Effects of Conditional Changes on High-Voltage Direct Current Transmission Line Characteristic

e high-voltage direct current (HVDC) transmission system, which links the Gurun Substation of the National Electric Authority of Malaysia (Tenaga Nasional Berhad, TNB) with the Khlong Ngae Station of the Electricity Generating Authority of ailand (EGAT), has been extensively researched to achieve the highest quality because it is the largest of its kind inailand, and there is a plant to expand its transmission power. However, the impact on the whole system is under-researched. To study and develop this system, the HVDC transmission line is modelled with the MATLAB/Simulink program and a laboratory setup to investigate the eect of transmission line distance, load power, and voltage on power loss, voltage drop, and waveform.eHVDC transmission line parameters are calculated from the actual transmission line parameters and converted to the simulation model parameters using the per-unit method. e model is veried and tested by the simulation program before creating the experimental setup. e simulation and experimental results demonstrate the eects of changing system conditions via the three aspects. All the three conditions directly aect the HVDC transmission line; nevertheless, they aect each aspect dierently.


Introduction
Currently, the high-voltage direct current (HVDC) transmission system is widely used because it has more advantages (such as low power loss, easy control of power ow, and long-distance power transmission) than the highvoltage alternating current (HVAC) transmission system. e technology of the HVDC transmission system is constantly evolving. Many countries adopt it to deliver a large amount of electricity. For example, the bipolar HVDC transmission system of the BritNed project connects power stations in Great Britain to those in the Netherlands. e BritNed project started in 2011 using a ±450 kV submarine cable with a capacity of 1,000 MW over a distance of 260 km. In the United States, the innovative HVDC PLUS transmission line technology from Siemens was rst applied to the Trans Bay Cable Project. It can deliver both active and reactive powers of 400 MW and ±170 Mvar, respectively, with a voltage of ±200 kV. In India, there are many HVDC transmission systems, including the Ballia-Bhiwadi project (2,500 MW, ±500 kV, 800 km) and the Mundra-Haryana project (2,500 MW, ±500 kV, 960 km). From 2010, China has been the country with the largest use of the HVDC transmission system in Asia. It also has plans of building approximately 35 projects, totalling 217 GW of power transmission [1].
e EGAT-TNB HVDC transmission system has begun to play a more crucial role in ailand. It is used to link the Gurun Substation of the National Electric Authority of Malaysia (Tenaga Nasional Berhad, TNB) with the Khlong Ngae Station of the Electricity Generating Authority of ailand (EGAT). ere exists another project that aims to connect China to ailand. Additionally, its usage will be expanded to cover its connection to distant or offshore alternative energy sources, such as wind and solar farms. In recent times, alternative energy is widely used commercially, and the technology of energy production is being developed worldwide. According to the International Energy Agency (IEA), an average industrial growth of 40% over the past ten years has resulted in a variety of photovoltaic technology. e IEA estimates that photovoltaic power generation will account for 5% of global electricity by 2030 and increase to 11% by 2050.
is will potentially reduce 2,300 million tons of carbon dioxide emissions per year. In India, the renewable energy trade is very successful because there are many large photovoltaic power plants.
Most of them are connected to the grid system [2], and their transmission lines run over a very long distance because they are distant from the load centre. One of the longest HVDC transmission lines in the world is built in India to supply a capacity of 2,000 MW from Talcher to Tamil Nadu over 1,400 km. Likewise, the HVDC transmission system is used in many developing countries to reduce power loss in longdistance transmission lines [3][4][5][6].
In ailand, the renewable energy capacity from 2013 to 2016 and its target in 2036 [7] in Table 1 show a continuous development of renewable energy production resulting from government policies. Among all the renewable energy types, the solar capacity has the largest increase. A solar power plant requires a large space; therefore, it must be built in a remote area. Power transmission over a long distance causes a significant power loss in the transmission line; nonetheless, if it is developed into the HVDC transmission line, the power loss will decrease substantially. Various aspects of the HVDC transmission system still need to be studied before they can be put into practice. e HVDC power transmission system has been developed in various research fields. One of the important areas is the development of converters [8].
