IMPEDANCE MATCHING TRANSFORMER SELECTION BASIC AND TUTORIALS

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Select a transformer with the correct turns ratio to match the 8- resistive load in Fig. 4.3 to the Thevenin equivalent circuit of the source.


Calculation Procedure
1. Determine the Turns Ratio
The impedance of the input circuit, Zi, is 5000 . This value represents the Thevenin impedance of the source. The load impedance, ZL , is 8 .

To achieve an impedance match, the required turns ratio is


Therefore, the impedance-matching transformer must have a turns ratio of 25:1.

Related Calculations. The maximum power transfer theorem (Sec. 1) states that maximum power is delivered by a source to a load when the impedance of the load is equal to the internal impedance of the source.

Because the load impedance does not always match the source impedance, transformers are used between source and load to ensure matching.

When the load and source impedances are not resistive, maximum power is delivered to the load when the load impedance is the complex conjugate of the source impedance.

RESISTANCE TYPE TRANSFORMER LOAD TAP CHANGER BASICS

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The LTC design that is normally applied to larger powers and higher voltages comprises an arcing switch and a tap selector. For lower ratings, LTC designs are used where the functions of the arcing switch and the tap selector are combined in a so-called arcing tap switch.


With an LTC comprising an arcing switch and a tap selector (Figure 3.3.1), the tap change takes place in two steps (Figure 3.3.2). First, the next tap is preselected by the tap selector at no load (Figure 3.3.2, positions a–c). Then the arcing switch transfers the load current from the tap in operation to the preselected tap (Figure 3.3.2, positions c–g).


The LTC is operated by means of a drive mechanism. The tap selector is operated by a gearing directly from the drive mechanism. At the same time, a spring energy accumulator is tensioned.

This operates the arcing switch — after releasing in a very short time — independently of the motion of the drive mechanism. The gearing ensures that this arcing switch operation always takes place after the tap preselection operation has been finished.

With today’s designs, the switching time of an arcing switch lies between 40 and 60 ms. During the arcing switch operation, transition resistors are inserted (Figure 3.3.2, positions d–f), which are loaded for 20 to 30 ms, i.e., the resistors can be designed for short-term loading.

The amount of resistor material required is therefore relatively small. The total operation time of an LTC is between 3 and 10 sec, depending on the respective design.


An arcing tap switch (Figure 3.3.3) carries out the tap change in one step from the tap in service to the adjacent tap (Figure 3.3.4). The spring energy accumulator, wound up by the drive mechanism actuates the arcing tap switch sharply after releasing. For switching time and resistor loading (Figure 3.3.4, positions b–d), the above statements are valid.

The details of switching duty, including phasor diagrams, are described by IEEE (Annex A [IEEE, 1995]) and IEC (Annex A [IEC, 2003]).

INSTRUMENT TRANSFORMER METERING IN METALCLAD SWITCHGEAR

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In 1958 the Meter and Service Committees of EEI and AEIC issued Guide for Specifications for Revenue Metering Facilities Installed in Metalclad Switchgear. This guide states the principal objectives to be attained:

1. That a separate sealable compartment be provided exclusively for revenue metering equipment when mounted within the switchgear;

2. That space be provided within the compartment sufficiently large to accommodate separately the installation of any standard current transformers and any standard voltage transformers required for metering;

3. That space be provided within the compartment for the installation of separate, isolated voltage transformer fuses, where required;

4. That, where required, adequate space and panel facilities be provided within the compartment to permit the installation of all necessary meters, instruments, auxiliary devices, or test facilities, of any type, whether they be front connected, back connected, surface mounted, or flush type;

5. That the arrangements be such that the secondary wiring may be installed in a manner to facilitate checking of connections.

By following these specifications, control of all metering transformers and conductors rests with the utility company.

When extremely high-capacity current transformers are used, it is essential that spacing of bus bars be adequate to avoid interference between individual transformers.

There are many advantages to be gained by mounting instrument transformers in the customer’s switchgear. Protection, appearance, and, in many cases, economy, may be the result.

TYPES OF FAILURES IN POWER TRANSFORMERS BASIC AND TUTORIALS

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The electrical windings and the magnetic core in a transformer are subject to a number of different forces
during operation, for example

a) Expansion and contraction due to thermal cycling
b) Vibration
c) Local heating due to magnetic flux
d) Forces due to the flow of through-fault currents
e) Excessive heating due to overloading or inadequate cooling

These forces can cause deterioration and failure of the electrical insulation of the transformer windings. Statistics for the causes of transformer failures experienced in U.S. utilities are not readily available.

The detection systems that monitor other transformer parameters can be used to indicate an incipient electrical fault. Prompt response to these indicators may help avoid a serious fault.

Some examples of actions taken to detect undesirable operating conditions are as follows:

1) Temperature monitors for winding or oil temperature are typically used to initiate an alarm requiring investigation by maintenance staffs. At this stage, the operators may start to reduce the load on the transformer to avoid reaching a condition where tripping the transformer would be required.

2) Gas detection relays can detect the evolution of gases within the transformer oil. Analysis of the gas composition indicates the mechanism that caused the formation of the gas, e.g., acetylene can be caused by electrical arcing; other gases are caused by partial discharge and thermal degradation of the cellulose insulation.

The gas detection relays may be used to trip or to generate an alarm depending on the utility practice. Generally, gas analysis is performed on samples of the oil that are collected periodically. A continuous gas analyzer is available to allow online detection of insulation system degradation.

