[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.7.9 Control block diagram, paralleling by usual circulating-current method.
3.7.8.3 Negative Reactance Method
The negative reactance method (Harlow, 2002) has a very distinct advantage over all others: it involves
no supplemental apparatus beyond the basic LTC control. It also involves no wiring between controls,
and it is very forgiving of mismatched transformers, including the built-in instrument transformers. It
fell from favor about 50 years ago because of an inherent inaccuracy in its operation. The error, in the
all-important voltage-regulation function, is a function of the power factor of the load. If the load power
factor changes from about 1.0 to 0.7 lag, there will be a maximum error of perhaps ±1.5 V (3 V total
swing) in the regulated voltage. This, coupled with the overall inaccuracy of controls of 50 years past
(the standard of the day allowed up to 5% error!), apparently made for an intolerable condition.
Today’s controls are far superior in accuracy, and the power system typically operates at a much
improved and consistent power factor. These two points combine to afford an order-of-magnitude
improvement in the operating accuracy of the technique. The technique is little known in the industry,
perhaps to the detriment of many who would find applications to benefit from the procedure.
3.7.9 Characteristics Important for LTC Transformer Paralleling
There are many transformer characteristics that must be known and evaluated when it is planned to
parallel LTC transformers. Some of the more notable follow:
1. Impedance and MVA. “Impedance” as a criterion for paralleling is more correctly stated as the
percent impedance referred to a common MVA base. Two transformers of 10% impedance —one
of 10 MVA and the other of 15 MVA — are not the same. Two other transformers, one 10%
impedance and 10 MVA and another of 15% impedance and 15 MVA, are suitably matched per
this criterion. There is no definitive difference in the impedances that will be the limit of acceptability,
but a difference of no greater than 7.5% is realistic.
2. Voltage rating and turns ratio. It may not be essential that the voltage ratings and turns ratios be
identical. If one transformer is 69–13.8 kV and the other 69–12.47 kV, the difference may be
tolerated by recognizing and accepting a fixed-step tap discrepancy, or it may be that the ratios
can be made more nearly the same using the de-energized tap changers.
3. Winding configuration. The winding configuration, as delta–wye, wye–wye, etc., is critical, yet
transformers of different configurations can be paralleled if care is taken to ensure that the phase
shift through the transformers is the same.
4. Instrument transformers. The transformers must have VTs and CTs that produce in-phase signals
of the correct ratio to the control, and they must be measuring the same phase in the different
transformers.
3.7.10 Paralleling Transformers with Mismatched Impedance
Very often it is desired to use two existing transformers in parallel, even when it is recognized that the
impedance mismatch is greater than that recommended for proper operation. This can usually be
accomplished, although some capacity of one transformer will be sacrificed.
If the impedances of the transformers in parallel are not equal, the current divides inversely with the
impedances so that the same voltage appears across both impedances. The impedances — effectively the
transformer impedance as read from the nameplate — can be taken to be wholly reactive. The problem
when dealing with mismatched transformers in parallel is that while the current divides per the impedances,
the control — if operating using the conventional circulating-current method — is attempting to
match the currents.
Realize that the LTC control and the associated paralleling equipment really have no knowledge of the
actual line current. They know only a current on its scaled base, which could represent anything from
100 to 3000 A. The objective of the special considerations to permit the mismatched-impedance transformers
to be paralleled is to supply equal current signals to the controls when the transformers are
carrying load current in inverse proportion to their impedances.
To illustrate, consider the paralleling of two 20-MVA transformers of 9% and 11% impedance, which
is much more than the 7.5% difference criterion stated earlier. We establish that Z 1 = k Z 2 , so:
And since I 1 = I 2 /k:
k = Z 1 /Z 2 = 9/11 = 0.818
I 1 = I 2 /k = 1.222 I 2
Transformer T1 will carry 22.2% more load than transformer T2, even though they are of the same
MVA rating. A resolution is to fool the controls to act as though the current is balanced when, in fact,
it is mismatched by 22%. A solution is found by placing a special ratio auxiliary CT in the control-current
path of T2 that boosts its current by 22%. This will cause both controls to see the same current when,
in fact, T1 is carrying 22% more current. In this way, the controls are fooled into thinking that the load
is balanced when it is actually unbalanced due to the impedance mismatch. Transformer T2 has effectively
been derated by 22% in order to have the percent impedances match.
The circulating-current paralleling described here is commonly used in the U.S. Another basis for
implementing circulating-current paralleling is now available. The new scheme does not require the
breakout of the circulating current and the load current from the total transformer current. Rather, the
procedure is to recognize the apparent power factors as seen by the transformers and then act so as to
make the power factors be equal. The control configuration used is that of Figure 3.7.10, where the phasor
diagram, Figure 3.7.11, shows typical loading for mismatched transformers. The fundamental difference
in this manner of circulating-current paralleling is that the principle involves the equalization of the
apparent power factors as seen by the transformers, i.e., the control acts to make 1 2. The benefit
of this subtle difference is that it is more amenable to use where the transformers exhibit mismatched
impedance.
References
American National Standard for Transformers 230 kV and Below 833/958 through 8333/10417 kVA,
Single-Phase; and 750/862 through 60000/80000/100000 kVA, Three-Phase without Load Tap
Changing; and 3750/4687 through 60000/80000/100000 kVA with Load Tap Changing, ANSI
Standard C57.12.10, National Electrical Manufacturers Association, Rosslyn, VA, 1998.
Beckwith Electric Co., Basic Considerations for the Application of LTC Transformers and Associated
Controls, Application Note 17, Beckwith Electric, Largo, FL, 1995.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC