ABSTRACT: Utilities are obliged to maintain the voltage within the declared voltage variation to prevent harmful effect on the connected equipments. An important aspect of supply quality is the correct application of voltage levels to all transmission and distribution network. Responsibility upon the supplier of electrical energy to ensure that the delivered level and quality of supply is always within the parameters set down by regulatory bodies. Different methods can be used to control the voltage.
This article shows the successful voltage control on SEC-COA network using reverse reactance method and the improvement on the design against the other methods.
1. INTRODUCTION
It becomes obligatory on the part of electrical undertakings to contain the variations of the voltages within the declared limits to prevent harmful effect on consumers equipment. For security purposes, multiple transformers are installed at transmission substations. Where
OLTC"s are used for voltage control, circulating currents will flow between transformers if the on load terminal voltages are not maintained to the same level at all times. For this reason it is normal voltage regulating scheme. One of these methods known as a reverse reactance scheme with on load tap changing system which is the subject of this article. The paper currently identifies the problems with most traditional voltage control systems and recognizes the advantages of the
Negative Reactive type system for the minimization of circulating currents between parallel transformers. The paper also points out a possible disadvantage with the use of negative reactance voltage control when the power factor is lower than unity.
2. ON LOAD TAP CHANGING SYSTEM
Automatic voltage control of the electrical network is implemented by use of voltage sensing practice for transformers in service at a location to have the same rating, the same number of tapping positions , tap interval and the same tap change control system However, voltage tend to vary as the demand varies in the network, such that, it decreases when the load increases and increases when the load decreases.
For the above reason techniques were developed to automatically keep the voltage within the desirable value using a different methods of
2.1 Regulated Relay
Tap changers are controlled automatically by the regulated relay which is normally supplied from a
P.T. in the case of Master Follower Method and relays which control motorized On Load Tap
Changers . Tap changers alter the transformer ratio in a number of steps, each step usually represent a change in voltage of the order of
1.5%. case of the Reverse Reactance Method and
Circulating Current Method. These CTs & PTs normally are located on the regulated bus. See
Fig. 1a and Fig. 1b.
Simultaneous Method; and from a PT & CT in

Regulating Relay
Fig.1a Regulating Bus
2.2 Time Setting In Regulating Relay
that only last for few seconds in the network.
Time delay setting are normally two types: t
Fig.1b Regulating Bus
1. Inverse Time Lag Characteristics, See Fig. 2a.
2. Independent Time Lag Characteristics, See
Fig. 2b. t
Regulating Relay
Fig. 2a Inverse time lag characteristic
Each of the above two time delay characteristics
X

V
Fig.2b Independent time lag characteristic has its merits. The inverse time lag characteristics is used when it is required to have a faster regulation during greater deviation in voltage.
The independent time lag characteristic is used when the next following transformer has a tap changer. Under such conditions regulating the upstream station is sufficient to restore the voltage to normal and the operation of the down stream station will not be necessary, hence time grading will be useful in this case and inverse time lag will not be a suitable setting. See Fig. No. 3.
Down stream Station
132kV
Relay
33kV
2.3 Blocking Unit in Regulating Relay
Fig. No 3
Regulating relays are also supplied with an auxiliary blocking unit which block the regulating pulse to the operating mechanism of the tap changer when the power transformer current or voltage exceeds a preset value which normally indicates a system fault rather than a normal loading condition.
2.4 Compensating Unit in Regulating Relay
In case where the voltage needs to be maintained at a remote point on the line a compensating unit will be needed to represent the line parameters and the voltage drop on the line. A compensation of line current and line parameters will be simulated and when compensated on the relay
Relay
13.8kV point the voltage can be maintained at a constant level irrespective of any changes in the load.
3. JOINT CONTROL OF PARALLEL
TRANSFORMER
The voltage control is further complicated by the need to operate transformers in parallel to meet the load demand and the security standard or both in the utility. Accordingly, it is not only the declared voltage to be maintained but also the parallel transformers has to be operating on the same tap or within not more than one tap difference to ensure proper load sharing between them.
Different methods are used for the joint control of tap changers of parallel transformers. The

selection of the suitable method depends on the prevailing system configuration. These methods are:
1. The Master Follower Method.
2. The Simultaneous Method.
3. The Reverse Reactance Method.
4. The Circulating Current Method.
3.1 Master Follower and Simultaneous Method
The master follower and the simultaneous methods use one regulating relay of the parallel units to control all the tap changers, the rest of the relays do not participate in the function. For this reason, each relay is provided with a selector switch with three positions controlling, controlled
3.2 Reverse Reactance and Circulating
Current Schemes
The reverse reactance and the circulating current methods, each tap changer is controlled by its own regulating relay.
The beauty of these two methods is that they sense the circulating current between the parallel units which indicates a difference in tap positions and
It requires a complicated switching arrangement between the breakers positions of both the parallel units and their bus section breaker for which these breakers have to have spare auxiliary contacts.
This means that the reliability of the method is
N
and individual control. These methods require a number of auxiliary contacts on each set of tap changer gear and also needs connections between the respective groups of these contacts on the different set of equipment. With the simultaneous method the tap changer receives a starting pulse simultaneously from the common relay while in the master follower a starting pulse is given to the first tap changer end when this has switched, a starting pulse is given to the next tap changer and so on.
These two methods do not cater for a stuck tap changer, hence, it is possible to have great tap differences between the parallel units unless such condition are prevented, for this reason they are less favorable nowadays use this circulating current to correct the situation.
This is of course in addition to maintaining the required bus voltage and requires the parallel units to be adjacent, more equipment and wiring between the units to get the actual circulating current. See Fig. 4. affected by these external factors in addition to its cost involvement.
The reverse reactance method is less complicated and proven to be satisfactorily operating in SEC-
Central system where no inter-wiring or auxiliary contacts are needed. See Fig. 5.