3) Sudden-pressure relays under oil respond to the pressure waves in the transformer oil caused by the evolution of gas associated with arcing.

4) Sudden-pressure relays in the gas space respond to sudden changes in the gas pressure due to evolving gases from an arc under oil.

5) Oil-level detectors sense the oil level in the tank and are used to generate an alarm indicating minor reductions in oil level and trip for severe reductions.

6) Online devices monitor bushings of the transformers, CTs installed in those bushings, and surge arresters installed on the transformers and generate an alarm indicating that repair is needed urgently so that major damage is avoided.

Details of the modern techniques for monitoring these components are given by Coffeen et al.

THE NATURE OF TRANSFORMER LOSSES BASIC INFORMATION

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Transformer losses are broadly classified as no-load and load losses. No load losses occur when the  transformer is energized with its rated voltage at one set of terminals but the other sets of terminals are open circuited so that no through or load current flows.

In this case, full flux is present in the core and only the necessary exciting current flows in the windings. The losses are predominately core losses due to hysteresis and eddy currents produced by the time varying flux in the core steel.

Load losses occur when the output is connected to a load so that current flows through the transformer from input to output terminals. Although core losses also occur in this case, they are not considered part of the load losses.

When measuring load losses, the output terminals are shorted to ground and only a small impedance related voltage is necessary to produce the desired full load current. In this case, the core losses are small because of the small core flux and do not significantly add to the measured losses.


Load losses are in turn broadly classified as I2R losses due to Joule heating produced by current flow in the coils and as stray losses due to the stray flux as it encounters metal objects such as tank walls, clamps or bracing structures, and the coils themselves. Because the coil conductors are often stranded and transposed, the I2R losses are usually determined by the d.c. resistance of the windings.

The stray losses depend on the conductivity, permeability, and shape of the metal object encountered. These losses are primarily due to induced eddy currents in these objects.

Even though the object may be made of ferromagnetic material, such as the tank walls and clamps, their dimensions are such that hysteresis losses tend to be small relative to eddy current losses.

Although losses are usually a small fraction of the transformed power (<0.5% in large power transformers), they can produce localized heating which can compromise the operation of the transformer. Thus it is important to understand how these losses arise and to calculate them as accurately as possible so that, if necessary steps can be taken at the design stage to reduce them to a level which can be managed by the cooling system.

Other incentives, such as the cost which the customer attaches to the losses, can make it worthwhile to find ways of lowering the losses. Modern methods of analysis, such as finite element or boundary element methods, have facilitated the calculation of stray flux losses in complex geometries.

These methods are not yet routine in design because they require a fair amount of geometric input for each new geometry. They can, however, provide useful insights in cases where analytic methods are not available or are very crude. Occasionally a parametric study using such methods can extend their usefulness beyond a specialized geometry.

AUTOTRANSFORMER CONNECTIONS BASIC AND TUTORIALS

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The autotransformer is both the most simple and the most fascinating of the connections involving two windings. It is used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity of a transformer.

Autotransformers are also used on radial distribution feeder circuits as voltage regulators. The connection is shown below:

The boosting autotransformer connection. The output terminals operateat a higher voltage than the input terminals.

The autotransformer shown above is connected as a boosting autotransformer because the series winding boosts the output voltage. Care must be exercised when discussing ‘‘primary’’ and ‘‘secondary’’ voltages in relationship to windings in an autotransformer.

In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage.

In the case of a boosting autotransformer, however, the primary (or high) voltage is associated with the series winding, and the secondary (or low) voltage is associated with the common winding; but the voltage across the common winding is higher than across the series winding.

The other possible connection for an autotransformer is shown below:

The bucking autotransformer connection. The output terminals operateat a lower voltage than the input terminals.

The autotransformer shown is connected as a bucking autotransformer because the series winding bucks, or opposes, the output voltage. The key feature of an autotransformer is that the KVA throughput of the transformer, i.e., its capacity, is different than the KVA transformed by the common and series windings. The common and series windings are wound on the same core leg.


1. The volts per turn in the common winding equal the volts per turn in the series winding. The common winding voltage divided by the series winding voltage is equal to the number of turns in the common winding divided by the number of turns in the series winding.

2. The sum of the ampere-turns of the common winding plus the ampere- turns of the series winding equal the magnetizing ampereturns.

The magnetizing ampere-turn are practically zero, so the magnitude of the ampere-turns in the common winding is approximately equal to magnitude of the ampere-turns in the series winding. The series winding current divided by the common winding current is equal to the number of turns in the common winding divided by the number of turns in the series winding.

3. The KVA transformed in the series winding equals the KVA transformed in the common winding. The capacity multiplication effect stems from the fact that the metallic connection between the input and output circuits allows part of the KVA to flow though the connection and bypass the transformation.

POWER TRANSFORMER TEMPERATURE RISE AND THE THERMAL CAPABILITY BASIC AND TUTORIALS

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Transformer KVA ratings have been alluded to on a number of occasions up to this point without explaining how the KVA rating is determined.

The KVA rating of a transformer is simply the steady-state KVA load applied to the output of the transformer at the voltage rating of the output winding that produces an average winding temperature rise (above the ambient temperature) equal to 65°C.

For older transformers, the rated average winding temperature rise was 55°C. Advances in insulating materials allowed a 10°C increase in average temperature.

The temperature rise of the winding is caused by all of the transformer losses that were previously discussed in this chapter. Therefore, the winding temperature is a function of load losses and no-load losses.


