MODELLING AND ANALYSIS OF THE ENHANCED TAPP SCHEME FOR

DISTRIBUTION NETWORKS

Maciej Fila Brunel University/EDF Energy, UK

maciej.fila@brunel.ac.uk

Gareth A. Taylor Brunel Institute of Power Systems

Brunel University, UK

gareth.taylor@brunel.ac.uk

Peter Lang EDF Energy, UK

peter.lang@edfenergy.com

Jonathan Hiscock Fundamentals Ltd

Long Crendon, UK

jhiscock@fundamentalsltd.co.uk

Malcolm R. Irving Brunel Institute of Power Systems

Brunel University, UK

malcolm.irving@brunel.ac.uk

Abstract - At present the main aim for

Distribution Network Operators (DNOs) is to

develop flexible, reliable and efficient networks, in

order to enable the connection of distributed

generation. Active control of distribution networks

is a feasible solution in order to reach that aim. One

of the first and most commonly used active control

devices is an on-load tap changer (OLTC) with its

automatic voltage control (AVC) relay. Even

though this technique for voltage control is well

established, traditional AVC schemes can be

unreliable particularly when the transformer

arrangement is complex and conditions of the

network variable. The main factors that undermine

the performance of AVC schemes are; intermittent

output of distributed generation, varying power

factor, difference in primary voltage or non-

identical paralleled transformers. The Enhanced

TAPP scheme can operate efficiently under the

above conditions. The first objective of this paper is

to present principles of the Enhanced TAPP scheme

and mathematical models for AVC schemes in

general. Then the functionality of this scheme will

be demonstrated using software simulation for a

range of distribution network case studies based

upon realistic EDF Energy network scenarios.

Results from the modelling and analysis of the

Enhanced TAPP scheme and conclusions are also

finally presented.

Keywords: advanced voltage control, parallel

transformer, TAPP scheme, distribution

network, voltage control relay

1 INTRODUCTION

Distribution Network Operators (DNOs) are

obliged to maintain voltage profile across the

networks within certain limits. These restrictions

are due to system constraints such as insulation

stresses but mainly due to statutory voltage limits

defined in the Electricity Safety, Quality and

Continuity Regulations 2002 (ESQC). Each

customer in the UK connected to LV network

needs to be supplied at 400/230 V with the

tolerance +10/-6% whereas HV customers (11kV

and 6.6kV) with the tolerance +/-6%. To satisfy

these requirements on-load tap changers (OLTC)

are used to produce appropriate output voltage

under dynamic load conditions and various supply

voltages as well as to reduce circulating current

when transformers run in parallel. To simplify,

automate and optimise performance of the

OLTC’s, automatic voltage control (AVC)

schemes are used.

Simple AVC arrangement is shown in the figure

1. The AVCs monitor voltage level VVT and the

current ICT on the secondary side of the

transformer. With the provision of these

measurements AVC relay adjusts tap position of

the transformer in order to maintain nominal

target voltage, provide load drop compensation

(LDC) boost and reduce circulating current when

two or more transformers are paralleled.

Figure 1: AVC arrangement.

As the distribution networks become more

complex and accommodate growing amount of

distributed generation (DG), conventional AVC

schemes become inefficient. To cope with the

voltage problems associated with the increasing

penetration of the distributed generation as well as

the rising load demand, distribution network

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 1

operators need more reliable and effective voltage

control devices.

The primary objective of this paper is to

investigate performance of the innovative AVC

relay SuperTAPP n+ and identify appropriate

voltage strategies for the distribution networks

with DG under varying load conditions.

This paper continues and completes the

introductory research study presented by the same

authors in [1], by extending the computer model

and analysis of the Transformer Automatic

Paralleling Package (TAPP) scheme and its

successor the Enhanced TAPP scheme with

distributed generation and load exclusion

functionality.

The paper structure is as follows. Section 2

describes common AVC schemes used in

distribution networks and their characteristics.

Section 3 presents a detailed model of the

Enhanced TAPP scheme and its principles as well

as distributed generation estimation technique and

the load exclusion functionality. Section 4

analyses voltage profile of the network under

various load and DG output conditions and AVC

performance. The results obtained from

simulation are presented. Finally, conclusions are

stated in Section 5.

2 REVIEW OF EXISTING AVC SCHEMES

In order to meet the engineering

recommendation for the security of supply and for

the higher reliability of supply, it is common

practice in distribution networks to parallel

transformers on one site or across the network.

