MAXIMIZING AUTOMATIC REVERSE POWER OPERATIONS
WITH LTC TRANSFORMERS & REGULATORS
E. Tom Jauch, Beckwith Electric Company, Inc.
Advanced LTC (Load Tap Changer) transformer and regulator controls automatically detect a
power flow reversal and allow several options of operation when power reversal is detected.
However, damaging system conditions can occur if there is an error in detecting proper
regulation direction or in the control action selected for the condition. These conditions occur
when the control action 1) blocks operation at a time tapchange action is required, 2) results in
running the tap position to tap position limits, 3) requires actions from other system components
of which they are not capable or 4) causes “hunting” of tap position.
This paper discusses six system operating conditions including radial feeds, distributed
generation (DG), radial feed/DG combination, transmission tie-transformers, networks, and
distribution substation paralleled transformers. Included also in the discussion are the desired
control direction detection methods, source-side voltage determination methods, selectable
operational control features and the consequences of improper operational choices for each.
Further, this paper discusses applications which could benefit from additional methods of
determining proper regulation direction.
Although other variations may exist, the selectable control operation choices discussed for
reverse power are:
• Ignore: The control will take the same action as in the forward direction, because it
does not use the power direction in the control decisions. This is the same as a control
which does not have the power direction knowledge.
• Block: The control will cease all operation as a controller as long as the power is in
the reverse direction and stays in the present tap position. Data recording and all nonregulating features continue as normal. Essentially, the control is in block auto condition.
• Regulate in Reverse: The control will use the source-side voltage (load-side in
reverse operation), use the reverse direction settings allowed and operate the taps
correctly to control the source-side (now load-side) voltage. It will begin this regulation
with no time delay for the initial operations. (“Regulate in Reverse” is only applicable for
single phase regulator applications.)
• Run to Neutral: The “run to neutral” operation is included as an alternate operation
option for use when different system conditions, which are not locally distinguishable,
could cause reverse power flow.
System Operating Conditions Causing Reverse Power Flow
Radial Feed (Figure 1)
The typical reverse direction regulator application is the alternate feed circuit shown in this
diagram. The “Load Break Switch” (LBS) is normally open and the power direction is left to
right in both regulators shown. (
) in Figure 1.
If maintenance or emergency work is required on or near Station A, 52-A could be opened
and the LBS closed. The power flow reverses in Reg A which, in this application, is a
positive indication of the circuit conditions.
Figure 1
Figure 1 Radial Feed System Example
The most desirable operation of the regulator affected is to automatically regulate the circuit
in the opposite direction, which is usually the source side. The control further needs to
recognize when the maintenance work is complete and the system is restored to the original
configuration so that the regulator can automatically return to normal operation. No
personnel visit would be required to the regulator site before or after the switching is
completed.
For proper operation, as described above, the control must be able to reliably determine 1)
the direction of the power flow with no effects of power factor, 2) the tap position for
impedance and voltage calculations and 3) the magnitude of the “source” side voltage for
regulation.
1) Typically, the quantity detected for directional comparison is the power flow. One
method used includes a “hysteresis effect” so that one power level point does not define the
direction, which could cause instability and confusion. This “hysteresis effect” is established
by requiring a power current level of 2% or 4 ma in the direction of power flow change.
When the power flow returns to normal, it again requires 2% or 4 ma of power flow in the
normally forward direction. The decision points for the power direction decision are then 4%
or 8 ma apart. The installation of a line regulator near the LBS could result in an extremely
low power flow in one direction when the LBS is open. To accommodate this application, a
bias is used to move the appropriate decision power current level to 0ma rather than 2% or 4
ma. This reduces the “hysteresis effect” and the decision points for the power direction
decision are then 2% or 4 ma apart.
2) The tap position information can be directly indicated to the control with external
equipment, such as a Selsyn position indicator, or by using a “keep track” logic to monitor
and compute the present tap position. A reliable “keep track” logic must be capable of
operating on both new and older regulators of different manufacturers and those with
somewhat “sloppy” counter contact mechanisms.
