VIABILITY OF 
FIXED TYPE POWER CAPACITORS 
FOR LOSS REDUCTION IN LOW VOLTAGE (400V) 
DISTRIBUTION NETWORKS OF POWER UTILITIES 
A dissertation submitted to the 
Department of Electrical Engineering, University of Moratuwa 
in partial fulfillment of the requirements for the 
degree of Master of Science 
L IBRARY 
UNIVERSITY OF MORATUWA, SRI LANKA 
MORATUWA 
by 
CHANDANA WARNAKULASURIYA 
Supervised by: Dr. JP Karunadasa 
Department of Electrical Engineering 
University of Moratuwa, Sri Lanka 
February 2009 
University of Moratuwa 
92426 
92426 
i i i 
DECLARATION 
The work submitted in this dissertation is the result of my own investigation, except 
where otherwise stated. 
It has not already been accepted for any degree, and is also not being 
concurrently submitted for any other degree. 
Name of Candidate: Chandana Warnakulasuriya 
Date: 05-02-2009 
I endorse the declaration by the candidate. 
. . SrglZr ' / b f r f W ' 
Name of Supervisor: Dr. JP Karunadasa 
TABLE OF CONTENTS 
DECLARATION i 
TABLE OF CONTENTS "" 
ABSTRACT iv 
ACKNOWLEDGMENTS v 
LIST OF FIGURES vi 
LIST OF TABLES viii 
1 RESEARCH STUDY 1 
1.1 Introduction 1 
1.2 Objectives 3 
1.3 Subject of the eresearch 4 
1.4 Area of research 6 
2 METHODOLOGY 8 
2.1 Base load Reactive power requirement 8 
2.2 Selection of the preferred location 8 
2.3 Selection of size of capacitor 12 
2.4 Selection of voltage rating of capacitors 13 
2.5 Selection of ambient temperature category 13 
2.6 Reduction of operating temperature of capacitors 13 
2.7 Influence of harmonics 16 
2.8 Overvoltage due to fixed capacitors 17 
3 DATA COLLECTION AND ANALYSIS OF RESULTS 18 
3.1 Power flow through Primary substations 18 
3.2 Comparison of costs and benefits 19 
ii 
3.3 Selection of size of fixed capacitor 27 
3.4 Selection of voltage rating of capacitors 30 
3.5 Assesment of ambient temperature APPLICABLE TO capacitors 31 
3.6 Reduction of operating temperature of capacitors 34 
3.7 Influence from system harmonics 39 
3.8 Overvoltage due to fixed capacitors under low load conditions 45 
4 CONCLUSION AND RECOMMENDATIONS 46 
Referneces 48 
Appendix A 49 
Appendix B 50 
ABSTRACT 
The viability of fixed type power capacitors for loss reduction in low voltage (400V) 
distribution networks of power utilities was studied. Usually, reactive power of the 
load is compensated at high voltage levels by utility operators, which requires high 
investment on equipment. This study reveals an alternative low cost reactive power 
compensation method, which will reduce the capacity requirements of HV reactive 
power installations. 
Fixed type power capacitors had been used at the secondary of few of earlier 
11 kV/415V distribution transformers in Colombo City distribution system. Most of 
these units are in operation, even after 20 years of service. But the practice of 
having fixed capacitors at distribution transformers was not continued thereafter. 
It was identified through model simulation and field measurements that installation 
of fixed value power capacitors at feeder pillars of the power distribution network is 
more economical compared to the earlier practice in Colombo City. The expected 
minimum energy saving is approximately 300 kWh/month with a 40kvar, 440V rated 
capacitor installed at the feeder pillar. 
The main advantages of the proposal are low capital requirement and shorter 
payback period. The expected maximum payback period is 7 months. In the 
proposed method, the temperature of operation of capacitors was more critical. 
The reliability of capacitors is to ensure by adapting measures to reduce the 
operating temperature of capacitor units, housed in steel enclosures. 
Encloses, housing capacitor units reduces the case temperature of capacitors. This 
arrangement was superior to the use of outdoor type capacitors without enclosures. 
Use of light coloured enclosure with holes for ventilation and thermal insulation will 
reduce the temperature of operation of capacitor unit at least by 10 °C, compared 
to dark green finished enclosure with louvers for ventilation and without thermal 
insulation on the internal surface. 
Field measurements confirmed that the lifetime of capacitors are not affected due 
to load harmonics. The maximum harmonic current absorbed by capacitors was 50% 
lower than the continuous over current rating of the capacitor unit. The simulation of 
the model with different fixed capacitor sizes shows that there is no risk that 
transformers would fall into resonance along with fixed capacitors under the load 
harmonics due to the selected size of capacitors and level of load harmonics. The 
voltage rise due to fixed type capacitors at no load condition is insignificant for the 
sizes of capacitors proposed in this study, Hence there is no risk that the power 
transformers would not fall into ferroresonance due to fixed capacitors. 
Practical difficulties of usage of LV fixed type capacitors were identified and 
solutions were recommended so that a cluster of fixed value shunt capacitors can 
be installed and operated effectively at low voltage distribution level for achieving 
greater economical benefits. 
iv 'o. 
ACKNOWLEDGMENTS 
I sincerely thank Dr. JP Karunadasa, Supervisor of this research study for his 
invaluable suggestions and guidance. I specially thank Professor JR Lucus, Dr. NK 
Wickramarachchi, Dr. SP Kumarawadu and Mr. WDAS Wijayapala for their 
constructive comments on this study . I also thank Dr. Udawatta for directing us in 
the proper path so that we could complete our research studies in time. 
I greatly appreciate Mr. FK Mohideen, AGM (R2), Ceylon Electricity Board who 
guided me to study the application of fixed type power capacitors for loss 
reduction in Colombo city. 
I also thank staff of various branches of Colombo City of Ceylon Electricity Board 
who helped me to implement the proposed system as a study sample. I also thank 
the Management of Ceylon Electricity Board for offering me the valuable 
opportunity of studying for my Masters Degree. 
LIST OF FIGURES 
• 
Figure 1: General arrangement of Colombo City underground distribution system.... 4 
Figure 2 : Comparison of distribution losses 6 
Figure 3 : Proposed locations for low voltage capacitors 7 
Figure 4 : Connection options of low voltage shunt capacitors 9 
Figure 5 : Fixed type capacitors in a room of a distribution transformer 9 
Figure 6 : Fixed type capacitors at a feeder pillar 9 
Figure 7 : Part of network considered for simulation 10 
Figure 8 : Total system to represent one transformer supplying two feeders 10 
Figure 9 : Sub system to represent one feeder pillar 11 
Figure 10 : Sub system for Measuring P, Q, V & I 11 
Figure 11 : Effect of enclosure 14 
Figure 12 : Use of thermal insulation with reflective foil 15 
Figure 13 : Ventilation arrangements with louvers and holes 15 
Figure 14 : Power quality analyzer with flexible current transformer 16 
Figure 15: Comparison of benefits 23 
Figure 16 : Pattern of power flow through a feeder pillar 24 
Figure 17 : Verification of energy saving in a feeder cable 24 
Figure 18 : Power loss across a feeder cable with capacitor at the end 25 
Figure 19 : Power loss across a feeder cable without capacitors 25 
Figure 20 : Power loss across a feeder, with and without capacitor 26 
Figure 21 : Load curve at the feeder, assumed for simulation 27 
Figure 22 : Simulation results for fixed value capacitor at the feeder pillar 28 
Figure 23 : Simulation results for fixed value capacitor at the transformer 29 
Figure 24 : Power saving under different capacitor values 29 
Figure 25 : Variation of lifetime under different voltage ratings of capacitors 30 
Figure 26: Thermometers used for field measurement 31 
Figure 27 : Distribution of internal air temperature of steel enclosures 31 
Figure 28 : Distribution of external air temperature of steel enclosures 32 
Figure 29 : Internal to external temperature difference 33 
Figure 30 : Indoor and outdoor type capacitors 34 
Figure 31 : Temperature logging with and without enclosure 34 
Figure 32 : Effect of enclosure 35 
vi 
Figure 33 : Temperature logging to evaluate the effect of exterior colour 35 
Figure 34 : Effect of colour on internal temperature 36 
Figure 35: Use of thermal insulation and temperature measuring set up :...36 
Figure 36 : Effect of thermal insulation 37 
Figure 37 : Comparing ventilation arrangements 37 
Figure 38 : Holes against louvers for ventilation 38 
Figure 39 : Current harmonics absorbed by capacitors 39 
Figure 40 : Current harmonics at a feeder pillar 40 
Figure 41 : Load harmonics 40 
Figure 42 : Level of current harmonics absorbed by capacitors 42 
Figure 43 : Model for analyzing the frequency response 43 
Figure 44 : Frequency response at a feeder pillar 44 
Figure 45 : Frequency response for various capacitor values at the feeder pillar 44 
Figure 46 : Frequency response for various capacitor values at the transformer 45 
vii 
LIST OF TABLES 
Table 1: Statistics of 11 kV & low voltage network 5 
Table 2: Daily active power flow from Primary substations in Colombo City 18 
Table 3 : Base reactive power flow from Primary substations in Colombo City 19 
Table 4 : Comparison of energy saving 23 
Table 5 : Upper limit of ambient temperature categories 32 
Table 6 : Comparison of harmonic current and THD values 41 
Table 7 : Voltage rise under no load condition 45 
viii 
1 RESEARCH STUDY 
1.1 INTRODUCTION 
Electrical network consists with generators, power transformers, transmission lines 
and distribution lines have resistive, inductive and capacitive effects. When the 
current passes through resistive elements of the network, it consumes active power, 
which is a loss of power. It is expressed as l2R, where I is the current through the 
network and R is the resistance of network components. The efficiency of power 
delivery is improved if the line current could be reduced for supplying the same 
active power. 