Under such system configurations another aim of

AVC schemes is to keep transformers at a desired

tap position in order to minimise circulating

current and prevent transformers from run away

tapping. Negative Reactance Compounding, True

Circulating Current, Master-Follower and TAPP

are standard voltage control schemes for parallel

transformers. There are several factors affecting

performance of the AVCs such as varying power

factor, presence of the DG (intermittent output),

difference in primary voltage or dissimilar

transformer impedances. Following section

presents advantages and weaknesses of the most

common AVC schemes under varying load

condition and network configuration.

2.1 Negative Reactance Compounding

One of the most common AVC schemes in the

distribution networks is Negative Reactance

Compounding (NRC) technique. NRC was

introduced to distribution networks in the 1960s

due to deficiencies of the Master-Follower

scheme. It uses LDC settings with the negative

value of reactance. The relationship between LDC

settings and NRC setting can be defined as

follows:

LDCLDCLDC jXRZ += (1)

LDCLDCNRC jXRZ −= (2)

The principles of NRC scheme are presented in

figure 2. In this strategy the transformer current is

used to create voltage drop IT·ZNRC and modify

measured voltage from VVT to VAVC. A circulating

current flows between transformers when they are

on different tap positions. This circulating current

rotates the transformer current and consequently

phasor of the voltage drop ITZNRC. When the

transformer is on the higher tap position, the

effective measured voltage VAVC is higher than the

measured voltage VVT and the relay tends to tap

down. When transformer is on the lower tap

position, the voltage VVT seen by AVC is reduced

and as a result tap position is increased. This

action is performed until circulating current is

minimised and the target voltage achieved.

Figure 2: NRC principle.

In this scheme connections between

transformers are not required as each AVC is able

to act independently. This feature makes NRC

scheme suitable for the paralleling transformers

across the network and paralleling non-identical

transformers. However this scheme has also its

disadvantages. The main one being that this

scheme is the most accurate at unity power factor,

and the error in the performance increases with

the power factor deviation. Another disadvantage

is that a compromise between strength of the

compounding and the susceptibility of the scheme

to produce voltage errors for varying power factor

must be found. The third is degradation of the

LDC performance due to the change in the

IT1

VVT IT1ZNRC

VAVCT1

Transformer at a higher tap position

VVT

IT2 VAVCT2

Transformer at a lower tap position

IT2ZNRC

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 2

polarity of the XLDC setting. To keep the same

boost, the value of RLDC must be increased [2].

2.2 Master-Follower

Another AVC scheme used by DNOs is master-follower. One of the parallel transformers

in the scheme monitors the voltage at the bus-bar

and adjusts tap position to provide desirable

voltage level. When master transformer finishes

the action all other transformers in the scheme

replicate it.

The scheme might be used along with LDC and

while settings are adjusted properly, the master-

follower scheme operates correctly under varying

power factor, reverse power flow and with

presence of distributed generation. However,

because connection between relays is required, it

is impractical to use this scheme to parallel

transformers across a network. Complex

switching is needed when one of the transformers

is taken out. Additionally, circulating current will

flow between paralleled transformers using this

scheme unless the transformers are identical, such

that they have the same impedance, number of

taps and incoming voltage [1].

2.3 True Circulating Current

The true circulating current scheme can be used in

order to control voltage at the bus-bar, eliminate

circulating current between transformers and

prevent transformers from run away tapping. In

this scheme interconnection between controllers is

used to deduce transformers currents in order to

calculate circulating current( )

2

21 TT

CIRC

III

−= (3)

For the transformer on the higher tap, ICIRC has a

positive sign, whereas on the lower tap it is

negative. ICIRC is used to create a voltage bias

which adjusts the relay target voltage such that

circulating current is minimised (e.g. transformer

on the higher tap position taps down).

Similar to the master-follower scheme, true

circulating current might be used with LDC and it

operates correctly under varying power factor,

reverse power flow and with presence of

distributed generation. Likewise, this scheme has

disadvantages such as:

- difficulty with paralleling transformers across

the network,

- necessary connection rearrangement while

taking one of the transformers within the

scheme out of service,

- transformers must be similar (impedance,

incoming voltage, connections),

- Inaccurate LDC in the presence of embedded

generator output.

2.4 TAPP scheme:

TAPP scheme is based on the negative

reactance compounding principle [3]. However in

this scheme two separate circuits are used, one for

the purpose of LDC and one for the purpose of

compounding, in order to eliminate the need of

trade-off between strength of the compounding

and the susceptibility to produce voltage errors for

varying power factor. Additionally this scheme

effectively reduces circulating current, which can

flow between parallel transformers, using

numerical techniques based on the target power

factor. Circulating current in the TAPP method is

evaluated by comparing the measured transformer

load current (ITR) with the target power factor

(pftarg) as is shown in figure 3.