The thermal capability of a transformer is defined in a slightly different way from the rated KVA. Thermal capability is the KVA load applied to the output of a transformer that causes the hottest area in the windings, called the winding hot spot, to reach some limiting temperature.

The hot-spot temperature determines the rate of loss of life of the transformer as a whole, which is a cumulative effect. Therefore, the hot-spot temperature limit is usually based on a loss-of-life criterion.

POWER TRANSFORMER SERIES IMPEDANCE AND REGULATION BASIC AND TUTORIALS

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The series impedance of a transformer consists of a resistance that accounts for the load losses and a reactance that represents the leakage reactance. This impedance has a very low power factor, consisting almost entirely of leakage reactance with only a small resistance.

As discussed earlier, the transformer design engineer can control the leakage reactance by varying the spacing between the windings. Increasing the spacing ‘‘decouples’’ the windings and allows more leakage flux to circulate between the windings, increasing the leakage reactance.

While leakage reactance can be considered a transformer loss because it consumes reactive power, some leakage reactance is necessary to limit fault currents. On the other hand, excessive leakage reactance can cause problems with regulation.

Regulation is often defined as the drop in secondary voltage when a load is applied, but regulation is more correctly defined as the increase in secondary output voltage when the load is removed. The reason that regulation is defined this way is that transformers are considered to be ‘‘fully loaded’’ when the secondary output voltage is at the rated secondary voltage.

This requires the primary voltage to be greater than the rated primary voltage at full load.

Let Ep equal the primary voltage and let Es equal the secondary voltage when the transformer is fully loaded. Using per-unit values instead of primary and secondary voltage values, the per-unit secondary voltage will equal Ep with the load removed. Therefore, the definition of regulation can be expressed by the following equations.


Regulation = (Ep - Es)/ Es


Since Es = 1 by definition,
Regulation Ep - 1 (3.8.2)
Regulation depends on the power factor of the load. For a near-unity power
factor, the regulation is much smaller than the regulation for an inductive load
with a small lagging power factor.

Example 3.4
A three-phase 1500 KVA 12470Y-208Y transformer has a 4.7% impedance. Calculate the three-phase fault current at the secondary output with the primary connected to a 12,470 V infinite bus. Calculate the regulation for a power factor of 90% at full load.

The three-phase fault is a balanced fault, so the positive-sequence equivalent circuit applies. The full-load secondary current is calculated as follows:

I 1.732 500,000 VA per phase/208 V 4167 A per phase
The per-unit fault current is the primary voltage divided by the series impedance:
1/0.047 = 21.27 per unit

The secondary fault current is equal to the per-unit fault current times the fullload current:
If 21.27 per unit 4167 A per phase 88,632 A per phase To calculate regulation, the secondary voltage is 1∠0° per unit by definition.

Applying a 1 per unit load at a 90% lagging power factor, I 1.0∠ 25.8°. Since the series impedance is mainly inductive, the primary voltage at full load Ep can be calculated as follows:

Ep 1∠0° + 1.0∠ 25.8° X 0.047∠90°
1.02 + j0.042 = 1.021 per unit
Regulation = Ep - 1 = 0.021 = 2.1%


ON LOAD TAP CHANGER DESIGN TEST FOR MOTOR DRIVEN MECHANISM BASIC INFORMATION

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Mechanical load test
If the LTC is operated by a separate motor-drive mechanism, the output shaft shall be loaded by the largest LTC for which it is designed or by an equivalent simulated load.

At such a load, 500 000 operations shall be performed at room temperature across the entire tap range. Additional cooling of the motor-drive is permissible during this test.

During this test, 10 000 operations shall be performed with the motor supply voltage at 85% of rated drive motor voltage. Also, 10 000 operations shall be performed at 110% of rated drive motor voltage. In addition, 100 operations shall be performed at a temperature of -25 °C.

The correct functioning of the tap position indicator, limit switches, restarting device, and operation counter shall be verified during this test. At the completion of this test, the LTC shall be operated manually, if applicable, through one cycle of operation.

The test shall be considered to be successful if there is no mechanical failure or any undue wear of the mechanical parts. Normal servicing according to the manufacturer's instruction book is permitted during the test. During this test, the heating system of the motor-drive mechanism shall be switched off.

Overrun test
It shall be demonstrated that, in the event of a failure of the electrical limit switches, the mechanical end stops will prevent operation beyond the end positions when a motorized tap-change is performed and that the motor-drive mechanism will not suffer either electrical or mechanical damage.

ON LOAD TAP CHANGER NAMEPLATE SAMPLE AND REQUIREMENTS

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The LTC nameplate shall be in accordance with ANSI C57.12.10-1987 and shall include the items listed below:

a) Number and year of this standard
b) Manufacturer's name
c) Serial number
d) Manufacturer's type designation
e) Year of manufacture
f) Maximum rated through current
g) Basic lightning impulse insulation level to ground

The nameplate shall be permanently attached to the LTC compartment.

TESTING OF PHASE SHIFTING TRANSFORMERS BASIC AND TUTORIALS

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Unless otherwise specified, all tests carried out at the factory should be made in accordance with IEEE Std C57.12.90-1993. Additional tests, particular to PSTs, are defined in 11.2, Special tests for PSTs.

Since the method of testing PSTs is dependent on the design, the testing methods will be mutually agreed upon by the user and manufacturer.

