Characteristics and Protection Methods for Double-Circuit HVDC Transmission Lines on the Same Tower Considering DC Line-Touching Faults

Abstract

In heavily loaded regional power grids, some AC transmission lines are confronting escalating pressures due to excessive short-circuit currents. To optimize AC channels, most research advocates for retrofitting existing AC lines into multi-line-commutated converter-based high-voltage direct current (LCC-HVDC) lines. However, there is a contradiction between limited land area for AC stations and the relatively large footprint of passive filters in LCC-HVDC; this paper introduces self-adapted LCC (SLCC) by replacing passive filter groups with a static var generator (SVG). Secondly, the reactive power compensation, harmonic filtering control methods of SVGs, and operation characteristics of the SLCC system are explored, and the harmonics of the grid-side current are reduced by nearly 14.6%. Then, to fill the gap of previous studies on solely AC or AC-DC line touching, inspired by emerging DC line-touching risks in double-circuit (LCC and SLCC) lines on the same tower, the equivalent models are formulated to elucidate the evolution mechanisms of voltage/current and extract fault features in various line-touching faults; it finds that the longitudinal differential current during line-touching faults can be capitalized. Based on the current feature, an effective protection algorithm tailored for the identification of DC line-touching faults is proposed. Finally, simulations are conducted to validate the efficacy of proposed control and protect methods, demonstrating the potential to enhance the reliability of AC to DC conversion projects.

1. Introduction

With the increase in electricity demand, the transmission channels in heavily loaded regional power grids are confronting significant pressures; especially some traditional AC lines approach their capacity limits and face escalating risk of exceeding short-circuit currents. Compared to constructing new transmission channels, a feasible solution is to convert existing AC lines into DC lines. Due to the fact that high-voltage direct current (HVDC) transmission technology features large transmission capacity, it is popular in long-distance transmission applications [1]. Some projects have adopted such schemes to enhance the transmission capacity of existing AC transmission lines [2,3,4].

In recent years, various types of HVDC technology have been developed. The line-commutated converter-based HVDC (LCC-HVDC) and flexible voltage source converter-based HVDC (VSC-HVDC) are two prevailing alternatives [5]. The LCC-HVDC possesses the merits of excellent voltage-withstanding capability, lower costs, etc., but the half-controllable thyristors cause commutation failures (CFs) and rely on the amount of AC filter groups [6]. Meanwhile, VSC-HVDC overcomes CFs but is more fragile to an overvoltage/overcurrent, which is solved by DC breakers, causing a relatively higher expense [7]. Thus, under the conditions of converting AC lines to DC lines in regional power grids, LCC-HVDC is the preferred option. In fact, it is applied in the actual AC to DC reconstruction project test in the Jiangsu Power Grid, China, which motivated the research of this paper. But it is feasible to apply VSC-HVDC from the perspective of harmonic filtering and flexible control in future work. However, as aforementioned, for an LCC-HVDC transmission system, there is a need to provide reactive power compensation devices to support AC voltage such as active power filters [8]. FACTS devices such as a static var generator (SVG) are preferable. But previous works, regardless of filters or FACTS devices, are linked with AC buses, facing the problem of an additional station area. To meet such demand, the first novel point of this paper is to introduce a self-adaption SVG-based LCC (SLCC). The main idea is to make use of space under the LCC valve in the converter station to place SVGs, which means that the SVGs are connected in parallel on the bus of the transformer and LCC converters. At the same time, SVGs aim to realize active power filters and reactive power compensation.