One disadvantage of the TAPP scheme is the

incurred voltage error as the load power factor

deviates from the set power factor. This is because

circulating current is considered a part of the load

current. This drawback is eliminated in Enhanced

TAPP scheme [3, 4].

Figure 3: Principles of TAPP scheme.

All the above AVC schemes have a common

drawback associated with the presence of DG.

This is because the voltage and current

measurements fed to the relay are taken locally as

shown in figure 1. Therefore, the AVC schemes

are unable to act appropriately when a voltage

increase occurs, at a remote point of the feeder as

a consequence of the presence of DG.

3 ENHANCED TAPP SCHEME AND

SuperTAPP n+ FUNCTIONALITY

There are several proposed solutions to improve

voltage profile in distribution networks with DG.

Solutions such as network reinforcements, line re-

conducting, building a dedicated line are currently

available and used in distribution networks.

Active voltage control with remote voltage

pftarget

ICIRC

VVT

ITR

β α

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 3

sensing units (i.e. GenAVC), line voltage

regulation, scheduling of distributed generation

and several other techniques are still in

development and trial.

The SuperTAPP n+ relay offers firstly, very

effective AVC performance based on the

Enhanced TAPP algorithm, secondly the

innovative technique for voltage control in the

distribution network with DG. The key benefit of

this scheme is that all measurements are taken

locally and there is no need for remote

communication with the generators.

To ensure effective voltage control at the

substation, to which DG is connected, the AVC

relay must be resistant to the intermittent

character of the DG and varying power factor of

the network. Enhanced TAPP scheme fulfils these

criteria as demonstrated in the [1]. This scheme is

the combination of the TAPP and Circulating

Current methods bringing together their

advantages stated in Section 2. Its principle is

presented in figure 4.

Figure 4: Enhanced TAPP principle.

As shown in figure 4, the circulating current is

calculated using the measured transformer load

current (IT1 for AVCT1 and IT2 for AVCT2

respectively) and summed load current (IT1 + IT2).

The voltage drop on the compounding setting

(ICIRCZT) is used to increase voltage seen by AVC

of the transformer on the higher tap position and

decrease voltage seen by AVC of the transformer

on the lower tap position. Such action is

performed until the voltage level at the bus-bar is

within the bandwidth of the target voltage and the

circulating current is minimised.

Figure 5 shows a system arrangement where

two parallel transformers are controlled by

SuperTAPP n+ relays. The innovative technique

employed in the SuperTAPP n+ relay is the ability

to estimate output of the generator which is

connected at remote point on the feeder. This is

achieved by the additional current measurement

IFG on the feeder with DG and ratio EST which

represents the load share between feeders with

embedded generation to those that do not have

generator [5, 6].

FGTL

FG

STII

I

I

I

generatorswithoutfeedersonload

generatorswithfeedersonloadE

−===

2

1

This ratio is calculated prior to the connection of

generator or when output of the generation is zero,

consequently IG=0 and IFG=I1. Representing sum

transformer currents as follows:

21

1

TT

n

N

TnTL IIII +== ∑=

(4)

the generator current IG can be determined as

follows:

( )( )FGFGTLSTG IIIEI −−⋅= (5)

Knowing the current output of the generator IG,

the voltage rise at the point of connection can be

evaluated and appropriate generator compensation

bias (buck) can be applied to the AVC. The

generator compensation bias is calculated in

reference to the voltage rise at the maximum

generator current IGMAX as shown in equation 6.

GMAX

GGMAXG

I

IVV ⋅= % (6)

This value corresponds to necessary voltage

reduction at the substation in order to bring

voltage level at the point of connection of DG

within statutory limits.

Additional advantage of this method is

improved performance of LDC. In order to

eliminate the error from the LDC performance

caused by DG, the generator current is removed

from the sum transformer currents ITL.

( ) ( ) ( )STFGTLGTLLOAD EIIIII +⋅−=−= 1 (7)

The LDC voltage boost calculation is based on

the true load current ILOAD not on the total

transformer current, as it is in standard AVC

relay.

The generator compensation bias and the

appropriate LDC voltage bias, along with the

circulation current voltage bias and target voltage

are used in the AVC relay to calculate effective

target voltage to control voltage profile in the

system with embedded generation.

Figure 5: SuperTAPP n+ relay arrangement.

