2019 12th International Conference on Intelligent Computation Technology and Automation (ICICTA)

Research on Three-phase Unbalanced Power Quality Control in Low Voltage

Distribution Platform
Yu Liu*, Yun Teng
Shenyang University of Technology, Shenyang, Liaoning Province, China, 110870
*corresponding author’s email: liuyu10183@163.com

Abstract—There is a problem of three-phase unbalanced Assuming that the power supply voltage is balanced in the
power quality for the power electronic interface of the low- three-phase load network, the three-phase load is connected
voltage distribution network. If reasonable and effective means ab bc
in a triangle, as shown in Figure. 1(a), where Y1 ǃ Y1
are not used to control the three-phase imbalance in the
distribution network, it may affect the safe and stable ca
and Y1 are vector expressions of the complex form of the
operation of the entire power system. Therefore, the problem
of three-phase unbalance in the low-voltage distribution three-phase admittance, and For unequal impedance. By
network has become a hot topic of research. Generally, for the means of the Y-Δ transformation, any ungrounded load of
treatment of three-phase load imbalance in the low-voltage the Y-connected form can be represented by the triangular
network segment, a reactive power compensation device is form in Figure. 1(a).
commonly used. In this paper, the vector analysis Assume:
compensation strategy is used to detect the active and reactive Y1ab G1ab  jB1ab
components of the fundamental current related to the
compensation capacitance value. The current detection Y1bc G1bc  jB1bc (1)
method of space vector transformation is designed. The
feasibility of the scheme is verified by simulation analysis. Y1
ca
G  jB
ca
1
ca
1
First, from the improvement of circuit characteristics, the
Keywords: Low voltage distribution area; Three-phase admittance of each phase is changed to pure conductance,
unbalance; Vector analysis; Vector transformation
that is, in parallel with the load susceptance value in each
phase, but the opposite direction of susceptance, that is, let
I. INTRODUCTION Brab  B1ab ǃ Brbc  B1bc and Brca  B1ca . Figure 1
In recent years, the State Grid Corporation has paid more (b) is a compensation illustration.
and more attention to the power quality problems of
distribution networks, especially low-voltage distribution
areas. It is proposed that by 2020, it is necessary to
comprehensively solve the problems of power grid weakness
and power quality in rural and remote areas, and strive to
provide low-voltage distribution[1]. The network will
basically eliminate the problem of "low power quality" and
effectively protect people's livelihood. This paper deeply
studies the power quality and change mechanism of low-
voltage power distribution station area, establishes the three- 
phase unbalance compensation network model of low- Figure 1. Three-phase unbalanced load ideal
voltage station area, studies the network topology compensation icon
optimization and compensation device optimization For the unbalanced network in Figure 1(a), the pure
configuration model of low-voltage station area, and bc ca
conductances G1 ǃ G1 are compensated one by one.
proposes vector analysis compensation strategy to
effectively solve the electric energy. Quality issues provide After the analysis of each parameter, it is finally determined
theoretical and technical support[2-3]. that the three branches of the delta connection form are
connected in parallel with three compensation admittances.
II. THREE-PHASE UNBALANCE COMPENSATION The expression of the compensation admittance of the ideal
NETWORK MODEL compensation network for the unbalanced circuit is as shown
Usually when analyzing the three-phase unbalance in formula (2). Shown as follows:
problem, the load and the compensator used are often given Brab  B1ab  (G1ca  G1bc ) / 3
in the form of a vector and solved by an ideal admittance
model. This method can not only improve the power factor Brbc  B1bc  (G1ab  G1ca ) / 3 ˄2˅
of the system, but also balance the three-phase load, in line
with the current three-phase unbalanced control Brca  B1ca  (G1bc  G1ab ) / 3
requirements. Through the derivation and analysis process of the
compensation admittance expression of the above ideal