Resonant frequency and transient voltage tests
These tests are normally performed on the core and coil assembly in air. However, they can also be performed inside the tank filled with oil and fitted with temporary bushings to give access to required test points.

For a two-core design in one or more tanks, the windings must be interconnected as for impulse testing. These tests are intended to verify the transient voltages and natural frequencies at various points in the windings at all tap combinations and connections that can be compared and evaluated with studies.

Temperature tests and loss distribution
In most cases temporary bushings must be installed for connections to windings, which are not normally accessible, in order to determine the various resistances for the temperature tests and to determine the losses and the distribution of these losses.

The location of these temporary bushings depends on the design and winding configuration and is subject to agreement between user and manufacturer. For two-tank designs, the tanks may be separate to determine the losses in the various cores and windings and the temperature test.

This information will be provided by the manufacturer to the user during preliminary discussions.

Dielectric test
For dielectric tests each tank with its corresponding core and windings should be connected electrically and mechanically together as for the service condition. In most cases, temporary bushings must be installed on lower voltage windings in order to perform the IEEE standard low frequency induced test on the higher source and load side windings.

In very high voltage PSTs, it is sometimes necessary to install an auxiliary winding next to the core for shielding purposes. This auxiliary winding can then be used for performing the low-frequency induced test through the use of temporary bushings.

PHASE SHIFTING TRANSFORMER USUAL SERVICE CONDITION BASIC INFORMATION

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These conditions shall be as stated in IEEE Std C57.12.00-2000, 4.1.1 through 4.1.7, and 4.1.9; 4.1.8 shall not apply. In 4.1.6.1 (a), the word secondary shall mean the L terminals of the PST.

a) The purchaser of the PST shall specify the switching arrangements that will be used to place the PST in and out of service. This shall include breaker or switch operations resulting from faults external and internal to the PST.

b) The PST shall be suitable for energization by voltage applied to either the S or L terminals.

c) The PST shall be capable of transferring rated kVA with the electrical source of power connected to the S or L terminals. Limited power transfer in the retard position has to be considered.

d) Seismic requirements shall be as specified in IEEE Std 693-1997. The seismic zone shall be provided by the purchaser. The foundation design shall be provided to the PST manufacturer by the purchaser.

The manufacturer shall provide for differential motion between the two tanks, if used, and in the case of remotely mounted radiators provide for their differential motion.

e) The manufacturer of the PST shall make provisions for differential alignments that will occur when two tanks are connected. The foundation tolerance shall be defined by agreement between purchaser and manufacturer.

f) Unless specified otherwise, the PST shall be manufactured for operation in the bypassed state with the source and load bushing connected through bus work. This shall require special consideration in design for lightning impulse and switching surges.

This condition will require additional testing with the terminals connected, as in operation, to demonstrate that the insulation level meets the specified BIL.

TRANSFORMER PERCENT (%) REGULATION BASIC AND TUTORIALS

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When a transformer is energized with no load, the secondary voltage will be exactly the primary voltage divided by the turns ratio (NP/NS). When the transformer is loaded, the secondary voltage will be diminished by an amount determined by the transformer impedance and the power factor of the load.

This change in voltage is called regulation and is actually defined as the rise in voltage when the load is removed. One result of the definition of regulation is that it is always a positive number.

The primary voltage is assumed to be held constant at the rated value during this process. The exact calculation of percent regulation is given in Equation



where cos 􀁕 is the power factor of the load and L is per unit load on the transformer.

The most significant portion of this equation is the cross products, and since %X predominates over %R in the transformer impedance and cos 􀁕 predominates over sin 􀁕 for most loads, the percent regulation is usually less than the impedance (at L = 1).

When the power factor of the load is unity, then sin 􀁕 is zero and regulation is much less than the transformer impedance.

A much simpler form of the regulation calculation is given in Equation


For typical values, the result is the same as the exact calculation out to the fourth significant digit or so.

TRANSFORMERS TAPPING BASICS AND TUTORIALS

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Transformers also provide the option of compensating for system regulation, as well as the regulation which they themselves introduce, by the use of tappings which may be varied either on-load, in the case of larger more important transformers, or off-circuit in the case of smaller distribution or auxiliary transformers.

Consider, for example, a transformer used to step down the 132 kV grid system voltage to 33 kV. At times of light system load when the 132 kV system might be operating at 132 kV plus 10%, to provide the nominal voltage of 33 kV on the low-voltage side would require the high-voltage winding to have a tapping for plus 10% volts.

At times of high system load when the 132 kV system voltage has fallen to nominal it might be desirable to provide a voltage higher than 33 kV on the low-voltage side to allow for the regulation which will take place on the 33 kV system as well as the regulation internal to the transformer.

In order to provide the facility to output a voltage of up to 10% above nominal with nominal voltage applied to the high-voltage winding and allow for up to 5% regulation occurring within the transformer would require that a tapping be provided on the high-voltage winding at about  13%.

Thus the volts per turn within the transformer will be: 100/87 D 1.15 approx. so that the 33 kV system voltage will be boosted overall by the required 15%. It is important to recognise the difference between the two operations described above.

In the former the transformer HV tapping has been varied to keep the volts per turn constant as the voltage applied to the transformer varies. In the latter the HV tapping has been varied to increase the volts per turn in order to boost the output voltage with nominal voltage applied to the transformer.

In the former case the transformer is described as having HV tappings for HV voltage variation, in the latter it could be described as having HV tappings for LV voltage variation. The essential difference is that the former implies operation at constant flux density whereas the latter implies variable flux density.

