps.

          Distribution Transformers

These are transformers installed in H.V. distribution feeders to meet consumer voltage requirements.  These are generally rated at 11KV and have a rating not exceeding 1000KVA.  These transformers are characterised by an intermittent variable load, which is usually considerably less than the full load rating.  They are therefore designed to have their maximum efficiency at between half and three quarter of full load.  These transformers are not provided with any OLTC gear but with only off circuit taps.

        Auto Transformers

An Auto Transformer is a transformer with a common winding for both primary and secondary. They are used in place of two winding power transformers where the ratio of transformation does not exceed 2 as they are cheaper than two winding transformers such as in a 132KV/66KV system or 66KV/33KV system.

TRANSFORMER TEST

Transformer Test

Transformer test are done to determine the electrical, thermal and mechanical suitability for the system where they will be applied or use. The major tests carried out in the transformers are:

Turn ratio test(open circuit test)

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ratio test is an essential type test of transformer. This test also performed as routine test of transformer. So for ensuring proper performance of electrical power transformer, voltage and turn ratio test of transformer is one of the vital tests.

This is also used to make sure that the turn ratio between the windings of the transformers is correct; with this information, you can decide what the output voltage of the transformer will be. The ratio is calculated under no load condition.

Simultaneous readings of voltage are taken to high voltage and low voltage windings area taken after the voltage is applied on one winding and on each taps if the transformer has a tap changer. The ratio is the division between the high reading and low reading. If it is a three phase transformer each phase is tested individually. Expected variation should be between 0.5%.

Procedures: The instrument use to test is multimeter. Assume the transformer we are testing is a 60MVA/132/33kv transformer with 10 taps. First of all note the nominal tap of the transformer before you proceed (but normally written on the name plate of the transformer) then a supply voltage of 415volt is connected at the primary side of the transformer 132kv while the secondary side of the transformer is open circuited, then with your multimeter take the line voltage of both the primary and secondary side of the transformer i.e. R-Y, R-B, B-Y .This is done in all the taps 1-10

Insulation resistance test

Insulation resistance test of transformer is essential type test, it is commonly known as megger test. This test is carried out to ensure the healthiness of overall insulation system of an electrical power transformer. It measures the quality of insulation within the transformer. Some variation will be obtainable depending on the moisture, cleanliness and the temperature of the winding. It is recommended that core and tank should always be grounded when the test is performed. Each winding should be short circuited at the bushing terminals. Resistances are  measured between each winding and all other winding to ground.

Procedure of Insulation Resistance test of transformer

1) First disconnect all the line and neutral terminals of the transformer.
2) Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance IR value in between the LV and HV windings.
3) Megger leads to be connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth.
4) Megger leads to be connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth.

NB : It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.

Measurements are to be taken as follows:
For Auto Transformer: HV-IV to LV, HV-IV to E, LV to E
For Two Winding Transformer: HV to LV, HV to E, LV to E
Three Winding Transformer: HV to IV, HV to LV, IV to LV, HV to E, IV to E, LV to E

Oil temperature should be noted at the time of insulation resistance test of transformer. Since the IR value of transformer insulating oil may vary with temperature.

Short circuit test

This test is done to determine the impedance voltage which gives an idea of how much load the transformer can carry.

Procedures: The instrument use to test is multimeter. Assume the transformer we are testing is a 60MVA/132/33kv transformer with 10 taps. First of all note the nominal tap of the transformer before you proceed (but normally written on the name plate of the transformer) then a supply voltage of 415volt is connected at the primary side of the transformer 132kv while the secondary side of the transformer is short circuited, then with your multimeter take the line voltage of both the primary and secondary side of the transformer i.e. R-Y, R-B, B-Y .This is done in all the taps 1-10

Magnetizing Current Test

Magnetizing current test of transformer is performed to locate defects in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problem in tap changers. These conditions change the effective reluctance of the magnetic circuit, thus affecting the electric current required to establish flux in the core.

1) First of all keep the tap changer in the lowest position and open all IV & LV terminals.
2) Then apply three phase 415V supply on the line terminals for three phase transformers and single phase 230V supply on single phase transformers.
3) Measure the supply voltage and electric current in each phase.
4) Now repeat the magnetizing current test of transformer test with keeping tap changer in normal position.
5) And repeat the test with keeping the tap at highest position.

Generally there are two similar higher readings on two outer limb phases on transformer core and one lower reading on the centre limb phase, in case of three phase transformers. An agreement to within 30 % of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory test, there is likelihood of a fault in the winding which needs further analysis.

