Automatic Three Phase Load Balancing System by
Using Fast Switching Relay in Three Phase
Distribution System
Sajid Ul Haq
Department of Electrical Engineering
COMSATS Institute of Information Technology
Islamabad Pakistan
engrsajid857@gmail.com
Bilal Arif
Department of Electrical Engineering
COMSATS Institute of Information Technology
Islamabad Pakistan
bilalarifmail@gmail.com
Asif Khan
Department of Electrical Engineering
ABASYN University Islamabad Campus
Islamabad Pakistan
khanasif109@gmail.com
Junaid Ahmed
Department of Electrical Engineering
COMSATS Institute of Information Technology
Islamabad Pakistan
junaid@comsats.edu.pk
Abstract—In three phase distribution system the unbalance
phenomenon occurs due to single-phase loads and unequal sharing of loads in each phase. To overcome this problem, distribution
systems require equal sharing of load in each phase. This is
possible by automatic three phase load balancing technique. The
automatic three phase load balancing system is possible by the
proposed hardware which is micro-controller and relay based
hardware. This hardware is installed in the incoming of three
phase lines and will accordingly switch the domestic load to the
least loaded phase using fast switching relays. The single full
house load is connected to the phase that is least loaded by
relay switching ON/OFF. The result of the proposed hardware
and simulation shows that microcontroller and relays switching
is effective in reducing the unbalancing in three phase lines and
also reducing the current in the neutral wire of the system. In
addition, it retains voltage regulation and stability in the all three
phase.
The phases are normally called red, yellow and blue. Three
phases distribution systems suffer from power fluctuations
and non-zeros neutral currents due to unbalanced loads. The
unbalanced loading in distribution systems mostly occur due
to the following reasons.
•
•
•
•
•
When there is a phase imbalance, due to the basic laws of
circuits, current increases in the neutral wire of three phase
system. The flow of neutral wire current is a hazard, it creates
safety problems and can cause fire. In addition it also decreases
the electrical power system efficiency, tripping (relays) protection devices of the system, and damaging network equipment.
The grids are designed as three phase electrical power systems.
Various problems arise in the electrical power distribution
grids because consumer loads are not perfectly predictable
and do not balance out when different loads are distributed
over the three phases. Faults in the distribution system can
cause over-loading in some areas and under-loading in others.
So to prevent these unbalance conditions, power and loads are
required to be controlled in real time. This real time controlling
technique is called load balancing technique. In other words,
load balancing is the process to avoid the power systems from
overloading and underloaded situation.
In [1] an intelligent consumers load transfer scheme is
proposed that dynamically reduces voltage unbalance (VU).
In this scheme to minimize VU in the distribution feeders,
consumers loads are transferred from one phase to another
without disconnection of phases. In [2] Scott transformers are
used in a low voltage radial feeder to balance the distribution
Keywords:Voltage unbalance, electric spring, electric vehicle, STATCOM.
I. I NTRODUCTION
Many domestic loads and small industries are supplied by
a single phase AC with a phase and neutral wire. However,
power is generated and transmitted in 3-phase AC. A single
phase generator has single coil rotating in magnetic field, while
3-phase generators have fixed coils that are separated by 120◦
from each other. These three voltages are out of phase from
each other by 120◦ . A set of balance 3-phase voltages form,in
terms of the time domain as,
√
VRed = 2Vm cos(wt + φ)
(1)
√
VY ellow = 2Vm cos(wt + φ − 120◦ )
(2)
√
◦
VBlue = 2Vm cos(wt + φ − 240 )
(3)
In three phase balance system the neutral current will be zero.
I N = IR + I Y + I B , I N = 0
Overloading of single phase.
Manual switching of the phases.
Unequal sharing of single phase loads on 3-phase system.
Unbalanced three phase loads.
Asymmetrical transmission impedance.
(4)
978-1-5386-5482-8/18/$31.00 ©2018 IEEE
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system. In this scheme, unbalanced 3 phase power supply is
converted to unbalanced 2 phase supply and then converted
back to a balanced 3 phase power supply using Scott transformers. In the scheme proposed by [3] the unbalancing of
a three phase system can be controlled by electric vehicle
(EV) chargers and photovoltaic (PV) inverters. A new 3phase electric spring (ES) circuit can also be used to decreases power unbalance in a 3-phase system as proposed
by [4]. In [5] a high realization control method is proposed
using cascade Static compensator (STATCOM) with (star)Y
configuration under unbalanced three phase conditions. In
conventional distributions, single-phase electrical loads are
equally distributed among three phases. However, daily loadflow variation is ignored, causing unbalancing in the three
phase distribution system. In [6] a new method is proposed
for equal sharing of load and balancing the three phases in
a 0.4 KV distributions grid. They also simulated daily load
flow using MATLAB/Simulink models. In [7] a new phasing
identification system is proposed that measures voltage phase
of underground distribution transformers at the secondary
side to determine the phase load and indicate unbalancing.