In most of the cases, the reactive power requirement of electrical loads is lagging, 
which means the load is inductive. Electricity generators shall operate to meet the 
active and reactive power demands of the load, which supplies through long 
transmission lines. Alternatively, it is possible to supply lagging reactive power with 
the use power capacitors, which is basically a passive device. Thus power 
generators can be used to supply active power, while power capacitors are 
supplying a major part of the lagging power requirement of the load. Power 
capacitors reduce the current through the electrical network as a portion of total 
reactive power is now supplied by capacitors, located closed to the load. 
Reduction in line current, due to power capacitors reduces network losses. 
Therefore, power capacitors are installed at high voltage and medium voltage 
levels in power utilities for compensation of lagging reactive power of loads and 
thereby improving the efficiency of power delivery. 
1.1.1 Proposed method of loss reduction & its uniqueness 
Reactive power compensation with power capacitors at high or medium voltage 
levels could be categories as bulk power compensation. In most of the cases, the 
target is to achieve unity power factor of operation. Lagging reactive power is 
injected to the electrical system of utilities as Breaker Switch Capacitors (BSC), where 
the total capacitance is controlled by circuit breakers or Static Var Controllers (SVC), 
where the capacitance is smoothly controlled by power electronics. 
Capacitors can be used in low voltage distribution network too for compensation of 
reactive power. The application of power capacitors in low voltage networks is not 
a common practice in electrical utilities due to several reasons. The low voltage 
network consists with many branch circuits and the level of reactive power flow is 
low. Therefore, it is required to connect large number of low power rated capacitors 
at many points for compensating reactive power of the load within the low voltage 
network itself. On the other hand, installation, monitoring and proper maintenance 
of a cluster of low voltage capacitors is a labour intensive program. 
Fixed type capacitors are superior to switching type LV capacitors for a similar 
application as fixed capacitors can be connected at large number of points in the 
low voltage network and operate economically. The application of capacitors in LV 
networks can be identified as a distributed method of compensation. The use of 
fixed value low voltage capacitors could make a viable project by identifying 
problems specific to the proposed method and formulating solutions for same. 
1.1.2 Advantages of the proposed method 
The equipment configuration is simple and thus capital requirement is lower 
compared to switching capacitor bank. With low voltage fixed type capacitors, 
lagging power of the load is compensated almost at the load itself and thus the 
effects of reduced line current is extended almost up to the final delivery point of 
the utility. On contrary, with high voltage power capacitors, the line current reduces 
only in sections upstream from the high voltage grid substation, where capacitors 
are installed. It will not improve power loses in the low voltage network. The reactive 
power compensation at low voltage level will reduce the burden on HV or MV 
reactive power compensation stations improving the overall efficiency. 
2 
1.1.3 Limitations of the proposed method 
In the proposed method, capacitors are installed as fixed units. The size of capacitor 
is selected considering the base reactive power of the load to avoid over 
compensation in off-peak time. Overcompensation increases the line current and 
thus has a negative effect on the intended purpose. The proposed method reduces 
distribution losses, but significant improvement of powerfactor cannot expect for all 
load condition as in the case of switching type capacitors. 
1.2 OBJECTIVES 
Some of 1 lkV/415V distribution transformers in Colombo city had been fitted with 
fixed type low voltage capacitors. This particular make of unit type distribution 
transformers with high voltage switching ring main unit (RMU) and low voltage 
distribution panel had been installed during the period from 1989 ~ 1990. The 
practice of having a fixed type capacitor at the secondary of distribution 
transformer had not been continued. Some of the capacitors installed as above are 
still in operation, after 20 years of service. 
The power capacitors installed as above had reduced the line current and the 
accumulated effect of reduced power loss over the long period of their operation is 
immense. The main purpose of this research is to identify problems specific to the 
proposal and formulate valid and proven guide lines to make it more viable the 
application of fixed type power capacitors for loss reduction in low voltage (400V) 
distribution networks of power utilities. 
Power capacitors, installed in an outdoor environment of a country like Sri Lanka 
would subject to very high temperature. Therefore the method of installation is 
critical for extended lifetime of capacitors. Hence suitable methodologies are to 
devise for determining followings. 
a) Preferred location in low voltage network 
b) Size of capacitor 
c) Whether to install with or without enclosure 
d) Enclosure selection 
Exterior colour, Use of thermal insulation & Provisions for ventilation 
e) Improvements to voltage profile 
f) Influence from system harmonics 
g) Overvoitage under light load conditions 
h) Conditions for resonance 
3 
1.3 SUBJECT OF THE ERESEARCH 
The low voltage network of electricity distribution system in Colombo City was 
chosen as the sample for the proposed research. 
1.3.1 Colombo City power distribution system 
The Colombo City power distribution system consists of eight number of Primary 
substation with 132kV or 33kV underground cables as power incoming feeders and 
the power is delivered through 1 lkV underground cables. 
1 lkV cables from primary substations are connected to 1 lkV substations, identified 
as ring or radial substations depending on the connectivity. The ring and radial 
substations feed power to large number of indoor type 11 kV/415V transformer 
substations named as Satellite substations. 
11 kV radial/ Q 
ring substation / ' \ 
132/11 kV / \ 
11 kV/415V Distribution 
Transformer 
Feeder pillar Distributor cable 
Figure l : General arrangement of Colombo City underground distribution system 
Low voltage cables from 11 kV/415V transformer substations to the consumer mains 
are identified as Feeder cable, Distributor cable and Service cable as shown in the 
Figure 1. 
4 
Summary of network points of the Colombo City system is given in Table 1 
Network nodes Quantity end of 2008 (Nos.) 
132/11 kV primary substations 4 
33/11 kV primary substations 5 
11 kV ring/radial substations 139 [1] 
11 kV/415V distribution transformers 1200 [1] 
Feeder pillars (Low voltage) 1500 [2] 
Table 1: Statistics of 11 kV & low voltage network 
1.3.2 Distribution losses of the Colombo City network [3] 
Electrical energy supplied to Colombo city system was 599 GWh in year 2008 up to 
June and the Energy sales during the same period was 555GWh. The corresponding 
Energy loss was 44 GWh and system losses was 7.4% for the first half in 2008. 
During the recent past years, several measures were implemented to reduce the 
system losses. In 2002, the distribution losses in Colombo City was 12.6% and as a 
result of various steps taken for reducing technical and non technical losses it was 
able to reduce distribution losses in Colombo City up to 7.4% in the year 2008. 
Prior to 2004, Non-technical loss was the major contributor of distribution losses in the 
Colombo City system. Non-technical losses were reduced from 6.5% (2004) up to 2.1% 
(Up to November 2008) due to the steps taken in the reason past by the Energy 
Management unit of the Colombo City. Currently technical losses had been the 
major contributor for the distribution losses in Colombo City. Technical and non 
technical losses of the Colombo City distribution system are compared in Figure 2. 
2008 (upto Nov) 2002 
Tech. 
losses 
5.3% 
Tech. 
losses 
6 . 1 % 
Non 
T&ch. 
losses 
6.5% 
• 
Total distribution loss 7.4% Total distribution loss 12.6% 
Figure 2 : Comparison of distribution losses 
Further improvement to distribution losses from the current level would be critical as 
most of the major steps had already been taken and further improvement to 
technical losses from the current level of 5.3% requires proper studies of the system 
and higher capital investment. In this respect, it would be very remarkable if the 
proposed method of having power capacitors is implemented fully in the low 
voltage system, which is a very cost method of injecting reactive power for 
reducing technical losses of the network. 
