This document analyzes voltage unbalance on Uganda's distribution network by measuring voltage and current profiles at distribution transformers on the Kigo feeder. Phase voltage unbalance rates were calculated and compared to phase currents. High unbalance was found at the Najja Central transformer likely due to transformer issues. Other transformers had unbalanced phase currents causing unbalance. Periodic audits are suggested to identify problematic transformers which can then have loads reconfigured or transformers replaced to mitigate unbalance. Automatic switching or power filters may also help but are more complex and costly to implement.

1. Determination of Voltage Unbalance on Uganda’s
Distribution Network and Suggesting Ways of
Mitigating it in the Highly Affected Areas
G. Mawanda, M. Kisuule
Department of Electrical and Computer Engineering, Makerere University,
Kampala, Uganda
kisuule01@gmail.com
Abstract—A study is done to determine the level of Voltage
Unbalance on Uganda’s Distribution Network with a case study of
Kigo Feeder taken. The voltage unbalance is determined by
measurement of the LV voltage and current profiles at the LV
terminals of sampled distribution transformers along the case
feeder. These measurements will be done at different times of the
day to capture the different day loadings. With these voltage
profiles, the IEEE Voltage Unbalance calculation is used to
ascertain the Phase Voltage Unbalance Rate (PVUR). These
PVUR values are compared to the prevailing instant phase
current readings to get the relation of the existing voltage
unbalance and the phase loadings. Causes of the existing voltage
unbalance, its effects on the network are cited and practical ways
of mitigating it are suggested.
Keywords—Voltage Unbalance, Phase Voltage Unbalance Rate,
Current Loading, Mitigation
I. INTRODUCTION
The Distribution network in Uganda, run by UMEME
roughly consists of over 60 33/11 kV substations, over 4000km
of 11kV and 33kV lines serving over 7000 distribution
transformers supplying up to 10,000km of LV network with a
customer base of 500,000 customers with a peak demand of
500MW and an annual energy consumption of 3000GWhrs.
Voltage Unbalance is a phenomenon in 3-phase systems where
the magnitudes of the voltages in each of the 3 phases is not
equal and phase angle displacement between the phases is not
120°. Voltage Unbalance is generally unmonitored on this
distribution network and yet vastly rampant arising from the
load side mainly and traversing through the entire distribution
network accounting for numerous technical losses, network
instability and damage of 3-phase equipment belonging to both
the utility, UMEME as well as customers.
Because a large percentage of the Ugandan distribution
network serves single phase customers, the largest source of the
unbalance is the uneven distribution of these single phase loads.
Additional causes of voltage unbalance include; asymmetrical
transformer winding impedances, open wye and open delta
transformer banks, asymmetrical transmission impedances as a
result of blown fuses.
Whenever an unbalanced voltage regime exists on the
network, unbalanced currents that are much higher flow causing
extra losses as well unbalanced voltages existing along the lines
presenting uneven impedance to loads and thus instability; thus
the system doesn’t respond to rapid changes in system
parameters suitably. The supplementary negative and zero
sequence currents that flow as a result of operating in
unbalanced voltage regimes cause additional power losses and
faults in power equipment as well as overheating of the 3-phase
asynchronous machines belonging to the various consumers.
II. METHODOLOGY
A. Project Scope
The project was done on Kigo 11kV Feeder chosen as the
case study. This is a 30km feeder covering the Najjanankumbi,
Masajja and Kigo load with over 30 distribution transformers.
This feeder is fed from Kampala South Substation located in
Najjanankumbi, 5km from Kampala City Centre along Entebbe
Road and belongs to the Najja Umeme District serving an
average load of over 2.5MW and a peak load of 4MW. This is
the most heavily loaded feeder at Kampala South Substation
and serves a suitable case feeder for the project study.
B. Gathering Data
Along the Kigo Feeder, distribution transformers were
located and instant voltage and current readings were taken
from the LV terminals. The study involved using a sample of 5
transformers out of the 30 with the first 5 transformers along the
feeder from the substation used for the study. These included
the list from Table 1 below;
These transformers are located in the urban areas of
Najjanankumbi, the first 5 km along Kigo Road and serve
mainly domestic households with a few 3-phase customers in
the form of millers and welders in the town centers.

2. Kolasi 500KVA
Najja Central 100KVA
White Angel 100KVA
Kasawe 500KVA
Delico 315KVA
Table 1: Sampled Distribution Transformer
The instantaneous voltage and currents flowing at the LV
terminals of the sampled transformers were taken by a
mutlimeter at different times of the day; morning (07:00-
11:00hrs), afternoon (14:00-18:00) and evening (19:00-23:00)
to reflect the different loading profiles as the day progresses.