TRANSFORMER CONSTRUCTION

Transformer construction consists of the followings: 

   - (a) The Tank or  Enclosure and associated   

           accessories or fittings;

   - (b) The Core and Winding  assembly;

   - (c) The Insulating and Cooling medium.

•         The Insulating and Cooling Medium  

        - a.  Air; 

        - b. Gas (Nitrogen or SF₆ gas);

        - c. Mineral (Transformer) Insulating Oil;

        - d. Synthetic Nonflammable Insulating    

              Liquid (Askarel);

        - e. Water (External Cooling)

. Type of Circulation 

     - a. N = Natural   : Self-Cooled, Natural Convection

     - b. F = Forced   :  Forced Circulation : Pumps

                                 :  Forced Cooling : Fans

•         Standard Type Designations 

   1. Air Natural Cooling       - - -   AN

   2. Air Forced Cooling       - - -   AF

   3. Gas Natural Cooling    - - -   GNS

   4. Gas Forced Cooling    - - -   GPS

  (The Gas could either be SF₆ gas or Nitrogen)

   5. Oil-immersed natural cooling  - - -  ONAN

   6. Oil-immersed water cooling    - - -  ONWN

   7. Oil-immersed forced-air cooling - - ONAF

   8. Oil-immersed forced-oil water cooling - - OFWN

   9. Oil-immersed forced-oil forced-air cooling - -OFAF

 10. Oil-immersed forced-oil natural cooling - - - OFAN

•         11.  Nonflammable liquid-immersed natural    

          cooling - - LNAN

   12.  Nonflammable liquid-immersed forced-air

          cooling - - LNAF

   13.  Nonflammable liquid-immersed forced-

          liquid water cooling - - LFWN

   14.  Nonflammable liquid-immersed forced-

          liquid forced-air cooling - - LFAF

   15.  Nonflammable liquid-immersed forced-

          liquid natural cooling - - LFAN

•         Transformer Circuits : Types

•         1. The Magnetic Circuits:

   - a. The Core Type: has one magnetic circuit  

   linking the ‘high’ and ‘low’ voltage windings.

   This circuit forms a ‘core’ through the coils.

   Cheaper than the Shell type and is used extensively in Power Distribution transformers.

   - b. The Shell Type: Has two or more magnetic circuits in the form of a ‘shell’ around the coils.

   Naturally requires more iron for the ‘extra-cores’ and costlier to construct than the Core-type. Used only in special applications and in small sizes.

   - c. The Torroidal Type: which is a ‘core-less’ circuit, with the ‘high’ and ‘low’ laying side by side and on top of one another. Very cheap and Compact in size, limited in usage to Accessory transformers.

•         2. The Electrical Circuits : 

•         There are basically two types of windings from a geometrical aspect, namely:

    - a. Layer (Barrel) Type  and

    - b. Disc Type.

    However manufacturers have enlarged these categories to make for easier identification as follows:

    - a. Layer (Barrel) type:

          - -(i)   Single-layered;

          - -(ii)   Multi-layered;

          - -(iii)  Helical winding;

          - -(iv)  Multi-section winding.

•         In both the single-layered and multi-layered design, each layer extends the full length of the winding and there are no spaces between the individual turns.

•         The Helical winding is similar to a layered-type winding with the addition of the spacers between turns. When higher currents are required, a double helix is used, consisting of two separate sets of conductors separated by radial spacers, but wound in parallel on the same cylinder.

    The Multi-section is a combination of the layer and helical design.             

    The winding is multi-layered with each layer divided into a number of sections. Secondly, each section is separated by spacers.

    By winding in sections, the voltage between layers is reduced so that the winding is suitable for higher voltages than the multi-layer winding.

•         - b. The Disc Type: 

          - -(i)   Disc and Continuous Disc Winding;

          - -(ii)   Interleaved Disc winding;

          - - (iii) Sheet (Strip) Windings;

          - -(iv) Pancake Winding.

•         The Disc winding is made up of several discs of rectangular conductors, i.e. each disc is a number of turns radially. A pair of discs can be wound on a special form in such a manner that no centre connection (crossover) is required. The full winding is then made by brazing together the outside connections between pairs of discs. To avoid brazing, a continuous disc winding is formed.

•         The Interleaved disc winding is not continuously wound. It is made up of pairs of discs which are joined to the next pair by a brazed connection.

•         Sheet (Strip) windings are generally made to have more than one section, each section being composed of several layers.

•         The ‘Pancake’ coil is used in large shell-type high voltage transformers. Pancake coils are usually wound with square or rectangular conductors, one or more conductors in parallel, depending upon the current. This winding exposes a large amount of conductor surface to the cooling medium.