1.4 AREA OF RESEARCH 
1.4.1 Locations for installation of power capacitors 
Power capacitors can be connected in the low voltage distribution system as Fixed 
type capacitors or Switching type capacitor banks at locations listed below and 
shown in Figure 3 for reducing distribution losses and improving the voltage profile of 
the LV network. 
1. At feeder pillars as fixed type capacitors. 
2. At 11 /0.415kV distribution transformers as fixed type capacitors. 
3. At 11/0.415kV distribution transformers as switching type capacitors. 
4. At the consumer's main as fixed type capacitors. 
Figure 3 : Proposed locations for low voltage capacitors 
The installation of switching type capacitor banks with regulators and control gears 
at distribution substations (above option 3) are expensive compared to fixed 
capacitors. Hence switching type capacitor banks cannot be used economically at 
most of distribution transformers of the selected sample when compared with the 
fixed capacitor system. The switching type capacitor banks are suitable for highly 
loaded distribution transformers with highly varying load patterns. 
The use of capacitors at the final power delivery point of the utility, i.e. at the 
consumers' main (Option 4 in above Figure 3) would be the ideal location for 
installing capacitors for improving system losses. But such a program would be 
extensively labour intensive. Proper monitoring and maintenance of such a system 
of small capacitors would be impracticable. Therefore, above locations 1 and 2 
were selected for this research study. 
7 
2 METHODOLOGY 
2.1 BASE LOAD REACTIVE POWER REQUIREMENT 
The method of reactive power compensation, proposed in this research is suitable 
to meet the base reactive power requirement of the network. The base reactive 
demand of the total low voltage network will give an indication of the maximum 
number of fixed type capacitors that could be installed at the distribution level. 
Daily load curves of primary substations in Colombo City were plotted in Annexure A 
based on load readings [4] taken on selected Wednesdays & Sundays in May 2006 
and September 2008. Base reactive power supplied by each primary substation was 
identified using those load curves and the maximum number of fixed capacitors 
that could be installed in respective area was determined. These quantities set a 
benchmark for the installation program. 
2.2 SELECTION OF THE PREFERRED LOCATION 
2.2.1 Low cost options 
Generalized installation of power capacitors at following low voltage locations of 
the selected sample were studied due to their low installation cost and the 
extended loss reduction ability in the low voltage network, which is not the case 
with high voltage capacitors. 
1. Secondary of distribution transformers as fixed units or switching units and 
2. Feeder pillars as fixed units in the low voltage network. 
8 
Above locations are to be further evaluated for the possible energy saving, cost of 
installation and cost recovery period for identification of the best option. The energy 
saving was determined by modeling typical cases of loads and part of the n&twork 
involved in the proposed methods of installation. The proposed locations are 
indicated in Figure 4. 
132 or 
33kV 
cables 
Primary 
Transformer 
11 kV Radial or Ring/ 
Satellite cables 
Distribution 
Transformer 
400V Feeder 
cable 
Up stream energy saving case 1 
Up stream energy saving, case 2 
T 
Feeder 
pillar 
Case 5: Case 2 
Fixed Cap at Fixed Cap at 
transformer feeder pillar 
Distributor 
cable 
' T 
[Load] 
T 
Figure 4 : Connection options of low voltage shunt capacitors 
2.2.2 Sample installations 
Figure 5 : Fixed type capacitors in 
a room of a distribution transformer 
) (b) 
Figure 6 : Fixed type capacitors at a 
feeder pillar 
Figure 5 and Figure 6 illustrate sample installations made for the study. Figure 5 shows 
the arrangement of low voltage capacitors proposed to install at 11 kV/415V 
9 
distribution transformers. Figure 6-a shows the fixing arrangement of a 30 kvar low 
voltage capacitor, in side of a steel enclosure, which is attached on to existing 
feeder pillar as in Figure 6-b. 
2.2.3 Building the simulink model 
a) System considered for simulation 
The part of the network considered for simulation is depicted in Figure 7. It represents 
an l lkV/415V distribution transformer supplying two equally loaded feeder pillars. 
Fixed type capacitors of the model at the transformer and feeder pillar are switched 
as per the case of study. 
11kV/415VTF Feeder cable F P 
T 
T Case 1 Case 2 
t 
Case 2 
Figure 7 : Part of network considered for simulation 
The Simulink model of the part of network considered is shown in Figure 8. The 
breaker shown in the model was for the simulation purpose only. 
Figure 8 : Total system to represent one transformer supplying two feeders 
10 
b) Simulink subsystem - Feeder pillar with fixed capacitors and the load 
Subsystem to represent one feeder pillar in simulink is detailed in Figure 9. Fixed type 
capacitors at feeder pillars are switched on as per the case of study and the 
breakers are for the simulation purposes only. 
C L > 
Irrt 
Feeder 
- M W — 
< { 5 ] 
FDin FDout 
M 
In 
V 
i 
LC 
CP 
FPout 
v 4-
I? m 
4 ® 
capacitor at 
feeder Pillar 
Load of 
feeder 
Figure 9 : Sub system to represent one feeder pillar, 
c) Simulink subsystem - Measuring module 
Figure 10 shows the subsystem for measuring active and reactive power, voltage 
and current in the simulink model. Three phase active & reactive power and line 
current & voltage are displayed in the scope. This sub system can be enabled or 
disabled for monitoring or logging of P, Q, V & I values at nodes of the total system. 
Q D 
Scope l 
Enable 
Figure 10 : Sub system for Measuring P, Q, V & I 
11 
2.2.4 Input data and assumptions made for the simulation 
a) Load at feeder pillar: Actual field measurements, logged over a period of 24 
hours at 15 minutes interval was used to model the load through the feeder 
pillar. 
b) lOOOkVA distribution transformer: Transformer was connected to two 
numbers of equally loaded feeders of which three phases were balanced. 
2.3 SELECTION OF SIZE OF CAPACITOR 
Once the preferred location was identified, the next step was to determine the size 
of the fixed type capacitor to be installed. 
2.3.1 Size of capacitor for overall maximum energy saving 
Simulink model was simulated for different capacitor values under the assumed load 
conditions over 24 hrs period. Energy saving results was tabulated to identify 
optimum capacitor value considering the maximum possible energy saving. Since 
the installation cost to include all materials, labour & transport is minimal and cost 
recovery period is only few months where the lifetime extends more than 20 years, 
the cost of capacitor has no significant influence on the selection of optimum 
capacitor size. 
2.3.2 Size of capacitor to avoid resonance at transformer under low load 
conditions 
Energized transformers, under no load condition draw lagging reactive power due 
to core magnetization. Reactive-power consumption of the transformer increases 
with the load current due to leakage reactance of transformer windings. Fixed type 
capacitors can be used at the transformers to compensate reactive power 
consumption by the transformer itself. 
However, if the connected capacitance at the transformer is higher, overvoltage 
would occur under no load condition and the transformer can fall into resonance 
due to saturation of the iron core. The recommended value [5] of fixed capacitor at 
a transformer is 5% of the rating of the transformer. For example, a 50 kvar fixed 
capacitor can be connected at the secondary of a lOOOkVA transformer. 
12 
The selected capacitor size, to be used at feeder pillars, which is th second option 
should not create overvoltage at the transformer under light load conditions, as 
there are 2 to 3 feeder pillars connected to a distribution transformer in general in 
the selected sample. 
Field measurements were carried out to identify the minimum load conditions at 
transformers and feeder pillars. 
2.4 SELECTION OF VOLTAGE RATING OF CAPACITORS 
Lifetime of capacitors depends on its operating voltage, ambient temperature and 
the maximum rms current through the capacitors, which is decided by the 
harmonics presence in the network. 
Standard shunt power capacitors have a voltage rating of 1.1 Un for 8hrs in every 
24h and the continuous admissible voltage is Un, where Un is the rated voltage [6], 
Lifetime of capacitors can be extended by selecting a higher value for its rated 
voltage than the actual operating voltage. 
2.5 SELECTION OF AMBIENT TEMPERATURE CATEGORY 
Steel enclosures were used to attach and protect fixed capacitors installed at 
feeder pillars. Although enclosures had provisions for ventilation, the ambient 
temperature applicable to capacitor could be much higher than the outside 
ambient air temperature. This was assessed with field measurements and the 
selection of ambient temperature category of capacitor units was confirmed with 
the results. 
2.6 REDUCTION OF OPERATING TEMPERATURE OF CAPACITORS 
Fixed type capacitors, to be installed at feeder pillar are proposed to house in a 
steel enclosure with provisions for better ventilation. Since enclosures are opened to 
direct sunlight every possible means shall be employed to reduce the heating of 
steel enclosure due to radiation of the sun. Followings matters were studied for 
identifying methods for reducing operating temperature of capacitors, which will 
prolong the lifetime of capacitor units. 