Because there are a number of circuits on a single distribution
transformer the current flowing in each phase was gotten by
summing up the currents flowing in each of the different
circuits for that given phase. This data was recorded in a form
as shown in Table 2 below;
Transformer Name: Time of the Day:
Current Voltage
Circuit
Red
1
2
3
Yellow
1
2
3
Blue
1
2
3
Table 2: Voltage and Current Measurements Recording Form
From the phase voltages measured, the Phase Voltage
Unbalance Rate (PVUR) according to the IEEE definition [5]
is calculated from the formula below;
%PVUR =
𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑑𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑝ℎ𝑎𝑠𝑒 𝑣𝑜𝑙𝑎𝑔𝑒
𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑝ℎ𝑎𝑠𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
The percent unbalance rate from the IEEE definition is then
calculated for each sampled transformer from the measured
voltage readings and recorded vis-a-vie the currents flowing in
each phase for that particular transformer at that given time.
This data was then entered in the table 3 shown below;
With the PVUR values calculated and recorded for each
transformer along with the phase currents flowing in each phase
for each of the times during which measurements were done;
Morning, Afternoon and Evening, the sampled transformers
along the feeder with high PVUR values above the acceptable
limits of 2% as per the IEC standard [3] were identified and the
reasons cited for this high unbalance rate as well as suggestion
of a practical mitigating solution to tackle the high unbalance-
ridden areas.
From the table below, the currents are recorded according to
the color band with the current in the red phase placed in the
red cell and the one in the yellow phase placed in the yellow
cell and so on for each given transformer and time.
DELICOKASAWE
WHITE
ANGEL
NAJJA
CENTRAL
KOLASI
CURRENTS
PVUR
CURRENTS
PVUR
CURRENTS
PVUR
TRANSFORMER
MORNING
AFTERNOOON
EVENINGTable 3: PVUR and Phase Currents Recording Form
III. RESULTS AND OBSERVATIONS
On observation, the transformer Najja Central was old and
filled with oil visibly on its exterior and had many loose
connections of the different LV circuits that it was supplying.
Some circuits supplied by this transformer had an unconnected
phase and thus suspected to be generating Voltage Unbalance
arising from the transformer condition.
The results obtained for the calculated PVUR values and the
phase currents for each transformer for each time band are
shown in the figures below;
A graph for each time band; Morning, Afternoon and Evening
is shown showing all the transformer values that were measured
for current and the PVUR calculated from the phase voltages.
For each transformer, columns of the currents in the red, yellow
and blue phase with the scale on the left reflect the measured
phase currents and the green trend line indicating the PVUR
values that were calculated with the graphs giving a graphical
representation of the relation between the two.

3. From the graphs, the following observations can be made;
 From morning through the afternoon to the evening, the
load goes on increasing as seen from the increase in phase
currents flowing in each transformer as the day progresses.
 The voltage unbalance levels as per the PVUR values also
increase as the day progresses from morning, through the
afternoon to the evening.
 Of the five sampled transformers, only the one, White
Angel has PVUR values below the acceptable limit of 2%
at all times of the day. This reflected fairly balanced phase
currents at all the different times of the day.
 The highest PVUR was recorded at Najja Central
transformer above 12% at all times of the day. The phase
current imbalance did not however reflect this unbalance
level.
 The transformers; Kasawe, Delico and Kolasi all had high
PVUR values that were coupled with high imbalance in
their phase currents. Because the phase currents increased
as the day progressed, so did the imbalance and this
reflected an increase in the PVUR values.
IV. ANALYSIS
Next a case was made for the cause of the high voltage
unbalance at the different transformer points along the feeder
where it was above acceptable limits and the possible effects
of such operation on the grid.
 For the Najja Central transformer, the unbalance was
suspected to be caused by the transformer itself. This is
because the load was fairly balanced as seen from the fairly
balanced phase currents.
Unbalance arising from a distribution transformer is mainly
due to asymmetrical winding impedance within the
transformer as a result of windings burning out or being
damaged as a result of arching and loss of transformer oil. The
unbalance could also be as a result of burning out of the
bushings and loose connections that present different
impedance per phase to the transformer load.
The effect of such unbalance operation is running of the
transformer at a lower efficiency as losses generated within the
transformer greatly increase. This is coupled with power factor
depreciation.
 For the Kasawe, Delico and Kolasi transformers, the
voltage unbalance was suspected to be caused by
unbalanced loading across the three phases as seen by the
unbalanced phase currents for each transformer.