The drawback in the reverse reactance method when the power factor is lower than unity and the only way of application is the estimation of the power factor of the load, however, this is not
4. APPLICATION OF REVERSE
REACTANCE METHOD :
4.1 Definitions
V g,
V y
, V
B
= Line to neutral voltage of the controlled bus.
IL = Load current.
T
Ic
V2
= Tap step.
= Circulating current.
= Upper operate voltage (the voltage at which the lower relay picks up during an increase in voltage).
V1 = Lower operate voltage (the voltage at which the raise relay picks up during a decrease in voltage).
A = Range of insensitivity = V2 -
V1.
Vs = Regulate value = V1 + V2
2
Vv
100
Vs
= Degree of insensitivity: = A/2 x
= [ V2 - V1] x 1_____ x
100
2 [V1 + V2 ]
2
Xt
= [ V2 - V1 ] x 100
[ V1 + V2 ]
= Transformer per unit impedance.
R normally a problem because the load characteristics is normally identifiable by the utility and the variation of the power factor is not that much pronounced.
Cos

R”
= Load power factor.
= The calculated per unit compensation resistance (+ or -).
X” = The calculated per unit compensation reactance (+ or -).
4.2 Selection of Insensitivity and Method of
Wiring
The tap changer alters the voltage of a transformer in a stepwise manner, the degree of insensitivity of the relay must be adopted accordingly. If the degree of insensitivity is too low, hunting can arise and if it is too high the small steps of transformer will not be utilized. Normally this is taken as 80% of the transformer step voltage as an appropriate figure.
Hence, if the tap changer step is 1% degree of insensitivity equals 0.8 x 1 = 0.8% should be selected. The relay can be connected from phase voltage and phase current of line voltage and line current of line voltage and phase current and by vector analysis the operate quantities can be decided.
4.2.1 Relay supplied from phase voltage phase current
If we consider the connection of Fig. 5b.
Vy
Vb
I
Y
In this figure, the relay is shown connected from yellow phase voltage and yellow phase current.
Assuming that due to the increase in load the bus voltage varies beyond the setting level. Then one of the transformers regulators will feel the variation and give a raise or lower command for the concerned transformer tap changer to bring the voltage to the desirable level. Because of this, one transformer will be on a different tap so a circulating current proportional to the difference of number of steps and the transformers reactance will flow “out” of the transformer at the higher tap and “in” for the others at the lower tap.

The yellow phase CT will carry all the current IL. This is shown vector ally as in Fig. 7 & 8.
I
C
V
Y

I TOTAL
IL

V
Y
I
C
I
C
I TOTAL
The total current will be flowing in the compensating unit R” + JX”.
The total current in the transformer at a higher tap
I
C
of load current has to be nullified in both is larger, hence the drop in the LDC unit will have tendency to tap up again while the one at the lower tap will have a tendency to tap down and due to this the tap changer will diverge which is not a desirable section. For this reason the effect transformers for a good voltage control.
To achieve this the load current is resolved into its vertical and horizontal components as shown in
Fig 9a and multiplied by the LDC components as shown Fig.9b.
I
L
COS

V
Y
I
L
I
L
SIN

As can be seen from the vectors, the (I and (I
L
L
Cos
 )(X”)
Sin
 ) (R”) have not effect on the voltage vector ally, if matched.
The (I
L
Cos
 )(R”) and (I
L
Sin
 ) (X”) have a direct effect on the voltage Vy, and if (R”) is reversed
(-R”) and the components are made equal.
I
L
SIN

(X”)
I
L
COS
 (R”)
I
L
COS
 (X”)
I
L
SIN

(R”)
V
Y
Fig. 9b Resolved quantity mult by R” & X”
(I
L
Cos
 )(R”) and (I
L
Sin
 ) (X”) the load component will be cancelled and the voltage regulation will not be affected by the load current and the effect of the Ic is left on the circuit as shown in Fig. 10a for transformer with higher tap and Fig.10b for transformer with lower tap.
(IC) (X”)
(IC) (R”)
V
Y
(IC)(X”)
(IC)(R
)
V
Y

4.3 Effect of Circulating Current
The two above diagram show that (+X)(Ic) will have the effect on the regulator and if correctly calculated the transformer with a higher tap will see a higher voltage and will not further tap while the other with the lower tap will see lower voltage and tend to raise hence the two transformers will be on same tap.
Computing Ic, R” & X” for two parallel units:
Ic = Per unit tap step Per unit current.
V
BR
V
R
X
T1
+ X
T2
Take degree of insensitivity = 0.8 x T =