However, no proposal is given to reduce the unbalancing. In
[8] a new technique is developed for identifying automatically
the phase each domestic loads is connected to, this information
can be used by the distribution systems for phase rebalancing.
In this technique voltage information from the energy meters
and phase information from the transformers is collected over
time and then correlated to determine each customer’s phase.
In [9] a stochastic method is proposed to calculate the increasing of a single-phase photovoltaic inverter (PVI) to voltage
unbalancing in LV distribution networks. The ambiguity in
location and phase is included in a serial number of stochastic
indicators. This technique is used in induction motors, electric
vehicle charging (EVC) and single-phase loads.
In this paper we present an automatic 3-phase load balancing device that balances 3-phase load using microcontrollers
and relays. This paper explains the details of automatic three
load balancing system and steps for how to design and
implement an automatic load balancing in power distribution
network.
PT otal
(6)
(7)
i
3Vp2
2R
By changing phase voltage to RMS votage,
√
Vrms = 2Vp
PT otal =
(8)
(9)
2
3V
(10)
R
2) Power for Non-Resistive loads: When the load is non
resistive, it may be written as,
PT otal =
Z = Zejϕ
(11)
The peak value of the current,
Ip =
Vp
|Z|
(12)
The instantaneous current of all phases are given as,
IR = Ip sin(θ − ϕ)
2
IY = Ip sin(θ − π − ϕ)
3
4
IB = Ip sin(θ − π − ϕ)
3
Calculating power from the above current equations,
PR = VR IR = Vp Ip sin(θ)sin(θ − ϕ)
(13)
(14)
(15)
(16)
2
2
PY = VY IY = Vp Ip sin(θ − π)sin(θ − π − ϕ) (17)
3
3
4
4
PB = VB IB = Vp Ip sin(θ − π)sin(θ − π − ϕ) (18)
3
3
Using trigonometric functions we simplified the above power
equtions, can be simplified as,
Vp Ip
PR =
[cos(θ) − cos(2θ − ϕ)]
(19)
2
4
V p Ip
[cos(θ) − cos(2θ − π − ϕ)]
(20)
PY =
2
3
8
V p Ip
[cos(θ) − cos(2θ − π − ϕ)]
(21)
PB =
2
3
Adding equations (19-21) we obtain,
3Vp Ip
PT otal =
cos(ϕ)
(22)
2
II. BALANCE AND UNBALANCE LOAD
DEFINITION
In 3-phase distribution sytem, electical loads are distributed
between the phases. The parameter power, voltage and current
are balanced among all 3-phases. The flow of the current in
the neutral wire is zero in balance condition.
PT otal =
3Vp Ip
2 [cos(3ϕ)
− [cos(2θ)
+cos(2θ − 43 π − ϕ) + cos(2θ − 83 π − ϕ)]]
A. BALANCE LOADS
3Vp2
cos(ϕ)
2
By changing phase voltage to RMS votage,
√
Vrms = 2Vp
In a three-phase balanced power system, the voltage phasor
components have same magnitudes but are 120 degrees differ
from each other.
1) Power for Resistive loads: In case of resistive loads the
instantaneous powers is constant all time,
P =VI
V L i2
R
PLi
=
PLi =
PT otal =
PT otal =
(5)
3V 2
cos(ϕ)
Z
(23)
(24)
(25)
2
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3) No neutral current in Balance 3-phase system: No
neutral current be obtained by equal sharing of loads on each
of the 3-phases. Therefore the net current is zero in the neutral
wire of the three phase balnce system,
IR =
VR − N
R
(26)
IY =
VY − N
R
(27)
IB =
VB − N
R
(28)
−IN = IR + IY + IB
Fig. 1. Transposition of 3-phase System
3) Neutral phase shift: Neutral phase shift is created when
unbalanced phenomena occurs in current of the power system.
