International Journal of Electrical and Computer Engineering (IJECE) Vol.
No.
October 2025, pp.
ISSN: 2088-8708.
DOI: 10.
11591/ijece.
Enhancing source currents and ensuring load voltage stability in railway electrification system via unified power quality conditions implementation Kittaya Somsai.
Jeerapong Srivichai.
Veera Thanyaphirak Department of Electrical Engineering.
Faculty of Industry and Technology.
Rajamangala University of Technology Isan.
Sakon Nakhon Campus.
Sakon Nakhon.
Thailand
Article Info
ABSTRACT
Article history:
In recent years, interest in electric railway system as a transportation solution for large urban areas has grown significantly.
This increased attention stems from several key advantages, including environmental friendliness, high performance, reduced maintenance costs, and lower energy expenses.
Railway electrification system rely on supplying power to trains through single-phase transformers.
However, these transformers can cause issues such as current imbalances and harmonics at the system connection point, which may impact critical loads.
Additionally, fluctuations in source voltage can influence the system's performance.
This study examines the causes of unbalanced loading in railway electrification system and introduces an innovative unified power quality conditioner (UPQC) specifically designed for integration into low-voltage railway electrification system.
The proposed UPQC aims to restore current balance, minimize harmonics, and enhance overall power quality.
Furthermore, it addresses the mitigation of voltage sags in the power distribution network.
The simulation results generated through MATLAB programming demonstrate the UPQC's effectiveness in enhancing system performance.
The findings reveal that the UPQC reduces source current imbalance to less than 1.
6% and total harmonic distortion (THD) to below 4.
89% across all test scenarios.
Additionally, the UPQC successfully maintains a load bus voltage of 25 kV during single-phase-toground and unbalanced three-phase-to-ground fault conditions.
Received Dec 20, 2024 Revised Jun 4, 2025 Accepted Jun 30, 2025 Keywords:
Current balancing Electric railway system Harmonic current Unified power quality Voltage enhancing This is an open access article under the CC BY-SA license.
Corresponding Author:
Kittaya Somsai Department of Electrical Engineering.
Faculty of Industry and Technology.
Rajamangala University of Technology Isan Sakon Nakhon Campus, 199 Village No.
3 Phang Khon-Waritchaphum Road.
Phang Khon subdistrict.
Phang Khon district.
Sakon Nakhon, 47160.
Thailand Email: kittaya.
so@rmuti.
INTRODUCTION
Railway electrification systems are critical to public transportation due to their ability to handle increasing passenger demand and transport goods over a wide range of distances.
However, ensuring their reliability and stability as efficient transportation solutions remains a challenge, particularly regarding power quality in the power supply.
These challenges are especially pronounced in long-distance, high-speed trains powered by 25Ae27.
5 kV, 50 Hz AC systems .
Ae.
In conventional AC traction systems, the conversion of a 115 kV three-phase grid to a 25 kV single-phase system is typically achieved using single-phase transformers .
, .
The V/V transformer is commonly used for this purpose due to its simple design, ease Journal homepage: http://ijece.
Int J Elec & Comp Eng
ISSN: 2088-8708
of installation, maintenance, and straightforward control.
Although V/V transformers offer certain advantages, they encounter major challenges, including negative sequence currents (NSC), harmonics, and a low power factor, all of which negatively impact the efficiency and power systemAo stability .
, .
These power quality issues are more noticeable when AC railway systems are connected to weak power grids, leading to higher operational costs and disruption to sensitive equipment, such as medical devices and database systems .
Problems such as voltage waveform distortion and harmonics reduce power supply efficiency and negatively affect the connected three-phase grid, especially during periods of high demand and frequent usage .
The single-phase, non-linear characteristics of AC railway traction loads further contributes to power quality issues, including NSC .
, .
low power factor .
, harmonics .
, .
, and reactive power consumption .
, .
These problems result in energy losses in feeders, reduced transformer capacity, malfunctioning of protective relays, and errors in transmission control systems.
To enhance the power quality (PQ) parameters of railway electrification system, researchers have explored various methods over the years.
Addressing NSC has been a focal point, with solutions such as specially designed transformers, including Scott.