Promoting the approach of converting three-phase AC lines to DC lines will result in multi-circuit DC transmission on a shared tower. Compared to conventional single-circuit lines, multi-circuit DC transmission on the same tower can reduce the number of transmission corridors and the occupied land area, significantly enhancing economic benefits. In fact, with the gradual maturation of DC transmission technology and the increasing scale of transmission, multi-circuit DC transmission on the same tower has been promoted and applied in China. Examples include the Xiluodu-Guangdong ±500 kV double-circuit DC transmission line on the same tower [9,10], where both circuits of the DC line are terminated at the same converter station, representing a fully shared-tower configuration. Another example is the Gezhouba-Nanqiao double-circuit DC transmission project, which features partial shared-tower construction [11]. Compared to conventional single-circuit lines, multi-circuit DC transmission on the same tower can reduce the number of transmission corridors, decrease land occupation, increase line transmission capacity, and significantly enhance economic benefits. It is worth noting that due to the high transmission voltage levels of these projects, the topology of the DC systems is symmetrically bipolar. Moreover, when converting low-voltage AC transmission lines to DC in a regional power system, the voltage level is lower; thus, a symmetrical monopolar topology is employed to reduce conversion costs. If both AC lines of a double-circuit transmission system on the same tower are converted to DC, a scenario with three symmetrical monopolar DC lines on a single tower emerges. The electrical distance between the conductors on the same tower is relatively small, and under the influence of external factors such as strong wind disturbances or short circuits caused by foreign objects [12,13,14], line-touching faults may occur, posing a significant impact on the system. Therefore, research on fault characteristics and protection algorithms for double-circuit DC transmission on the same tower is essential to ensure safe and reliable system operation.

Obviously, the second challenge is that there is a unique DC line-touching fault and the characteristics are complicated. In terms of a fault characteristic analysis, line-to-line faults in double-circuit AC transmission lines on the same tower are primarily analyzed based on symmetrical component methods or fault transient characteristics. The magnitude of fault currents is related to the neutral point connection of the two AC lines [12,15]. For AC-DC cross-line faults, the analysis is mainly based on fault transient characteristics, and the traveling wave features after the fault are related to the voltage phase of the AC line at the time of the fault [16,17]. However, compared to line-to-line faults or AC-DC line-touching faults in double-circuit AC transmission lines [18], fault circuits in double-circuit DC transmission lines on the same tower exhibit significant differences, and fault traveling wave characteristics are independent of the fault timing [19,20]. Furthermore, double-circuit DC transmission lines are equipped with converter stations at both ends, and the characteristics of DC line-to-line contact faults are significantly influenced by converter station control.

As for protection algorithms, existing DC line protection schemes are primarily designed for balanced single-circuit DC transmission line faults [21,22,23]. The mainstream approach employs traveling wave protection as the primary protection, with differential undervoltage protection and differential protection serving as backup. However, due to the differences in loop topology and fault characteristics between the touching faults of double-circuit DC lines on the same tower and single DC faults, the current protection schemes and settings tailored for single-circuit DC lines may lose accuracy or even fail in the context of double-circuit DC line-touching faults. Therefore, it is necessary to propose a protection algorithm that can accurately and rapidly identify touching faults based on the characteristics of double-circuit DC faults on the same tower.

In general, this paper is motivated by three aspects: (1) to introduce a novel SLCC-HVDC technology serving as a satisfying option for “AC to DC lines conversion” application, benefiting from proper appropriate space occupancy and reactive power support of SVGs replacing conventional AC passive filters; (2) previous research focus on the touch faults of either AC lines or AC/DC lines and the emerging LCC-SLCC DC line touching on the same tower need further exploration and mathematical interpretation; (3) recent protection methods focus on single-circuit DC transmission, and this limits the effectiveness in double-circuit line-touching faults of different types of DC topology.

To fill in the gap of interesting SLCC fault analyses and the protection of double-circuit LCC-SLCC HVDC lines on the same tower, the main work and contribution of this paper was conducted in three aspects:

(1)

The design and operation of SLCC-HVDC are investigated, including the topology and reactive power and harmonic filtering control strategy.

(2)

The DC line-touching fault mechanism and characteristics of double-circuit LCC-SLCC HVDC lines on the same tower are analyzed by equivalent models, and the different polarities’ line touching and transmission power are considered.

(3)

The applicable protection methods for a line-touching fault of double-circuit LCC-SLCC HVDC lines on the same tower are proposed and verified.

The organization of this paper is as follows: Section 2 designs the control methods of the SLCC and presents the configuration of LCC-SLCC double-circuit HVDC lines on the same tower. Section 3 investigates the fault characteristics of a line-touching fault in the LCC-SLCC system. Section 4 presents the protection methods. Finally, Section 5 concludes this paper.