V.
This voltage will equal , Ic x X” = 
V.
Hence X” can be quantified.
R” = X” Tan  and also R” can be quantified
V
B
Ø I
Y
V
Y
Fig. 11 Taking connection of fig. 5b
In this case current is fed by line voltage V
BR
and yellow phase current I y
. Similarly resolving vector ally.
I
Y
SinØ
I
Y
Cos

V
Y
I
Y
Sin

(X”)
IL Cos
 (R”)
I
C
I
Y
I
Y
Sin

(R”)
I
Y
Cos

(X”)
V
BR
FIG. 12
Again taking (I y
Sin
 )(R”) and (I y
Cos
 ) (X”) will cancel the load components in line with V
BR
.
V
Y

4.
(I
C
)(R”)
(I
C
)(X”)
V
Y
V
BR
Fig. 13 Circulating Current Components
In conclusion the negative reactance method is
The effect of Ic is similar to the previous effect but now the resistance component is the effective cheap and proved to be effective in SEC-COA system. one and not the reactance.
Ic x R” = 
V
Then R” is computed.
R”Tan 
= X”.
Hence X” is computed.
While the requirement of the voltage regulating functional aspect are seen on the previous pages, it is also essential to match the relay in a perfect order to the primary system. For establishing the above object, it is very much essential to identify the mode of design of the compensating units i.e.
R & X.
Manufacturers resort to different methods of design for providing the U
R
and U x
components.
As one example relay type MVGC01 of
ALSTOM make is being dealt with as follows:
V
R
1.
V
B
Apply V ref leading the current by 90 o .
2. Set V
R
= 24V, V c
= V
XL
= Zero.
3. Apply I
N
.
Addition of 24V will be seen with V s
.
FIG. 14
I
Y
V
Y
I
V
FIG. 15a s
24
Vr

5. Apply V ref and I
Y
in anti phase with the voltage with V
XL
switch (direct).
Set V
XL
= 24V.
I
V
S
Y

V s
+ 24V will be the voltage measured.
6. Set the V
XL
switch “Reverse”.
Inject the current so that the V s and the current injected are co-phased.
I V
S
FIG. 16
Method of matching this design with our primary requirements.
V
S
Vrb
Iy
I
Y
Cos

(R) = equivalent U
R
Fig. 17
I
Y
Sin

(X) = equivalent U x with the selector switch in forward position.
Y

U
R
U x
For balancing the load put the switch reversed.
U x
(-)
U x
Sin

= U
R
Cos

For 0.85 power factor.
U x
Sin

= U
R
Cos

U x
(0.526) = U
R
(0.85)
U x
= 1.6159U
R
------------------------------- (1)
(Selector Switch (Reverse).
Circulating current check.
U
R
V
S
24V
24V

FUNCTION OF VOLTAGE REGULATING RELAY MVGC01
Vref 100 Ur=24
Irated uc=0 ux=0 ur=24
total volts under the above condition 124 volts
I rated vref 100
Uc=0 ux=24 ur=0
total volts under the above condition 124 volts
total volts under the above condition 124 volts
Uc=0 Ux=24 Ur=0

Figure 18

Figure 19

OPERATION OF THE TRANSFORMER L-N MAGNITUDE
Figure 20
PERFORMANCE OF VOLTAGE REGULATING RELAY MVGC01
REF
1 Ip= the power component of current Iy
2 Iq= the reactive component of current Iy
3 Vrb= No-load line to line voltage to the relay ref voltage.
4 Ixt= Reactance drop of the transformer at the given load.
5 Xe= the reactance of the compensator in ohms
6 Re= the resistance of the compensator in ohm

(Selector Switch (Reverse).
Circulating current check.
Note 1
FIG21
Ic when the transformer in question pushes out the circulating current Ic
Ic*Xe which is the voltage in the Xe element in the compensator reduces the control voltage and thus not allowing the Ic beyond a limit and that is to be decided within two tap difference.
Selector switch already (reversed)
(I
CX
) i.e. equivalent U x
will be in anti phase with
V rb
which will make the relay to lower the voltage which is the real requirement for adjusting the voltage by the concerned voltage regulator relays.
Note 2
Ic when the transformer in question receives out the circulating current Ic c*Xe which is the voltage in the Xe element in the compensator increases the control voltage and thus not allowing the Ic beyond a limit and that is to be decided within two tap difference.
Circulating current check.

Conclusion
The most problem areas with traditional schemes such as Complex control circuitry associated with the parallel operation of transformers , Inadequate performance under varying load conditions , High skill requirement for installation, operation
&maintenance, and Operational limitations have been overcome & solve with a new modern the requirement for installation, operation
&maintenance between the old traditional schemes and the new modern scheme will be more than 50%
Reference:
1. ALSTOM Manual ( Protective Relays for
Power Systems ). method of the reverse reactance scheme . The expectation of the cost reductions on the control circuitry associated with the parallel operation and
Relay Type Chosen - MVGC01.
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