Woodbridge, impedance-matching balance, and LeBlanc transformers .
Ae.
Reactive power compensation has been another area of focus, with static VAR compensators (SVC) .
Ae.
and static synchronous compensators (STATCOM) .
, .
widely utilized.
While SVCs dynamically compensate reactive power, they generate harmonics, which STATCOMs can Single-phase STATCOMs are well-suited for railway feeders with low isolation needs, while threephase STATCOMs can dynamically address NSC but require large step-down transformers.
Additional methods, such as railway power conditioners (RPC) .
Ae.
and active power quality compensators (APQC) .
, have demonstrated potential in addressing power quality issues.
While RPCs effectively compensate for NSC, harmonics, and low power factors, they are limited by their complexity and lack of flexibility.
The hybrid railway power compensator (HRPC) .
Ae.
, which integrates RPCs with passive filters or SVCs, offers some improvement in power quality.
However.
HRPCs still face challenges, such as control complexity, overcompensation, and resonance, which degrade system performance .
Notably, most of these solutions fail to mitigate voltage sags resulting from single-phase or three-phase faults in railway systems.
The unified power quality conditioner (UPQC) is a promising alternative for resolving power quality problems in the AC electrification system of railway traction.
By employing series and parallel converters.
UPQC can address voltage and current disturbances while mitigating harmonic distortions.
Recent studies have focused on designing V/V-connected UPQC to eliminate harmonics and NSC in railway systems .
Other research has emphasized integrating renewable energy sources and energy storage systems into UPQC designs to enhance power quality and utilize energy recovered during braking operations .
Modern research trends also highlight the potential of UPQC for improving railway power quality through intelligent control systems.
Artificial intelligence (AI) based control techniques have shown promise in enhancing system performance, reducing harmonics, and improving overall power quality .
, .
However, the high cost of UPQC, especially for high-voltage three-phase systems, poses a significant barrier to widespread This study introduces a novel UPQC application tailored for railway systems, specifically designed for connection to the low-voltage side near the load to enhance control and compensation efficiency.
The proposed UPQC effectively reduces current imbalance and harmonics while maintaining voltage stability during single-phase and three-phase faults in the power grid.
An in-depth analysis and control system design for the UPQC demonstrate its ability to efficiently mitigate power quality issues with high precision.
The results of MATLAB simulation validate the performance of the proposed UPQC in improving power quality and operational stability in railway systems.
PROPOSED SYSTEM
Electric railway system with UPQC Figure 1 shows schematic diagram of the proposed compensator designed to balance currents and regulate voltage within the railway electrification system.
It depicts the connection of traction substations to the power grid at the point of common coupling (PCC) through a V/V transformer.
The compensator employed in this study is the UPQC, which is installed between the traction side and the low-voltage side of the V/V transformer to achieve current balancing and voltage regulation.
Figure 2 provides a detailed view of the UPQC configuration tailored specifically for these purposes in the railway system.
In conventional practice, the V/V transformer in traction substations steps down the three-phase 115 kV from the source system to two single-phase traction feeders operating at 25 kV.
The UPQC comprises two converters that share a DC-link capacitor, which helps maintain a stable DC-link voltage.
One converter, called the series converter, is linked to the traction feeder through a three-phase transformer, while Enhancing source currents and ensuring load voltage stability in A (Kittaya Somsa.
A ISSN: 2088-8708 the other, the shunt converter, is connected in parallel to the feeder via a separate three-phase step-down As illustrated in Figure 2, the shunt converter is strategically placed close to the load, which serves as the power supply station for trains, while the series converter is positioned near the V/V transformer.
This configuration is intentional, as the shunt converter is well-suited for mitigating current imbalances caused by variable loads, while the series converter is designed to shield the loads from voltage fluctuations originating from the power source.
By employing an effective control strategy, the UPQC can dynamically adjust the output currents and voltages of both converters.
This dual capability allows the series converter to address voltage sags at the load while the shunt converter compensates for current imbalances, thereby significantly enhancing the overall system efficiency and stability.
Figure 1.
Schematic diagram of the electric railway system with UPQC Figure 2.
Configuration of the electric railway system with UPQC Int J Elec & Comp Eng.
