Abstract
The paper reports the investigation of the dynamic operation of novel 24-pulse HVDC transmission system for eliminating the 11th and 13th harmonic currents from spreading out to the AC system and improving power quality. The complete simulation of the 24-pulse HVDC system is performed using the MATLAB/Simulink. Simulation results show that the Total Harmonic Distortion (THD) of AC system in 12-pulse system is 8.51%. Whereas, the THD of AC system in 24-pulse system is 4.49%. The waveforms of AC current have improved without employing 11th and 13th tuned filters at AC systems.
Keywords
This work was supported in part by the Universiti Sains Malaysia through the Bridging Grant under Grant 304.PELECT.6316098.
1 Introduction
The HVDC transmission involves the use of converter for the conversion of AC to DC (rectifier) at the transmitting end, and converting back the DC to AC at the receiving end (inverter). In order to meet the requirements of power system security and power quality, a HVDC converter station has the major components such as converter valves, converter transformers, AC filters and reactive power compensators, DC smoothing reactance, control and protection systems, and telecommunication system [1]. A 12-pulse line commutated converter (LCC) HVDC transmission based on thyristor is an economic option in terms of its cost and other advantages. The harmonics generated in AC line currents of thyristor converters have often caused serious problems in HVDC system. The distortion factor of AC current of 12-pulse LCC configuration is more than 15% [2]. Therefore, the 12-pulse does not meet power quality standard requirement determined by IEEE-519 standard [3]. Thus, passive filters have been installed for compensation, which are complex in design and bulky in size [4]. The alternative way for improving power quality without filters installation is to utilizing high pulse number converters, such as 24-pulse and 36-pulse [5, 6]. Miyairi et al. [2] have proposed a new method to reduce harmonics at input and output of the rectifier. It is reported that the distortion factor of input AC current of 24-pulse system is 7.57%. These results meet the power quality requirements. However, this method cannot be applied to the rectifier without an interphase transformer and sub-rectifier elements constitute the tap-changer. Adding these elements to the rectifier may increase operational cost.
Therefore, this paper proposes a new 24-pulse system utilizing thyristor valves for eliminating the 11th and 13th harmonic currents from spreading out to the AC system and improving power quality. In this paper the harmonics characteristics of 24-pulse and 12-pulse are analysed. By using MATLAB/Simulink, two simulations of the conventional 12-pulse and proposed 24-pulse LCC HVDC systems are carried out with similar power rating. The 24-pulse system is fully validated by digital simulation.
2 System Configurations
The block diagram of 12-pulse and 24-pulse HVDC systems are shown in Figs. 1a and 1b, respectively.
In Fig. 1a, the 12-pulse system is monopolar and interconnecting two AC systems of different frequencies through 300 km DC line. It has two six-pulse bridges \( Y1\_R \) and \( D1\_R \) connected to the AC system 1 at busbar \( a1, b1, c1 \) through a two transformer one connected in star-grounded/star \( Yg/Y1 \) and other connected in star-grounded/delta \( Yg/D1 \) formation. Similarly, at busbar \( a2, b2, c2 \) the two six-pulse bridges \( Y1\_I \) and \( D1\_I \) connected to the AC system 2 through two transformers one connected in \( Yg/Y1 \) and other connected in \( Yg/D1 \). The AC filters at each side are: 11 Tuned Filter (\( TF_{11} \)), 13 Tuned Filter (\( TF_{13} \)), High-Pass Filter (\( HPF \)), and Capacitance Bank (\( CB \)). These filters are connected to AC systems to provide a low impedance path to ground for removal of harmonics current 11, 13, 23, and 25 and provide the AC reactive power compensation.
In Fig. 1b, the 24-pulse system is modified based on Fig. 1a. The difference between these two configurations are: (i) at busbar \( a1, b1, c1 \), the six-pulse bridges \( Y1\_R, D1\_R, Y2\_R, D2\_R \) are connected to the AC system 1 through transformers \( Yg/Y1, Yg/D1, Yg/Y2, Yg/D2 \) with phase-shifting \( + 7.5^\circ , + 7.5^\circ , - 7.5^\circ , - 7.5^\circ \), respectively. Similarly at busbar \( a2, b2, c2 \), the six-pulse bridges \( Y1\_I, D1\_I, Y2\_I, D2\_I \) are connected to the AC system 2 through transformers \( Yg/Y1, Yg/D1, Yg/Y2, Yg/D2 \) with phase-shifting \( + 7.5^\circ , + 7.5^\circ , - 7.5^\circ , - 7.5^\circ \), respectively. (ii) The AC filters are divided into two groups such as \( HPF \) and \( CB \) at each side.