ICIRC

VVT

IT1

β αT1

- ICIRC

αT2

IT2

IT1+IT2

ICIRCZT

VAVCT2

VAVCT1

- ICIRCZT

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 4

4 SOFTWARE SIMULATIONS

MODELLING AND RESULTS

4.1 AVC modelling and implementation into

OCEPS

Operation and Control of Electrical Power

Systems (OCEPS) software includes load flow

package. It is based on the Newton-Raphson

algorithm and uses a partitioned-matrix approach

to the Jacobian equations. This algorithm has been

tested on a variety of systems and proved to be a

very robust tool for load flow studies in

distribution and transmission networks [7]. To represent the tap position of the OLTC and

its changes, the model of the transformer with

variable turns-ratio “T” is implemented in the load

flow software. The AVC monitors and compares

actual voltage and target voltage magnitudes. If

the difference between these two values exceeds

the band width settings, then AVC calculates

incremental turns-ratio change “∆T”. The

calculation for ∆T depends on which AVC

scheme is used. The following equation represents

how ∆T is evaluated for the SuperTAPP n+ relay:

CIRCGLDCTARGVT VVVVVT +++−=∆ (8)

where:

VTV - measured voltage at bus-bur

TARGV - relay voltage setting

GV - the generator compensation bias

LDCV - the LDC bias

CIRCV - the circulating current bias

The new turns-ratio is calculated in order to

produce desirable voltage output of the

transformer as follows:

TTT OLDNEW ∆+= . (9)

To reflect the effect of the changes in the turns-

ratio on the currents and voltages in the network

the admittance matrix needs to be recalculated as

follows:

( )

⋅

⋅⋅−

⋅−=

S

P

NEWNEW

NEW

S

P

V

V

YTYT

YTY

I

I2

(10)

The system shown in figure 5 was used to

investigate and analyse functionality of the

SuperTAPP n+. The system consists of two 33/11

kV parallel transformers, both rated 20 MVA with

reactance 12% as well as two feeders and the

generator connected at the remote point on feeder

1. The CT is located on the feeder 1 and the load

ratio of feeder 1 to feeder 2 is set EST=0.1.

4.2 Case 1 - SuperTAPP n+ performance with

LDC

In this case the LDC performance of the

SuperTAPP n+ was investigated. The distribution

network under maximum load condition

experienced unacceptable voltage drop on the

feeder 2. To improve voltage profile the 2% LDC

boost at the 11 kV bus-bar was required. This

strategy was effective until DG was added. When

the generator was producing 3 MW the LDC

effectiveness was significantly degraded and

voltage magnitude at Bus 5 was below lower

statutory limit at 0.931 pu. This voltage profile of

the system is presented in figure 6 by the dashed

line.

1.000

1.036

1.052 1.049

1.000

1.036

0.946

1.000

1.022

1.038 1.035

1.000

1.022

0.931

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

Distance

Vo

ltag

e [

pu

]

SuperTAPPn+

TAPP

Bus 3

Bus 5

Bus 2

Bus 1

Bus 4

Figure 6: SuperTAPP n+ performance with LDC.

In order to amend this undesirable state the

output current of the generator was deducted from

transformers’ load currents and LDC boost based

on the true load current was applied. This

SuperTAPP n+ functionality enable AVC to keep

voltage profile of the distribution network

presented in figure 6 within statutory limits.

4.3 Case 2 - SuperTAPP n+ performance with

Generator Bias and LDC

In the second case the system was under low

load condition. The generator at its maximum

power output of 4 MW caused voltage rise at the

point of connection above upper limit. The AVC

scheme was unable to detect this situation on the

network with voltage magnitude at Bus 3 of 1.066

– dashed line in the figure 7.

1.000

0.980

1.043 1.041

1.000

0.980

0.954

1.0001.005

1.066 1.064

1.0001.005

0.979

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

Distance

Vo

ltag

e [p

u]

SuperTAPPn+

TAPP

Bus 5

Bus 3

Bus 2Bus 1

Bus 4

Figure 7: SuperTAPP n+ performance with

Generator Bias and LDC.

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 5

When SuperTAPP n+ was used with the 2%

generator compensation bias setting at the

maximum output of the generator the AVC was

able to reduce voltage level at 11 kV bus-bar in

order to bring voltage at the Bus 3 within the

limit. The performance of this AVC scheme is

presented in figure 7 by the continuous line.

4.4 Study System

Figure 8 shows the one-line diagram of the

132/11 kV local distribution network. The

substation is equipped with two 30 MVA

transformers with OLTCs. These transformers can

apply voltage variations in the range of +/- 10% in

32 steps and are controlled by AVC relay. Five

units of DG with the total rated capacity of 5 MW

are connected to the distribution network. All

units are represented as the one generator

connected at the Bus 3. The generator is requested

to operate at unity power factor. The distribution

network supplies residential and commercial

customers with a load demand of maximum 50

MVA, minimum 15 MVA and overall power

factor of 0.96. Feeder 1 supplies Load 1 and has

the generator connected along its length. Feeder 2

represents the feeder of the network with the

lowest voltage profile. Load 3 relates to the rest of

the load of the substation.