In this case, the distribution phases are not balanced to
each other; at load side the line to neutral voltages have
different magnitudes and phases. The magnitudes of current
in this situation in all three phases are different, which results
in different voltage drops which further cause the neutral
phase shift. Arcing and dielectric breakdown of insulation is
produced by neutral phase shifts.
(29)
As we know non-dimensionalized current, can be written as
i=
IN R
Vp
2
2
inon−dimension = sin(θ) + sin(θ − π) + sin(θ + π)
3
3
(30)
2
(31)
inon−dimension = sin(θ) + 2sin(θ)cos( π)
3
inon−dimension = sin(θ) − sin(θ)
III. DECOMPOSITION OF ASYMMETRICAL
COMPONENTS IN LV DISTRIBUTION NETWORK
In an unbalanced 3-phase system, the magnitudes and
phases of the voltage phasor components are different from
each other in LV network. Decompsing the voltage phasor
component into a complete set of symmetrical component
helps evaluate the system as well as any imbalance.
A vector of three phase voltage is,
⎡
⎤
Vr
Vryb = ⎣ Vy ⎦
Vb
⎡
⎤ ⎡
⎤ ⎡
⎤ ⎡
⎤
Vr
Vr,0
Vr,1
Vr,2
⎣ Vy ⎦ = ⎣ Vy,0 ⎦ + ⎣ Vy,1 ⎦ + ⎣ Vy,2 ⎦
Vb
Vb,0
Vb,1
Vb,3
(32)
inon−dimension = 0
B. UNBALANCED LOADS
When the amplitude of three phase voltages and current is
different and the angle between the phases is shifted by 120◦
it is called unbalanced or asymmetrical. The power system is
called unbalanced or asymmetrical. In balanced condition no
neutral current flows. Unbalance system occurs due to nonlinear loads.
1) Neutral Current: By adding the three phase currents
together as complex numbers and then converting form rectangular to polar co-ordinates form which determined neutral
current of the 3-phase unbalanced system . If the three-phase
root mean square (RMS) currents are,
The subscripts 0, 1 and 2 shows respectively 0, +ive and -ive
sequence components. The zero sequence components have
same magnitude and are phase with each other,
I R , IY , IB
V0 = Vr,0 = Vy,0 = Vb,0
the neutral RMS current is:
IN = IR + IY cos( 23 π) + jIY sin( 23 π)
+IB cos( 43 π) + jIB sin( 32 π)
But phase sequences of the positive and negative sequences
have the same magnitudes, but their phases difference by 120◦
(33)
V1 = Vr,1 = αVy,1 = α2 Vb,1
V2 = Vr,2 = α2 Vy,2 = αVb,2
which simplify to
√
1
1
3
− IB + j
(IY − IB )
(34)
2
2
2
2) Asymmetrical transmission impedances: The impedance
of the transmission line varies therefore faults occur in
the balance three phase power system. Due to this fault
the system goes to unbalanced condition. Without transposition of the transmission line, asymmetrical phenomena
increases which creates asymmetrical impedance in power
systems.Transposition show in given ”Fig. 1”.
alpha is a phasor rotation, which rotates a phasor vector by
120◦ with counterclockwise,
IN IR − IY
2
α = e 3 πi
(35)
after decomposition the vector in three symmetrical components gives the following form,
⎡
⎤ ⎡
⎤ ⎡
⎤
V0
V1
V2
Vryb = ⎣ V0 ⎦ + ⎣ α2 V1 ⎦ + ⎣ αV2 ⎦
V0
αV1
α 2 V2
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⎡
Vryb
1
=⎣ 1
1
1
α2
α
⎤⎡
⎤
1
V0
α ⎦ ⎣ V1 ⎦
V2
α2
= AV012
where,
⎡
Vryb
⎤
⎡
V0
1
= ⎣ V1 ⎦ , A = ⎣ 1
V2
1
⎡
1 1
1
A−1 = ⎣ 1 α2
3
1 α
1
α2
α
⎤
1
α ⎦
α2
⎤
1
α ⎦
α2
by these equations we convert an asymmetrical set of three
phasors to symmetrical components set of three phasors. By
this conversion VU phenomene is rare in symmetrical components. Three phase connected load are efficiently working
during symetrical conditon and flow of neutral current in the
neutral wire is vanish and the stability of the power system is
attained.