2.6.1 Use of enclosures 
The temperature rise of the capacitor container was monitored on two capacitor 
units, while one capacitor was kept inside of a ventilated steel enclosure and the 
other was without an enclosure. Both the specimens were exposed to sunlight, 
during the measurement as indicated in Figure 11. The purpose of this measurement 
was to establish the effectiveness of enclosures for reducing the heating of outdoor 
capacitor unit. 
Figure 11 : Effect of enclosure 
2.6.2 Exterior colour of enclosure 
Feeder pillars of the Colombo City distribution system had been painted with light 
gray colour for a longer period. But in the recent pass, the colour of feeder pillar had 
been changed to dark green, mainly considering the aesthetic view. Then the steel 
enclosures housing capacitors also had to be matched with the feeder pillar colour. 
The effect of exterior colour of steel enclosure was evaluated by simultaneously 
logging internal air temperature of the two cases, where one enclosure was painted 
with silver colour and the other with dark green colour. 
14 
2.6.3 Use of thermal insulation 
Effectiveness of reflective foil for reducing the heat due to sun's radiation was 
monitored. Thermal insulation can be affixed on to three internal surfaces of outdoor 
enclosures, especially on the front side, which is the largest area on which the sun 
light falls directly. One side of the thermal insulation has a laminated aluminum foil, 
which is very effective in reflecting the sun's radiation out of the enclosure. 
Figure 12 : Use of thermal insulation with reflective foil 
2.6.4 Provisions for ventilation 
Ventilation arrangements are provided on three sides of the steel enclosure as 
shown in Figure 13. There are two possible arrangements for ventilation as louvers 
and holes. The use of ventilation louvers or holes on the front surface is not suitable 
as the sunlight could directly fall on to the internal capacitor unit and it could be 
covered by posters with the time. 
Figure 13 : Ventilation arrangements with louvers and holes 
15 
2.7 INFLUENCE OF HARMONICS 
When a capacitor bank is added to a power system, it is effectively connected in 
parallel with the system's impedance, which is primarily inductive. As far as the 
harmonic source is concerned, it sees a capacitor in parallel with an inductor. There 
is a frequency at which these two parameters will be equal. This frequency is called 
the system's natural resonant frequency. At this frequency, the system's impedance 
appears to the harmonic source to be very large. Therefore, a harmonic current at 
the resonant frequency flowing through this impedance will result in a very large 
harmonic voltage [2], Therefore frequency response of fixed capacitors in low 
voltage system was studied to identify resonance conditions. 
2.7.1 Frequency response of fixed capacitors in the system. 
The two possible locations for connecting low voltage capacitors were considered 
for the analysis in frequency domain to evaluate possible resonance conditions. 
Those locations are fixed value capacitor at the feeder pillar and at the distribution 
transformer. The power network was modeled using simulink for the above two 
cases and the frequency response was studied to identify resonance condition for 
various capacitor values. 
2.7.2 Current & voltage harmonics in low voltage network 
Harmonics levels of feeder loads and that of the capacitor current were monitored 
using LEM make single phase power quality analyzer, illustrated in Figure 14. These 
readings were taken routinely at 120 numbers of feeder pillars, at which capacitors 
were installed. 
Figure 14 : Power quality analyzer with flexible current transformer 
16 
2.8 OVERVOLTAGE DUE TO FIXED CAPACITORS 
The model without the load was simulated for various capacitor values to asses 
overvoltage possibilities due to fixed value capacitors. The voltage rise at the 
respective points of the network was monitored for the two cases, where the fixed 
type capacitor is connected at feeder pillar and transformer respectively. 
17 
3 DATA COLLECTION AND ANALYSIS OF 
RESULTS 
3.1 POWER FLOW THROUGH PRIMARY SUBSTATIONS 
Table 2, below summarizes the daily active power flow of Primary substations in 
Colombo City. Total active power demand of Colombo City shows an annual 
growth rate of 4.3% compared to the year 2006. It was also observed that load 
connected to primary substation E and F had been transferred to the new primary 
substation I, which was energized in 2008. 
Year/Primary Daily A B C D E F G H 1 Total 
September 
2008 (MW) 
Max 33 15 19 22 53 40 27 19 25 244 
Min 8 4 8 8 14 ' 2 10 7 6 82 
Avg-H 20 9 15 16 31 24 21 16 15 168 
Avg-L 11 5 11 11 18 14 12 11 8 100 
May 2006 
(MW) 
Max 28 14 18 22 70 42 26 16 
N
ot
 b
e
e
n 
e
ne
rg
ize
d
 234 
Min 9 5 9 8 19 11 13 9 
N
ot
 b
e
e
n 
e
ne
rg
ize
d
 
85 
Avg-H 18 9 15 16 43 26 20 14 
N
ot
 b
e
e
n 
e
ne
rg
ize
d
 
161 
Avg-L 12 6 11 11 25 16 16 11 
N
ot
 b
e
e
n 
e
ne
rg
ize
d
 
108 
Note: 
Max - The maximum recorded in a high demand day of the week (Wednesday) 
Min - The minimum recorded in a low demand day of the week (Sunday) 
Avg-H - The average value of a high demand day of the week (Wednesday) 
Avg-L - The average value of a low demand day of the week (Sunday) 
Table 2: Daily active power flow from Primary substations in Colombo City 
18 
L IBRARY 
UNIVERSITY OF MORATlF"A, SRI LANKA 
MORATUWA 
Base reactive power measured at each primary substation within the year 2008 and 
2006 are summarized in Table 3. It gives an indication of the quantity of fixed type 
capacitors that could be concentrated in area, supplied by respective primary 
substations. 
Year/Primary A B C D E F G H 1 Total 
September 
2008 (MVAr) 2.3 1.1 3.1 1.2 6.3 2.6 1.5 1.8 2.4 27.0 
May 2006 
(MVAr) 
1.5 0.7 3.1 1.2 9.1 5.4 4.4 3.7 Not been energized 33.0 
Table 3 : Base reactive power flow from Primary substations in Colombo City 
Referring to base reactive power flow in Colombo City distribution system in 2008, it 
is possible to have a maximum of 750 numbers of 40 kvar fixed power capacitor 
units. The power demand at feeder pillars was considered when deciding the 
actual number of installations. 
3.2 COMPARISON OF COSTS AND BENEFITS 
The financial benefit of the two options, considered for fixing low voltage power 
capacitors with minimal capital requirement was calculated as detailed below. 
Part of the low voltage network to include feeder pillar, feeder cable, distribution 
transformer and 11 kV cables up to the primary transformer was considered for 
evaluation of saving of losses in both the cases. 
3.2.1 Determination of resistance of components of selected network: 
Transformer parameters: 
Capacity 
Rated primary voltage 
Rated secondary voltage 
Vector group 
Rated secondary line current 
VATF = 1,000 kVA 
VHV = 11,000 V 
VLV = 415V 
D y n l l 
= WATF = 
LV -J3VLV 
9 2 4 > . L, J 
; 3 R A R Y 
Base impedance referred to secondary 
Losses at Full load 
Losses at No load 
No load current at the rated voltage 
referred to secondary 
Impedance referred to secondary 
Equivalent resistance of primary & 
secondary windings 
Cable parameters: 
Temperature coefficient of resistance - Al 
Temperature coefficient of resistance - Cu 
Low voltage Feeder cable 
Type of cable 
Resistance @90 °C to BS5467 
Operating temperature of the cable. 
Resistance @70 °C 
Typical cable length 
Resistance of a typical feeder cable 
11 kV Satellite cable 
Type of cable 
Resistance @20 °C to BS6480 
Operating temperature of the cable. 
Resistance @90 °C 
Typical cable length 
Resistance of a typical 11 kV Satellite cable 
Z b = - ^ - = 0.1722 fl 0 VATF 
Wpi = 10,020 W 
WNL = 1,410 W 
10 = 0.6% = 8.49 A 
Z = 4.6% = 0.0079224 n 
wFL/ 
RCU = — = 0 . 0 0 1 7 3 Q hv 
aAI = 0.00403 
aCu = 0.00393 
LT/240mm2/PILC/AL/4C 
Ri = 0.162 Q/km/phase 
02=70 °C 
R 2 = R l [ l + a A 1 ( 0 2 - 9 0 ] 
= 0.0175 Q/km/phase 
= 125m 
R Fd = 0.02188 Q/phase 
1 lkV/70mm2/PILC/Cu/3C 
Ri = 0.268 Q/km/phase 
62=90 °C 
R 2 = R l [ l + a C u ( 9 2 - GJ] 
= 0.0175 Q/km/phase 
= 750m 
R sat = 0.24050 Q/phase 
20 
11 kV Radial cable 
Type of cable 11 kV/240mm2/XLPE/Cu/3C • 
Resistance @20 °C to BS6480 Ri = 0.0754 Q/km/phase 
Operating temperature of the cable. 