e essential devices can be categorized into two types: rectifiers and inverters [9]. e rectifier is installed at the sending side of the HVDC transmission line to convert AC to DC power. Alternatively, the inverter is installed at the receiving side to convert DC to AC power. At the beginning of the development, the line-commutated converter (LCC), also known as the current source converter (CSC), was popularly applied using the thyristor technology. is can support a high value of power, voltage, and current. Subsequently, a switching device, that is, the insulated-gate bipolar transistor (IGBT), that can quickly turn on and off was created. e IGBT was applied to develop a new converter type known as the voltage source converter (VSC) [10]. e VSC has more advantages than the LCC [11]. For example, the VSC can independently control both active and reactive powers; however, the LCC cannot control the reactive power [12]. Moreover, the VSC is smaller than the LCC because it does not need a trigger guard, and its filter is smaller [13]. A number of studies have focused on improving the performance of the VSC [14] and the LCC [15]. Furthermore, the fuzzy logic has also been applied to the control system to allow more power transmission. Recently, the multiterminal converter (MTC) has been developed to improve the overall efficiency of power transmission [16]. In addition, it has been developed by combining LCC and VSC as a hybrid HVDC system [17], which can take advantage of both converters. e hybrid HVDC system can be divided into 4 types: a pole hybrid HVDC system, a terminal hybrid HVDC system, a series hybrid HVDC system, and a parallel hybrid HVDC system. e HVDC transmission system has been researched in several studies to deliver a large amount of electricity over a long distance and reduce power loss [18] in the HVDC transmission line. ese studies have proposed a method for calculating the power loss in the HVDC transmission line. e computation of the entire system is very complex. erefore, it is necessary to separate the power loss calculation from the converter station [19,20] and the HVDC transmission line [20][21][22][23]. In a study by Jovcic [23], the power loss of each device in the HVDC transmission line using the VSC was calculated and simulated in two models. e first model (±150 kV, 350 MW, 100 km) had a power loss of 4.6%, whereas the second model (±300 kV, 1000 MW, 200 km) lost 4.9% of its power. e results suggested that the transmission line distance, voltage, and load power affected the power loss. Furthermore, a power loss arising from the corona phenomenon is calculated [24]; the current harmonics are considered and possibly analysed using the PSCAD/EMTDC program [25]. Most studies have been conducted to reduce the power loss in the HVDC transmission line by enhancing the functionality of the converter [26] and improving the transmission line connection [27]. Generally, the model of HVDC transmission systems should be selected to suit the analysis method. e most popular models are the π-equivalent circuit, T-equivalent circuit, universal line, and Bergeron circuit models [28][29][30][31]. Khatir studied the π-equivalent circuit model using the MATLAB/Simulink program [29,32]. e π model is suitable for analysis at low frequencies [29], whereas the universal line model is most appropriate for underground and submarine cables [33][34][35]. Other researchers used different methods for calculating the transmission line parameters [36][37][38][39][40][41][42][43][44][45][46][47] and the PSCAD/EMTDC program based on the CIGRE Benchmark to study the simulation models.
In ailand, studies on the HVDC transmission system have attracted scholarly attention. For example, the EGAT-TNB HVDC transmission system, which increases the stability of the network in Southern ailand and connects the ASEAN network in the future project of "ASEAN Power Grid," was studied using the ATP program to control powers. It was also used to analyse fault events [48][49][50][51][52][53], fault signals [54][55][56], and fault location [57]. e power flow of this system was analysed using dynamic simulation to validate the stability and reliability of the operation of the HVDC transmission line [58][59][60][61][62][63].
However, the simulations in these studies were done using computer programs, which might differ from the actual electrical system. e EGAT-TNB HVDC transmission system should be extensively researched, making it efficient both theoretically and practically. erefore, this study focuses on the possibility and impact of changing the relevant factors in the HVDC transmission line. In this research, the EGAT-TNB HVDC transmission line is modelled using the MATLAB/Simulink program and a laboratory setup to study the e ects of transmission line distance, transmission line voltage, and load power on power loss, voltage drop, and waveform. e simulation and experimental results can be used to predict the impacts and prevent possible damage. In the future, this system will be studied on the topic of renewable energy connections and faults in HVDC transmission lines. Some electrical parameters cannot be studied through the simulation model only, so a comparative study of both the computer program and the laboratory setup is needed. is study is bene cial to researchers, government agencies, students, and those interested in the HVDC transmission system. e contributions of this study are summarized as follows: (1) is paper studies the possibility of simulating the real HVDC transmission line system to be the HVDC transmission line experiment setup in the laboratory (2) is paper designs the HVDC transmission line model in both the computer program and the experimental setup to determine the accuracy and use it as a tool for further analysis of electrical characteristics (3) is paper studies the e ect of the transmission line distance, size load, and voltage on power loss, voltage drop, and waveform using the MATLAB/Simulink program and the experimental setup for consideration of preventing damage to the HVDC system is study is organized as follows: Section 2 explains the HVDC transmission line that is designed in MATLAB/ Simulink and applied to the experimental setup; Section 3 highlights the results of the simulation and the experimental setup in three subsections according to the conditions; and nally, Section 4 concludes the study.

