Voltage-current Double Loop Control Strategy for Magnetically Controllable Reactor Based Reactive Power Compensation

XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE
Voltage-current Double Loop Control Strategy for
Magnetically Controllable Reactor Based Reactive
Power Compensation
Zhou Li
School of Electrical Engineering
Southeast University
Nanjing, China
lizhou@seu.edu.cn
Xuan Yang
Hangzhou Electric Power Company
State Grid
Hangzhou, China
cyxark@163.com
Kashif Mehmood
Department of Electrical Engineering
University of Lahore
Lahore, Pakistan
kashif_mehmood@seu.edu.cn
Yu Qin
Jiangsu Electric Power Company
State Grid
Nanjing, China
bsp25t@126.com
Ruopei Zhan
School of Electrical Engineering
Southeast University
Nanjing, China
zrp_seu@126.com
Abstract—Voltage regulation depending on reactive power
compensation is the main feature of the AC power supply
system. Magnetically controllable reactors (MCR) are
becoming a growing demand for this purpose. The structure
and working principles of MCR are analysed in this paper
while a simulation model is established. The reactive power
compensation strategy based on a single loop voltage control
system (SLVCS) is presented and a double loop
voltage-current control system (DLVCCS) is proposed. A
comprehensive scenario is developed to mitigate reactive power
compensation by using the proposed controls. Simulation
results substantiate that proposed controls of MCR has a faster
response in comparison to the traditional control, and it
provides the least voltage variation at the line end. It also
shows that the proposed control on MCR meet the desired
objective of the voltage regulation and provides flexibility to ac
transmission system.
Keywords—flexible ac transmission systems, magnetically
controllable reactor, reactive power compensation, saturable
reactor, power quality.
I. INTRODUCTION
Electricity demand is increasing due to the rapid
development of the World’s economy and population [1].
Various renewable and non-renewable energy sources used
to meet the ever increasing demand that disrupts power
system quality and stability [2]. Ultra-high voltage grid
development has become a promising measure to address the
increasingly prominent energy issue[3]. However, the
ultra-high voltage grid and non-linear load cause reactive
power imbalance and poor voltage regulation [4]. Many
reactive power compensation devices under flexible AC
transmission system (FACTS) like Static Var Compensator
(SVC), Fixed Capacitor (FC), Static Synchronous
Compensator (STATCOM), Unified Power Flow Controller
(UPFC) and so forth have been used previously to improve
voltage regulation at customer side [5]. FACTS are
becoming increasingly popular in high voltage power
transmission system as it replaces the traditional control
means and improves the power controllability of the AC
transmission system. Moreover, it reduces the power
transmission cost and system network loss while improving
the reliability and stability of the system [6][7]. SVC is the
most commonly used device that plays an important role in
voltage regulation and stability [8]. However, traditional
SVCs which include Thyristor switch Capacitor (TSC) and
Thyristor Controlled Reactor (TCR) are not much feasible
for number of reasons which includes: high voltage
application due to large quantities of harmonics [9], Failure
of Thyristors due to high Voltage Stress [10], High Cost of
SVC due to large amount of Power electronic devices [11].
FC are another useful reactive power compensation device
in case of low voltage profile, but it suffers from
overcompensation because of no control mechanism of
reactive power [12]. Similarly, Traditional, STATCOM is
adapt for ultra-high voltage system, but it is not much
economical and operational [13]. Among the aforementioned
FACTS devices, Magnetically controllable Reactor (MCR)
gains much importance because of its relatively simple
control, low cost, and a wide range of applications [14-16]. It
can not only improve transmission power but also improves
power quality and voltage regulation in HV and UHV
applications [17]. Furthermore, it limits system overvoltage,
requires little maintenance, provides reactive power
compensation and has low harmonics [18-20].
MCR can work on a wide range of voltages and can
restrain the short circuit fault currents [21]. It is widely used
in the electrical industry, for example, for power flow control,
voltage regulation, reactive power compensation, and
restricting inrush currents. By using MCR, it is possible to
get continuous variable reactance. MCR includes better
dynamic behavior without introducing a lot of harmonic
contents [22]. The MCR provides low triple harmonics and
therefore allows control of reactive power in a single phase
or single three-phase system as required in the transmission
system.
Besides the benefits of using MCR, MCR suffers from a
low response due to the inherent hysteresis effect as
compared to SVC. To the best of our knowledge, no previous
study has addressed this issue. Therefore, to fill this
technological gap, this work presents a single loop voltage
control system (SLVCS) and proposes a double loop
National Natural Science Foundation of China under Grant 51707031