Resistance @90 °C 
02= 90 °C 
Typical cable length 
R 2 = R l [ l + a C u ( 6 2 - 0 0 ] 
= 0.090 Q/km/phase 
= 1500m 
Resistance of a typical 11 kV Radial cable RRad = 0.13532 Q/phase 
3.2.2 Evaluation of energy and financial saving 
Following assumptions were made for the calculation of monthly energy saving. 
i. Peak and off-peak power demand of each section of the network including 
feeder pillar are 60% and 10%, respectively of the rate power through it. 
ii. Duration of peak demand is 8hrs. 
iii. Typical lengths of l l k V radial cable, l l k V satellite cable and low voltage 
feeder cable are 1500m, 550m and 125m, respectively. 
iv. Unit cost of energy is Rs 10.50 
Energy saving as a result of reduced l2r losses due to capacitors was determined for 
each section of the network from feeder pillar up to 132kV or 33kV/ l l kV primary 
transformer for the assumed peak and off-peak load conditions of the day. Then the 
possible energy saving of the day was calculated by summing corresponding 
energy saving in each section and it was used to get the possible monthly energy 
saving under both the cases. Steps of the calculation are given in Table 4. 
21 
Network section 
Unit 
LV Feeder 
cable 
lOOkVA, 
415/11 kV 
Distribution 
TR 
11 kV Satellite 
cable 
11 kV Radial 
cable 
Resistance of network 
elements 
Q/ 
phase 0.02188 0.00173 0.24050 0.13532 
Rated current of 
network elements A 400 1391 190 400 
Power factor 
(assumed) pf 0 8 0 8 0.8 0. 8 
Typical load 
characteristics 
Peak Off-peak Peak 
Off-
peak Peak 
Off-
peak Peak 
Off-
peak 
Load as a 
percentage of rated % 60% 10% 60% 10% 60% 10% 60% 10% 
power 
Duration of peak & 
off-peak loads hrs 8 16 8 16 8 16 8 16 
Line current A 240 40 835 139 114 19 240 40 
Active power flow kW 138 23 480 80 1,738 290 3,658 610 
Reactive power flow kvar 104 17 360 60 1,303 217 2,744 457 
Case 1 Fixed value capacitor at Feeder pillar (Ratings: 40kvar@440V operating at 415V) 
Reactive power flow 
with the capacitor at 
feeder pillar 
kvar 68 -18 324 24 1268 182 2708 422 
Line current new A 214 41 806 116 113 18 239 39 
Reduction in line 
current A 26 -1 
29 23 1.11 1.06 1.12 1.09 
Reduction in power 
loss in 3 phases W 775 -5 244 30 182 28 
217 35 
Energy saving per day kWh 6 0 2 0 1 0 2 1 
Total energy saving 
per month kWh 
300 
Value of the total 
energy saving 
Rs per 
month 
3,150@ 10.50 unit cost 
22 
Case 2 Fixed value capacitor at Distribution transformer (Ratings: 50kvar@440V, operating voltage is 415V) 
Reactive power flow 
with the capacitor at 
transformer 
kvar 104 17 316 16 1259 173 2699 
• 
413 
Line current new A 240 40 799 113 113 18 239 39 
Reduction in line 
current A 0 0 36 26 1.39 1.30 1.39 1.36 
Reduction in power 
loss in 3 phases 
w 0 0 301 34 226 34 271 43 
Energy saving per day kWh 0 0 2 1 2 1 2 1 
Total energy saving 
per month kWh 
190 
Value of the total 
energy saving 
Rs 
per 
mont 
h 
1,995 @ 10.50 unit cost 
Table 4 : Comparison of energy saving. 
Possible energy saving, installation cost and cost recovery period, with respect to 
the two cases of having shunt power capacitors at feeder pillar and at the 
distribution transformer are compared in Figure 15. 
Figure 15: Comparison of benefits 
As per the comparative values given in Figure 15, the installation of fixed type 
capacitors at the feeder pillars is more beneficial than having fixed type capacitors 
at the distribution transformer. 
23 
3.2.3 Verification of feeder load pattern with field measurements 
Field measurements were taken to justify the assumptions made in above section 
3.2.2 for calculation of financial benefit of energy saving. The resulting power flow in 
a typical feeder pillar is shown in Figure 16. It was observed that the typical load, 
supplied by the feeder pillar is having a peak duration of 10.5hrs, an average peak 
time power flow of 80% of its rated power and a base load of 25% of the rated 
power. Thus the power flow pattern assumptions made in section 3.2.2 is justifiable 
and it will represent the majority of feeder loads in the selected sample. 
T y p i c a l l o a d p a t t e r n a t a f e e d e r p i l l a r 
250 
Figure 16 : Pattern of power flow through a feeder pillar 
3.2.4 Assessment of monthly energy saving by field measurements 
Two loggers were used to log power flow simultaneously at both the ends of a 
feeder cable as shown in Figure 17. In this particular case of field measurement, the 
length of the feeder cable was 75m, where the length of feeder cable would be in 
the range of 100m to 150m in the general case. 
Feeder cable: LT/240mm7PILC/AL/4C 
Length: 7S~200m 
l l k V / 4 1 5 V 
— ( 3 D - ? 
Logger 1 
Feeder pi l lar 
T* 
Figure 17 : Verification of energy saving in a feeder cable 
Data were logged with and without capacitor at the feeder pillar over a known 
period of time. The power loss across the feeder cable with and without capacitor 
against various input power values are shown in Figure 18, Figure 19 and Figure 20. 
Power loss (%) Vs input power 
With cap with correct compensation 
3.0% 
_ 2.5% 
r 2.0% 
CO 
-2 1.5% 
a> 
5 1.0% o 
0.5% 
0.0% 
40 60 80 100 120 140 160 180 200 220 
Input power (kW) 
• Loss-% — L i n e a r (Loss-%) 
Figure 18 : Power loss across a feeder cable with capacitor at the end 
Power loss (%) Vs input power 
Without capacitors 
4% 
g 3% 
</> C/> 
-2 2% 
0) 
o 0% 
Q_ 
- 1 % 
40 60 80 100 120 140 160 180 200 220 
Input power (kW) 
• Loss-% ——Linear (Loss-%) 
Figure 19 : Power loss across a feeder cable without capacitors 
25 
Power loss (%) Vs input power 
With & without capacitors 
2.0% 
1.5% 
1.0% 
Loss-% Loss-% - — L i n e a r (Loss-%) Linear (Loss-%) 
60 80 100 120 140 160 180 200 220 
Input power (kW) 
Figure 20 : Power loss across a feeder, with and without capacitor 
Following results were derived with reference to Figure 20. 
Total energy supplied during the period 
Duration of energy monitoring 
Corresponding monthly energy supply 
From Figure 20, the average energy saving 
Corresponding energy saving in a month 
Financial saving @ Rs 10.50 unit price 
(The length of the feeder cable was 75m) 
12,324 kWh 
94.45 hrs 
93,946 kWh/ month 
0.3% 
282 kWh/ month 
2,959 Rs/month 
The above field measurement shows an energy saving of 228 kWh per month. The 
calculated energy saving using network parameters and assumed load conditions 
indicates an energy saving of 300 kWh per month when a 40 kvar, 440V capacitor is 
connected to feeder pillar. The result obtained by field measurement confirms the 
validity of calculated results and assumptions made. 
26 
3.3 SELECTION OF SIZE OF FIXED CAPACITOR 
The size of fixed type capacitor to be connected at feeder pillar was determined by 
simulation and the results were compared with base reactive power requirement at 
a typical feeder pillar. 
3.3.1 Base reactive load at feeder pillars 
The Figure 16 in 3.2.3 depicts typical base reactive power requirement at a feeder 
pillar, which was plotted by taking field measurement. The base reactive load at the 
feeder pillar had been 30kvar approximately. In this typical case, 30kvar fixed type 
capacitor can be installed at the feeder pillar so that it will not contribute to 
increase system losses under light load conditions. 
3.3.2 Use of simulink to determine the optimum size of capacitor 
A typical distribution transformer in the selected sample supplies to 2 to 3 numbers of 
feeders. This network was modeled in Simulink and was simulated under different 
values for the fixed capacitor at feeder pillar and at distribution transformer for 
sampled load data over a period of 24 hours. The daily load pattern of the feeder 
pillar shown in Figure 21 was used as input variable of the modeled system. The 
simulation results are given in Figure 22 and Figure 23. 