2. The Topology and Control of Double-Circuit HVDC Lines on the Same Tower

2.1. The Topology of LCC-SLCC HVDC Lines on the Same Tower

The topology of the double-circuit HVDC lines on the same tower is shown in Figure 1. In our study, two types of HVDC lines are investigated, including conventional LCC-HVDC and meaningful SLCC-HVDC transmission lines. Specifically, a 6-pulse converter based on thyristors is connected on both sides of the neutral point to form positive and negative lines. Then, bipolar symmetric operation constitutes a 12-pulse LCC converter, which serves as the rectifiers and inverters of both LCC and SLCC. Notably, in a conventional LCC-HVDC transmission system, plenty of passive AC filters are linked with an AC bus to support reactive power for LCC converters during commutation. It is worth noting that such filters are put into use in groups; thus, the reactive power cannot be adjusted continuously, and the footprint is a significant issue. However, in the SLCC-HVDC system, we configure an SVG in parallel between each converter and the transformer; this topology helps remove the original passive filters. Consequently, the SVG is responsible for reactive power compensation and harmonic filtering.

As for the connection mode of the converters, both LCC-HVDC and SLCC-HVDC operate in the symmetrical monopolar mode. The neutral point of the rectifier side is not grounded, while the neutral point of the inverter side is grounded. The two polar buses are grounded through a large resistance to maintain the voltage balance between the positive and negative polars.

It is obvious that compared with the traditional symmetrical single-circuit single-tower HVDC transmission system, the structure of Figure 1 connects two DC lines on a common tower; then, the number of corridors can be significantly reduced, and the land occupation area is also reduced accordingly. Furthermore, the transmission capacity of its line corridor can be regarded as the sum of the transmission capacities of several circuits, which can significantly improve the economic benefits of transmission. However, the accompanying issue is that there will be mutual coupling between each DC line under fault conditions, resulting in more complicated and indistinct fault characteristics than a conventional AC-DC line-touching fault. There is a need to elucidate it.

2.2. Control of LCC-HVDC Transmission System

The LCC-HVDC transmission system in Figure 2 adopts a traditional control strategy, where the rectifier side employs constant DC current control (CCC), while the inverter side includes constant voltage control, voltage-dependent current order limit (VDCOL) control, and constant extinction angle control (CEA). In Figure 2, R_V is the damping coefficient to ensure resistive characteristic during VDCOL stage. d_x is the coefficient of the commutation voltage drop in the CEA control. Two sets of LCCs are cascaded to form a 12-pulse LCC, sharing a set of polar control systems. During steady-state operation, the rectifier-side LCC operates in the CCC mode, while the inverter operates in the CEA mode.

In CCC, the LCC-HVDC system adopts hierarchical control. The main control pole issues a current command I_{dc_ord}, which is compared with the current value output from the VDCOL control. The smaller value is taken as the current command for both the rectifier and inverter sides. The difference between the commanded value and the measured current value at both ends is calculated. After PI control, the lead angle β at both ends is obtained. On the rectifier side, the obtained β is subtracted from π to obtain the firing angle αrec of the rectifier side. The inverter side takes the larger lead angle output from the CCC and CEA to calculate the firing angle. The CEA employs predictive extinction angle control [5], where the extinction angle for control is calculated using simulated parameters such as DC current, voltage, and commutation reactance. The advantage of predictive constant extinction angle control lies in its fast response time.

2.3. Topology and Control Performance of SLCC Transmission System

Compared to the LCC-HVDC system, the original passive filter and reactive power compensator on the inverters are replaced by SVG. The DC side of the SLCC consists of two six-pulse LCCs in series to form a twelve-pulse LCC. A set of 2/6/12 filters is installed between the two poles and the ground wire, respectively, and a flat-wave reactor is sympathetically arranged on the two poles. Under this scheme, the symmetric unipolar SLCC-HVDC has unipolar operation capability. In unipolar operation, the SLCC has only one six-pulse LCC and SVG running on a single end. The 6 harmonics generated on the DC side of the six-pulse LCC are filtered by the DC filter, and the 6k ± 1 harmonics generated on the AC side are filtered by the SVG, to ensure that the harmonics generated on the AC/DC side during the unipolar operation of the SLCC remain at a relatively low level.

To undertake the tasks of reactive power compensation and active filtering, the SVGs adopt a Y-connected cascaded H-bridge, owing to the advantages of a small volume as well as easy assembly and maintenance, and it is suitable to retrofit AC to DC lines. Due to the direct connection between SVG and a single six-pulse LCC, SLCC mainly needs to filter out the 6k ± 1st harmonic current on the AC side. The control strategy of SLCC is basically the same as that of the LCC. The control strategy of SVG mainly includes four parts: dq current control, in-phase voltage balancing, inter-phase voltage balancing, and active filtering. The dq current control adopts a dual-loop decoupling control with feedforward voltage, mainly responsible for maintaining the DC side voltage of the SVG and controlling the reactive power or voltage effective value on the AC side, as shown in Figure 3.