voltage-current control system (DLVCCS) to solve the slow
response of widely used MCR.
The main contributions of this work are:
i) Working principle, hysteresis characteristics of
multi-valve MCR and its mathematical model are
presented.
ii) The SLVCS and proposed DLVCCS strategy are
explained and tested. The analysis are carried out to
check the response of the whole system with both
control strategies.
iii) A simulation environment is created based on the
mathematical model and the proposed control strategy.
A typical scenario of reactive power compensation
problem is developed to test the credibility of proposed
control strategies on MCR.
iv) Finally, based on the simulation analysis, results are
compared to show the superiority of the proposed
DLVCCS over SLVCS and traditional control.
II. PRINCIPLE AND CHARACTERISTIC OF MAGNETICALLY
CONTROLLABLE REACTOR
MCR is a device that changes the reactance of windings
by changing the permeability of a ferromagnetic core in
which the coil is wound by the application of direct current.
The biasing of direct current causes a direct current flow in
the coil as a result of dc flux developed in the core.
Similarly, because of the alternating voltage, alternating
current flows in the coil and AC flux developed in the core
in the direction opposite or the same as a DC flux [22]. Flux
interaction determines the state and point of magnetic core
operation and determines the permeability and reactivity of
the coil. There must be an adequate ampere turns by the
direct current that balances the ampere-turns of the
alternating current winding in order to regulate the core
saturation [23].
MCR is based on magnetic saturation. By controlling the
direct current through the control coil, the MCR's iron core
becomes saturated and its reactance varies depending upon
the saturation degree of the iron cores. The schematic
diagram of the MCR is shown in Fig. 1.
B1 B2
Vs
Is
N
Nk
GND
T1 T2
AC Grid
D
Ab
Fig. 1 Schematic Diagram of MCR.
The body of MCR includes cores, yokes, working and
control windings and necessary switching devices [14].
The control windings are connected by winding taps every
half of the iron core and thyristors are connected between
these taps. High power thyristors are not needed because the
ratio of the turn is typically 5%. The principle of operation
is attained by varying the force of the magnetic field in the
iron heart using a constant or controlled current through the
control coil [16].
The equivalent, simplified working circuit and the
working states when thyristors T1 and T2 are conducted in
MCR is shown in Fig. 2.
Fig. 2 Equivalent simplified Circuit and the working states of MCR.
In Fig. 2, ideally, the DC bias and the alternating
magnetic flux in the yokes are used to change the saturation
degree (β) of magnetic-valves flows in the two cores. From
Faraday’s Law of electromagnetic induction, the
relationships between dc voltages and the alternating
voltage of MCR satisfy the turn ratio in two windings
transformer. These turns ratio correspond to the different
values of the dc voltages and can be used as the excitation
powers for the magnetization and demagnetization[23].
The inductance of the coil “Lcoil” in MCR is controlled by
regulating the direct current magnitude ‘ik’ through the
control winding ‘Nk’, which changes the magnetic field
strength (H) in the magnetic valves. The relation between
the Lcoil and ik is represented in (1). When the ik increases
the resulting Lcoil decreases. So as the reactive power of the
MCR increases [24].
2
s
MCR
coil
V
Q
2 fL
=

(1)
The Fundamental Equations of equal circuits of MCR are
as follows
1 2
s m b
1 2
k k k k b
k k e1
k k e2
e1 1
e2 2
dB dB
V V sin t NA ( )
dt dt
dB dB
V R i N A ( )
dt dt
N N i I.H
N N i I.H
H f (B )
H f (B )
 
=  = + 
 
 = + −
 
 
+ = 
 − =
 
= 
 = 
(2)
where Ab is the area of the iron core, B1 and B2 are the
magnetic flux densities on two cores, He1 and He2 are the
magnetic field intensities of cores as a function of B1 and B2
respectively.

