Filtering Characteristics of Parallel-Connected Fixed Capacitors in LCC-HVDC

Researchers from Guangzhou and Shanghai Universities, China, published an article in Frontiers in Energy Research Journal on the filtering characteristics of parallel-connected fixed capacitors in LCC-HVDC line-commutated converter (LCC) high voltage direct current (HVDC) transmission technology, considering system strength variations.

The AC power system strength exhibits time-varying characteristics during operation, thereby affecting the filtering performance of the system’s filters. Failure to account for this variability may result in harmonic levels exceeding permissible limits under specific power system strength, thereby affecting the power system’s normal operation.

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 is important 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 severe threat to the safe operation of the power system.

Power grid companies have established the corresponding harmonic content standard to address this issue, specifying that the Total Harmonic Distortion (THD) value of harmonic voltage and current should not exceed 1.5%. To meet the standards, adequate measures need to be taken. AC filters are essential components in the 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) to 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 average 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 better-filtering effects.

For LCC-HVDC systems, both the reduction of AC harmonics and the compensation of reactive power must be considered. 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 ample space requirements and poor filtering effectiveness due to the resonance points shifting. Consequently, research and application of active filters 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 considers 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 enhances the harmonic suppression performance of LCC-HVDC and 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 addressing issues such as losses and maintenance. Therefore, novel filtering techniques have been proposed. Huang introduced a hybrid active power filter with a 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 controlling 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. This 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 addresses the issues associated with traditional filters. Nevertheless, the above studies do not consider 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 is vital 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 the problems above, this paper analyzes the impact of system strength on filtering. A tuning method for parallel-connected capacitors is proposed, considering power system strength and establishing a capacitance range that meets filtering standards. This method contributes to the fault and transient analysis of novel filtering techniques involving parallel-connected fixed capacitors and provides 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 proposes 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 L_s is the smooth inductance, L_c is the transformer equivalent inductance, R_s 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. C_a is the DC filtering capacitor, Z_inv is the equivalent impedance of AC system at the inverter side, and Z_sys(n) is the grid impedance at the n_th 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.

Table 1 shows that the 11th and 13th harmonic content of the novel filtering technique 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 novel filtering technique’s filtering effect is superior to that of the passive filter.

For a 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: 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: The 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 and novel filtering techniques can effectively suppress harmonic interference. However, due to limitations such as capacitor impedance, resonance issues, and the filtering frequency range, the filtering effects vary under different capacitances. In practical engineering, choosing different filtering methods based on actual requirements and grid conditions is advisable.

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 calculates the required capacitance range 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 systems reduce 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.