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Principle of operation of the UPQC The test setup for the electric railway system configuration integrated with the UPQC is shown in Figure 2.
Further details are presented in the single-line diagram presented in Figure 3.
This diagram represents the system components, including the power system, the V/V transformer, a railway load consisting of two trains, the right-shunt UPQC device, and the UPQC control system.
The main components of the UPQC in the described configuration are the series and shunt converters, which share a DC-link capacitor.
The operational mechanism of the UPQC for achieving current balancing and voltage enhancement in the railway electrification system is outlined as follows.
The series converter operates as a voltage source converter (VSC) and is connected in series with the traction feeder.
Its role is to inject an unbalanced series voltage into the traction feeder to compensate for unbalanced load voltages and to regulate the load voltage to the required level.
Similarly, the shunt converter, also a VSC, is connected in parallel with the traction feeder.
Its function is to inject unbalanced currents into the traction feeder, thereby compensating for unbalanced source currents at the PCC, while also maintaining the DC-link voltage at the desired level.
The effectiveness of the UPQC depends heavily on the control strategies implemented for its The roles of both the series and shunt converters have been clearly defined, providing the basis for their respective control strategies.
The UPQC employs two distinct control strategies: one for the series converter and another for the shunt converter.
By effectively coordinating these strategies, the UPQC ensures proper compensation of unbalanced source currents and maintains regulated voltages within the railway electrification system.
As shown in Figure 3, the current originating from the source and passing through the source resistance ycIyc and inductance yayc is denoted as ycnyc .
The voltage provided to the traction station, denoted as ycycya , is reduced via a V/V transformer with a transformation ratio of ycayc .
For the series converter, the current passing through the series transformer to the load is denoted as ycnycyc , while the current entering the series converter is represented by ycnycycya .
The voltage from the series converter that is supplied to the load is ycycycya .
This voltage, ycycycya , is related to the output voltage of the series converter, ycycycya , by the series transformer ratio ycaycyc .
It can be regulated by modifying the control signal ycycyc .
For the shunt converter, the voltage at the PCC is represented as ycycEa , while the current of the shunt converter that passes through its resistance ycIycEa and inductance yaycEa , is denoted as ycnycEa .
The current entering the shunt converter is also represented as ycnycEa .
This current, ycnycEa , is utilized to compensate for unbalanced source currents and to reduce harmonics in the supply current.
The relationship between ycnycEa and ycnycEaya is defined by the shunt transformer ratio ycaycEa .
The adjustment of ycnycEa is achieved by controlling the voltage across the shunt converter, ycycEaya , through the control signal ycycEa .
Nonetheless, to control the voltage across both the series and shunt converters .
cycycya and ycycEaya ), it is essential to depend on the DC voltage, ycyccyca , on the DC side.
However, controlling the voltage across both the series and shunt converters .
cycycya and ycycEaya ) relies on the DC voltage ycyccyca on the DC side.
Figure 3.
Single-line diagram of the system
CONTROL STRATEGIES OF UPQC
The UPQC control system employs various theoretical approaches, including instantaneous power theory .
, instantaneous symmetrical components theory .
, and synchronous rotating reference frame theory .
This study, however, focuses specifically on reducing unbalanced source currents using instantaneous power theory and achieving voltage regulation through the synchronous rotating reference frame theory.
Enhancing source currents and ensuring load voltage stability in A (Kittaya Somsa.
A ISSN: 2088-8708 Control of shunt converter The shunt converter is in charge of correcting unbalanced source currents and maintaining the DClink voltage at the desired level.
Additionally, it can filter out harmonic currents.
The instantaneous power theory or the p-q theory, which has been previously applied in D-STATCOM devices for harmonic filtering and improving power quality, is now utilized to control the shunt converter in this UPQC.
One of the primary uses of the p-q theory is to address unwanted currents, such as harmonic and unbalanced currents.
[ yayu ] = Oo
cyayca ] ycyayca
Oo3
Oe ] Oo3
cnyayu ] = Oo 1
c ] = [ Oeycnyayu
ya Oe
ycnyayca ] Oo3 ycnyayca
Oe ] Oo3
ycnyayu ycyayu
][ ]
ycnyayu ycyayu Figure 4 illustrates the control strategy designed for shunt converter.