00 
i— 
a) 
i 
o 
a 
180 
150 
120 
> 90 
o > 
u < 
Assumed daily load curve at the feeder pillar 
T V * T K J i ^ 
I l A * * a 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
Time of day 
W-3Ph 
— var-3Ph 
Figure 21 : Load curve at the feeder, assumed for simulation 
27 
a) Reactive power at the source c) Load increase due to improved voltage 
Figure 22 : Simulation results for fixed value capacitor at the feeder pillar 
The Figure 22-a), above shows the reduction in reactive power, supplied from the 
source end due to capacitors at the load end. Figure 22-b), shows the reduction in 
line current with 20 kvar capacitor at the feeder pillar. 
When a 20 kvar capacitor was connected to the feeder, the voltage at that point 
had been improved by 1 to 2V, which had slightly modified the load of the 
simulated model. Figure 22-C), shows the minute change in active power of the 
load end under the above condition. 
Location: Feeder pillar Si2e of Fixed capacitor: 20 kvar Energy saving: 124 kWh/month 
sso 
600 
SSO 
5G0 
450 
400 
360 
0:00 .12:00 24:00 
0:00 12:00 24 :00 0:00 12:00 24:00 
d) Reduction of line losses 
1 0997 
1.0996 
1 0995 
1 0994 
1 0993 
b) Reduction of line current 
24:00 
Power loss across the feeder was monitored separately for the two cases of with 
and without the capacitor under the varying load condition of the day. The resulted 
power saving is shown in Figure 22-d). The accumulated energy saving within a 
period of one month was 124 kWh for a 20kvar capacitor. 
Figure 23, below shows the corresponding results for the fixed value capacitor at the 
distribution transformer. In this case, monthly energy saving, possible with a 20 kvar 
capacitor was only 30 kWh. 
a) Reduction o f reactive power at source c) Load increase due to improved voltage 
17:00 
b) Reduction of line current 
12:00 
£ 200 
24:00 
17:00 
dI Reduction of line losses 
0:00 
Location: T r a n s f o r m e r S i ze o f F ixed capacitor : 2 0 kvar 
12:00 24:00 
Energy saving: 3 0 k W h / r n o n t h 
Figure 23 : Simulation results for fixed value capacitor at the transformer 
Results obtained by simulation as above are compared in the Figure 24 below. It 
can conclude that the optimum size of capacitor for connecting at a feeder pillar is 
in the rage of 50 to 60 kvar and it would save 220 kWh units per month under the 
assumed load conditions of simulation. The corresponding optimum fixed value 
capacitor at the transformer is 40 kvar, approximately whereas the possible energy 
saving was only 60 kWh per month. 
Energy saving for different capacitor values 
250 
c 
o 
E 
CO 
c 
> 
CD 
CO 
— FP(Fix) 
— TF(Fix) 
20 30 
Figure 
40 50 60 
Size of capacitor (kvar) 
24 : Power saving under different capacitor values 
70 
29 
3.4 SELECTION OF VOLTAGE RATING OF CAPACITORS 
The characteristic lifetime of film type capacitors is mathematically expressed by 
Equation 1 [7], 
6 = Lifetime at name plate rating X 
Rated voltage 
Applied voltage 
Vscale 
Equation 1 : Characteristic lifetime of a.c. metalized film capacitors 
0 is the characteristic life of the capacitor in hours. Characteristic life is defined as 
the point in time, where 63.2% of the population fails. 
Vscale is the scaling factor assigned for voltage stress. The voltage-scaling factor 
varies by capacitor manufacture and ranges from 7 to 9.4. 
Rated Temp is the case temperature for which the capacitor is rated 
Variation of lifetime for different voltage ratings of capacitor is shown in Figure 25. 
Capacitors with rated voltage of 440V were selected for the proposed study, where 
the operating voltage is 415V, when connected at distribution transformers or 
feeder pillars. This will extend the characteristics lifetime by 50% to 75% of the lifetime 
at name plate rating of capacitors. This corresponds to improvement of lifetime up 
to 17 years to 20 years, whereas the lifetime at name plate rating of capacitors was 
11 years. 
Chacteristic lifetime of capacitors 
under different voltage ratings (System voltage - 4X5V) 
2 3 4 5 6 7 8 
Voltage stress factor (design value) 
Figure 25 : Variation of lifetime under different voltage ratings of capacitors 
30 
3.5 ASSESMENT OF AMBIENT TEMPERATURE APPLICABLE TO CAPACITORS 
Fixed type capacitors are housed in steel enclosures, which are attached to feeder 
pillars. The internal and surrounding air temperatures of those enclosures were 
measured using digital thermometers and mercury thermometers as shown in Figure 
26. Temperature readings were taken during the period from 0900h to 1500h to 
cover all the capacitors, installed at feeder pillars. 
Figure 26: Thermometers used for field measurement 
3.5.1 Internal air temperature of enclosures 
Internal temperature of enclosures with Capacitors 
Internal temperature (°C) 
Figure 27 : Distribution of internal air temperature of steel enclosures. 
The internal air temperature of the steel enclosure is the ambient temperature 
applicable to capacitor units. As per Figure 27, the capacitors had been subjected 
to maximum of 55°C temperature. The average value of the internal air 
temperature was 39°c with 6.3% standard deviation. The high temperature of 
operation affects the life of capacitors. Therefore, it was required to select a higher 
temperature rating for capacitors. And also it was necessary to identify suitable 
31 
means to reduce the internal temperature of steel enclosures, wherein capacitors 
are installed. 
3.5.2 Selection of ambient temperature category 
Upper limit of the ambient temperature, applicable to capacitors for general usage 
is divided into four categories as detailed in Table 5 [8], 
Ambient temperature 
Symbol °C 
Maximum 
Highest mean over any period of 
24 h 1 year 
A 40 30 20 
B 45 35 25 
C 50 40 30 
D 55 45 35 
Table 5 : Upper limit of ambient temperature categories 
The temperature class of capacitors was selected to be -25/D, which can withstand 
following ambient conditions, referring to standard IEC 60831-1. 
• Maximum ambient temperature of 55 °C 
• Highest mean ambient temperature over 24 hrs period of 45 °C and 
• Highest mean ambient temperature over 1 year period of 35 °C 
3.5.3 External air temperature of enclosures 
External temperature of enclosures (Ambient) 
^ 20 r • 
I 10 I o 
o 0 L— - m - ~ j m — — • — ™ — - - H Count 
O 28 29 30 31 32 33 34 35 36 37 38 39 40 
z External temperature (°C) 
Figure 28 : Distribution of external air temperature of steel enclosures. 
The external air temperature of the steel enclosure is the ambient temperature 
applicable to enclosures housing capacitors. The average value of the external air 
temperature was 33°c with 1.7% standard deviation. It was noted that in some 
cases, the external air temperature measurements had been affected due to the 
heat of feeder pillar itself and concreted pavements etc. 
3.5.4 Internal to External temperature gradient 
Internal to External temperature difference 
10 I 
GO 
0 5 10 15 20 25 
Temperature difference (°C) 
Figure 29 : Internal to external temperature difference. 
The average value of internal to external air temperature gradient was 7°c with 5.4% 
standard deviation. As per Figure 29, enclosures containing capacitors had been 
subjected to a maximum of 20°C temperature difference, measured with the 
outside environment. It is a very high value when compared to the size of the 
enclosure. Thus it was required to redesign the enclosure to improve the ventilation. 
The enclosures, installed with capacitors are having limited space for heated air to 
trap. High temperature difference measured from internal air to surrounding air of 
steel enclosure reveals that the holes or louvers provided were not sufficient. It 
needs to improve the circulation of air inside of enclosure. Further, other means shall 
be employed to reduce the rate of heating of enclosure due to radiation of the sun. 
Since the capacitor is hermetically sealed and only the terminal end has to protect 
against the splash of water, it is possible to improve the ventilation by having more 
and large louvers or holes around the enclosure. 
33 
3.6 REDUCTION OF OPERATING TEMPERATURE OF CAPACITORS 
3.6.1 Advantage of enclosure 
Outdoor type capacitors are available for low voltage applications and these 
capacitors have sealed terminals to prevent ingress of moisture. Figure 30 
compares the arrangements of indoor and outdoor type capacitors. 
a) Indoor capacitor b) Sealed type outdoor c) Interal terminals are 
capacitor sealed with resin against 
moisture 
Figure 30 : Indoor and outdoor type capacitors 
Field measurements were taken with a dual channel temperature logger to assess 
the advantage of having an enclosure for reducing the operating temperature of 
capacitor. The measuring setup is shown Figure 31 and the results are shown in 
Figure 32. The enclosure had been spray painted with light gray colour and it was 
without any thermal insulation attached to the internal surface and louvers had 
been provided for ventilation. 