Except in very exceptional circumstances transformers are always designed as if they were intended for operation at constant flux density. In fixing this value of nominal flux density some allowance is made for the variations which may occur in practice.

50 MVA 138KV AUTOTRANSFORMER SPECIFICATION SAMPLE

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Type:    Outdoor use, Oil-immersed, OA/FA/FA, 3 Windings with rubber diaphragm
             conservator vented via silica gel dehydrating breather, On-load-tap changer,  
             manufactured according to ANSI C57.12.00 Std., All Copper Windings,                                                        
            For use as a Step-down transformer in an electric utility transmission substation.
           Complete with standard accesories.

Rating:
              HV    -     30/40/50 MVA
              LV    -      30/40/50 MVA
             TV    -      12/16/20 MVA

Cooling Method:  OA/FA1/FA2

Rated Voltage:      HV -  138KV
                            LV -    69KV
                            TV -   13.2KV

Tap Voltage:   HV Side OLTC: 138 KV + 8, - 12 x 1.0%,  21 Taps


OLTC: ABB type UZFRT 550/300, 138,000 Volts,
             3 Phase, 60 Hz, 21 positions,

            With Motor Drive Mechanism type BUF 3,
            Motor : 460 Volts, 3 Phase, 60 Hz
            Contactors: 230 Volts AC,
            Position Transmitter: 230 Volts AC
            Heating Element      : 230 Volts AC
            With Manual/Automatic Change over-switch,  Raise & Lower pushbuttons.
           
BIL:
      Winding             :    HV      –  650 KV
                                  LV       -  350 KV
                                  Neutral – 150 KV
                                TV       -  110 KV

     Bushing               : HV        -  650 KV
                              LV        -  350 KV
                                Neutral   - 150 KV
                                   TV          - 150 KV

Frequency : 60 Hz
Connection:
            HV  -  Star with Neutral (Auto-Star) brought out to a bushing
            LV  -   Star with Neutral (Auto-Star) brought out to a bushing
            TV  -   Delta

Vector Group : Yyna0d1

Guaranted Losses at rated voltage, frequency, unity pf & @ 85 deg C (50 MVA):        
          No -Load Loss:               18.6 KW
          Load Loss @ 50 MVA:   157.6 KW
         Efficiency :                       99.65% @ 50 MVA( Without Auxiliary Loss)

Temperature Rise Limits:
             Oil         -  65 deg C
             Winding – 65 deg C

% Impedances  @ 85 deg
                  HV – LV@ 50 MVA        HV - TV@ 20 MVA       LV - TV@ 20 MVA
         8L    -   149,040 V   -   10.28            -       10.45
         N     -   138,000 V   -   10.50            -       10.31              -            5.6          
       12R   -   121,440 V   -   11.09            -       10.52
     
           
Audible Sound Level @ 50 MVA with all fans running: 72dB

Service Condition:
                Maximum ambient air temperature:                                             40 deg C
                Average ambient air temperature for any 24h period:                  30 deg C
                Maximum altitude above sea level:                                             1000 meters
               Maximum ambient relative humidity:                                            88%
                Mean annual rainfall:                                                                  2400 mm
               Maximum wind velocity:                                                             220 km/hr
               Maximum seismic factor:                                                            0.45g

CONSTRUCTIONS
     
a) Core:
The core of the transformer will be constructed of the highest quality, non-aging high permeability, cold-rolled gain-oriented  silicon steel sheet especially suitable for the purpose. Every care will be taken during slitting and cutting process to avoid burrs. Both sides of each sheet will be special glass film insulated on to minimize eddy current losses. The cores will be carefully assembled and rigidly clamped to ensure adequate mechanical strength to support the windings and also reduced vibration to minimum under operating conditions.
      b) Windings:
                       The winding of the transformer shall be made of  high tensile strength electrolytic copper of a high conductivity (Class A, in accordance with ANSI)
                       and insulation material of high quality shall be used. The windings shall be  free from burrs, scales and splinters.
 
The insulation material of windings and connections shall not shrink, soften or collapse during service. Thermally upgraded paper shall be used for conductor insulation. The design, construction and treatment of windings shall give proper consideration to all service factors, such as high dielectric and mechanical strenght of insulation, coil characteristics, uniform electrostatic flux distribution, prevention of corona formation, and minimum restriction to oil flow.

Moreover, under any load condition, none of the material used shall disintegrate, carbonizer or become brittle under the action of hot oil.

The coils must be capable of withstanding movement and distortion caused by abnormal operating conditions. Adequate barriers shall be provided between windings and core as well as between high voltage and low voltage windings. All leads or bars from the windings to the terminal boxes and bushings shall be rigidly supported. Stresses on coils and connections must be avoided.

Due to very unfavorable short-circuit conditions and numerous short-circuits in the network, special measures have to be taken to increase the capability of the winding to withstand short-circuit currents. Winding and arrangement of coils shall be designed so as to unify the initial potential distribution caused by impulsive traveling waves, as much as possible, to avoid potential oscillation and in order to withstand abnormal high voltage due to switching.

To increase the capability of the transformer windings to witstand electromagnetic forces under short circuit conditions, modern technology in design and construction shall be applied. (e.g. low current density, provision of pressure limiting devices and spring elements, use of perfectly dried pre-compressed pressboard, maintaining a balance of ampere-turns between windings, ets.)

Measures against coil displacement as generated by the radial and longitudinal forces shall be considered. Computation of strength against these forces including the description of the method being applied shall be submitted in detail.

