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Efficiency comparison of alternating current (AC) and direct current (DC) distribution system at residential level with load characterization and daily load variation

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The war of currents between alternating current (AC) and direct current (DC) is decades old. The DC has proved itself in the fields of generation and transmission; now it is making its way at distribution level in quite an efficient fashion. The DC seems to be challenging the AC, as a result of efficient DC to DC as well as to and from AC conversion with the development of efficient power electronic converters. This paper presents efficiency comparison of the two paradigms for a residential colony. The model is separately simulated for AC and DC supply from the grid keeping the infrastructure for both schemes unchanged. The residential loads are divided in to three categories depending upon their supply requirements. The loads are further classified according to their time of usage during the whole day. The efficiency comparison between AC and DC system is established on the basis of daily load variation.
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Efficiency Comparison of Alternating Current (AC) and Direct
Current (DC) Distribution System at Residential Level with Load
Characterization and Daily Load Variation
Hasan Erteza Gelani1*, Mashood Nasir2, Faizan Dastgeer1, and Haseeb Hussain1
1Department of Electrical Engineering, University of Engineering and Technology Lahore,
Faisalabad Campus, Faisalabad, Pakistan
2Department of Electrical Engineering, Lahore University of Management Sciences,
Syed Babar Ali School of Science and Engineering, opposite Sector U, Defence Housing Authority,
Lahore 54792, Pakistan
Abstract: The war of currents between alternating current (AC) and direct current (DC) is decades old. The
DC has proved itself in the fields of generation and transmission; now it is making its way at distribution
level in quite an efficient fashion. The DC seems to be challenging the AC, as a result of efficient DC to
DC as well as to and from AC conversion with the development of efficient power electronic converters.
This paper presents efficiency comparison of the two paradigms for a residential colony. The model is
separately simulated for AC and DC supply from the grid keeping the infrastructure for both schemes
unchanged. The residential loads are divided in to three categories depending upon their supply
requirements. The loads are further classified according to their time of usage during the whole day. The
efficiency comparison between AC and DC system is established on the basis of daily load variation.
Keywords: AC, DC, efficiency, residential, load categories, load variation
1. INTRODUCTION
The so called war of currents between alternating
current (AC) and direct current (DC) has its roots
from the very start of electricity. The pioneers of
electricity generation Edison and Tesla both had
their own views of mode of electricity. Edison
supported DC as mode of electricity whereas Tesla
supported AC. In the beginning AC winning the
battle of currents due to invention of transformers,
an easy and cost effective way of stepping up and
stepping down voltages as required.
Alternating current was considered the
champion in all sections of electricity, i.e.,
generation, transmission and distribution.
However AC enjoyed dominance and superiority
over DC for not a long period of time. Researchers
were constantly putting efforts to find new modes
of generation and that lead to the invention of
renewable resources like solar panels and fuel
cells that produced power in the form of DC.
Direct current has proved itself in transmission
area better than AC due to lesser losses. With the
advancement of research in the relative field DC
made its way in the distribution section. Direct
current challenging AC at distribution level due to
the invention of highly efficient power electronic
converters and several DC operated electronic
loads.
At residential level DC has entered our
bedrooms, kitchen and garage. Many of residential
Research Article
Proceedings of the Pakistan Academy of Sciences: Pakistan Academy of Sciences
A. Physical and Computational Sciences 54 (2): 111–118 (2017)
Copyright © Pakistan Academy of Sciences
ISSN: 2518-4245 (print), 2518-4253 (online)
————————————————
Received, January 2016; Accepted, June 2017
*Corresponding author: Hasan Erteza Gelani; Email: erteza.gelani@uet.edu.pk
loads are DC operated; our microwave ovens,
computers, laptops, phones, lighting and electric
vehicles. The feasibility of DC distribution and its
comparison with AC has been the interest of many
researchers since years. The comparison is made
on the basis of efficiency, cost and losses [1].