Fig. 3. Proteus simulation Design
IV. P ROPOSED H ARDWARE
The design of the proposed hardware will made using
Proteus software. First of all we implemented this automatic
three phase load balancing system in Proteus simulation and
took the results from it and then designed hardware of the
proposed system which is shown in ”Fig. 3”. The hardware
is divided into three main circuits and each circuit consists of
electrical components which are given in ”Fig. 2”,
• Current sensing circuit
• Relay operating circuit
• Controller circuit
In ”Fig. 3” shows the whole simulation of automatic three
phase load balancing system and also shows each component and his electrical symbol and names. The simulation is
possible with proteus software This simulation is start from
three phases input and terminated with single phase output.
Our desired to swiping the three phase loads and our single
phase full house load is connected to that phase which is least
loaded among it. The display show any instant of the parameter
variation like voltage, current and connected output status of
phases. This hardware is prototype and experimental design
for three phase distribution system show in ”Fig. 4”. In currently this hardware design is very helpful to overcome three
phase unbalancing, near distribution transformer. In nowadays
mostly urban area is feeding a long radial distribution network,
LV side is connected with single full house loads which creates
the problem. By this hardware our single phase load is swiping
to the least loaded phases among three phase in LV network,
maintain the power system stability and also improve the
efficiency of the distribution transformer.
Fig. 4. Hardware Design
Fig. 2. Proposed Design
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V. C ALCULATION OF R ELAY OPERATION
Where PS is plug sitting, so PS is 200%,when the fault current
is 1000A in the CT primary. Hence, fault current is calculated
on the secondary side of CT is,
1000 * 5/200 = 25A,
The relay used in hardware design is Inverse Definite
Minimum Time (IDMT) overcurrent relay. In an over current
relay, the ideal inverse time characteristics can not be achived.
The secondray current of current transformer (CT) is directly
proportionally to the system current. The actuating quantity
in IDMTis only current, there is current operated element
in the relay, no voltage coil are required to construct this
protective relay. In IDMT,near pick-up value the operating
time is inversely proportional to the fault current of the
relay. Above the pickup value of relay becomes considerably
slightly constant, as shown in the ”Fig. 5”. This is obtained
when current slightly greater than the pick-up current and
using a core of the electromagnet which gets saturated. The
mathematical relation between the the operating time and
current of IDMT characteristic can be written as
toperating =
0.14 ∗ (T M S)
(P M S)
0.02
−1
25
= 12.5.
(44)
2
An electromechanical relay when we set the time of travelling
distance is called time setting. This adjustment is commonly
known as time setting multiplier (TMS) of relay. The TMS
calibrated range is from 0 to 1 in steps 0.05 sec .The fastest
operation is obtaind when TMS is selected as 0.1. Putting the
values in equation no (2).
P SM =
toperating =
(36)
VI. R ESULTS
The single phase output load is keept constant and the loads
of three phase input line varied. The single full house load
is connected to that phase which is least loaded by relays.
The varying loads range from 0 W to 1000 W in each phase.
We continuously checked the display phase swiping between
output load and incoming three phases according to load
situation at different loads and noticed different phase swiping
at different loads. Tables given below show the phase swiping,
and load currents of each phase. Table I and table II show
results. We took experimental data from display show in ”Fig.
6”, our single phase full house load is connected by default
Red phase show in display denoted by phase1 and A1, A2,
A3 show load of each phase.
In ”Fig. 7” the load connected in each phase is given, this
load variation is sensing by Arduino nano and give signal to
the fast switching relay to connect our desired single full house
load to least loaded phase among three phase LV network.
There is no flow of neutral current to variation of load from
0W to 1000W on each phase, in ”Fig. 8” load variation is
(37)
The value of IDMT is 10A; the CT ratio is 200:5 the PSM
and TMS is given below,
P ick up current = 10A,
PS =
pick up current
∗ 100%,
Rated secondray current of CT
Current sitting =
P SM =
pick up current
,
Rated secondray current
(45)
The operating time of the relay is most important because
when the domestic load switched to the least loaded phase, it
required sufficient delay time for its operation. The 0.27s is
more suitable relay operating time and the power system will
be stable during this time.
where PSM is plug-sitting multiplier, TMS is time multiplier
sitting The maximum load current of IDMT is 10A, assuming
25 % overload, maximum overload can be calculated as
10A + (0.25 + 10A) = 12.5A
(0.14) ∗ (0.1)
= 0.27sec
(12.5)0.02 − 1
(38)
(39)
(40)
F ault current in relay coil
,
Rtaed CT secondray current ∗ current sitting
(41)
10
∗ 100% = 200%,
(42)
PS =
5
10A
= 2,
(43)
Current sitting =
5A
Fig. 5. Various characteristics of over current relays
Fig. 6. Result on Display
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TABLE I
C ONNECTED LOADS ON EACH PHASE .