The tank, conservation, coolers and bushings shall be adequately braced to withstand ocean shipment, and earthquake with seismic coeffecient of 0.45 g (horizontal)

c)   Short Circuit Withstand Capability
The transformer shall withstand the combined effects of thermal, mechanical and electromagnetic stresses arising under short-circuit conditions based on the maximum durations of fault:

Primary  Winding: 2 seconds

Secondary  Winding: 2 seconds

Tertiary    Winding                    2          seconds

The maximum sustrained short-circuit current in each windings shall be stated by the manufacturers. The maximum temperatures of the windings shall not exceed 250 deg C within the seconds duration of fault. All transformer accessories, parts, components (CT's, bushings, tap-changer, etc.) shall be capable of withstanding the cumulative effects of repeated mechanical and thermal over-stressing as produced by short-circuits and loads above the nameplate rating.

For design purposes, the following network data shall be take into consideration. The available system fault currents as as follows (in rms):

138 KV: Ik"  60 KA         69 KV : Ik" = 50 kA 13.2 KV : Ik" = 40 kA

The transformer shall be capable of withstanding the resulting successive short-circuits, without cooling to normal operating temperature between successive occurence of the short-circuit, provided the accumulated duration of short-circuit does not exceed the maximum duration permitted for single short-circuit defined above.

The upper limits of the symmetrical overcurrent due to such short-circuits as a multiple of rated current shall also be specified by the manufacturer.

      d)  Overload Capability
The short-time overload rating and operation of the transformer shall be in accordance with ANSI C57.92 or IEC 354. All other auxiliary equipment (bushings, CT's, etc) affected shall be rated to match the transformer overload rating.

e) Transformer Tanks:
The tank should have sufficient strength to withstand full vacuum and internal pressure of 1.0 kg/cm2, with cooling equipment & conservator connected. The tank cover will be clamped with bolts and nuts, and will be provided with handhole or manholes of suitable size. All seams and jointwill be oil tight. Guides within the tank will be furnished to facilitate tanking and untanking, and to prevent movement of the core and coil assembly, in transit. The casing will be provided with suitable lugs for lifting the completely assembled transformer filled with oil. All gaskets will be synthetic rubber bonded cork.

f) Radiators:
The transformer will be provided with a number of sufficient radiators for self-cooled (OA) operation. The radiator will be installed on the tank via radiator valves, so that each radiator can be detached from the tank independently of the oil in the main tank. The radiator valves will have the open and close positions clearly marked. Radiators will be equipped with provisions for draining. Radiators shall be made of galvanized steel.

g) Forced-air-cooling system:
For forced-air-cooled (FA) operation, the transformer will be provided with automatically controlled three phase motor-fans actuated from winding temperature. Fan motor, weather proofed, three phase, Hz, and will be thermal protected. The cooling-fans will be mounted on the radiators and the control box will be mounted on the wall of the tank. Motor Voltage: 460 VAC, 3 phase, 60 Hz.

h) On-load tap-changer:
The following tap-changer will be equipped on H.V. side for the regulation of voltage under loading conditions.
Type Type UZFRT 550/300
                        3 phase,60 Hz, 21 positions
Number of tap positions           21 taps positions
                                                                                   (138KV + 8 X 1380 V, - 12 X 1380 V)
                                   Manufacturer ABB

                                    Motor Drive Mechanism:
                                    Type:                                        ABB type BUF 3
Motor Voltage: 460 Volts, 3 Phase, 60 Hz
                                    Contactors Voltage:                  230 VAC
                                    Position Transmitter:                 230 VAC
                                    Heating Element:                       230 VAC
                             
                            Motor-Drive Mechanism Accessories:
                                   1. Standard Accessories
                                   2. Phase Failure Relay
                                   3.Circuit Breakers for Control & Auxiliary circuits
                                   4. Accessories for paralleling with 2 transformers using MASTER-FOLLOWER method.

                          OLTC Accessories:
                                   1.  Oil Conservator
                                   2.  Oil Level Indicator with contacts for Alarm
                                   3.  Dehydrating Breather
                                   4.  Pressure Relief Valve/Device with contacts for tripping
                                   5.  Pressure Relay with contacts for tripping
                                   6. Oil Flow Controlled Relay with contacts for alarm
                                   6. Thermoswitch Housing
                                   7. Valve for oil filtration mounted on the top
                                   8. Valve for oil filling, draining & filtration
                                   9. Earthing terminal
                                  10. Prepared for on - line oil filter unit



i) Oil preservation system:
Conservator system with sealed diaphragm will be used. Conservator with low-profile design having a moisture-proof barrier made with an oil-resisting diaphragm will be applied and placed at the level slightly higher than the transformer tank.

j) Bushings:
Primary:          ABB type GOB 650-1250-0.3 Brown, Cat # 123 193-K
                                              1250 Amps, Nominal Voltage: 170 KV rms,
                                               Phase to Earth Voltage:145 KV rms, BIL: 650KV,
                                               Creepage Distance: 4080 mm
                                               Porcelain Color: Brown
                                               Short end shield

Secondary:       ABB type GOB 380-800-0.3 Brown, Cat # 123-185-K
           800 Amps, Nominal Voltage: 100 KV rms,
                                                Phase to Earth Voltage:72.5 KV rms, BIL: 380 KV
                                                Creepage Distance:2210 mm
                                                Porcelain Color: Brown
                                                Short end shield