This approach utilizes the p-q theory, which incorporates the yuyu0 transformation or Clarke transformation .
This transformation uses a real matrix to convert three-phase voltages and currents into yuyu0 axes, as defined in equations .
The load real power .
cyya ) is divided into its mean component .
cyIya ) and oscillating component .
cyEya ).
Similarly, the reactive power of the load .
cya ) is divided into its mean .
cIya ) and oscillating .
cEya ) components.
For simplicity, this study focuses on a three-phase, three-wire system, where zero-sequence voltage and current components are absent.
Under this condition, the ycyya and ycya of the load can be derived from .
In this research, the shunt converter of the UPQC corrects for the components ycyEya and ycEya , ensuring that only the ycyIya is taken from the power system.
As a result, the constant instantaneous power control strategy provides optimal compensation for power flow, even in the presence of non-sinusoidal or unbalanced system currents.
The real power of the electric railway system, represented as a nonlinear load in Figure 2, must be continuously monitored and divided into ycyIya and ycyEya components using a mean function.
Additionally, the inclusion of a DC voltage regulator into the shunt converter control strategy, as shown in Figure 4, is essential.
A small quantity of power .
cyIycoycuycyc ) must be drawn continuously from the power system to compensate for the losses in the PWM converter.
If not, the DC capacitor would supply this energy, leading to its gradual discharge.
This highlights the need to maintain the DC voltage at a constant level higher than the peak AC-bus voltage, which is achieved using a PI controller to control the DC voltage.
Thus, the shunt converter must supply the oscillating components of the instantaneous active and reactive currents of the load.
These currents can be determined using .
for their reference values in the yuyu The reference currents are then transformed back into the abc reference frame, as described in .
, for further use in the control system.
Figure 4.
Control block diagram of shunt converter of UPQC ycyayu ycnycEayu [ O ] = 2 2 .
c ycyayu ycyayu ycnycEayu yayu ycyayu OeycyE ycy i ycoycuycyc Oeycyayu ] [ Oeyc Int J Elec & Comp Eng.
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ycnycEayca
cnycEayca ] = Oo Oe2
ycnycEayca
[Oe 2
Oo3
Oo3
ycnycEayu
[O ]
ycnycEayu The gate pulses which required for shunt converter are produced by hysteresis current control.
Comparing between the fundamental current .
cnycEa.
) and reference signal current with the hysteresis band cnycEa.
A E.
is used to generate the gate pulses.
The gate pulses required for the shunt converter are generated using hysteresis current control.
This method compares the actual fundamental current ycnycEa.
O with the reference current signal within a specified hysteresis band .
cnycEa.
A E.
to produce the gate pulses.
Control of series converter The series converter is in charge of compensating the load voltage and regulating it to the desired To manage voltage imbalances and maintain stable operational voltage, this study applies the synchronous rotating reference frame (SRF) theory.
The SRF theory involves transforming three-phase signals .
into the dq0 rotating reference frame using the Park transformation.
This transformation relies on the initial alignment of the dq0 frame at time t=0, corresponding to the position of the rotating reference frame determined by the angular velocity .
This approach facilitates the conversion of AC waveforms into DC signals.
However, in this study, the zero-sequence component is neglected, as the focus is on a threephase three-wire system.
The unbalanced voltage in the electric railway system is primarily due to the NSC within the synchronous rotating reference frame, posing a significant challenge for voltage compensation.
To mitigate this issue, the series converter of the UPQC is employed to compensate for the NSC in the dq domain, while the PSC in the dq domain is employed to control the load voltage to the desired level.
Figure 5 presents the diagram of the series converter control.
In this diagram, the three-phase load voltages .
cyayca , ycyayca and ycyayca ) are transformed into dq-PSC .
cyaycc( ) and ycyayc( ) ) and dq-NSC .
cyaycc(O.
and ycyayc(O.
) using .
, respectively.
The orientation of the rotating reference frame .
in these equations is determined by a phase-locked loop (PLL), which calculates the angular velocity of the voltage ycyc.
ycyaycc( ) .
cyayc( ) ] = sin .
ycya0( ) cos (Oeyuiy.
ycyaycc(O.
cyayc(O.
