Tap Changer Controls In the Presence of

Distributed Generation

Lixi Zhang

ECE Department

University of Western Ontario

London, Ontario, Canada

Abstract — This paper discusses the effect of distributed

generation (DG) on conventional load tap changer (LTC) control.

This discussion covers two types of cases: a single LTC with a

reverse power flow control methods, in which several control

options will be compared; and two LTC operating in parallel.

For the latter case, both the reverse power flow control methods

and parallel operating methods will be discussed. Based on the

study of the two cases, an algorism for operating LTC in presence

of DG is presented.

Keywords-load tap changer (LTC); distributed generation

(DG);voltage stbility;reverse power flow control

I. INTRODUCTION (HEADING 1)

A typical load tap changer (LTC) control scheme for step-down transformer only considers the unidirectional power flow, which is from transformer‟s transmission side to distribution side [1]. It usually regulates the voltage in favor of the distribution side. In some emergency cases, such as a voltage collapse scenario in which the transmission side voltage drops to a low level or a maintenance case in which the power flow reverse its direction, a reverse power flow control may be applied to assure the voltage stability.

However, as the increasing interconnection of distributed generation (DG) units into distribution system, other reverse power flow conditions may exist. One scenario is that the generation of DG units exceeds the local load capability.

Another possible scenario is that DG units generate reactive power exceeds the local requirement. Reactive power compensation is a major measure to control the distribution voltage stability.

In this paper, I present emergency control of LTCs focusing on reverse power flow. Following this, a discussion about the reverse power flow control of LTCs in a paralleled-transformer will be presented.

II. CONTROL OPERATION OPTIONS FOR REVERSE POWER

FLOW ON LTCS

The reverse power flow control options discussed in this paper includes:

A. Ignore

The LTC will take the same action as in the normal condition, because it does not use the power direction knowledge in the control operation decisions. The LTC will regulate the original distribution side voltage from the source side.

B. Block

The LTC will cease all control operation as long as the reverse power condition exists and stays in the current tap position. The blocking signal can be initiated from a local sensor which monitors the power flow direction and the transmission side voltage, or from a remote control center. All other features of LTC will continue as normal, and only the automatic control is in block condition.

C. Regulate Point Reverse

The LTC will use the transmission side (source side) voltage (which will turn into „load side‟ in a reverse condition), and use the reverse power flow direction settings to operate the taps correctly control the source side voltage. The process will be initiated with no time delay, and change back to normal operation when a forward power flow condition is restored.

D. Regulate Logic Reverse

The LTC will still use the load side voltage, but reverse the logic of the tap control. This situation usually happens when only the direction of the reactive power changes. It will restore the normal operation when the direction of reactive power changes back.

E. Return to Neutral

The „run to neutral‟ operation mean the LTC will set the tap to neutral. It is included as an alternate operation option for use when different system conditions, which are not locally distinguishable, could cause reverse power flow. It can be a safe measure in some applications.

III. THE IMPACT OF DISTRIBUTED GENERATION ON TAP

CHANGER CONTROL

The impacts of DG on tap changer control mainly focus on two issues: power flow direction and voltage drop.

Considering different types of DG: unity power factor, leading power factor and lag power factor. The unity power factor DG does not exchange reactive power with the system,

while the leading power factor DG absorb reactive power from the system, and the lagging power factor DG generate reactive power to the system. The direction of reactive power will change the regulation direction of LTC.

Consider the one line diagram shown in Fig.1 with DG present in the feeder. The DG does not control its terminal voltage. When the DG causes a voltage rise between bus-3 and bus-4, an overvoltage will happen at bus-4. Hence, the feeder capacitor set point voltage may need to be decreased with the presence of DG, in order to prevent overvoltage at DG buses.

Furthermore, with the LTC keeping U2 nearly constant, the increase of the voltage profile along the feeder due to the DG may cause U3 higher when the feeder capacitor is still expected to generate reactive power to compensate the reactive power demand and minimize losses. Hence, the LTC set point voltage may need to be decreased with the presence of DG, in order not to cause feeder capacitors unnecessarily disconnected when they are still expected to generate reactive power.

Figure 1. DG's impact on voltage drop

∆𝑈 = 𝑈1 − 𝑈2 ≈𝑅𝐿𝑁 𝑃𝐿−𝑃𝐷𝐺 +𝑋𝐿𝑁 𝑄𝐿− ±𝑄𝐷𝐺

𝑈2 (1)

Equation1 indicates that if the DG generates reactive power

or the DG does not exchange reactive power with the grid, the DG will always decrease the voltage drop along the feeder. If the generated power is larger than the feeder load, power will flow from the DG to the substation and causes a voltage rise.

Further, Equation1 indicates that, if the DG absorbs reactive power, the DG can either increase or decrease the voltage drop. This depends on the DG active and reactive power relative to the load active and reactive power and the X/R ratio of the line.

IV. REVERSE POWER FLOW CONTROL ON LTCS

A. Reverse Power Flow without DG Connection

First, consider a scenario shows in Fig. 1. Two radial systems are fed from two separate substations. The “Load Break Switch” (LBS) is normally open and the power direction is left to right in both LTCs. If maintenance or emergency work is required on or near Station A, 52A will open and the LBS will close. Therefore, the power flow reverses in LTC A.