Conversion loss comparison was made by Seo et
al. [2]; they suggested that with local generation
DC is better than AC as far as conversion losses
are concerned. It was further proved that converter
efficiency is directly proportional to loading. In
case of DC distribution, DC-DC converter is the
heart of the system and its losses were studied by
Gelani and Dastgeer [3]. The authors proposed that
the conversion losses of the main DC-DC
converter have serious impact on the overall
efficiency of DC distribution system of a
residential colony.
Starke et al. [4] created two models and
simulated them with AC and DC separately. It was
proved that for the same conduction losses DC is
1.22 times better than AC. DC proved to be a
better choice within buildings considering losses,
safety and power quality [5]. The authors also
presented a model with different operating
voltages for DC distribution system and it was
suggested that a voltage level of 326V is best
suited keeping in view the power loss and voltage
drop. Another topology presented by Nilsson and
Sannino et al. [6] considered three designs; DC,
AC and mixed AC/DC. The mixed system was
proved to be worst as regards to system losses and
DC system was proved to be better than AC only
when semiconductor losses are considered half.
With DC as medium of power transfer; several
research efforts are made involving distributed
generation and microgrids for colonies. The losses
involved in power transfer comprising conversion
losses have been an interest to many researchers
[7-10]. The authors presented different models at
microgrid level, compared AC with DC and
deduced their results favoring DC over AC.
Besides DC gaining success over AC in many
areas yet DC lags AC in many fields, the major
one being protection. As there is no null point in
DC, employing DC at distribution level is a threat.
As well as the deciding feature in the efficiency
calculation is the DC-DC converter whose
efficiency varies with the load therefore at low
loads the efficiency of DC system will be lower
than that of AC system [3].
This paper presents a model of a residential
colony comprising 50 homes. The model is
separately simulated for AC and DC supply from
the grid. The infrastructure of the colony is kept
the same for both simulations. By infrastructure;
the conductor type, resistance and load demand is
meant. The loads are divided in to three categories
depending upon their supply demand. A further
classification of the loads is made on the basis of
their time of usage. Six periods, each period of
four hours is developed and the system is
simulated for each period with AC and DC supply.
2. LOAD CLASSIFICATION
The common residential loads are divided in
following categories:
DC Loads: The loads that require DC power for
their operation.
AC Loads: The loads that require AC power for
their operation.
AC/DC Loads: The loads that can be run both on
AC and DC for their operation. These loads
include majorly heating loads. When the system is
simulated with DC supply, the supply to these
loads will be DC and for system’s AC supply,
these loads will be run on AC.
Table 1 shows classification of residential
loads according to their time of usage. A full day
is divided into six periods, each period of four
hours. Obviously loads do not operate for twenty
four hours; the loads are divided in sections of
their time of usage. Table 2 presents classification
of AC, DC and AC/DC loads according to their
time of usage. An average home consumes 30kWh
112 Hasan Erteza Gelani et al
per day [3]; hence the loads are allotted power
according to the percentage of 30 kWh. From the
pie chart of Fig. 1 [11], the percentage share of
each residential load helps in calculating wattage
of each load consumed throughout the day e-g
share of air-conditioning unit is 17% and 17% of
30kW is 5.1KW. This 5.1kW is distributed
according to the time when air conditioning unit is
operational.
3. MODELING AND SIMULATION
As stated earlier that a residential colony of fifty
(50) homes is modeled. DC power from the grid
reaches DC-DC converter which steps down the
grid voltage to 325V. The reason of delivering
325V at homes is to fully utilize the conductors.
The peak value of generally employed 230V RMS
is approximately 325V.
The effect of efficiency variation of DC-DC
converter with loading is shown in Fig. 2 [12].
Each main DC-DC converter supplies ten homes
as shown in Fig. 3. The homes are equipped with
AC, DC and AC/DC loads. Each home has its own
DC-DC converter to further step down the voltage
suitable for DC loads and an inverter for AC loads
as shown in Fig. 4.
Table 1. Period load classification in kW.