Furthermore, the greater the imbalance in phase currents,
the greater the PVUR value giving more evidence.
0
2
4
6
8
10
12
14
16
0
20
40
60
80
100
120
140
Kolasi Najja
Central
White
Angel
Kasawe Delico
PVUR
PhaseCurrentsinA
Distribution Transformers
Morning TX Phase Currents and VUF
RED YELLOW BLUE VUF
0
2
4
6
8
10
12
14
16
0
20
40
60
80
100
120
140
160
180
200
Kolasi Najja
Central
White
Angel
Kasawe Delico
PVUR
PhaseCurrentsinA
Distribution Transformers
Afternoon TX Phase Currents and VUF
RED YELLOW BLUE VUF
0
2
4
6
8
10
12
14
16
18
20
0
50
100
150
200
250
300
350
Kolasi Najja
Central
White
Angel
Kasawe Delico
PVUR
PhaseCurrentsinA
Distribution Transformers
Evening TX Phase Currents and VUF
RED YELLOW BLUE VUF

4. Unbalanced loading could be caused by a variety of
scenarios including but not limited to;
1) On connection of new customers, special attention is
not taken by the technical personnel to know the loading
conditions of the phases and tend to connect customers basing
on their convenience. This in time leads to phase loading
imbalance.
2) Downstream in the LV network, only a single phase
may be taken far from the transformer on the assumption that
a few customers will be supplied at these points. However,
many times new customers are connected in these far areas and
they are placed on this single phase and it ends up being
overloaded compared to the others that were not extended
further.
3) When customers are disconnected for any reason
from faults to bill payment defaulting, many times when
reconnecting them, care is not taken to place them on the phase
they were on before so they end being shifted on the more
convenient phase many times the lowest one (Yellow phase).
This in time will create a load disparity across the phases as it
will become overloaded compared to the others.
4) Many times non-technical personnel are hired by
customers to rectify some problems where the utility charges
for the works required or has bureaucratic hurdles and these
non-technical people after doing their work don’t place the
customer on the phase they were on before. Many times they
are hired to reconnect the customer after being disconnected
and also caused uneven single phase loading across the three
phases.
5) During connection of new customers to the network,
no care is taken to connect loads so as to maintain a balance
across all three phases and with time the most convenient
phases to add loads end up being overloaded compared to the
other ones.
6) Over time the load demand of customers changes due
to current situation with some loads growing and others
diminishing and this is not taken into consideration by the
utility as a balanced system can become unbalanced overtime
as loads on the different phases have changed.
The effect of operating under such unbalanced regimes is
increased unbalanced currents due to the voltage unbalance
and this causes losses in the parts of the networks which they
flow increasing as they go further upstream with higher
voltage levels. Any 3-phase equipment placed at these points
will suffer from overheating and associated problems of
unbalanced voltage operation. [10]
The network under unbalanced conditions also suffers from
less stability as the ability to adapt to a rapid change in
conditions such as rapid loss of load due to fault or planned
switching, lightning among others is compromised as in such
conditions, the network under unbalanced conditions will
suffer greatly. There are increased technical losses on the
network when operating under unbalanced conditions and the
higher the unbalance levels, the higher the technical losses
experienced.
V. MITIGATION TECHNIQUES
Next a case to case basis for each transformer condition is
made to derive practical mitigating solutions for the voltage
unbalance situation on the case feeder.
For Najja Central transformer where the voltage unbalance
is identified to be caused by the transformer itself, replacement
of the transformer or repair in the transformer workshop is
suggested. This should be done with special care taken on the
winding impedance that is suspected to be asymmetrical. This
will surely solve the voltage unbalance problem that exists
there.
For the three transformers; Kolasi, Kasawe and Delico that
had high unbalance levels as a result of phase loading
imbalance, the loads downstream the LV network should be
reconfigured to balance the loads across the phases. This can
be done by disconnecting customers from the heavily loaded
phases with high currents and transferring them equally to the
phases with lower currents flowing. This can be done till the
currents flowing in the different phases is roughly equal. This
can go a long way in reducing the voltage unbalance levels at
these points.
The major mitigating solution that is suggested is a periodic
audit of all the distribution transformers on the network in
order to ascertain those which have unbalanced voltage
regimes. This can be done every year or half year by the
UMEME Districts Operations and Maintenance technical
team.