No
Red(W)
Yellow(W)
1
0W
0W
Blue(W)
0W
2
400W
315W
320W
3
100W
215W
60W
4
300W
215W
260W
5
100W
315W
120W
Fig. 8. Three Phase Current (A)
loaded phase using fast switching relays. The single full house
load is connected to that phase which is least loaded by relay
switching ON/OFF. The hardware and simulation result shows
that the microcontroller and relays switching is effective in
reducing the unbalancing in three phase line. In addition, it
retains voltage regulation and stability in the all three phase.
Fig. 7. Connected loads on each pahse
R EFERENCES
[1] F. Shahnia, P. Wolfs, and A. Ghosh, Voltage Unbalance Reduction in Low
Voltage Feeders by Dynamic Switching of Residential Customers among
Three Phases, vol. 5, no. 3, pp. 13181327, 2013
[2] Y. Li and P. a Crossley, Voltage balancing in lowvoltage radial feeders using Scott transformers, IET Gener. Transm. Distrib., vol. 8, no. February,
pp. 14891498, 2014
[3] S. Weckx and J. Driesen, Load Balancing with EV Chargers and PV
Inverters in Unbalanced Distribution Grids, IEEE Trans. Sustain. Energy,
vol. 6, no. 2, pp. 635643, 2015
[4] Q. Wang, M. Cheng, and Y. Jiang, Harmonics Suppression for Critical
Loads Using Electric Springs with Current-Source Inverters, IEEE J.
Emerg. Sel. Top. Power Electron., vol. 1, no. c, pp. 18, 2016.
[5] L. Tan, S. Wang, P. Wang, Y. Li, Q. Ge, H. Ren, and P. Song,
High Performance Controller with Effective Voltage Balance Regulation
for a Cascade STATCOM with Star Configuration Under Unbalanced
Conditions, IEEE Trans. Ind. Appl., vol. 5, no. June, pp. 110, 2015.
[6] K. Mansouri, M. Ben Hamed, L. Sbita, and M. Dhaoui, Three-phase
balancing in a LV distribution smart-grids using electrical load flow
variation: L.F.B.M., 16th Int. Conf. Sci. Tech. Autom. Control Comput.
Eng. STA 2015, no. 1, pp. 427431, 2016
[7] C. S. Chen, T. T. Ku, and C. H. Lin, Design of phase identification
system to support three-phase loading balance of distribution feeders,
IEEE Trans. Ind. Appl., vol. 48, no. 1, pp. 191198, 2012.
[8] H. Pezeshki and P. Wolfs, Correlation based method for phase identification in a three phase LV distribution network, IEEE ISGT Eur. 2012,
no. July 2011, pp. 17, 2012.
[9] M. Osama, B. Shakeel, S. A. Jaffar, M. F. Ali, and S. Sajjad, 74 . LV
Three Phase Automatic Load Balancing System, vol. 2016, no. Eesd,
2016.
TABLE II
E ACH PHASE CURRENT AND CONNECTED STATUS OF SINGLE PHASE LOAD .
No
Red(A)
Yellow(A)
Blue(A)
Connected Output Status
1
0A
0A
0A
Red
2
1.86A
1.41A
1.47A
Yellow
3
0.45A
0.97A
0.27A
Blue
4
1.42A
0.96A
1.19A
Yellow
5
0.46A
1.52A
0.54A
Red
give with taken by different readings. With the above range of
variation there will be no neutral flow of current, the system
will be stable.This hardware is design upto 5A load variation,
we also change equipments and its rating for higher load
variation, according to our requirements.
VII. C ONCLUSION
Domestic load is single phase while connected to three
phase distribution network. The three phase distribution system
suffers from unbalancing due to overloading of one phase as
comparing to the remaining two phases. To overcome this
problem, distribution system requires equal sharing of load
on each phase. When equal sharing is obtained, due to which
in 3-pahse unbalancing, energy losses, overload situation and
return current flow in neutral is reduced. The automatic three
phase load balancing system is possible by the proposed
hardware which is micro-controller and relay based hardware.
The hardware is installed in the incoming of three phase lines
and will accordingly switch the domestic load to the least
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