Appliances
Period-1
(0:00-
4:00)
Period-2
(4:00-8:00)
Period-3
(8:00-12:00)
Period-4
(12:00-
16:00)
Period-5
(16:00-20:00)
Period-6
(20:00-0:00) Total
Air Conditioning
1.5
1.5
0.2
0.2
0.5
5.1
Space heating
0.8
0.5
0.4
0.3
0.6
3.3
HVAC Appliances
0.2
0.27
0.25
0.23
0.25
1.5
Cooking
0.5
2.2
1.2
2
1.5
8.7
Water Heating
0.2
0.8
0.4
0.5
0.7
3
Lighting
0.1
0.2
0.2
0.6
1.1
3
Electronics
0.1
0.2
0.6
0.5
0.6
2.4
Laundry Appliances
0.1
0.4
0.6
0.3
0.5
2.1
Others
0.05
0.06
0.3
0.19
0.2
0.9
Fig. 1. Residential electricity consumption.
Efciency Comparison of Alternating Current (AC) and Direct Current (DC) Distribution System 113
Fig. 2. Efficiency vs loading for DC-DC converter.
Fig. 3. Residential colony for DC grid.
114 Hasan Erteza Gelani et al
Fig. 4. Inside home circuitry for DC colony.
Fig. 5. Residential colony for AC grid.
Fig. 6. Inside home circuitry for AC colony.
Efciency Comparison of Alternating Current (AC) and Direct Current (DC) Distribution System 115
The same model of fifty homes is simulated
for AC grid with suitable changes in fig. 3 as
shown in Fig. 5. The main DC-DC converter is
replaced by transformer; as DC-DC converter in
the previous case. Due to the fact that
transformer’s efficiency variation with loading is
very low therefore transformer is also assumed to
operate at its maximum operating efficiency. Fig.
6 depicts each home is now equipped with a
rectifier for DC loads and no need of another
transformer because AC loads can run on 230V
from the main transformer.
In order to make the simulation as near to
practical as possible; conductor and converter
(DC-DC, transformer and inverter) losses are
given their due portion. The losses are
characterized in to three categories: constant,
linear and quadratic according to their relationship
with the output power. Of these linear losses
making up major portion the lot, comprise of
switching loss [3].The switching losses occurring
with their proportion are shown in table 3 as
included in the simulation. Inductor and Capacitor
power loss is modeled by placing series resistance
with ideal inductor and capacitor respectively. On
state switch loss is modeled by taking product of
switch voltage and current in on state; transient
switch loss is modeled by measuring power loss in
fall time and tail time of the switch. Diode power
loss is calculated by multiplying on state voltage
and current whereas gate driver loss is adjusted by
pulse count and time for pulses for a fixed period.
Both models are simulated for six periods of time
and results are tabulated. The efficiency for each
period is found by using the simple
relation P
out P
in
. Standard Drake [13] ACSR
(Aluminum Conductor Steel Reinforced) is
employed with 0.025Ω/mile dc resistance and
Table 2. Period wise supply load classification in kW.
Categories Period-
1
(0:00-4:00)
Period-2
(4:00-8:00)
Period-3
(8:00-12:00)
Period-4
(12:00-
16:00)
Period-5
(16:00-20:00)
Period-6
(20:00-0:00) Total
AC
1.8005
2.1706
1.053
0.7319
1.252
1.701
8.709
DC
0.2005
0.4006
0.803
1.1019
1.702
1.201
5.409
AC/DC
1.5005
3.5006
2.003
2.8019
2.802
2.401
15.009
Table 3. Converter loss contribution.