Design of the Simulation Model and the
Experimental Setup e HVDC transmission system in this study is modelled after a real system. e system links the Gurun Power Station of the Tenaga Nasional Berhad (TNB) to the Khlong Ngae Power Station of the Electricity Generating Authority of ailand (EGAT). e HVDC transmission line structure is a monopolar model with a metallic power return path as shown in Figure 1.
is system is studied using both a simulation model and an experimental setup. In the simulation model, the MATLAB/Simulink program is used to simulate the HVDC transmission system. e e ect of the change in the transmission line distance, load power, and voltage on power loss, voltage drop, and waveform is observed. e simulation model and results are applied to design the experimental setup in a laboratory room to con rm that the results can be practically used.

Simulation Model Design Using MATLAB/Simulink
Program.
e EGAT-TNB HVDC transmission line is modelled in the MATLAB/Simulink program to compare the simulation results with actual data and with other studies to validate the system design and verify its reliability. is simulation is divided into two steps: First, the parameters of the EGAT-TNB HVDC transmission line are calculated using the per-unit method to assemble the simulation model and to assess the reliability of the circuit model; and second, the model is customized based on the experimental setup.

Simulation to Verify Reliability.
is study uses a simple 6-pulse 250 kV 500 MW HVDC transmission system model [64] as the base model in the MATLAB/Simulink program as shown in Figure 2    in the recti er control to control the trigger angle (alpha). e parameters: resistance (R), inductance (L), and capacitance (C), are calculated from the real HVDC transmission line with a distance of 300 km. Insulated-gate bipolar transistors (IGBT) are employed in an inverter to convert the power from DC to AC. e inverter uses a pulse width modulation (PWM) controller and an optimal modulation index to keep a constant output voltage. e test of the experimental setup has a limit on the electrical capability. e voltage, current, and power must be within the range of safety controls, and it is impossible to create the experimental setup based on a 250 kV simple model. We choose 450 V and 500 V design models for testing and building the experimental setup as shown in Figures 2(b) and 2(c). is is because both voltages are close to the common laboratory voltage and high enough for the test to show obvious e ects. e design models employ parameters transformed from the 250 kV simple model by the per-unit method according to Table 2. e powers of both design models are limited to a capacity of 750 W.
To ensure the reliability of a base model and both design models, they are tested under ve conditions to study the response characteristics of the system in each system change condition. e test is divided into 5 conditions: a low current operation, a normal operation, a fault situation, a trigger angle change, and a normal operation. e simulation results are shown in terms of current, alpha angle, fault current, and voltage in Figures 3-6, respectively. Each gure compares the results of the three systems: 250 kV, 450 V, and 500 V models. First, when the system responds to a lowcurrent operation in ranges of 0 to 0.3 s, the reference current is set at a constant value of 0.5 pu. In Figure 3, the 250 kV, 450 V, and 500 V models reach their respective steady states at 0.1, 0.075, and 0.055 s, respectively, whereas all of their trigger angles are approximately 60°. Second, when the system responds to a normal operation during 0.3 to 0.5 s, the reference current is set to 1.1 pu. e response current takes approximately 0.13, 0.1, and 0.075 s, respectively, to enter a new steady state. In Figure 4, the trigger angle decreases from 60 to 45, 50, and 50°, respectively, but there is no change in fault current and voltage, shown in  Figures 5 and 6. ird, after a fault occurs in the system at 0.5 s, the respective maximum fault current is approximately 5 kA, 6 A, and 5 A as shown in Figure 5, respectively. Fourth, during the interval from 0.55 to 0.57 s, the trigger angle that is forced by the protection system is changed to 165°.
us, the recti er operates in the inverter mode and delivers power back to the power source, and the fault current is eliminated from the system. Finally, between 0.57 and 0.8 s, the system resumes its normal operation and enters a steady state at 0.75, 0.67, and 0.67 s,  Journal of Electrical and Computer Engineering respectively. e system's responses to all 5 conditions are similar in direction and time responses. is makes it possible to conclude that both the 450 V and 500 V models can be used to represent the 250 kV model. erefore, the experimental setup is designed based on both models, which are presented in the next subsection.