] = sin (Oeyuiy.
ycya0(O.
uiyc Oe Oesin .
uiyc Oe uiyc Oesin .
uiyc 2yuU cos (Oeyuiyc Oe Oesin (Oeyuiyc Oe ) .
cyayca ] ycyayca cos (Oeyuiyc 2yuU ) Oesin (Oeyuiyc .
) .
cyayca ] ycyayca Figure 5.
Control block diagram of series converter of UPQC Certainly, converting a three-phase abc signal into a dq0 rotating reference frame in an unbalanced system will result in a dq0 signal that is an imperfect DC signal.
In other words, this signal will oscillate at Enhancing source currents and ensuring load voltage stability in A (Kittaya Somsa.
A ISSN: 2088-8708 twice the frequency of the fundamental frequency.
To achieve a perfectly DC signal, it is necessary to apply a process that averages out the given signal.
To achieve balance in the two-phase load voltage .
cyaycayca and ycyaycayca ).
it is essential to regulate the dq-NSC components to zero .
cyaycc(O.
= 0 and ycyayc(O.
= .
In the meantime, the dq-PSC is utilized to adjust the load voltage to the preset level, where the desired voltage magnitude depends O AEycyaycc( ) on .
cya | = Oo.
cyaycc( ) )2 .
cyayc( ) )2 and the voltage angle is related to ycaycuyci.
cya ) = tanOe1 .
cyayc( ) O Oesin .
ycycycyca( ) 2yuU cycycyca( ) ] = .
uiyc Oe 3 ) Oesin .
uiyc Oe 3 ) O ycycycyca( ) cos .
uiyc ) Oesin .
uiyc ) O ycycycycc( ) O cycycyc( ) O 1 ycycyc0( ) .
cos (Oeyuiy.
Oesin (Oeyuiy.
1 ycO ycycycyca(O.
ycycycc(O.
cycycyca(O.
] = .
os (Oeyuiyc Oe 3 ) Oesin (Oeyuiyc Oe 3 ) .
cycycyc(O.
] O
2yuU 2yuU ycycycyca(O.
cos (Oeyuiyc ) Oesin (Oeyuiyc ) 1 ycycyc0(O.
The series converter control employs four proportional-integral (PI) controllers for its operation.
The outputs of the PI controllers serve as the reference signals for the dq-PSC and dq-NSC components of O the series converter .
cycycycc( ) , ycycycyc( ) , ycycycycc(O.
and ycycycyc(O.
The referenced signals of the dq-PSC and dq-NSC are converted back to positive and negative components three-phase AC signals through .
Note that for a three-phase three-wire system, the zero-sequence component can be neglected O cycyc0( ) = ycycyc0(O.
= .
Subsequently, the three-phase AC referenced signals of the series converter O .
) can be acquired by summing these transformed reference signals.
The series converter requires gate pulses which are generated using pulse width modulation (PWM).
System parameter and PI controller design Based on the single-line diagram illustrated in Figure 3, the system's equivalent circuit, including the right-shunt UPQC, is presented in Figure 6.
In this equivalent circuit, the railway load is modeled as a series combination of resistance and inductance.
The parameters for both the system and the UPQC are provided in Table 1.
Figure 6.
Equivalent circuit of the system Table 1.
System and UPQC parameters Parameter name Source voltage V/V transformer ratio Source resistance Source inductance Norminal load voltage Load train 1 Load train 2 Shunt transformer ratio Fundamental frequency Symbol PL1 jQL1 PL2 jQL2 Value 25/115 0 j0.
5 j0.
1/25
Unit
MVA
MVA
Parameter name Series transformer ratio Series resistance Series inductance DC losses resistance Norminal DC voltage DC capacitance Shunt resistance Shunt inductance Switching frequency Int J Elec & Comp Eng.
