Protection scheme for VSC-MTDC based on low-frequency reactive power

VSC‐MTDC system integrity protection scheme: Backup protection based on virtual traveling wave power

“…First, analyse the virtual traveling wave power in terms of physical meaning [19]. The traveling wave is the energy transmission state of these distributed parameters.…”

“…(1) The amplitude of $${Z_{C1}}$$ and $${Z_{C2}}$$: First, analyse the line wave impedance and calculate it using detailed parameters of an actual project [19]. Analysing the distribution parameters of the line, the amplitude‐frequency characteristics of the line wave resistance can be obtained as shown in Table 1 [18].…”

“…$${f_{j,k}}$$ and $${e_{j,k}}$$ represent scale factor and wavelet coefficient. The scale coefficients and wavelet coefficients obtained from formula (8) are [19]: $$$$\begin{equation}\left\{ \def\eqcellsep{&}\begin{array}{l} {f_{j,k}} = \displaystyle\sum_m {{h_0}(l - 2k){c_{j,m}}} \\[12pt] {e_{j,k}} = \displaystyle\sum_m {{h_1}(l - 2k){c_{j,m}}} \end{array} \right.\end{equation}$$$$where l represents the sampling sequence number; k represents the sequence number of the coefficient; h 0, 1 is the filter coefficient.…”

“…(1) Traditional backup protection [19]: This is the most typical DC protection scheme. Its criterion is [19]: $$$$\begin{equation}\left\{ \def\eqcellsep{&}\begin{array}{l} {I_{op}} \ge {I_{act.\min }}\\[6pt] {I_{op}} \ge {I_{res}}\\[6pt] {I_{res}} = {I_{act.\min }} + m({I_{res1}} - {I_{_{res}.\min }}) \end{array} \right.\end{equation}$$$$where $${I_{op}}$$, $${I_{act.\min }}$$ and $${I_{res}}$$ are the differential current, minimum operating current and braking current.…”

“…The comparison results are as follows: Figure 12a: The distributed capacitance current causes the fault current to oscillate. At this time, the braking current is higher than the operating current, and the current differential protection (protection [19]) fails. Figure 12b: Under different fault resistance and noise interference, the ratio of virtual traveling wave power is still less than the threshold (50), which indicates that the proposed scheme can identify the interference of 800 Ω + 10 dB. Figure 12c,d: Figure 13c represents protection [21], and Figure 12d represents protection [20]. When the interference reaches the level of 300 Ω+20 dB, the protection [21] and the protection [20] cannot operate correctly. …”