Figure 31 : Temperature logging with and without enclosure 
34 
T1 T2 
2 Tue Dec 2008 Time 
Figure 32 : Effect of enclosure 
Accordingly, the cose temperature of the capacitor can be reduced 
approximately by 2.5 °C by having an enclosure to house the capacitor unit. 
3.6.2 Effect of exterior colour on heating due to sun's radiation 
Internal air temperature of the steel enclosure was logged simultaneously for the 
two enclosure types as shown in Figure 33, using thermocouples with Fluke 54 II 
temperature logger. The type of louvers and dimensions of the two enclosures were 
the same and one enclosure had been painted with dark green colour and the 
other had been coloured with light gray. 
(a) (b) 
Figure 33 : Temperature logging to evaluate the effect of exterior colour 
35 
27 Fri Jun 2008 Time 
Figure 34 : Effect of colour on internal temperature 
The internal air temperature of the enclosure finished with dark green colour had 
reached a maximum temperature of 59 °C and the light gray enclosure had 
reached a maximum of 52 °C. The average of difference of internal air temperature 
of two cases was 6 °C. Thus the exterior colour of the enclosure is significant for 
reducing inside air temperature of the enclosure when it is open to sunlight, directly. 
Figure 35: Use of thermal insulation and temperature measuring set up 
3.6.3 Thermal insulation 
The arrangement used for logging internal air temperature with and without thermal 
insulation is shown in Figure 35 and the results are shown in Figure 36. 
36 
T1 12 
W i t h o u t t h e r m a l 
i n s u l a t i o n 
-•yA/ 
/ .Y 
b i 
1 Mon Dec 2008 
/ W i t h t h e r m a l i n s u l a t i o n 
Figure 36 : Effect of thermal insulation 
S t e a d y r e g i o n 
The thermal insulation, applied internally of the enclosure had reduced the inside air 
temperature by 2.5 °C, approximately. 
3.6.4 Ventilation arrangement 
The internal air temperature was logged simultaneously of the two enclosure types 
as shown Figure 37, where one enclosure had holes for ventilation and the other 
with louvers. The temperature was measured with Fluke 5411 dual channel logger. 
The results are shown in Figure 38. 
Figure 37 : Comparing ventilation arrangements 
T1 T2 
47 
46 
45 
44 
43 
42 
41 
P 
38 
38 
37 
35 
34 
13:05 13:10 13:15 13:20 13.25 13:30 13 35 13 40 
2 Tue Dec 2008 Time 
Figure 38 : Holes against louvers for ventilation 
It is clear from the above figure that holes are more effective for ventilation than 
louvers. The internal air temperature of enclosure with holes had been lower by l°C 
to 1.5 °C, approximately when compared that with louvered enclosure. 
3.6.5 Selection of enclosure 
Following options were considered for deciding the most suitable type of enclosure. 
1. Use of capacitors without enclosures. 
2. Light gray finished enclosure without thermal insulation and louvers for 
ventilation. 
3. Light gray finished enclosure with interior thermal insulation and holes for 
ventilation. 
4. Dark green finished enclosure without thermal insulation and louvers for 
ventilation. 
The second option had lowered the body temperature of capacitor units by 2.5 °C, 
when compared with option 1, above. 
38 
The third option hod lowered the internal air temperature, which is the ambient 
temperature applicable to capacitor units by 4 °C when compared with option 2, 
above. 
The third option, when compared with option 4 had reduced the internal air 
temperature by 10°C. 
The dark green colour had a major impact on the internal air temperature. 
Therefore the use of dark green for feeder pillars in Colombo City was discontinued. 
Referring to above results, the third option is recommended for continuation of 
capacitor installation works. 
3.7 INFLUENCE FROM SYSTEM HARMONICS 
3.7.1 Current harmonics absorbed by capacitors 
Currents Harmonics absorbed by a 40 kvar and a 30 kvar capacitor are shown in 
Figure 39. Quadruple harmonics had not been absorbed by the capacitor, as it was 
connected in Delta. 
H a r m o n i c a b s o r p t i o n by capaci to r s 
60 7 — - — — 
1 3 S ' 7 9 11 13 I S 17 
Harmonic component 
Figure 39 : Current harmonics absorbed by capacitors 
3.7.2 Harmonics presence of the load at feeder level 
Current harmonics of the load were measured at feeder pillars using LEM Q70 power 
quality analyzer. Sample readings taken under high and low load conditions are 
shown in Figure 40. 
39 
Load harmonics measured in feeder 
i High load 
I L o w load 
• L - • _ , mm—, _ 
3 5 7 9 
Harmonic c o m p o n e n t 
Figure 40 : Current harmonics at a feeder pillar 
Harmonics of the order of 3rd, 5th, 7th and 11th were significant. Total harmonic 
distortion (THD) level of the load current during the peak time was 5%. The 
corresponding THD level in off-peak time was 23%. 
3.7.3 THD levels in peak & off-peak duration 
Current harmonics of load measured at the feeder pillars are shown in Figure 41. 
When comparing severity of harmonic under different load conditions, the THD 
figure would not express the correct status of the problem. 
Harmonic content of the load current 
High 
load 
Fundamental current (A rms) 
Figure 41 : Load harmonics 
40 
Following results in Table 6 were derived referring to Figure 41 for comparison of THD 
values with the harmonic current under different load conditions. 
Low load High load 
Average fundamental current (A rms) 50 300 
Average total harmonic current (Arms) 12 22 
THD value 24% 7.3% 
Table 6 : Comparison of harmonic current and THD values 
The THD value was higher (24%) under low load condition, where the total harmonic 
current was only 12A compared with corresponding values under high load 
conditions, which were 7.3% and 22A, respectively. 
Therefore, harmonic current in rms shall be recorded of different harmonic orders for 
assessing the harmonic problem, correctly. Then, the total harmonic current shall be 
compared against the allowable over current rating of capacitors for assessing the 
influence of harmonics on capacitors than mere comparison of THD values. 
3.7.4 Influence from system harmonics on capacitors 
Current harmonics absorbed by fixed capacitors installed at feeder pillars were 
monitored and results are shown in Figure 42. Power capacitors manufactured to 
I EC 60831 have an overcurrent rating of 1.5ln, when taken into account the 
permissible tolerance of capacitance [6], 
41 
L 
Harmonic current absorption by capacitors, installed at feeder pillars 
20 
E X-
c 
<u 
3 
U 
u 
c 
o 
E 
15 
10 
Cap rat ing @> 440V :40kvar 
in (5> 415V (operat ing vo l tage) ; 50A 
Max. al lowable over current :26A 
Cap rating g? 440V :30kvar 
In @ 415V (operating vol tage): 37A 
Max. al lowable over current :20A 
• 
• • • • • • 
• • • • 
* • 
• # • • • • • • • • • 
Partially failed units • • • • • • • • • 
• • • • • « • • • 
• • • # • • • • • 
• • • • 
• 
r 1 « 
20 25 30 35 40 45 
Fundamental current (A rms) 
50 55 60 
Figure 42 : Level of current harmonics absorbed by capacitors 
Figure 42 shows three separate regions. When compared with nominal current of 
capacitors that were used in the sample system, those regions represent partially 
failed capacitor units, 30 kvar healthy units and 40 kvar healthy units in the given 
order. The vertical axis shows the harmonic current absorbed by each capacitor 
units. According to the allowable overcorrect rating of capacitors (1.5ln), 30 kvar 
and 40 kvar capacitors have overcurrent ratings of 20 A and 26 A, respectively. 
According to field measurements depicted in Figure 42, the maximum current 
drawn by 30 kvar and 40 kvar capacitors, over their rated current is less than the 
corresponding allowable overcurrent ratings. Thus it can conclude that fixed 
capacitors, installed in the selected sample have no influence from load harmonics. 
It is clear from the above figure that capacitors had absorbed certain components 
of current harmonics of the system. Thus the capacitors installed as a cluster in the 
distribution system has contribute to improve the quality of power to a certain 
degree. 
42 
3.7.5 Frequency domain analysis of the simplified model 
Fixed capacitor along with inductances of the transformer and line reactances 
would create a resonance condition. Transformer terminal voltage would rise under 
resonance conditons causing the transformer to fall into ferroresonance condtion 
damaging the transformer and other switchgears. Transformer under ferroresonance 
condition operates at high flux levels causing to overheat and vibrate the 
laminated sheets of the Iron core leading to insulation failure damaging the 
transformer [9-12], Thus it should be ensured that fixed type capacitors installed at 
feeder pillars or at transformers would not form a resonance circuit. 
The model representing two feeders connected to a 1000 kVA transformer was 
considered here. The model was simulated for different capacitor values 
connected to feeder pillar and transformer, separately under no load condition. 