                       Tertiary:           CEDASPE s.p.a. Italy type Dt 30 Nf 1000
                                              1000 Amps, Nominal Voltage: 36 KV,
                                               Maximum Voltage to Ground: 30 KV, BIL:170 KV,
                                               Creepage Distance: 640 mm
                                               Porcelain Color: Brown
                                               Threaded Extended Rod

                      Neutral:            CEDASPE s.p.a. Italy type Dt 52 Nf 1000
                                              1000 Amps, Nominal Voltage: 52 KV
                                              Maximum Voltage to Gound: 52 KV, BIL 250 KV,
                                              Creepage Distance: 1080 mm
                                              Porcelain color: Brown
                                             Threaded Standard Rod


Complete with the following accessories:

31.  One (1) Buchholz Relay with 2 contacts for alarm & tripping
32.  Two (2) Dial type Oil Level Indicators for Main Tank & OLTC with contacts for alarm.
33.  One (1) Oil Temperature Indicator & Relay type AKM OTI series 34 for alarm.
34.  Three ( 3) Winding Temperature Indicators & Relays for HV, LV & TV windings with
        3 contacts each for alarm, tripping & fan control, AKM type WTI series 35.
35.  Qualitrol type self resetting mechanical Pressure Relief Device with contacts for tripping
36.  Conservator for main Tank - Sealed Diaphragm constant pressure type.
38.  Breather type conservator for OLTC.
39.  Annunciators (Marshalling Kiosk)
40.  Bushing Current Transformers
         HV:     300/200/100:5A;    0.6 - B 0.5
         LV & Neutral:  600/500/400/300/200/100:5A;   C-400
         TV :    1200/1000/900/800/600/500/400/300/200/100:5A;  C-400
       

41.     Galvanized Steel Radiators

42.     Bushing Terminals

         HV- Universal 4 hole NEMA Flat Terminals
         LV- Universal 4 hole NEMA Flat Terminals
         TV – Universal Multi -hole NEMA Flat Terminals

43.   Sets of Surge Arresters mounted nearest to the HV, LV & TV transformer bushings,
       with Surge Counters & 4/0 AWG THW Copper conductors connected
       to grounding terminals.                                                                                                                                                                              


HV     : 120 KV Voltage Rating, 98 KV MCOV, Station Class, Polymer housing,      
      Metal Oxide, Line Discharge Class 4 per IEC, 12 KJ/KV Energy capability
      65 KA Pressure Relief Capability, Grey Silicone Insulator
      ABB type PEXLIM-P
      Complete with top clamps to hold a 336.4 MCM Aluminum Conductor
and 4/0 AWG THW Copper green wire ground conductor connected to ground terminal.

       LV     : 60 KV voltage Rating, 48 KV MCOV, Station Class, Polymer housing, Metal
      Oxide, Line Discharge Class4 per IEC, 12 KJ/KV Energy Capability
      65 KA Pressure relief Capability, Grey Silicone Insulator,
      ABB type PEXLIM-P
      Complete with top clamps to hold a 795 MCM Aluminum Conductors
      and  a 4/0 AWG THW Copper green wire ground conductor connected
      to ground terminal.

       TV     : 18 KV Voltage Rating, 15 KV MCOV Station Class, Polymer housing,
      Metal Oxide, Line Discharge Class 3 per IEC, 9.0 KJ/KV energy capability
      65 KA pressure relief capability, Grey Silicone Insulator,
      ABB type POLIM-S 15N
                   Complete with top clamps to hold a 795 MCM Aluminum Conductors
      and a 4/0 AWG THW Copper green wire ground conductor connected to
      Ground terminal.

44.    Neutral Conductor: 4/0 AWG THW Copper wire colored green connected to
         ground pad.

45.   Insulating Oil – Shell Diala B or equivalent

46.  With provision for Built-in OLTC Insulating Oil Filter Machine, such as mounting
        brackets, connecting flange, connecting valves, etc.

47.  Cooling Fans must be 3 Phase, 460 VAC, 60 Hz, Winding Temperature Controlled
        for Automatic Operation, with automatic/manual change over switch.
       With Circuit breakers for motor overload & short circuit protection.

48.  Grounding Pads for HV Arresters, LV Arresters, TV Arresters & Neutral Cables.

49.  Steel Ladder with caution marking

50.  All External Power & Control cables must be flexible, multicore,  PVC insulated
       & enclosed in conduit pipes & flexible hoses.

51.  All power & control circuits must be protected by circuit breakers.

52. Welded Tank Cover.

53. All wiring connections & terminations must be ANSI standard using crimp type
       terminal lugs with insulator caps.

54.  All wirings must be color-coded.

55.  With replacement gaskets

56.  Anchor Bolts

57. One (1) Spare bushing each for HV, LV & TV

58. One (1) Spare OLTC Tap Position Indicator for remote use

59. With provisions for parallel operation of existing power transformer using
      Master - Follower method of OLTC Control.
       
