Researchers from Guangzhou and Shanghai Universities, China published an article in Frontiers in Energy Research Journal on filtering characteristics of parallel-connected fixed capacitors in LCC-HVDC line-commutated converter (LCC) high voltage direct current (HVDC) transmission technology considering the variations of system strength.
The AC power system strength exhibits time-varying characteristics during operation, thereby affecting the filtering performance of filters in the system. Failure to account for this variability may result in the harmonic levels exceeding permissible limits under specific power system strength, thereby affecting the normal operation of the power system.
Consequently, building upon the existing filtering technique based on parallel-connected fixed capacitors for LCC-HVDC systems, a method for tuning the parameters of parallel-connected capacitors is proposed, thereby the capacitance range meets the filtering requirements under various system strengths.
Introduction
High voltage direct current (HVDC) transmission technology plays an important role in large-capacity and long-distance transmission applications. However, with the increasing number of converter stations in the power system, the harmonic levels in the power grid are also rising. Excessive harmonics can lead to distortion of the AC waveform and reactive power deficits, posing a serious threat to the safe operation of the power system.
To address this issue, power grid companies have established the corresponding harmonic content standard, specifying that the Total Harmonic Distortion (THD) value of harmonic voltage and current should not exceed 1.5%. To meet the standards, effective measures need to be taken. AC filters are essential components in HVDC transmission system for mitigating harmonics. In addition, the AC power system strengths have a significant impact on the operation of the power grid and the strengths can be quantified using the short-circuit ratio (SCR) provide the typical variation of the SCR and its impact on the frequency and voltage stability. Lower AC strength corresponds to lower resonance frequencies and an increasing risk of resonance, leading to the degradation of the system’s normal performance and distortion of voltage. In summary, weak AC power system strength can weaken the system’s robustness and stability. Increasing the AC power system strength is beneficial for raising the system’s resonant frequency and stability, while achieving the better filtering effects.
For LCC-HVDC systems, it is necessary to consider both the reduction of AC harmonics and the compensation of reactive power. Traditionally, active or passive filters are commonly employed to achieve these objectives. The circuit topology of passive filters is relatively simple, but they suffer from drawbacks such as large space requirements and poor filtering effectiveness due to the resonance points shifting. Consequently, research and application of active filter have become more extensive.
Various design approaches for active filters have been proposed, addressing aspects such as optimizing the multi-topology structure of passive filters, parallel filtering structures based on hybrid systems, and adopting two-parallel single-tuned LC structures while considering the time-delay effect of controllers. The designed filters can effectively reduce harmonics and simultaneously reduce the size of capacitor banks to reduce reactive power compensation. Liu consider the coordination between hybrid active filters and existing reactive power compensation devices. They introduce a novel AC filtering system that utilizes a series-connected passive resonance topology and a control scheme for active filtering. This not only enhances the harmonic suppression performance of LCC-HVDC, but also optimizes the reactive power compensation between different filter groups thereby reducing HVDC costs.
The above filtering technologies typically require the addition of AC filter stations. This entails significant space occupation, and the necessity to address issues such as losses and maintenance. Therefore, novel filtering technique have been proposed. Huang introduce a hybrid active power filter with simple combination of passive filters and IGBT valves. The selected passive filter capacity and topology effectively enhance the filtering efficiency while ensuring independence between the filter and AC system, preventing resonance. However, the system configuration cost is high and the control of IGBT valves is difficult. A novel induction filtering technique based on the field-circuit coupling calculation method is proposed by Li, which greatly reduces the harmonic current content, enhances the excitation performance, improves the electromagnetic environment, and reduces the harmonic losses of HVDC converter transformers.
Zhai, Zhao, and Xue propose a novel filtering technique based on parallel-connected fixed capacitors in HVDC converters, which effectively suppresses harmonics without external AC filters and reactive power compensation devices. It also provides reactive power compensation and suppresses the commutation failure. This filtering method successfully address the issues associated with traditional filters. Nevertheless, the above studies do not take into account the impact of AC power system strength on the filtering performance of the filters. Power system strength is a crucial indicator of power system stability and play a vital role in power system operation. For the novel filtering technique involving the addition of parallel-connected fixed capacitors, it is crucial to consider whether the capacitance of the parallel capacitor can still maintain the system stability and meet the harmonic requirement under the varying system strengths.
To address aforementioned problems, this paper analyzes the impact of system strength on the filtering. Additionally, a tuning method for parallel-connected capacitors is proposed considering power system strength, establishing a capacitance range that meets filtering standards. This method not only contributes to the fault and transient analysis of novel filtering technique involving parallel-connected fixed capacitors but also provide valuable guidance for the configuration of capacitance parameters in practical engineering.
