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.

FERRORESONANCE IN DISTRIBUTION TRANSFORMERS BASIC INFORMATION AND TUTORIALS

Ferroresonance is the name given to the phenomenon where the exciting reactance of the transformer can become nearly equal to the capacitive reactance of the line to ground, forming a resonant circuit. Such a resonant circuit can distort the normal line impedance to ground so that one line of a 3-phase circuit can rise to a destructive voltage.


Distribution transformers are generally considered as transformers of 500 kVA, and smaller 67,000 V and below, both single-phase and 3-phase. Older installations are primarily pole-/platform-mounted units. Newer installations are frequently pad-mounted units.

Typical applications are for supplying power to farms, residences, public buildings or stores, workshops, and shopping centers. Distribution transformers have been standardized as to high- and low-voltage ratings, taps, type of bushings, size and type of terminals, mounting arrangements, nameplates, accessories, and a number of mechanical features, so that a good degree of interchangeability results for transformers in a certain kVA range of a given voltage rating. They are now normally designed for 65 C rise.


Such a ferroresonance practically never occurs in a normal circuit configuration with the transformers loaded, but it can exist under a combination of the following circumstances which usually occur only during switching of a 3-phase bank or blowing of a fuse in one line:


1. System neutral grounded, ungrounded transformer neutral

2. No load on the transformer

3. Relatively large capacitance line-to-ground such as may exist in cable circuits (underground distribution) or very long overhead lines (although ferroresonance can be and has been corrected by adding still more capacitance which presumably throws the combination out of resonance again)

Although ferroresonance has been studied at some length, it still does not seem possible to reliably predict its occurrence. Experience indicates that it is possible to prevent ferroresonance during switching on a transformer bank if all three transformers are resistance-loaded to 15% or more of their rating, or if special switches are used to assure that the three lines close simultaneously.