III. PROPOSED DOUBLE LOOP VOLTAGE-CURRENT CONTROL
The current loop control based method proposed in [14] is
mainly applicable to control the current of MCR and is not
stable for the voltage regulation application. Based on
voltage regulation, a single-loop voltage control system of
MCR is proposed. The control system is shown in Fig. 3
Fig. 3 Single-loop voltage control system (SLVC) for MCR.
MCR suffers from a low response due to the inherent
hysteresis effect. At present, there are many solutions for
fast excitation of MCR, such as adding capacitors and using
other excitation methods for MCR. However, most of them
need to change the structure of MCR, which increases the
cost of equipment and the difficulty of debugging.
Reference [14] physically modelled and analyzed the
reason for the slow response of self-excited MCR. In short,
the reason is that MCR is an AC-DC co-excitation of
electromagnetic equipment, and DC excitation is generated
by an auxiliary circuit, which can be equivalent to
inductance and resistance in series. The inductance value is
generally larger, the resistance value is smaller, and the time
constant of the circuit is larger. Even if the control voltage
from the thyristor rectifier changes greatly at a certain time,
the control current of the circuit must go through a transition
process to achieve stability, so the DC excitation can reach
stability and MCR can realize the control objective.
However, when the current of the DC excitation circuit
needs to be increased, the strong excitation can be carried
out in a short time. The excitation current can be increased
to the appropriate value and then the excitation intensity can
be reduced, which makes the excitation current increase
rapidly to the appropriate value. This method does not need
to add series resistance or shunt capacitor and other
equipment but can increase the change of trigger angle in a
short time. The DLVCCS is shown in Fig. 4. It provides a
fast response of the MCR to achieve the desired goal.
Fig. 4 MCR control method based on (DLVCCS).
IV. REPRESENTATION, SIMULATION, RESULTS AND ANALYSIS
A. Power System Representation
To verify the implementation of the proposed
compensation system, several simulations were carried out
with the following allocations. The electrical power system
is shown in Fig. 5.
The transmission line is represented by the inductor Lline
and resistance Rline. The load (RLoad and LLoad) and the fixed
Capacitor (FC) is connected in parallel with the MCR at the
end of the line using a circuit breaker. the transmission line
is represented by the L-line inductor and R-line impedance.
Loads (RLoad and LLoad) and fixed capacitors (FC) are
connected at the end of the channel using circuit breakers
parallel to the MCR. This configuration represents the actual
scenario in the electrical power system.
Fig. 5 Power system for Simulations.
TABLE I PARAMETERS OF POWER SYSTEMS AND CONNECTED MCR
The parameters of the Power system including Load, FC
and MCR are presented in Table I.
The Block Diagram of MCR contains Saturable and Ideal
Transformers, Saturable transformer is used to control the
variable inductance and ideal transformer is used for the
switching circuit which consists of Thyristors is as shown in
the Fig. 6.
Fig. 6 Schematic of the MCR.
In the power system shown in Fig.5 circuit breaker and
pure reactive fixed load as FC are set at the end of the line to
simulate the sudden change of load. Another normal load is
used in the simulation. The circuit breaker is initially
Parameters and their Value Descriptions
Rated Power 500KVA
Primary Voltage 5KV
Secondary Voltage 5KV
Rater Frequency 50Hz
Non-Linear Transformers (T1 and
T2)
Rated Power 500KVA
Primary Voltage 10KV Idea Transformer for Thyristor
Circuit
Secondary Voltage 1000V
Voltage 10KV Voltage Source
Line Resistance, Rline=8 Ω
Line Inductance, Lline=0.0254H Line Parameters
Real Power 0.3MW
Inductive Power 0.05MVAR
Load
Reactive Power 0.8MVAR Fixed Capacitor

