PHASE SHIFTING TRANSFORMER SHORT CIRCUIT CHARACTERISTIC

Short Circuit Requirements 

General
PSTs shall comply with the short circuit requirements of IEEE Std C57.12.00-2000, unless otherwise agreed upon by the purchaser and manufacturer.

Transformer categories
The kVA rating to be considered for determining the category should be the equivalent to the rating according to IEEE Std C57.12.00-2000.


Short-circuit current magnitude
The manufacturer shall determine the most onerous conditions for short circuit on every winding or active part in accordance with IEEE Std C57.12.00-2000.

These conditions should take into account the large impedance swings that can occur as the tap position is changed from the extreme positions to the mid position.

Since the system short-circuit levels are critical to the design of PSTs, the user shall specify the maximum system short-circuit fault levels expected throughout the life of the unit.

If a short-circuit test is performed, it shall be done in accordance with IEEE Std C57.12.90-1993.

The test shall be carried out on the tap position that produces the most severe stresses in each winding. This may require more than a single test depending on the type of construction.

For two-core PSTs this usually requires a test on the zero phase-shift position, as this position involves only the series transformer, and a second test on a position to be agreed upon between customer and manufacturer.

PHASE SHIFTING TRANSFORMER CONSTRUCTION

In general, construction requirements for PSTs should be in accordance with the requirements for power transformers, as covered in IEEE Std C57.12.00-2000 and other applicable IEEE standards based on kV and kVA ratings, with the following exceptions or additions.


Enclosed throat connections
Enclosed throat connections in fully assembled condition must meet the pressure and vacuum requirements of PST tanks for all designs that subject the enclosed throat connection to the same operating pressures and vacuum levels as the transformer tank.

Liquid insulation and preservation system 
Liquid insulation and preservation systems shall be in accordance with IEEE Std C57.12.00-2000 with the following addition.

Two tank designs with enclosed liquid-filled throat connection
Enclosed liquid-filled throat connections may be either sealed from each tank or opened to the insulating fluid from one or both tanks. Enclosed throat connections shall be designed for installation or removal without the need to jack or move either or both of the transformer tanks and shall accommodate thermal expansion and contraction of the throat assembly and both tanks. For a sealed throat system that isolates the insulating fluid, the throat connections require a separate conservator system.

For a system where the throats are not directly connected to a main tank and the isolation of the insulating fluid in different compartments is not important, the throats may be connected to the conservator system of the main tank. If this approach is used, the user should be aware that the use of oil and gas analysis to isolate problems will be complicated.

For throat connections that place barriers between both the tanks and the throat, the throat shall be equipped with the following accessories:

— Gas accumulation relay
— Pressure relief device/relay
— Liquid filling and draining valves
— Rapid rate of rise relay
— Liquid level gauge

PHASE SHIFTING TRANSFORMER RATING DATA REQUIREMENT BASIC INFORMATION

In general, rating data for PSTs should be in accordance with the requirements for power transformers as covered in IEEE Std C57.12.00-2000 with the following exceptions or additions.


Polarity, angular displacement, and terminal markings
Terminal markings unique to PSTs
The designations H and X shall not be used and shall be replaced by S and L to indicate the source and load. The S terminals shall be marked S1, S2, S3, and (if applicable) S0. The L terminals shall be marked L1, L2, L3, and (if applicable) L0. Y and Z designations shall be used for additional windings that are brought out of the tank.

Enclosed throat connection terminal markings
Enclosed throat winding terminal connections shall be marked in any manner that will permit convenient reference and cannot be confused with the markings of the external transformer terminals.

Impedance
Impedance shall be in accordance with IEEE Std C57.12.00-2000 with the following additions.

General
Rated impedance shall be at zero phase-shift connections.

Change in impedance with phase-angle regulation
The impedance of PSTs can vary substantially over its range of phase-angle regulation. The user must specify the acceptable ranges of impedances and the manufacturer shall calculate and provide a matrix of impedances as required by the user. The extent of test verification of impedance values other than rated impedance should be specified and agreed upon by the purchaser and manufacturer.

Nameplates
Nameplates shall be in accordance with IEEE Std C57.12.00-2000 with the following addition: The nameplate of the PST shall show the phase shift in degrees from the S to the L terminals starting at the zero phase-shift tap and for each tap position in the advance and retard direction while operating at no-load.

The nameplate shall also show the phase shift in degrees from the S to the L terminals while operating at maximum rated kVA output at unity power factor at the S terminal for all tap positions which result in acceptable service conditions.

Intermediary phase shifts at varying loads may be specified by the purchaser for inclusion on the nameplate. The user may request impedance changes be indicated on the nameplate for any tap position.