LBS

G 52A

LoadA1 LoadA2

G 52A

LoadB1 LoadB2

Station A

Station B

Reg. A

Reg. B

Figure 2. Radial system

The most desirable operation of the LTC A is „regulate point reverse‟ which will automatically regulate the circuit in the reverse direction. The LTC also needs to recognize when the reverse condition is eliminated and the system is restored to the original configuration so that the LTC can automatically return to normal operation.

Another LTC reverse operation case is related to voltage stability emergency control [2]. It is primarily concerned with the problem of stopping the evolution of an unstable scenario before its conclusion toward a voltage collapse. In this sense, timing is a critical aspect: time to identify the instability and time to apply the emergency control is essential. For this reason many emergency control measures, such as undervoltage load shedding are based on extensive offline computations.

In addition, emergency voltage control aims at providing an acceptable stable operating point of the power system. If the system has avoided voltage collapse, there is usually more time available for restoring a stable operating point, allowing for instance the start-up of back-up generators.

While checking the reverse control operation, the regulate point reverse performs better than other options, which will give the system operators longer time to take emergency measures and restore the voltage stability.

B. Reverse Power Flow with DG Connection

An example of an application of DG on a distribution system is shown in Fig. 2.

G 52A

LoadA1 LoadA2

DG

Station AReg. A

Figure 3. DG interconnection system

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.

Considering the principle of LTC operation, it must providing reactive power (leading) into the system reactance to achieve a voltage increase or consumer reactive power (lagging) through the system impedance to lower the voltage.

Because of the relatively high distribution system X/R ratios, the effectiveness of reactive power is 3 to 5 times higher than that of real power for regulating voltage [1]. Therefore, the only way for the DG to raise or lower the voltage is to generate and transmit large amounts of reactive power.

When the DG is a unity or leading power factor (DG absorbs reactive power from the system) unit, the direction of reactive power will not changed. If the direction of real power changes, LTC still need to regulate the voltage from the source side. Therefore, the most desirable operation of the LTC is to ignore the power reversal and continue to regulate the voltage from the stronger transmission system.

However, when the DG is a lag power factor unit, it will generate reactive power into the system and may exceed the local demand which will result in a reverse reactive power direction. The operational selection becomes more difficult.

This result of the case shows the fact that measurement of power flow only may not be a good indication of which side is most capable of regulating voltages.

On a distribution system with the addition of a DG anywhere beyond the LTC, 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.

The 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 distribution side load, but it also may be blocked in a position which may cause extreme load side voltages.

Since neither of those operations is acceptable, „Run to Neutral‟ may be applied. The taps are dispatched to neutral and the control remains blocked from operation until the power direction reverses again. 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.

C. Comparisonsand Discussion

In a radial system, the reverse power flow control can use „Regulate Point Reverse‟ to change the regulate point from load side to source side.

DG interconnection condition is very difficult to determine the operational options. Only the power flow direction is not enough to make a decision. The main challenge is to determine where should be the regulate point.

When recognize a reverse power flow condition with DG interconnection, the change of system is also important.

V. REVERSE POWER FLOW CONTROL ON PARALLELED LTCS

A. LTC Parallel Operation

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, see Fig. 4.

G 52A

LoadA1 LoadA2

G 52A

LoadB1 LoadB2

Station A

Station B

Reg. A

Reg. B

Figure 4. Paralleled LTC operation

There are three premises for transformers operating in parallel [3-4]:

The transformers must continue their basic function of controlling the load bus voltage as prescribed by the setting on the control.

The transformer must act so as to minimize or control the current that circulates between them, as would be due to the tapchangers operating on inappropriate tap positions.

Actions A. and B., above, must operate correctly in multiple transformer applications regardless of system configuration changes or station breaker operations and resultant station configuration changes.

Generally, if there was some variation that makes one tapchanger operate first, that same variation would cause the same transformer to operate first the next time a voltage adjustment is needed. For these reasons, additional paralleling equipment is required to maintain the taps in the most beneficial position.

Four LTC parallel operation methods are presented in the paper:

The master/follower paralleling method assumes that, under all system operating configurations, the desired operation objectives are met by maintaining the same physical tap position on all paralleled transformers. The operation consists of one active LTC commanding additional transformers‟ tapchangers to follow.

The power factor paralleling method assumes that the most desirable combination of tap positions on paralleled transformers is one that maintains equal power factors in the transformers.

The circulating current paralleling method assumes that a continuous circulating current path is maintained

for all system operating configurations, and that any changes in the circulating current magnitude are a result of an undesirable change in the relative tap positions of the paralleled transformers. The circulating current method biases all paralleled controls to operate next in the direction that minimizes the circulating current. This is similar to the bias of the LDC unit described in the negative reactance method.

The reactive power balancing paralleling method‟s theoretical basis is that paralleled transformers are meant to share the reactive power load (as well as the real power load) of the load bus. Since the real power sharing of the paralleled transformers is determined by the relative transformer and system impedances and not the tap position, real power flow should not be able to affect tap position choice.