978-1-7281-4284-5/19/$31.00 ©2019 IEEE 622

DOI 10.1109/ICICTA49267.2019.00138
compensation network, it can be seen that any unbalanced BˊVector analysis compensation model and its principle
load in the three-phase circuit can be converted into a Figure 2 shows an unbalanced load-compensating circuit
balanced three-phase network by paralleling the model of a low-voltage distribution network constructed
compensating admittance associated with it. The using only capacitors of unequal capacity between phases.
compensation network is incorporated without affecting the
active power between the power supply and the load[6].
III. LOAD COMPENSATION STRATEGY BASED ON VECTOR
ANALYSIS
The above ideal compensation network strategy is based
on the expression of load susceptance when performing
compensation. In actual engineering circuits, the load of the
load cannot be directly measured by the measurement tool
like current or voltage. Therefore, the above-mentioned
ideal compensation is based on The compensation algorithm
of the network is only for the theoretical one, and can
provide a theoretical basis for further deriving the
compensation algorithm below. Therefore, it is necessary to Figure 2. Low Voltage Unbalance Compensation
further derive a compensation scheme that compensates by Network
using the easily measurable electric quantity. The
symmetrical component method is used in the literature [4], Assume that the three-phase voltage of the power supply
and the compensation susceptance expression can be output (low-voltage output end of the transformer) is
introduced. The process is based on the compensation balanced (that is, the amplitude is equal, the difference
current of the instantaneous current and voltage. The between the phases is 120 ), and the voltage of each phase is
DŽ

process coordinate transformation is cumbersome, and the U, and the phase angle of phase A is 0 . The relationship
DŽ

load current and voltage must be sampled periodically. between the voltages of the phases can be shown by equation
Literature [5] proposed balancing compensation based on (3):
instantaneous power theory. The proposed theory has U A U ‘0
‘0
improved the response time and compensation effect
compared with the theoretical compensation of symmetrical UB U ‘  120 (3)
components. However, this method requires the positive
sequence and negative sequence of the fundamental wave.
UC U ‘12
‘120
In time detection, it is necessary to design a response Then U AB 3U ‘30 , I AB ǃ I BBC ǃ I CA leads the
detecting device, and the method is costly. This paper will
derive the compensation algorithm of three-phase unbalance U AB ǃ U BBC ǃ U CCA 90 ˈ I C 1 ǃ I C 2 ǃ I C 3 respectively
based on vector analysis from another direction. This
before U A ǃ U B ǃ U C 90 and respectively. The phasor
method is based on vector decomposition of current and
voltage at the load end, and compensates by cross- relationship between the voltage and current of each phase in
connecting reactive devices. Compensation mathematical the network is shown in Figure 2.
model of the network, the program not only reduces the The current I AB across the capacitor between the A
calculation process, but also uses the proposed phase and the B phase can be changed by the vector
compensation network to design a corresponding intelligent decomposition to be an active current parallel to the A phase,
grouping composite switch based on full capacitance 1
compensation, which can greatly Reduce the cost of I ABP I AAB ‘180 and a reactive
compensation. 2
A. Compensation basis based on vector analysis 3
The principle of load balancing compensation in this current I ABQ I AAB ‘90 perpendicular to the A phase.
paper is because a large number of inductive loads exist in 2
the load type of three-phase lines. Therefore, in the three- I ABQ is opposite to the direction of the active current of
phase system, the principle of transferring active and
phase A itself, thereby canceling part of the active power on
reactive power by using the corresponding electronic
phase A, and I ABQ is ahead of phase A voltage 90 , thereby
DŽ
components between phases is selected. By integrating
unequal capacitors between the phase lines, the active compensating for the reactive power of phase A. Based on
balance and reactive power compensation of the load are the above, it is shown that the capacitor elements can be
achieved [6]. transferred between phases in phase-to-phase[7].