PHASE SHIFTING TRANSFORMER UNUSUAL SERVICE CONDITIONS

The unusual conditions shall be the same as those listed in IEEE Std C57.12.00-2000, 4.3.1 through 4.3.3. Additional unusual service conditions that may apply to PSTs are as follows:

Operation with two or more PSTs in parallel or in series
The purchaser shall ensure the manufacturer has all nameplate data, test data, and applicable system information necessary to design the PSTs for proper load sharing. The purchaser must specify in detail to the manufacturer the LTC’s controls that will be provided by the purchaser.

If the manufacturer provides the LTC’s controls, the purchaser shall provide the control scheme used with any existing PSTs to the manufacturer to ensure a compatible system.

Operation of PSTs in series with series capacitor banks
If the PST is, or may be, operated in series with a series capacitor bank, this operating condition shall be pointed out to the manufacturer by the purchaser. The operating conditions shall be specified and the protection scheme used by the purchaser to prevent series resonance shall be provided to the PST manufacturer for review and for considerations in design.

Unbalanced current flow through the PST
The purchaser must provide details of operating conditions that will subject the PSTs to unbalanced phase currents and voltages that may exceed allowable standard limits. The manufacturer will provide for these conditions during the design of the PSTs. The following are examples of operating conditions that could produce such problems:

a) Unbalances resulting from operation of parallel transmission lines in close proximity to the PST connected lines, where line transpositions are unequal resulting in unbalanced voltage at the PSTs and unequal current flow through the series windings.

b) Single-pole operation of the circuit breakers following line faults where single-pole reclosing is utilized


Transient recovery voltages
Transient voltage may exist circuit breakers are operated. These conditions may be between the PST and the circuit breaker.

Surge protection
Any condition where the PST may operate without surge protection applied at all S and L terminals.

USUAL SERVICE CONDITION OF PHASE SHIFTING TRANSFORMER

Usual service conditions
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.

Loading at other than rated conditions
This subclause shall be the same as IEEE Std C57.12.00-2000, 4.2, with the exception that additional limits must be observed for retard operation under overload. These limits must be defined by the manufacturer and agreed upon by the purchaser prior to completion of the PST design.

PHASE SHIFTING TRANSFORMER DESIGN CRITERIA

Phase angle
The rated phase angle is defined under no-load conditions. However, it should be noted that the unit is unlikely to operate at this phase angle under load in the advanced position due to the effect of the voltage drop in the unit.

In the retard position the no-load phase angle should not be exceeded (unless the unit has been designed for that), as overexcitation will occur in parts of the PST. In the retard position the power that can be transferred is usually lower than the rated power in the advanced position.


Dielectric design of the two-core type
The transmission of transient voltages in the two-core design is rather complex. When applying impulse tests to either the S or the L terminals of the series transformer, the connected exciting winding of the main transformer will also be exposed to a high voltage.

There may be high-voltage oscillations of the connecting leads, depending on the capacitive voltage control of the series winding. High voltages may be transferred to other windings coupled to the series winding or to the excitation winding. Therefore, rather complex computer models may be required to compute the transient voltages for this configuration.

Special considerations for a two-tank design
When the two-core design is used with two tanks, special precautions must be taken to design connections between the two tanks. The connection operates at the system voltage level so that the leads must be insulated for the overvoltages that may occur under both transients and power frequency conditions.

A short-circuit between the connections of the two units has to be considered as an internal fault, which would cause severe damage or even destroy the PST. A short-circuit proof design for this special case would result, if possible at all, in a significant increase in cost. Therefore, it is strongly recommended to use metal enclosures to protect the connections against lightning strikes and other possible sources of a short circuit.

Overload conditions (loading above nameplate rating)
Overloading of a PST in the sense of operating it with a current beyond the name-plate rating increases the internal phase angle β [see Equation (2)] and consequently also the load phase-shift angle α∗ (r) in the retard position.

This may result in a load phase angle that exceeds the maximum rated no-load phase angle. The voltage across the regulating winding and consequently also the voltage per step of a single-core type, as well as the voltage across the series winding of a two-core type will, in this case, exceed the rated voltage.

Furthermore, in a two-core design, the main transformer also will experience a certain degree of overexcitation with the same consequences for the regulating winding. The degree depends on the ratio of the impedances of series and main transformer.

It must—beside the effect that parts of the core(s) may be overfluxed—therefore also be checked whether the parameters’ voltage per step, current, and switching capability are still within the limits of the LTC design.

TESTING OF PHASE SHIFTING TRANSFORMERS BASIC AND TUTORIALS

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

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.