Simulation for Experimental Setup with the MATLAB
Program.
e simulation model of the experimental setup is designed based on the 450 V and 500 V models; nevertheless, each device circuit is condensed into a device box for easy understanding as shown in Figure 7(a) and 7(b). e main devices in Table 3

Experimental Setup Design.
e experimental setup is built using a π-equivalent circuit in the HVDC transmission line as shown in Figure 8(a). It consists of an adjustable three-phase transformer, an electrical protection device, a recti er, two DC circuit breakers, an HVDC transmission line, a three-phase inverter, and an electrical load. e threephase power supply sends electric power through the AC circuit breaker to the adjustable three-phase transformer A and the electrical protection device B, respectively. is power is supplied from the protection device to the recti er Journal of Electrical and Computer Engineering power, and voltage are measured using the Fluke 435 Series II Basic Power Quality, Energy Analyser, and Teledyne LeCroy WaveSurfer 3024 as shown in Figure 8. e parameters of the experimental setup are simulated from the parameters of the actual system. e resistance of the experimental setup is calculated by (1) [36] and converted using the per-unit method and used in the adjustable resistor as shown in Figure 9. e resistance is calculated in di erent voltages and distances as shown in Table 4. e transmission line circuit is divided into two parts (two pi      14 Journal of Electrical and Computer Engineering circuit models). e sum of the parameters of both parts must be exactly the same as the HVDC transmission line value. erefore, in the experiment setup, the resistance of each part is set as 50% of the HVDC transmission line value according to Figure 8(a) and Table 4.
Here, r is an HVDC transmission line resistance (ohm/km), ω is a resonance frequency, and K is a computational constant. e inductor of the experimental setup is made in the form of an adjustable inductor as shown in Figure 10. e inductance of the system is calculated using (2) [36]. Each inductance of the inductor in Table 5 is connected to the selector switch of the experimental setup to be used as the      HVDC transmission line parameter. Similar to the resistance, the transmission line circuit is divided into two parts, and the inductance of each part is set as 50% of the HVDC transmission line value according to Figure 8(a) and Table 5.
Here, l is an HVDC transmission line inductance (henry/km), r is an HVDC transmission line resistance (ohm/km), g is an HVDC transmission line conductance, ω is a resonance frequency, K is a computational constant, L line is a line length (km), R ch is a load resistance (ohm), and C ch is a load capacitance (farad). For the capacitance of the experimental setup in Figure 11, the lm capacitor that can support a voltage of 630 V is selected to connect between the line and the neutron of the HVDC transmission line. e transmission line circuit is divided into two parts, and each part has two sides of the capacitor. erefore, the capacitance of each side is set to be 25% of the HVDC transmission line value, making the sum of all the sides the same as the HVDC transmission line value as shown in Table 6. e capacitance of the HVDC transmission line is calculated according to (3) [36]. Each capacitance of the capacitor is also connected to the selector switch of the experimental setup to be used as the HVDC transmission line parameter.
Here, c is an HVDC transmission line capacitance (farad/km), l is an HVDC transmission line inductance (henry/km), ω is a resonance frequency, and K is a computational constant.  e di erence in power loss and voltage drop between the source and destination of the HVDC transmission line is presented to compare the resulting changes as shown in [65] P loss,line P Sen − P Rec ,