Figure 8: One-line diagram of 132/11kV

distribution network.

Currently the generator is connected to the

distribution network with an overvoltage

protection relay. When the voltage at the point of

connection increases to an unacceptable level, the

generator is requested to reduce its output. The

generator has experienced several tripping due to

high voltage, especially under low load

conditions.

The OCEPS software was used to analyse

SuperTAPP n+ relay functionality in the above

system. Enhanced TAPP algorithm with the

generator output calculation technique was used

to control AVCs of the transformers. An

additional CT was employed on the feeder with

the generator in order to provide current

measurement to the relays. The load ratio of the

feeder with generator to the feeders which do not

have generators was calculated based on the

historical data and set EST=0.18. The LDC was set

to provide 2% boost at the maximum load, and the

generator compensation bias was set at 2% at the

5 MW generator output.

The simulation was performed under various

load and generator output conditions. The results

are presented in figure 9.

Figure 9: Performance of the SuperTAPP n+ in

the distribution network.

The AVCs maintained the voltage profile of the

system within statutory limits at the substation as

well as at the remote points of the feeders. The

voltage magnitude at the point of connection of

the generator at Bus 3 was kept below 1.06 pu

while voltage level along feeder 2 and at Bus 5

was kept above 0.94 pu.

This voltage improvement prevented

undesirable voltage rise at the Bus 3 and helped to

avoid unnecessary tripping of the generator.

5 CONCLUSIONS AND FURTHER WORK

In this paper the functionality of the innovative

AVC scheme was presented. The performance of

the SuperTAPP n+ relay in realistic distribution

network was demonstrated and analyzed. The

study results of this paper confirm that this AVC

technique can be an accurate method of voltage

control at the substation bus-bar as well as at the

remote points of the feeders in the distribution

network with DG. The main advantage of this

scheme is that all measurements are taken locally.

With the accurate LDC settings, the proper

voltage profile of heavily loaded network with

significant penetration of DG can be maintained.

Also with the appropriate settings of the generator

compensation bias the system where DG causes

unacceptable voltage rise at some point of the

feeder can be controlled.

The numerical results presented in this paper

correspond to the specific system however the

load flow software created to perform it can be

used for variety of distribution network cases. The

software can be used as a universal tool for

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 6

network planning engineers in order to investigate

opportunities to accommodate additional DG and

for the voltage profile improvements using the

SuperTAPP n+ relay.

It is important to note that as long as one feeder

attached to the substation under consideration

remains without DG then it can be used as a

reference feeder the SuperTAPP n+ scheme. In

such circumstances the scheme can then be

applied generally in order to consider networks

with DG present in several feeders. Further work

will demonstrate the application of the

SuperTAPP n+ scheme in such scenarios based

upon real network models with varying load

conditions.

REFERENCES

[1] M. Fila, G.A Taylor, J. Hiscock, “Systematic

modelling and analysis of TAPP voltage

control schemes”, 42nd

International

Universities Power Engineering Conference,

UPEC 2007, pp. 327-334, Brighton, UK, 4-6

September 2007

[2] M. Thomson, “Automatic-voltage-control

relays and embedded generation”, Power

Engineering Journal, No. 14, pp. 71-76, 2000

[3] V.P. Thorney, N.J. Hiscock, “Improved

voltage quality through advances in voltage

control techniques”, IEE Seventh

International Conference on Developments in

Power System Protection, pp. 355-358, 2001

[4] "Technical Specification for Advanced

Voltage Control Relay - SuperTAPP n+”,

Fundamentals Ltd, 2006

[5] J. N. Hiscock, D. J. Goodfellow, “A Voltage

Control Scheme for High Voltage Power

Transformers”, UK Patent Application GB

2417376, 2004

[6] J. N. Hiscock, D.J. Goodfellow, “Voltage

Control of Electrical Networks with

Embedded Generation”, UK Patent

Application GB 2421596, 2004

[7] M.R.Irving, M.J.H.Sterling, “Efficient

Newton-Raphson algorithm for load flow

solution in transmission and distribution

networks”, Proc IEE, C,134,5,1987, pp325-

328

[8] “Automatic Voltage Control” EI 05-1050,

EDF Energy Networks, 2007

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 7