One “keep track” method of tap position knowledge is incorporated into controls for single
phase regulators with a range of +/- 16 taps with a central neutral position. For complete
reliability, the motor power source for manual, automatic and external (SCADA) initiated tap
changes must be the same as the motor power input to the control and with operational
counter and neutral position contacts.
This method allows the user to define the counter contacts as open-close-open or openclose/close-open as used by different manufacturers. In cases of less secure contact operation,
a counter time window can be designated to assure single counts for single tap operations.
The internal algorithms of this “keep track” method maintain a uninterrupted knowledge of
the present tap position.
3) The “source side” voltage can be monitored by an additional potential transformer or
regulator winding tap indicating the voltage to be regulated. It can also be calculated from
the normal regulated voltage input and the tap position (ratio) information and regulator
impedances. The control also uses the information on Type A or B regulators and typical
regulator impedances in the determination of source voltages.
The choice of this selected operational option is generally named “Reverse Regulation.”
Note: In this application, if the regulation direction is chosen incorrectly, the regulator will
be run to its tap limits in its effort to regulate the true source voltage. This can, of course,
result in damagingly high or low distribution system and customer voltages.
Distributed Generation (DG) (Figure 2)
An example of an application of DG on a
distribution system is shown in Figure 2.
As the distribution loads reduce at night
and the DG keeps generating, the power
flow on a feeder regulator can reduce and
ultimately reverse at the regulator location.
DG
Figure 2 Distributed Generation System Example
Let’s begin with an analysis of the capabilities of the equipment on the circuit. If the
regulator does not regulate the voltage towards the DG installation, how could the DG
regulate the voltage? It must “push” VArs (leading) into the system reactance to achieve a
voltage increase or “pull” VArs (lagging) through the system impedance to lower the voltage.
Because of the relatively high distribution system X/R ratios, the effectiveness of VArs is 3
to 5 times higher than for Watts for regulating voltage.
Therefore, the only way for the DG to raise or lower the voltage is to generate and transmit
large amounts of VArs. Usually three items prohibit this action: 1) the inability of the DG to
generate those amounts of VArs, 2) the contractual obligations enforced by many utilities
that the DG only effect KWs on the system and 3) the heavy financial burden of using DG to
regulate distribution voltages.
The most desirable operation of the regulator, in this application, is to ignore the power
reversal and continue to regulate the voltage from the stronger utility system. The decision
of how to regulate the voltage may rely on the relative “driving point impedance” (DPI) to
the individual sources. When one direction has a substantially lower DPI, as in this example,
it should be the direction regulating the voltage.
This application illustrates the fact that measurement of power flow only may not be a good
indication of which generation is most capable of regulating voltages. In intertie or DG
applications, the most capable of regulating the voltage is the source with the lowest driving
point impedance (highest fault duty or capability to ship VArs.) Another example of this
phenomena is the application of transmission tie transformers with comparable low
impedances from both sides. The changing of taps does more to adjust system VAr flow
than to adjust system voltages. This will be discussed in more detail later.
The choice of this selected operational option is generally named “Ignore.”
Note: In this application, if the regulation direction is chosen incorrectly, the regulator will
be run to its tap limits in its effort to regulated the true source side voltage. This can of
course, result in damagingly high or low distribution system and customer voltages. It is also
possible for chosen “direction” to be unstable due to load variation near the point of zero
regulator power current.
Radial Feed & DG Combination
The combination of the two previous applications creates this third category. On a
distribution system which is shown in Figure 1 with the addition of a DG anywhere beyond
the regulator, the problem of operational selection becomes more difficult.
The problem stems from the fact that the system condition causing the reverse power is
indeterminable at the regulator site. Therefore, if it is set for “Reverse Regulation” for the
radial situation and the DG is the actual condition, the regulation direction is chosen
incorrectly. If, on the other hand, it is set for ignore for the DG case and it is in a radial feed
condition, the regulation direction is chosen incorrectly.