18
Journal of Electrical and Computer Engineering e P loss, line is the power loss of the HVDC transmission line, which is the di erence between the active power of the sending side (P Sen ) and the active power of the receiving side (P Rec ). e P loss, line is also calculated using the line conductance of the HVDC transmission line (G) multiplied by the square of the voltage drop (V drop, line ). e voltage drop is the voltage di erence between the voltages of the sending side (V Sen ) and the receiving side (V Rec ).

Transmission Line Distance Condition.
e HVDC transmission line is considered in three distances: 150, 300, and 600 km. All the distances are tested based on a capacity of 500 V and 750 W. e electrical parameter is considered by both voltage and current waveforms at the sending side (Sen) and the receiving side (Rec) of the HVDC transmission line. e voltage and current waveforms of the MATLAB/Simulink program and the experimental setup are compared at di erent transmission line distances as shown in Figures 12 and 13. e voltage waveforms in Figure 12 have vertical and horizontal axes as the voltage value (V) and operating time (s), respectively. e voltage waveforms are compared between the transmission line distances of 150 km, 300 km, and 600 km. However, the results of the simulation and the experimental setup are also analysed, in which the sending side of the transmission line is shown in Figure 12(a) and the receiving side of the transmission line is shown in Figure 12 the impedance in a transmission line can behave to some extent like a power system ller circuit. However, this will result in higher power loss and voltage drop. e comparison of the voltage waveforms between the sending side ( Figure 12(a)) and the receiving side (Figure 12(b)) shows that the voltage waveforms at the receiving side are generally the smoothest. is is because the voltage waveform on the receiving side is less oscillating than on the sending side. Moreover, the longer the transmission line distance, the lower the harmonic voltage on the receiver side. e simulation and experimental results have similar waveform characteristics; however, the experimental results demonstrate smoother waveforms and more noticeable DC properties. It was found that the voltage waveform of the simulation result was more oscillating than that of the experimental result due to the frequency limitation of recording devices of laboratory instruments that cover less frequencies and distances than the simulation. e current waveforms shown in Figure 13 have vertical and horizontal axes as the current value (A) and operating time (s), respectively. e results are organized in the same manner as in Figure 13. It is found that the 600 km distance has the smoothest waveform, whereas the other distances have more oscillation.
e current waveforms at the receiving side (Figure 13(b)) are smoother than those at the sending side (Figure 13(a)), and both the simulation and the experimental setup provide similar waveform characteristics.   Figure 14. e results demonstrate that the longer the transmission line distance, the higher the transmission line impedance and the greater the power loss as shown in Figure 14(a). Because the HVDC transmission line distance is doubled, the power loss and voltage drop are twice higher. e results of the MATLAB/Simulink program and the experimental setup show the same tendency. e power loss and voltage drop of both systems have similar values, but the simulated ones are slightly lower as shown in Figure 14(b) because of the impedance in the experimental wiring. is denotes that the power loss and voltage drop are directly proportional to the HVDC transmission line distance owing to the line's impedance.