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Rdcsh
Cdc
Rsh
Lsh
Value 100, 100
80,000
Unit
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When examining the equivalent circuit of a series converter shown in Figure 6, the formula for the output voltage, ycycycya , at the series transformer in dq-PSC and dq-NSC can be expressed in the following manner:
ycycycyaycc(A) = ycaycyc ycycycycc(A) = ycaycyc .
cycycyaycc(A) Oe ycIycyc ycnycycyaycc(A) yuiyaycyc ycnycycyayc(A) ) .
ycycycyayc(A) = ycaycyc ycycycyc(A) = ycaycyc .
cycycyayc(A) Oe ycIycyc ycnycycyayc(A) Oe yuiyaycyc ycnycycyaycc(A) ) .
The subscribe (A) means the positive and negative components.
The both dq-PSC and dq-NSC axes of the voltage ycycycya at the series converter can be modified by controlling the signal ycycyc in dq-PSC and dq-NSC axes as well.
However, the voltage ycycycya is used to enhance the load voltage by combining it with the supply voltage ycycya .
Consequently, the load voltage, ycya , in dq-PSC and dq-NSC axes can be represented by the following equations.
ycyaycc(A) = ycycyaycc(A) = ycaycyc .
cycycycc(A) ycyccyca Oe ycIycyc ycnycycyaycc(A) yuiyaycyc ycnycycyayc(A) ) .
ycyayc(A) = ycycyayc(A) = ycaycyc .
cycycyc(A) ycyccyca Oe ycIycyc ycnycycyayc(A) yuiyaycyc ycnycycyaycc(A) ) .
As shown in .
, the signal ycycyc functions as the input, while the voltage ycya is regarded as the output of control systems.
The diagram illustrating the control mechanism for adjusting the load voltage of the series converter as shown in Figure 7.
As mentioned earlier, the unbalanced transformation of three-phase voltage on the dq-PSC and dq-NSC axes results in the creation of oscillating signals twice the fundamental frequency.
Hence, utilizing averaging methods becomes essential to regulate this load voltage.
This averaging is expressed through a constant-time transfer function, characterized by a time constant of ycNycoyceycaycu = 0.
Based on the control diagram depicted in Figure 7 and the system parameters outlined in Table 1, the transfer function of the load voltage control system, neglecting any disturbances .
cnycycya ycIycyc , ycnycycya yuiyaycyc , and ycycya ) can be expressed by the .
ycOya .
ycOycyc .
12,000 .
005yc 1 Through the design of a PI controller aimed at ensuring robust response time with a phase margin (P.
) exceeding 60 degrees and a gain margin (G.
) surpassing 6 dB, the controller's P and I component parameters are established at yaycE = 0.
00008 and yaya = 0.
To design the PI controller for DC voltage regulation, we employ the mathematical model of the DC voltage of the UPQC, detailed in .
, which can be expressed by .
=Oe yayccyca ycIyccycaycEa ycyccyca Oe yayccyca ycIyccycaycyc 2 yayccyca ycyccyca ycycEaya ycnycEaya In this context, yayccyca represents the DC link capacitor, while ycIyccycaycEa and ycIyccycaycyc denote losses in the shunt and series converters, with ycIyccycaycEa = ycIyccycaycyc simplifying the design.
The control system is based on a fixed current ycnycEaya = yaycEaya with the shunt converter voltage ycycEaya = ycycEa ycyccyca .
Using these parameters, the DC voltage control system schematic as shown in Figure 8 is developed.
Based on this diagram and Table 1, the transfer function for the system, under nominal conditions with yaycEaya =50 A, is given in .
ycOyccyca .
ycOycEa .
50y100 2 .
,000y10Oe6 y100yc .
Figure 7.
Block diagram of load voltage control Figure 8.
Block diagram of DC voltage control Enhancing source currents and ensuring load voltage stability in A (Kittaya Somsa.
A ISSN: 2088-8708 To achieve a robust and stable response in the DC voltage control system, a proportional-integral (PI) controller is meticulously designed.
The primary objective of this design is to ensure a phase margin (P.
greater than 60 degrees and a gain margin (G.
) exceeding 6 dB, which collectively contribute to improved system stability and dynamic performance.
Through an iterative tuning process that considers the system's dynamics and control objectives, the proportional and integral gains of the PI controller are determined as yaycE = 0.
1232 and yaya = 0.
16645, respectively.