disconnected and closed at any time to simulate the sudden
increase of reactive power.
B. Comparison between SLVC and DLVCCS
When MCR is not used, the terminal voltage is shown in
Fig. 7(a). The simulation results of single-loop control are
shown in Fig. 7(b). From the terminal voltage, MCR has
good control effect and can stabilize the voltage near the end.
In addition, it is obvious that the disadvantage is that the
MCR response speed based on the control method is slow.
The first reason is that the MCR circuit is an LR circuit with
a large time constant.
The second reason is that the control method is relatively
rough and there is no special solution for the slow response
of MCR. Fig. 7(b).shows that the response speed is 2.375
seconds which is rather slower to achieve the desired
voltage regulation. Fig. 8 shows the change of conduction
angle of MCR in the simulation above under the single-loop
control method of MCR based on voltage loop. The
changing trend of the trigger angle is similar to that of
voltage, and the change degree is small.
(a)
(b)
Fig. 7 The terminal voltage of the MCR (a) terminal voltage without MCR
(b) terminal voltage with MCR under SLVC
The simulation of double-loop control is carried out, and
the results are shown in Fig. 9. The voltage regulation speed
of this method is better than that of single-loop control
method based on voltage
Fig. 8 Conduction angle variation of the MCR on SLVC
Fig. 9 shows that the desired voltage regulation is
achieved in 0.25 seconds with stability.The output voltage is
calculated by the voltage outer loop, and the predicted
working current is obtained. The actual working current is
tracked. The output regulator is fine-tuned to get the final
predicted working current. Then the trigger delay time is
calculated according to the trigger angle.
Fig. 9 The terminal voltage of the MCR under DLVCCS
The advantage of this method is that it makes full use of
DC excitation voltage and achieves strong excitation effect
by increasing the trigger angle in a short time. The change
of the trigger angle of MCR in the simulation test is shown
in Fig.10.
Fig. 10 Conduction angle variation of the MCR on DLVCCS.
Because of the strong excitation effect of double-loop
control, this method has a remarkable effect in the face of
small range load fluctuation. As shown in Fig. 11, Fig. 11(a)
shows the system voltage fluctuation without MCR and Fig.
11(b) shows the system voltage fluctuation after MCR. It
can be seen that MCR can quickly adjust the output reactive
power and stabilize the voltage when the load fluctuation is
small.
(a)

(b)
Fig. 11 Voltage variation at line end (a) without MCR (b)with MCR.
However, if the load fluctuation in a wide range of the
system reaches the requirement of setting MCR as the
minimum trigger angle or 180 degrees for reactive power
regulation, then the speed of regulation of this method will
not be significantly better than that of MCR control method
based on single-loop voltage loop.
V. CONCLUSION
The physical structure and circuit of MCR are analyzed in
this work. Moreover, equivalent simulation circuit of
parallel MCR is built on PSCAD/EMTDC based on the
mathematical model. A novel reactive power compensation
strategy for MCR driven by a DLVCCS has been
successfully proposed. Compared with the SLVCS, the
simulation results corroborate that the proposed strategy
improves the response speed of MCR by 2.125 seconds.
This method makes full use of DC excitation voltage and
achieves strong excitation effect by increasing the trigger
angle in a short time. The proposed control allows an
accurate and flexible reactive power compensation scheme
under the load requirement. The use of DLVCCS provides a
smooth ac voltage control within the required response time
in comparison to SLVCS.
ACKNOWLEDGMENT
This work was supported by National Natural Science
Foundation of China under Grant 51707031.
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Zhou Li received his B.S. and Ph.D. degrees,
all in Electrical Engineering, from the School of
Electrical Engineering, Southeast University,
China, in 2007 and 2014, respectively. He is
now a lecturer of the School of Electrical
Engineering, Southeast University, China. His
research interests include power system
operation and control, AC/DC hybrid power
transmission, and also renewable energy
generation and control.
KASHIF MEHMOOD received B.Sc. and
M.Phil. Degrees (Hons.) in Electrical
Engineering from The University of Lahore,
Pakistan in 2011 and 2015, respectively, he is
currently pursuing the Ph.D. degree from
Southeast University Nanjing, Jiangsu China.
His research interests include the areas of power
system operation and control, flexible AC
transmission and distribution systems (FACTS),
the application of metaheuristic optimisation
(artificial intelligence) techniques in power
system's problem and the UHV magnetically
saturable controllable reactor for reactive power
compensation.
Ruo-pei Zhan received his B.Eng. degree in
Electrical Engineering from Southeast
University, China, in 2018. He is currently
pursuing the M.Eng. in Electrical Engineering
at Southeast University. His research interests
include the voltage source converter based high
direct current.
Xuan Yang received his B.S. and Ph.D.
degrees, all in Electrical Engineering, from the
Huazhong University of Science and
Technology and the University of Birmingham,
in 2009 and 2014. Currently, he is an electrical
engineer in State Grid Hangzhou Power Supply
Company. His research interests include power
system planning and dispatching.
Yu Qin received his B.S. and M.S. degrees, all
in Electrical Engineering, from the School of
Electrical Engineering, Southeast University,
China, in 2007 and 2010, respectively.
Currently, he is an electrical engineer in State
Grid Jiangsu Electric Power Co., Ltd. His
research interests include power system
operation and control.

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