Electrical Load Condition.
e electrical load is studied under two conditions: 750 and 1500 W. Both conditions are tested with a voltage of 500 V and a distance of 600 km. e voltage and current waveforms at the sending and receiving sides of the HVDC transmission line are compared for the different electric load powers as shown in Figures 15 and 16. e voltage waveforms in Figure 15 have vertical and horizontal axes as the voltage value (V) and operating time (s), respectively. e voltage waveforms are compared between the electrical loads of 750 W and 1500 W. However, the results of the simulation and the experimental setup are also analysed, in which the sending side of the transmission line is shown in Figure 15(a) and the receiving side of the transmission line is shown in Figure 15(b).
In Figure 15, the waveforms of the 750 W load are very much similar to those of the 1500 W load in both the simulation and experimental results because the change in load power does not affect the impedance in the transmission line but will result in more power loss due to more current flowing through the transmission line. A comparison of the voltage waveforms between the sending and the receiving sides shows that the voltage waveforms at the receiving side are generally smoother. However, the waveforms of the 1500 W load at the receiving side sway the most. e current waveforms shown in Figure 16 have vertical and horizontal axes as the current value (A) and operating time (s), respectively. e 750 W and 1500 W loads have the same characteristics, whereas the 1500 W load has a higher current value. e current waveforms at the receiving side are smoother than those at the sending side, and the experimental setup results are smoother than the simulation results. e corresponding power losses and voltage drops are shown in Figure 17. e simulation and experimental results show the same tendency: e higher the electrical load, the higher the current and the greater the power loss as shown in Figure 17(a). When the electrical loads are doubled, the power losses increase 2 times (in pu unit). e power loss in the experimental setup is similar to that in the simulation. e voltage drops of both systems have similar values; however, the simulated ones are slightly lower as shown in Figure 17(b). As the electrical loads are doubled, the voltage drops are twice higher. However, when considering the results of the simulation and the experimental setup, it was found that the experimental setup involved a number of external elements such as cables, terminals, and electrical protection devices. is results in a slightly greater power loss and voltage drop than those in the simulation.

Voltage Condition.
e 450 V and 500 V voltage conditions are considered in this subsection. Both conditions are tested based on a capacity of 750 W and a distance of 600 km. e voltage and current waveforms at the sending and receiving sides of the MATLAB/Simulink program and the experimental setup are compared at different voltage conditions as shown in Figures 18 and 19. e voltage and current waveforms of the 450 V and 500 V voltage conditions are very similar in both systems; nevertheless, the receiving side has smoother waveforms than the sending side. Moreover, in the 450 V simulated system, the voltage and current waveforms are more oscillating. is may be the result of the rectifier's limited ability to adjust the voltage. e results are compared in two aspects: the power loss and voltage drop. It is observed that the 450 V model has higher power losses than the 500 V model because at the same load power, the lower the voltage, the greater the current and power loss as shown in Figure 20 From the three subsection tests, the preliminary results showed that the power loss and voltage drop are directly proportional to the length of the HVDC transmission line because of the resistance of the line, whereas the power of the load is directly proportional to the power loss and voltage drop as well due to the greater current flowing through the power line. Yet, the change in transmission line voltage has no significant effect on the transmission line system as shown in Table 7.

Conclusion
is study focuses on the effect of transmission line distance, load power, and voltage on power loss, voltage drop, and waveform using the MATLAB/Simulink program and the experimental setup for consideration of preventing damage to the HVDC system. e results show that the longer the transmission line distance, the higher the transmission line impedance and the greater the power loss. e power loss and voltage drop are directly proportional to the HVDC transmission line distance because of the line's impedance. For the different load power conditions, the higher the electrical load, the higher the current and the greater the power loss. e increase in the power loss is many times greater than that in the load power. e power loss and voltage drop of both systems have similar values. Finally, for the different voltage conditions, the effects on power loss and voltage drop are rather small because the voltage conditions are not much different. Nevertheless, the waveforms of the 450 V model show more pronounced oscillation. is may 22 Journal of Electrical and Computer Engineering be as a result of the rectifier's limited ability to adjust the voltage. is study can be used to describe the electrical phenomena of an HVDC transmission line in a real system under the effects of each condition. e results of this study can be applied to the design of high-voltage direct current transmission lines. In addition, it is also possible to simulate the power transmission conditions according to the design to consider the impact before the actual construction and estimate the impact of renewable energy and faults in the HVDC transmission line.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.