These parameters are selected to optimize both transient response and steady-state error correction, ensuring the controller meets the desired performance criteria.
SIMULATION RESULTS
This section presents the results of testing a UPQC for improving unbalanced currents and voltage stability in railway electrification system using MATLAB simulations.
Figure 9 illustrates the test system simulated using MATLAB, with the parameters detailed in Table 1.
Four case studies were evaluated:
integration of UPQC, i.
increased train load, .
single phase-to-ground fault, and i.
unbalance three phase-to-ground fault.
Results highlight UPQC's superior performance, with detailed analysis supported by graphs and numerical data demonstrating its effectiveness in enhancing power quality and ensuring stability across all scenarios.
Figure 9.
Test system simulated using MATLAB Integration of UPQC This test focuses on evaluating the capability of UPQC in managing unbalanced currents during the initial startup phase of compensation.
During the time interval from t=0.
1 Ae 0.
2 seconds, the system operates with only the train 1 (TL) active.
As depicted in Figure 10, the train 1 draws current from phase-a of the system and returns it through phase-b, while phase-c remains unloaded.
During this period, the three-phase supply currents exhibit a significant imbalance, reaching 100%.
Furthermore, the system experiences substantial harmonic distortion, with the total harmonic distortion (THD) measured at 10.
Figure 10.
Managing unbalanced currents during the initial startup phase of compensation At t=0.
2 seconds, the UPQC is activated in the system.
Simulation results reveal that the UPQC effectively addresses the current imbalance by injecting three-phase compensating currents through its shunt converter, as illustrated in Figure 11.
Consequently, the three-phase supply currents are restored to a balanced state, reducing the imbalance percentage to 3.
025%, as depicted in Figure 10 at t=0.
2 Ae 0.
4 seconds.
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Moreover, the UPQC significantly reduces harmonic distortion, lowering the THD to 4.
These findings highlight the UPQC's capability to enhance power quality by mitigating current imbalances and harmonic distortions, even under challenging system conditions.
Figure 11.
Shunt converter currents during the initial startup phase of compensation Increased train load This study examines the effects of increased load on the system, specifically at t=1.
0 seconds, when the second train (TR) is introduced into the network.
The simulation results shown in Figure 12 indicate that the system current increases from about 9A to 14A at t=1.
0 seconds.
During this interval, the UPQC demonstrates its effectiveness in compensating for both current imbalance and harmonic distortion.
The compensating current generated by the UPQC under these conditions is illustrated in Figure 13.
While the magnitude of the compensating current remains comparable to the previous case, its waveform exhibits slight variations due to changes in the current consumption behavior of the train.
The UPQC's compensation leads to a substantial decrease in the current imbalance, achieving an imbalance percentage of 6%, and a substantial decrease in the THD to 2.
A detailed analysis of harmonic currents is highlighting the UPQC's capability to enhance power quality in the presence of dynamic load variations.
Figure 12.
Managing unbalanced currents for increasing train load Figure 13.
Shunt converter currents for increasing train load Single phase-to-ground fault This section describes the simulation of a single-phase-to-ground fault occurring on the source side at phase-a at 1.
5 seconds.
During this period, the rms voltage of phase-a on the source side decreases from 115 kV to 45.
30 kV, as illustrated in Figure 14.
Meanwhile, the rms voltages of the remaining phases remain at the normal value of 115 kV.
This voltage sag causes the rms voltage supplied to the left train load (RL) to drop from 25.
00 kV to 18.
80 kV.
Enhancing source currents and ensuring load voltage stability in A (Kittaya Somsa.
A ISSN: 2088-8708 When the electric railway system is compensated using the series converter of a UPQC, the train 1 (TL) load voltage can be maintained at the desired 25kV.
The voltage waveforms demonstrating this compensation are shown in Figure 15.
Figure 16 presents the output voltage waveforms of the UPQC injected into the system to stabilize the load voltage.
It is evident that the injected voltage is applied only to phase-a.
Regarding source current quality improvement during this fault, the results are depicted in Figure 17.
The UPQC not only compensates the load voltage but also mitigates supply current issues, addressing both unbalance and harmonic distortions simultaneously.