The audit can be done by measuring the instantaneous voltages
and currents at the transformer LV terminals. The ones with
high voltage unbalance levels can be identified and if it
correlates to unbalanced currents flowing in the phases it can
be dealt with by balancing the loads across the phases as
described above. This can go a long way in determining the
level of unbalance and thereafter dealing with it. Furthermore
automatic switching configuration can be employed at the LV
network where the currents flowing in each of the phases is
monitored in real time in relation to the voltages. When the
currents become imbalanced past the limit, the controller
switches loads automatically from the heavily loaded phase to
the lightly loaded phase to balance the loads. This however
involves a lot of extra circuitry including controllers and
change-over switches that are expensive and quite costly to
install proving challenging to install and maintain.
Also, some power equipment can be installed on the MV
network to compensate for the negative and zero sequence
currents that flow as a result of the voltage unbalance and can
deal with it consequently. These include passive power filters
that balance the load impedance [13], shunt connected
thyristor-controlled static VAR compensators where the load
current is balanced by adding reactive elements in parallel to
the load [14] and additional power electronic devices like

5. active line conditioners that dynamically correct voltage
unbalances through the injection of a correction voltage in one
phase. [15] These are however disadvantageous as they inject
unwanted harmonics into the ac system on top of their being
costly and complex to install and maintain and thus not readily
desirable.
VI. RECOMMENDATIONS
To get a clearer picture on the energy that can be saved in a
given time period say a month with a more balanced network
compared to the existing unbalanced one by applying some
mitigation solutions can be done by simulation using a power
simulation software like ETAP, DigSilent Power Factory and
the like.
The actual energy consumed at present voltage unbalance
conditions on this feeder can be gotten from the meters placed
at the substation for the case feeder. For the same time period,
a simulation of the case feeder can be run with considerations
made for a balanced network and the energy that is consumed
within the same period derived. The energy saving can then be
calculated in monetary terms and this can validate the need for
these mitigating solutions to be applied on the entire grid.
A further study can be done to show the saving in terms of
energy if these mitigation techniques are applied to the entire
distribution network.
VII. CONCLUSIONS
Operating under unbalanced voltage regimes is a big cause of
technical losses and a deliberate effort taken by UMEME to cut
down on the levels of unbalance on the grid can go a long way
in reducing the technical losses that are a major challenge faced
by the distribution network utility.
The cost of the suggested mitigations is much less than the
energy lost by operating the network under unbalanced
conditions as can be evidenced by the simulation under
recommendation.
VIII. REFERENCES
[1] Electric Power Systems and Equipment—Voltage
Ratings (60 Hertz), ANSI Standard Publication no. ANSI
C84.1-1995.
[2] EPRI Power Electronics Applications Center, “Input
performance of ASDs during supply voltage unbalance,” Power
quality testing network PQTN Brief no. 28, 1996
[3] IEEE Recommended Practice for Electric Power
Distribution for Industrial Plants, ANSI/IEEE Std. 141-1993,
(Red Book).
[4] IEEE Recommended Practice for Electric Power Systems
in Commercial Buildings, ANSI/IEEE Std. 241-1990, (Gray
Book).
[5] D. R. Smith, H. R. Braunstein, and J. D. Borst, “Voltage
unbalance in 3- and 4-wire delta secondary systems,” IEEE
Trans. Power Delivery, vol. 3, no. 2, pp. 733–741, Apr. 1988.
[6] Tsai –Hsiang Chen, Cwng-Han Yang, Ting-Yen Hseih,
“Case Studies of the Impact of Voltage Unbalance on Power
Distribution Systems and Equipment”
[7] A. Robert and J. Marquet, ‘Assessing Voltage Quality
with relation to Harmonics, Flicker and Unbalance’, WG 36.05,
Paper 36-203, CIGRE 92
[8] Annette von Jouanne and Basudeb Banerjee, ‘Assessment
of voltage unbalance’, IEEE Trans. on Power Delivery, Vol. 16,
No. 4, pp. 782-790, Oct. 2001
[9] Emmerton Associates, “Distribution Loss Reduction
Report on Technical Losses prepared for UMEME” 04 August
2013.
[10] Tsai-Hsiang, Chena Chwng-Han and Yang Ting-Yen
Hsieh, ‘Case Studies of the Impact of Voltage Imbalance on
Power Distribution Systems and Equipment’, Proceedings of
the 8th WSEAS International Conference on Applied Computer
and Applied Computational Science 2008
[11] J. D. Kueck, D. A. Casada, and P. J. Otaduy, “A
comparison of two energy efficient motors,” IEEE Trans.
Energy Conversion, vol. 13, no. 2, pp. 140–146, June 1998.