A choice of “Block” for reverse power operation would result in the regulator locking onto
its present position. Not only would no regulation be effective for the downline load, but it
also may be blocked in a position causing extreme load-side voltages.
Since neither of those operations is acceptable, an additional choice is offered. The choice of
this selected operational option is generally named “Run to Neutral.” The taps are dispatched
to neutral and the control remains blocked from operation until the power direction reverses
again. The control MUST have tap position knowledge to perform this action. There are no
system conditions where this is the “ultimate” setting; however, when contradictory
conditions may exist, it is a compromise setting without extreme consequences.
Transmission-Tie Transformer Applications (Figure 3)
On an electric power network, there are numerous connections (transformers) between
different levels of transmission and sub-transmission voltages. These connections may be
electrically near each other or far apart. The effect of tap changes on those tie transformers
can be quite varied, depending on the amount of active generation or loading on one side
versus the other. These effects also are a function of the driving point impedances (DPI)
directionally from each side relative to the transformer impedances themselves.
Figure 3 Transmission-Tie Transformer Example
Figure 3 is an extremely simplified diagram for a transmission-tie transformer application. It
ignores the many lines from each bus to multiple other busses, the numerous power sources
and generators present on the system and switching stations located on the both voltage
systems as well as other parallel paths for power flow. By its simplicity, it highlights the
complexities of the overall power system and the many factors needed to be included in
analysis including the “automatic” changes that occur during operation.
The analysis of a few system conditions with LTC transformer tap operations will be
informative.
Definitions:
DPI(1) is the driving point impedance at the transformer bus T1 towards Station A.
DPI(2) is the driving point impedance at the transformer bus T2 towards Station B.
X is the transformer impedance on the same base. Taps are 1% voltage change each.
Designated regulated voltage is VT2. – Designated source voltage is VT1.
Initial condition: LTC neutral tap position– Voltages are each 100% - 0 VAr flow (100% PF)
Case 1:
DPI(1) = DPI(2) = 2%; X =10%
Action:
LTC changes 1 tap (1%) to raise VT2
Effects:
∆I(VArs) = V/Z = 1%/(2%+10%+2%) = 1/14 = 0.0714 PU amps
VT2 = 100% + (0.071 X 2%) = 100.143
VT1 = 100% - (0.071 X 2%) = 99.857
Turns ratio change = 1%; Voltage drop in X = 0.0714 X 10% = 0.714%
Result:
The 1% turns ratio change (tapchange) raised the VT2 by only 0.143 %.
A “side effect” lowered VT1 by 0.143%.
The internal transformer voltage drop negated 71.4% of the turns ratio change.
The tapchange was more effective for VAr control than voltage control.
Case 2:
DPI(1) = 2%; DPI(2) = infinite (no generation) X =10%
Action:
LTC changes 1 tap (1%) to raise VT2
Effects:
∆I(VArs) = V/Z = 1%/(2%+10%+infinite) = 0.0 PU amps
VT2 = 100% + 1% = 101%
VT1 = 100% - (0 X 2%) = 100%
VAr flow does not change except for possible load increase
Result:
The 1% turns ratio change (tapchange) raised the VT2 by 1%.
Case 3:
DPI(1) = infinite (no generation); DPI(2) = 2%; X =10%
Action:
LTC changes 1 tap (1%) to raise VT2
Effects:
∆I(VArs) = V/Z = 1%/(infinite+10%+2%) = 0.0 PU amps
VT2 = 100% + (0 X 2%) = 100.00
VT1 = 100% - 1% = 99.0%
Result:
The 1% turns ratio change (tapchange) had no effect on VT2 (the only source).
A “side effect” lowered VT1 by 1%.
A voltage control to regulate VT2 would continuously repeat the attempt.
The analysis of these extreme cases—any of which could automatically occur in varying
degrees due to system switching—introduces the possibility that the direction of power flow
may not be an appropriate condition to determine voltage regulation direction in all
applications. In some cases, it may be more conclusive to compare the voltage changes on
both sides of the regulating devices in response to a tapchange. In other cases, the magnitude
of the VAr flow change may be more significant.