During this period, the supply current's unbalance percentage is reduced to 1.
30%, and the THD is minimized to 2.
Nevertheless, a slight rise in source current is observed due to the increased power losses in the UPQC while compensating during fault conditions in the system.
The waveforms of the compensated currents provided by the UPQC are shown in Figure 18.
Figure 14.
Source voltage for single phase-to-ground fault Figure 15.
Load voltage for single phase-to-ground fault Figure 16.
UPQC voltage for single phase-to-ground fault Figure 17.
Source currents for single phase-to-ground fault Int J Elec & Comp Eng.
Vol.
No.
October 2025: 4430-4444
Int J Elec & Comp Eng
ISSN: 2088-8708
Figure 18.
UPQC current for single phase-to-ground fault Unbalance three phase-to-ground fault This section details the simulation of an unbalanced three-phase-to-ground fault occurring on the source side at 1.
5 seconds.
During this fault, the rms voltage of phases-a, phases-b, and phases-c on the source side decreases from 115 kV to 76.
00 kV, 71.
13 kV, and 64.
88 kV, respectively, as shown in Figure This voltage sag results in a reduction of the rms voltage supplied to the left train load (RL) and the right train load (TR) from 25 kV to 15.
70 kV and 15.
36 kV, respectively.
When the electric railway system is compensated using the series converter of a UPQC, the voltage levels for both the left train load (TL) and the right train load (TR) are maintained at the set value of 25 kV.
The voltage waveforms illustrating this compensation can be seen in Figure 20.
Figure 21 displays the output voltage waveforms from the UPQC that are injected into the system to stabilize the load voltage, with compensation applied to phases a and c.
Figure 19.
Source voltage for unbalance three phase-to-ground fault Figure 20.
Load voltage for unbalance three phase-to-ground fault Figure 21.
UPQC voltage for unbalance three phase-to-ground fault Enhancing source currents and ensuring load voltage stability in A (Kittaya Somsa.
A ISSN: 2088-8708 For improving the source current quality during this fault condition.
Figure 22 presents the The UPQC not only maintains the load voltage stability but also corrects supply current imbalances and reduces harmonic distortions.
The unbalance percentage of the supply current is brought down to 0.
85%, and the THD is minimized to 2.
A slight increase in the source current is observed due to the additional power losses incurred by the UPQC while compensating for under three-phase fault These losses are more significant compared to those associated with single phase-to-ground The current waveforms compensated by the UPQC are shown in Figure 23.
Figure 22.
Source currents for unbalance three phase-to-ground fault Figure 23.
UPQC currents for unbalance three phase-to-ground fault
CONCLUSION
This research addresses critical power quality issues in railway electrification system, particularly those connected to low-voltage systems, by implementing a UPQC.
The study highlights the UPQC's significant advancements in managing unbalanced currents, reducing current harmonics, and ensuring voltage stability, thereby enhancing the reliability and efficiency of electric railway operations.
Railway electrification system, favored for their environmental benefits and cost efficiency, face challenges such as unbalanced currents and voltage fluctuations due to the use of single-phase transformers in a three-phase power supply The proposed UPQC, consisting of two three-phase power converters .
hunt and serie.
connected in a back-to-back configuration with a shared DC capacitor, effectively mitigates these issues.
The shunt converter addresses current imbalances and improves current harmonics by injecting compensating currents, while the series converter manages voltage fluctuations by injecting compensating voltages.
The control strategies for the UPQC are based on instantaneous power theory for the shunt converter and synchronous rotating reference frame theory for the series converter, enabling real-time compensation and dynamic response to load changes and fault conditions.
Comprehensive simulation results validate the UPQC's effectiveness in various scenarios, including increased train load and fault conditions, demonstrating significant reductions in THD and current imbalance percentages.
The study also emphasizes the cost-effectiveness of the UPQC, which can be installed on the low-voltage side of the V/V transformer without extensive infrastructure modifications.
The findings underscore the UPQC's potential for widespread adoption in railway electrification system, offering a robust solution for managing power quality challenges and improving system stability and efficiency.
This research contributes valuable insights into the application of advanced power quality conditioners in transportation systems, paving the way for future developments in this field.
REFERENCES