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 The reactive power of the load is not only generated in
the system, but also includes the reactive power generated by
the compensation of the phase-to-phase capacitor and the
reactive power generated by the capacitance to the ground[8].
The vector analysis diagram of Fig. 3 shows that the value of
the reactive current between the phases is known. After that,
the reactive current provided by each relative capacitance
can be represented by (8) as a matrix form:
ª I C1 º ª I Laq º ª I AB º
«I » « » 3« »
« C2 » « I Lbq »  « I BC » ˄8˅

«I » 2
Figure 3ˊ vector analysis diagram «¬ I C 3 »¼ «¬ I CA »¼
¬ Lcq ¼
Let I AP ǃ I BBP and I CCPP be expressed as the active
current relationship of phases A, B, and C after phase-
crossing capacitor compensation, which can be expressed by Ⅳ. SIMULATION ANALYSIS
formula (4): Simulation software is usually used to analyze some
1 1 theoretical or algorithmic feasibility, and can also use the
I AP I Lap  I AB  I BC simulation module that comes with the software to analyze
2 2
the influence of various parameters in the system on the
1 1
I BP I Lbp  I BC  ICA ˄4˅ system. Therefore, the use of simulation software can easily
2 2 complete academic research activities and become an
1 1 indispensable part of the academic research neighborhood.
ICP I Lcp  ICA  I AB The theoretical research in this paper is based on
2 2 Matlab/Simulink software, and the feasibility of the
In order to balance the compensated active power and proposed scheme is analyzed by constructing a system
calculate the average value of the above equation, the active
simulation model[9-10].
current of each phase should be equal (the currents of the
Simulation Analysis of Three-phase Unbalanced Load
phases are equal, and the phase angles are different from
Compensation Based on Ideal Admittance Network
each other by 120 ) as shown in equation (5):
DŽ

In order to simulate and analyze the ideal compensation

PLa  PLb  PLc network of Figure 1, the following figure 4 shows the
I AP I BP ICP ˄5˅
simulation analysis model of the compensation network built
3U
The formula (4) and the formula (5) are combined to by the system.
obtain the following formula:
2 PLa  PLb  PLc
I AB  I BC 2I Lap 
3 U
2 PLa  PLb  PLc
I BC  ICA 2I Lbp  ˄6˅
3 U
2 P P P
ICA  I AB 2I Lcp  La Lb Lc
3 U
In formula (6), PLa ǃ PLb ǃ PLc represent the active
power of each phase in the system before compensation.
When the PLa ǃ PLb ǃ PLc voltage and the active Figure 4. Compensation network simulation model
current are displayed in a product form, the formula (7) can
be converted into the following form: First, the susceptance that is compensated to become
4 2 2 pure conductance is calculated according to each known
I AB  I BC I Lap  I Lbp  I Lcp inductance, which is Cab 202.642P F ǃ
3 3 3
4 2 2 Cbc 101.321P F and Cca 50.661P F . By using the
I BC  ICA I Lbp  I Lap  I Lcp ˄7˅ formula for deriving the ideal compensation network
3 3 3
susceptance value, the compensation susceptance value of
4 2 2
ICA  I AB I Lcp  I Lap  I Lbp
3 3 3

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each phase can be obtained, and it can be known that the analyzed. According to the above theory, by analyzing the
susceptance value at this time is one of capacitive and compensation of single-phase pure conductance load, the
inductive. By calculating the formulas according to formulas three-phase load compensation model of ideal compensation
2, the calculated susceptance values are Lab 0.110H ǃ network is derived, and the corresponding compensation
algorithm is designed. The simulation system diagram is
Cbc 275.664P F and Lca 0.0551H respectively. built by Simulink in MATLAB to verify the feasibility of the
Finally, the susceptance value of each phase compensated algorithm. It can be seen from the simulation results that the
susceptance is integrated, and the compensation admittance algorithm can optimize the three-phase unbalance of the
value of each phase of the compensation network is system. However, the compensation principle is represented
determined, so as to determine the reactance value of each by the load susceptance. In practical applications, the load
phase. By setting relevant parameters in the model, the susceptance cannot be accurately measured, in order to
following figure 6 is obtained as follows: improve the deficiency of this measurement compensation
method. In this paper, a load compensation model based on
vector analysis is proposed and the corresponding
mathematical model is derived. The simulation analysis
shows that the three-phase unbalance compensation scheme
based on the compensation network is feasible. For the
known load, the reactive compensation can achieve better
results.
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č. CONCLUSIONS
This paper mainly introduces two compensation strategies:
ideal compensation network and vector analysis for power
compensation. Firstly, the principle of reactive power
compensation and three-phase power phenomenon are

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