Network Applications (Figure 4)
Network applications imply different sources—either multiple paths from a single source or
interconnected distribution from separate sources.
52A
LA1
52A
LA1
52A
LA1
52A
LA1
52A
LA1
Figure 4 Network Example
With the use of regulators in a network situation, extreme care must be taken to assure
enough impedance is between the “paralleled” regulators to limit the off-tap circulating
current that can be established. These currents can destroy the regulators. Usually regulators
are not initially used in this manner but when distribution automation (DA) methods are
employed, possible resultant systems may create similar conditions.
LTC transformers used in this manner can also be considered “in parallel.” Because of the
relatively high impedances of transformers, they are not subject to damaging circulating
currents as are regulators. Some in-field tests are being planned to assist in determining the
criteria for the need for paralleling equipment in these applications. The proper operation in
this application may be a combination of paralleling methods and reverse power operation
settings. The need for paralleling logic in an LTC control scheme depends on the impedance
between the units. When paralleling logic is used, care must be taken so that sensitivities to
off-tap positions are not too high and cause hunting of the taps.
If reverse power logic is used on these transformers, the usual setting is to block rather than
to ignore, regulate in reverse or run-to-neutral since these are usually unexpected occurrences.
Distribution Substation Paralleled Transformers (Figure 5)
For reliability considerations, many utilities are paralleling transformers to a distribution bus
that are fed from different lines. That application creates the need for some special analysis.
Although advanced paralleling techniques are required, beyond the normal circulating
current methods, they will not be addressed in this paper.
Line or system A
Line or system B
Figure 5 Distribution Substation Paralleled Transformer Example
Notice the similarity of this application to the system or intertie application of Figure 3.
Instead of one tie transformer, two back-to-back transformers are acting similarly. Even if
the two systems or lines are the same system voltage, which they usually are, the effects of
tap changes will be similar to those of Figure 3 conditions.
If the magnitude of the two voltages are different, a circulating VAr current is established
because most of the impedance of the combined systems and transformers is reactive. It is
quite possible, depending on that magnitude difference, to have one transformer with leading
power factor and the other with lagging power factor. Proper paralleling equipment can
prevent this unbalanced loading condition.
On the other hand, if the magnitudes are equal but the voltage angles are different, a KW
circulating current is established and the power could actually reverse in one transformer.
Comparing the cases illustrated for Figure 3, we see that automatic equipment operations
could cause the same problems. These include removing the source from one side or the
other or substantially changing the DPIs of either system.
To understand the potential operations and problems of this application and the reverse
power or other settings required for controls, knowledge of both the system operation and the
control functions is required.
Conclusion
This paper has illustrated some applications of advanced LTC transformer and regulator
controls that have the capability of a selected response to changes in power flow direction.
This has included the consequences of an error in detecting proper regulation direction
correctly or in the control action selected for the condition.
It is apparent that when system configurations can change, causing a reversal in the desired
voltage control direction, care must be taken to investigate the effects of any preset strategy
of operation. Suggestions have been made, that for some applications, criteria other than
power flow may be better suited for the decision of voltage control direction. It is imperative
that if system conditions can occur which change the voltage control direction, that they be
recognized and included in the overall operating strategy.
Discussions include the fact that the final result of a tapchange for voltage control is a
function of the driving point impedances of the systems connected. At times, the effect on
system VAr flows may be much greater than on voltage levels. In some applications,
unintended voltage reductions may be experienced as a result of a tapchange in another
location on the system. It is possible that operations at one point on the system may negate
operations at another location. This could result in “hunting” for an appropriate solution to
interconnected problems.
Although it is beyond the scope of this paper to discuss, there is a method of combining
paralleling algorithms and voltage controlling algorithms to create a VAr-biased voltage
operation beneficial for some intertie or generator bus transformer applications.