It can be realized from Fig. 4 that if the magnitude of the two voltages are different, a circulating reactive power current is established. It is also quite possible, depending on that magnitude difference, to have one transformer with leading power factor and the other with lagging power factor.

On the other hand, if the magnitudes are equal but the voltage angles are different, a real power circulating current is established and the power could actually reverse in one transformer.

B. Reverse Power Control in Paralleled LTCs

Power Factor method of paralleling — if the intersystem flow was reactive power, the power factor method would block the operation of the appropriate tapchanger to attempt to minimize the difference in power factor. This would result in operation at different tap positions for the transformers which would cause equal reactive power flow in the transformers.

If the intersystem flow was real power, the power factor method would block the operation of the appropriate tapchanger to attempt to minimize the difference in power factor. This would result in operation at different tap positions for the transformers which would cause unequal reactive power flow in the transformers. The result is that the transformer with the highest real power load will now be forced to also have the highest reactive power loading.

Circulating current method of paralleling — if the intersystem flow was reactive power, the circulating current method would bias the operation of the tapchangers to attempt to offset the flow. This would result in proper operation at different tap positions for the two transformers and proper sharing of the reactive load from the two sources. That is, the tap difference would equal the voltage level difference thus stopping the flow through reactive power.

If the intersystem flow was real power, the circulating current method would again bias the operation of the tapchangers to attempt to offset the flow. However, the real power flow cannot be corrected with tapchanger operations. The result is fairly unpredictable but does result in circulating reactive powers in one direction and circulating real power in the other. This condition usually results in „hunting‟ between tapchangers.

The reactive power balancing method of paralleling ignores all real power flows; therefore it has only one purpose under all system conditions with the transformers paralleled. That purpose is to equalize the transformer reactive power flows to the substation load of the paralleled transformers.

C. Comparison and Discussion

Since Master/follower method only consider one LTC reverse power flow condition and ignore the rest paralleled LTCs, it will need more information to determine a reverse power flow condition caused by DG.

Both power factor and circulating current methods will be affected by the real power flow in the LTCs. Under a reverse power flow condition, the result becomes undeterminable. These two methods need to be avoided during a reverse power flow condition.

The reactive power flow method only considers the reactive power flow in the LTCs and therefore will eliminate the affection of real power flow. By using this method, it is easy to determine the regulation direction.

D. Proposed Reverse Power Flow Control for LTCs

Based on the discussion above, a proposed LTC control under reverse power flow condition is presented, see Fig. 5.

Start

Calculation

Load-side voltage

Load-side current

Tap position

Line Drop Compensation

Transformer impedance

Parallel operation mode

Reverse power

flow

Transformer protection schemes

Transient disturbance

Source-side

voltage

calculation

Load-side voltage

Load-side current

Tap position

Line Drop Compensation

Transformer impedance

Parallel operation mode

Reverse

regulation

options

Cause of reverse power

Network topology

Location of generators

Parallel operation

Var flow

Passive reverse

regulation

Normal

Operation

BlockReturn to

Neutral

Calculation

and Monitor

Calculation

and Monitor

Power direction

Tap position

Gap increase

Tap Hunting

Tap limits

Capacity limits

Current limits

Voltage limits

Communication

Errors

Tap position

Gap increase

Tap Hunting

Tap limits

Capacity limits

Current limits

Voltage limits

Communication

Monitor

Current limits

Voltage limits

Capacity limits

Communication

Block / Alarm

Errors

Change back

to normal

operation

YES

Regulate

reverseIgnore forward

Forward

power

Change back

to normal

operation

YES

Forward

powerYES Errors

YES

Figure 5. propose rever power flow control for LTCs

VI. CONCLUSIONS

This paper has presented some applications of LTC regulator control operations that have the capability of regulating a transformer under a reverse power flow condition. Several cases are studied and discussed, including traditional radial system, DG interconnection system and paralleled operation LTCs.

The results shows that in order to determine the reverse power flow condition on LTCs, only the direction of power flow is not enough. In the presence of DG, when a system configuration has changed, it will cause a reversal in the desired voltage control direction.

When control the paralleled LTC under a reverse power flow condition, ignoring the real power flow can be an easier way to satisfy the parallel operation requirements. However, the real power flow is still required to determine the regulating point.

A proposed control option for LTC in reverse power flow condition is presented but more details will be considered in the latter work, including the reverse power flow capacity of LTC, the limitation of reverse power flow, and the coordination between LTCs and other voltage stability devises.

REFERENCES

[1] C.W. Taylor, Power System Voltage Stability, Mc Graw Hill, 1994.

[2] T. Gönen, Electric Power Distribution System, McGraw-Hill Book Company, 1986.

[3] “Introduction to Paralleling of LTC Transformers by the Circulating Current Method”, Tapchanger Control Application Note #11, Beckwith Electric Co., Inc., February 1998.

[4] “Advanced Paralleling of LTC Transformers by the Circulating Current Method”, Tapchanger Control Application Note #13, Beckwith Electric Co., Inc., 1999.