60. TESTS:
      The following tests shall be carried out at the factory with the presence
      of user representative and records of testing will be submitted.
         a. Winding  Resistance Tests
         b. Turns Ratio Tests
         c. Polarity & Phase Relation Tests on rated voltage
        d. Measurement of no-load losses and excitation current @ 90%, 100%
           & 110% of rated secondary voltage @ rated frequency
           rated voltage connection.
        e. Measurement of impedance voltage and load loss tests at rated current
           and rated frequency.
        f. Low frequency tests (Applied Voltage & Induced Voltage) including
           partial discharge measurement in terms of RIV.
        g. Leak Test.
        h. Routine test certificate for the bushings, current transformers and surge
            arresters shall be submitted.
        i. Temperature Rise Test at OA, FA1 & FA2 ratings on the tapping
          with maximum losses.
       j. Lightning Impulse Test on HV, LV , Neutral & Tertiary terminals.
        (Full wave, Chopped wave & reduced full wave)
      k. Audible Sound level Test at no-load and rated frequency and
         with all fans operating.
            l.  Measurement of zero-sequence impedance.
           m. Insulation Power factor
           n.  Insulation Resistance Tests at ambient temperature.
           o. Vacuum test on transformer tank, conservator & radiators; and pressure
               test on tank and oil-filled compartments.
           p. Determination of Capacitances (windings to ground & between windings)
          q.  Tests on auxiliary equipment & accessories ( functional tests
               including cooling control)
           r.  Voltage regulation
          s. Measurement of the power taken by the fan and oil pumps motors.
          t. Functional tests on Tap Changer
          u. Test on Current Transformers ( Check on polarity, ratio & wiring)
          v. Mechanical inspection, ( check of layout, dimensions, nameplate
             data, clearances, etc)
         w. Oil tests
         x. Efficiency at principal tap and full load for unity & 0.8 power factor.
       
61.  SPECIAL TEST
       Certificate of Short Circuit Test on power transformers of similar rating
       shall be submitted.
   
62.  Other Accessories, Tools

    a. Pressure gauge with nitrogen tube and automatic filling device which
         fill the transformert through the tube in case of any leakage shall
         be supplied.
    b. Three-dimensional impact recorder with time period recording chart of
        at least 3 months for use during transport of the transformers.
    c. Silica -gel breathers for main and OLTC conservators.
 
63.  Painting
      Special attention should be given to the protection of all iron-work.
      The methods propised and the means adopted should be fully described
      in the offer.

     All surfaces shall be thoroughly cleaned of rust, scale, grease and dirt and
    other foreign matters and all imperfections shall be removed by means
    of approved methods.

     The following treatments shall be applied:

     a.  External surfaces

           All steel surfaces shall be sand-blasted in accordance with SIS 055900,
          and shall then be painted in the following sequence:
       
            1. two (2) primer coats:                      2 x 35 um (micrometer)
           
            Binder  : epoxy resin hardened with polyamide              
                  Pigment: titanium dioxide, zinc oxide, zinc phosphate, tinting additives

                  2. one (1) intermediate coat:               35 um
 
                 Binder:  epoxy resin hardened with polyamide
                 Pigment: titanium dioxide, micaceous iron oxide, tinting additives

                 3. one (1) top coat (polyurethane base):   35 um

                 Binder:  epoxy resin hardened with isocyanat
                 Pigment: titanium dioxide, micaceous iron oxide, tinting additives

                 Coating thickness:                   Total        140um

                 The color code shall be Munsell Gray No. N7.0

        b. Internal surfaces
   
                Inside the transformers vessel, sand-blasting shall be performed in
               accordance with SIS 055900. After that solvent-free,
               oil-resistant coating shall be applied.

               The minimum dry film thickness shall be 40 um.

HIGH PERMEABILITY STEEL USED IN TRANSFORMER CORE BASIC INFORMATION

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Use of cold-rolled grain-oriented steel as described above continued with only steady refinement and improvement in the production process until the late 1960s.

However, in 1965 the Japanese Nippon Steel Corporation announced a step-change in the quality of their electrical steel: high-permeability grainoriented silicon steel.

Production is simplified by the elimination of one of the coldrolling stages because of the introduction of around 0.025% of aluminium to the melt and the resulting use of aluminium nitride as a growth inhibitor.

The final product has a better orientation than cold-rolled grain-oriented steel (in this context, generally termed ‘conventional’ steel), with most grains aligned within 3° of the ideal, but the grain size, average 1 cm diameter, was very large compared to the 0.3 mm average diameter of conventional material.

At flux densities of 1.7 T and higher, its permeability was three times higher than that of the best conventional steel, and the stress sensitivity of loss and magnetostriction were lower because of the improved orientation and the presence of a high tensile stress introduced by the so-called stress coating.

The stress coating imparts a tensile stress to the material which helps to reduce eddy-current loss which would otherwise be high in a large-grain material.

The total loss is further offset by some reduction in hysteresis loss due to the improved coating. However, the low losses of high-permeability steels are mainly due to a reduction of 30 40% in hysteresis brought about by the improved grain orientation.

The Nippon Steel Corporation product became commercially available in 1968, and it was later followed by\ high-permeability materials based MnSe plus Sb (Kawasaki Steel, 1973) and Boron (Allegheny Ludlum Steel Corporation, 1975).

OPEN DELTA CONNECTION FOR LIGHT AND POWER BASIC AND TUTORIALS

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Standard Connection
When the secondary circuits are to supply both light and power, the open-delta bank takes this form. In addition to the applications listed above for the open-delta bank for power, this type of bank is used where there is a large single-phase load and only a small three-phase load.


In this case, the two transformers would be of of different kva sizes, the one across which the lighting load is connected being the larger. This is also the connection that should be used when protected transformers are employed in a three-phase bank supplying both light and power.


Simplified Connection
This is similar to the connection above but gives a nonstandard 180° angular displacement. Otherwise the information given above is applicable to this connection.



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