Operation principle of novel filtering technique
In response to the shortcomings associated with the traditional filters, Xue propose a novel filtering technology by adding parallel-connected fixed capacitors to the inverter side of the converter station.
The circuit diagrams for the novel filtering technique are shown in Figures 1, 2, where Ls is the smooth inductance, Lc is the transformer equivalent inductance, Rs is the DC resistance, TY(D)1∼TY(D)6 are the thyristors of the converter bridge, CapY(D)ab, CapY(D)bc, CapY(D)ac are the parallel-connected fixed capacitors, LY(D)abc are series inductance, mainly used for absorbing surplus reactive power. Ca is the DC filtering capacitor, Zinv is the equivalent impedance of AC system at the inverter side, and Zsys(n) is the grid impedance at the nth harmonic frequency.
It can be seen from Figure 2 that the impedance of the capacitor branch will be small at high-frequencies, allowing a large AC current to flow through this branch. This causes the high-frequency AC harmonics to be “short-circuited” by the capacitors, achieving harmonic reduction. This further achieves the dual purpose of harmonic filtering and reactive power compensation. Xue et al. (2018) compare the simulation results between traditional and novel filtering technique, demonstrating that the latter can achieve similar filtering effects.
The novel filtering technique not only addresses the limitations of traditional filtering but also achieves filtering effort with a reduced amount of reactive power compensation. Therefore, studying the influence of power system strength on filtering effectiveness based on this novel filtering technique is of significance.
Results
Comparative with existing passive filtering techniques
Table 1. shows the harmonic content using different filtering techniques, with the novel filtering technique provided for capacitor capacitance of 7.35 μF and 8.8 μF. The choice of 7.35 μF is based on the fact that, at this capacitance, the reactive power compensation generated by the capacitor is comparable to the reactive power compensation achieved with a passive filter. This selection allows for a more meaningful comparison of the filtering effects of different techniques.
From Table 1, it is observed that the 11th and 13th harmonic content is higher than that of the passive filter, but the high-frequency harmonic content is lower for the capacitance of 7.35 μF. In terms of THD values, with the same reactive power compensation, the filtering effect of the novel filtering technique is superior to that of the passive filter.
For capacitance of 8.8 μF, the reactive power generated by the capacitor is much larger than the reactive power compensation of the passive filter. However, the filtering effects for some harmonics orders is worse than the passive filter, the main reasons are as follows:
A. Impedance of the parallel-connected fixed capacitor itself: The fixed capacitor has a certain internal impedance in the system. When the capacitance is large, the internal impedance also increases, leading to higher losses and increased harmonic content.
B. Resonance issues: When the capacitance is large, it requires series or parallel-connected grounding inductance to absorb the surplus reactive power. In this case, a resonant circuit is formed by the capacitor C and inductor L. With a large capacitance, the resonance frequency decreases, and if it becomes close or equal to the resonance frequencies of other circuits in the system, it may cause resonance issues, leading to a reduction in filtering effectiveness.
C. High-performance broadband of the passive filter: Passive filter can choose different parameters according to requirements to meet the harmonic reduction. By adjusting the parameters of circuit components, better harmonic suppression can be achieved. In contrast, the filtering effect of parallel-connected fixed capacitors can only be effective within a specific frequency range, making it difficult to filter out harmonic signals at other frequencies, and adjustment is challenging.
In summary, both traditional filtering and novel filtering techniques can effectively suppress harmonic interference. However, due to limitations such as the impedance of capacitors, resonance issues and the filtering frequency range, the filtering effects vary under different capacitances. In practical engineering, it is advisable to choose different filtering methods based on actual requirements and grid conditions.
Conclusion
This paper proposed a tuning method for the parameter of parallel-connected fixed capacitors considering the system strength based on the novel filtering technique. The method enables the calculation of the range of required capacitance for filtering under varying power system strengths. By analyzing the filtering performance under various power system strengths, this paper reveals that the variation in power system strength affects the filtering performance of the novel filtering technique.
Specifically, within a certain range of power system strengths, weaker AC system leads to a reduction of filtering performance. Consequently, a tuning method for the capacitor parameters’ range is proposed for the novel filtering technique considering the power system strengths. This method is effective for LCC-HVDC systems with varying power system strengths. It provides guidance for the selection of filtering capacitance in practical applications.
Read the full paper under the reference link:
Front. Energy Res., 06 March 2024, Sec. Sustainable Energy Systems, Volume 12 – 2024 | https://doi.org/10.3389/fenrg.2024.1370585