The model given in Figure 43 was used to analyze the frequency response when 
capacitors are connected at feeder pillars. The low voltage capacitor would form a 
resonance circuit with saturable core inductance of transformer. Simulink model 
was used to study the resonance conditions that would occur at the transformer. 
r * 0 r * 0 
zs ZTF 
P o w e r g u i 
- C o n t i n u o u s 
11/0.415 kV Capacitor 
Transformer a t TF 
Mag 
Core2 Feeder 
+-WV--
capacitoi at Feeder 
feeder Pillar total load 
Feeder pilars 
Figure 43 : Model for analyzing the frequency response 
43 
The result obtained when a 200 kvar capacitor was connected at feeder pillars is 
shown in Figure 44 as an example. It depicts that a parallel resonance would occur 
for harmonic of the order of 7th. 
F r e q u e n c y ( H z ) 
P h a s e 
Figure 44 : Frequency response at a feeder pillar 
Summary of results corresponds to various capacitor values at the feeder pillar are 
shown in Figure 45. The frequency of resonances for capacitor values lower than 20 
kvar is higher than the 21st harmonic. On the other hand, the presence of harmonics 
of the order higher than 21st in the low network is negligible. 
Frequency response 
Impedance at feeder pillar fo r di f ferent capacitor values 
450 550 650 
Frequency (Hi ) 
Figure 45 : Frequency response for various capacitor values at the feeder pillar 
44 
Summary of results obtained for different capacitor values at the feeder pillar are 
shown in Figure 46. The frequency of resonances for capacitor values lower than 60 
kvar is higher than the 21st harmonic, where the presence of harmonics of the order 
higher than 21st is negligible. 
Figure 46 : Frequency response for various capacitor values at the transformer 
3.8 OVERVOLTAGE DUE TO FIXED CAPACITORS UNDER LOW LOAD CONDITIONS 
The model was simulated for various capacitor values with no load connected to 
feeder pillars. The voltage rise at respective points of the network was monitored for 
the two cases that the fixed type capacitor is connected at feeder pillar and 
transformer. The results, tabulated in Table 7 indicate that the rise in voltage under 
no load condition even for higher fixed capacitors values are not a critical issue. 
Size of 
capacitor 
Fixed capacitor at 
feeder pillar 
Fixed capacitor at 
11/0.145kV transformer 
20kVA +1 +1 
60kVA +2 +2 
80kVA +3 +3 
Table 7 : Voltage rise under no load condition 
Frequency response 
Impedance at TF secondary far d i f ferent capacitors sizes 
4 CONCLUSION AND RECOMMENDATIONS 
Capacitors can be used to reduce system losses in low voltage networks too. This 
cannot be achieved with HV or MV capacitors as the reduction of line current due 
to capacitors is only in upstream sections of the network from the point of reactive 
power injection. 
The proposed method of using fixed power capacitors, which can be considered as 
a distributed type of compensation method of reactive power is more reliable 
compared to bulk reactive power compensation techniques used at higher voltage 
levels. Failure of HV shunt power capacitor system would affect a major load. 
Fixed type power capacitors, which require only minimal accessories, are highly 
effective and very economical in reducing system losses of the low voltage 
distribution network. The payback period of the proposed method is only 6 months. 
Followings are recommended when implementing a cluster of Fixed type 
capacitors for loss reduction purposes in a LV distribution system. All possible means 
shall be employed to reduce the operating temperature of capacitors, which was 
found to be the major reason to reduce the lifetime of capacitor units. 
• Use capacitors close to the final distribution point of the low voltage network. 
In the selected sample of underground system having fixed type low 
voltage capacitors at feeder pillars is more effective than having them at 
distribution transformers. 
• When capacitors are installed outside and exposed to direct sunlight used 
enclosures. Enclosures will reduce temperature of the capacitor tank at 
least by 2.5 °C. 
• The exterior colour of enclosures shall be a light colour. For example the 
internal air temperature of a light gray coloured enclosure is lower by 7 °C 
compared to that of Dark green finished enclosure. 
• Thermal insulators with reflective foil shall be affixed on to internal surfaces. 
Internal air temperature of enclosures could be reduced by 2.5 °C by 
thermal insulators at least on top and the front sides of the enclosure. 
• Provision of sufficient number of holes for ventilation of enclosures is more 
effective than having louvers. Louvers do not allow air flow in all directions 
with the flow surrounding natural air. It is possible to achieve a temperature 
difference of 1~1.5°C of internal air inside of the enclosure by using holes 
instead of louvers. 
• Cluster of low voltage capacitors installed in the distribution system shall be 
monitored periodically after installation. 
47 
REFERNECES 
[1] Colombo City Ring diagram, Planning Branch, CEB, Colombo City, 2008. 
[2] Feeder pillar details, Operation Unit, CEB, Colombo City, 2008. 
[3] Report on loss reduction activities in Colombo City, Energy management unit, 
CEB, Colombo City, 2008. 
[4] Primary substation load readings, Operation Unit, CEB, Colombo City, 
2006,2008. 
[5] Power factor correction and harmonic filtering, Spain,Circutor, 2006. 
[6] "Over loads," in Shunt power capacitors of the self healing type for a.c . 
systems having a rated voltage upto and including 1000V, IEC 60831-part 1, 
pp.39, 1996. 
[7] Avoiding AC Capacitor Failures in Large UPS Systems, American Power 
Conversion, Revision 0, pp. 10-11, 2003. 
[8] "Service condtions," in Shunt power capacitors of the self healing type for a.c . 
systems having a rated voltage upto and including 1000V, IEC 60831-part 1, 
pp. 17-19, 1996. 
[9] Surya Santoso, Roger C. Dugan, Peter Nedwick, Modeling Ferroresonance 
Phenomena in an Underground Distribution System, The IPST Conference, pp.2, 
2001, Available http://www.ipst.org/TechPapers/2001 /IPST01 Paper034.pdf 
[10] Zia Emin, Yu Kwong Tong, Ferroresonance Experience in UK: Simulations and 
Measurements, The IPST Conference, pp.1, 2001, Available http://www.ipst.org 
/TechPapers/2001/IPST01 Paper006.pdf 
[11] "Symptoms of ferroresonance", Wiki: Ferroresonance (1/2), Available 
http://wapedia.mobi/en/Ferroresonance#5 
APPENDIX A 
Sub A 24(Wed) & 7(Sun)-09-
40 T — 2008 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Sub C 24(Wed) & 7(Sun)-09-
20 2008 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
60 
40 
20 
0 
Sub E 24(Wed) 8. 7(Sun)-09-
2008 
A 
§ \ J J\\ 
/ . 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Sub G 24(Wed) 8. 7(Sun)-09- " 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Sub B 24(Wed) & 7(Sun)-09-2008 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Sub D 24(Wed) & 7(Sun)-09-2008 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Sub F 24(Wed) & 7(Sun)-09-2008 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Sub H 24(Wed) & 7(Sun)-09-2008 
0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 
Mvar_H (On a high demand day) 
Mvar_L (On a low demand day) 
MW_H 
MW L 
49 
APPENDIX B 
STEPS OF MATLAB PROGRAM 
Initialize model parameters 
Import Load data (P & Q) 
Case of simulation ? 
Casel: Fix capacitor at 400V feeder pillar (20kVA @ 2xFP) 
Case2: Fix capacitor at 11/0.415kV transformer (40kVA @ 1xTF) 
Caee3: Switching capacitors at 11/0.415kV transformer (40kVAx5 steps ( 1xTF) 
Steps - Case 1 or 2 
• System is simulated for the imported load data, where the Load is switched on in all three 
r 
feeder pillars at t=0 s. 
• P, Q, V & I at the source and the load are measured at t=0.25 s. 
• Fix capacitors at three feeder pillars or fix capacitor at the transformer are switched on at t=0.3 
s, depending on the case. 
• P, Q, V & I at the source and the load are measured at t=0.5 s. 
Steps - Case 3 
• System is simulated for the imported load data, where the Load is switched on in all three 
feeder pillars at t=0 s 
• Q flow at Transformer secondary is measured at t=0.25 s 
• No. of possible capacitor to switch is calculated to compensate above Q. 
• System is again simulated for the imported load data as before 
• P, Q, V & I at the source and the load are measured at t=0 25 s. 
• All required capacitor steps are switched on at t=3 s. 
• P, Q, V & I at the source and the load are measured at t=0.5 s. 
Calculated results 
• Power loss between the source and the load without capacitor as 
[Source Power 3 X Load Power ] with capacitor 
• Power loss between the source and the load with capacitor as 
[Source Power ~ 3 X Load p o w er ] without capacitor 
• Saving of loses is calculated as the difference of above results. 
• Energy saving over a period of one month is calculated by integrating above results 
MORATUi 
50 
IB R A R l