3 Characteristic Harmonics Analysis
The harmonics characteristics are related to the pulse number of the converter. These harmonics are of order \( kp \pm 1 \) on the AC side and of order \( kp \) on the DC side, where \( p \) is the pulse number and \( k = 12 \) or \( k = 24 \) for 12-pulse or 24-pulse, respectively. For a six-pulse bridge with a \( Y_{g} /Y1 \) transformer connection, the Fourier series expansion for the alternating current \( i_{Y1} \) is:
For a six-pulse bridge with a \( Y_{g} /D1 \) transformer connection, the Fourier series expansion for the alternating current \( i_{D1} \) is:
For a six-pulse bridge with a \( Y_{g} /Y2 \) transformer connection, the Fourier series expansion for the alternating current \( i_{Y2} \) is:
For a six-pulse bridge with a \( Y_{g} /D2 \) transformer connection, the Fourier series expansion for the alternating current \( i_{D2} \) is:
where, \( \omega \) is the fundamental repetitive frequency in unit of \( rad/s \), \( i_{d} \) is DC current. In the 12-pulse system, the total AC-side current of the 12-pulse converter is the sum of the two six-pulse currents \( i_{Y1} \) and \( i_{D1} \). From Eqs. (1) to (2), the harmonics of opposite polarity per cycle is cancel out and the following equation can be written:
Whereas in the 24-pulse system, the total AC-side current of the 24-pulse converter is the sum of the four six-pulse currents \( i_{Y1} ,i_{D1} , i_{Y2} , i_{D2} \). From Eqs. (1) to (4), the harmonics of opposite polarity per cycle is cancel out and the following equation can be written:
4 Simulation Verification
The conventional 12-pulse and proposed 24-pulse systems in Figs. 1a and 1b, respectively, are simulated using MATLAB Simulink. The detail parameters of 12-pulse and 24-pulse systems are listed in Table 1. Both simulations of the 12-pulse and 24-pulse systems are carried out at same power rating condition to compare their performance. In each case, the steady-state waveforms of the AC side currents are obtained using the MATLAB/Simulink.
5 Results and Discussion
Figure 2 shows the results of power quality indices obtained from the simulations of 12-pulse and 24-pulse systems. Figure 2a, b and c shows the respective input of 3-phase AC current measurements at rectifier side (AC system 1) of 12-pulse system. Whereas, Fig. 2d, e and f shows the respective input of 3-phase AC current measurements at rectifier side (AC system 1) of 24-pulse system.
Figure 3a, b shows the Total Harmonic Distortion (THD) result for 12-pulse and 24-pulse systems, respectively. The THD results of Fig. 3a, b are obtained from the AC current in Fig. 2a. d, respectively by using FFT analysis. It is seen from Fig. 3a, the THD of current waveform at AC system 1 in 12-pulse system is 8.51%. Whereas, Fig. 3b depicts 4.49% THD of current waveform at AC system 1 in 24-pulse system. The waveforms of AC current have improved remarkably without employing 11th and 13th tuned filters at AC systems. The AC current waveform is almost sinusoidal as shown in Fig. 2d. The current waveform has dominant 23th and 25th harmonics as shown in Fig. 3b.
6 Conclusion
Test results have shown that power quality has significantly improved by employing the proposed 24-pulse LCC HVDC configuration. Therefore, the THD for AC current in the proposed 24-pulse HVDC transmission system meets the IEEE-519 Standard requirements of power quality.
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Mohammed, S.R., Teh, J., Kamarol, M. (2019). Power Quality Improvements in a Novel 24-Pulse Line Commutated Converter HVDC Transmission System. In: Zawawi, M., Teoh, S., Abdullah, N., Mohd Sazali, M. (eds) 10th International Conference on Robotics, Vision, Signal Processing and Power Applications. Lecture Notes in Electrical Engineering, vol 547. Springer, Singapore. https://doi.org/10.1007/978-981-13-6447-1_28
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