Loss Type Proportion per Converter
Inductor Power Loss
0.5%
Capacitor Power Loss
0.7%
Switch Loss- On state
0.15%
Switch Loss- Transient state
1.8%
Diode Power Loss
0.8%
Gate Driver Loss
0.25%
Table 4.Period wise efficiency of AC and DC colony
Efficiency
Period-1
(0:00-
4:00)
Period-2
(4:00-8:00)
Period-3
(8:00-12:00)
Period-4
(12:00-
16:00)
Period-5
(16:00-
20:00)
Period-6
(20:00-0:00)
AC
91.36%
92.63%
88.91%
83.99%
90.11%
91.09%
DC
77.65%
82.33%
85.23%
91.21%
95.41%
92.31%
116 Hasan Erteza Gelani et al
0.1172Ω/mile ac resistance. The pole spacing in
system is kept at 30 meters and ac/dc resistance
values are adjusted accordingly.
4. RESULTS AND DISCUSSION
The results tabulated in Table 4 present the
efficiency values throughout the day. Different
loads operate at different periods of time. For
periods 1, 2 and 3; the efficiency of AC grid is
higher than that of DC grid. The reason is
dominance of AC loads during these periods. The
difference between AC and DC grid efficiency for
periods 1 and 2 is large due to the fact that DC
loading is quite low in these periods and
considering the curve of Fig. 2 the DC-DC
converter efficiency is very poor at light loads.
The loss coefficients evaluated in [3] for DC-DC
converter revealed the maximum dependency of
DC distribution system on main DC-DC converter
therefore with poor efficiency values for DC-DC
converter at light loads has a direct impact on
system’s efficiency.
As DC loads increase operating during day
time, the efficiency of DC-DC converter increases
and in turn the efficiency of DC grid enhances. DC
dominates AC in periods 3, 4 and 5 but the
difference between AC and DC grid efficiency
values are not to an extent as it was for periods 1,
2 and 3. With DC loads increasing in our society
this difference will however be overcome and
there is a chance that DC dominates AC
throughout the day.
Despite the fact that power electronics has
gained so much success that highly efficient are
available [3]; a conversion stage still holds
responsible for efficiency evaluation of a system.
Another reason for excellent efficiency of AC grid
as compared DC grid as well as better average
efficiency of AC grid is the fact that AC grid lags
a conversion stage inside home than DC grid. The
inside home circuitry of DC grid has two
converters as compared to AC grid having only
one.
5. CONCLUSIONS
In the light of above discussion, it is concluded
that in order to improve the efficiency of DC grid,
the efficiency of DC-DC converter needs to be
improved particularly at light loads because this is
one of the major reasons that DC lags AC in
efficiency in three out of six periods presented in
this research effort. The DC-DC converter is the
“backbone” of the system therefore; its efficiency
decides the efficiency of the system. Finding
appropriate solution to the problem highlighted in
this research effort may lead to a new world of DC
distribution research. As specified, DC is entering
the area of power distribution after proving itself
in power transmission and there is a chance that
DC regains its fame that it had at the origin of
electricity era.
6. ACKNOWLEDGEMENTS
Mr. Suleman and Mr. Mohsin are highly appreciated for
their support and assistance.
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This paper analyzes the feasibility of direct current for the supply of offices and commercial facilities. This is done by analyzing a case study, i.e. the supply to the Department of Electric Power Engineering, Chalmers University of Technology, Gothenburg, Sweden. Voltage drop calculations have been carried out for different voltage levels. A backup system for reliable power supply is designed based on commercially available batteries. Finally, an economic evaluation of AC versus DC is performed and protection of the proposed system is briefly addressed.
AC vs. DC distribution: A loss comparison
  • M R Starke
  • L M Tolbert
  • B Ozpineci
Starke M.R., L.M. Tolbert, B. Ozpineci. AC vs. DC distribution: A loss comparison. IEEE PES Transmission and Distribution Conference and Exposition, 21-24 April 2008, Chicago, USA, p. 1-7 (2008).
Power Supplies: a Hidden Opportunity for Energy Savings. A NRDC report
  • N Horowitz
Horowitz N. Power Supplies: a Hidden Opportunity for Energy Savings. A NRDC report;
Power System Analysis
  • J J W D Grainger
  • Stevenson
Grainger J.J. & W.D. Stevenson. Power System Analysis, McGraw Hill (1994).