In the outer loop control, the active power component utilizes the average value of the measured DC voltage V_{dc_avg} of the three-phase arms as the feedback, which is then processed through a PI controller to output the active current reference value i_dref. For the reactive power component, either the measured reactive power at the valve side of the converter transformer Q_Lref or the effective value of the AC voltage V_acref serves as the feedback, and outputs the reactive current reference value i_qref through the PI controller. Under the normal operation mode, the reactive power control of the SVG adopts a strategy to maintain zero reactive power. In case of an AC fault, it switches to the AC voltage control mode. For the inner loop of AC current control, a decoupling control method with feedforward voltage is employed. Compared to the conventional feedforward decoupling control used in voltage source converters (VSCs), due to the significant harmonic currents generated by LCC, the SVG requires additional low-pass filtering of the measured dq-axis components of the voltage and current.

Due to the application of cascaded H-bridge topology in SVG, there is no direct connection between the DC side of each phase bridge arm, and the DC voltage control only controls the overall average voltage of each phase bridge arm submodule, which cannot ensure the balance of voltage between each phase bridge arm submodule. Therefore, it is necessary to introduce a voltage balancing algorithm for inter-phase submodules. This paper adopts a phase-to-phase voltage balancing algorithm based on negative sequence voltage, as shown in Figure 4. The DC sides of the submodules in its three-phase arms are not directly connected. If there is asymmetry in the external circuit of the SVG or in the output voltages and currents of the three-phase arms, significant imbalance and fluctuations may occur in the capacitor voltages of the three-phase arms. To maintain the inter-phase balance of the capacitor voltages in the SVG’s three-phase arms, an inter-phase voltage balancing algorithm is necessary. The basic implementation of this algorithm involves injecting an additional zero-sequence component into the three-phase modulation waves.

It is well known that the harmonics filter is of great importance of LCC, because the switching of power electronic devices will introduce a large amount of harmonics. In SLCC, to avoid harmonics flowing into converter transformers and the power grid, the switching harmonics are mainly absorbed by SVGs. Therefore, SVGs use the active filtering algorithm. The control block diagram of the filtering algorithm is shown in Figure 5.

The characteristic harmonic component i_Chj from the AC current i_Cj can be extracted based on a fast Fourier transform as the reference current for the harmonic control. Considering the limitation of algorithm complexity, we focus on extracting characteristic harmonics from the 5th to 49th order. Then, the I_p − I_q method is used to extract the harmonic component i_shj from the SVG compensation current i_sj, which serves as the input current for the harmonic control to ensure the speed of the control algorithm. Finally, i_Chj and i_shj are used as reference currents and control inputs, respectively, and quasi-PR (QPR) control is introduced to suppress the harmonic current of characteristic frequencies.

The simulation models of LCC-HVDC and SLCC-HVDC are established on PSCAD/EMTDC V4.6.2 software. The parameters are given in

Table 1. Parameters of the unipolar symmetrical HVDCs.

Parameters	Value
Rated DC voltage	±200 kV
Rated DC power capacity	1200 MW
Rated capacity of the transformers	750 MVA
Leakage inductance of the transformers	0.18 pu
Ratio of the sending-end LCCs	230 kV/173 kV
Ratio of the receiving-end LCC (Line 1)	230 kV/168 kV
Ratio of the receiving-end SLCC (Line 2)	230 kV/145 kV

and

Table 2. Parameters of the SVGs.

Parameters	Value	Parameters	Value
Rated capacity	300 MVA	Capacitance of the submodule	13 mF
Rated voltage/current of the IGBT	2 kA, 4.5 kV	Number of the submodules in each bridge arm	72
Average voltage of the submodules	2.3 kV	Inductance of the connecting reactor	8.5 mH

. Figure 6a,b compare the effect harmonics on the transformer current. The total harmonic distortion (THD) under the symmetrical operation mode is as shown in Figure 6c,d.

In the LCC-HVDC system, with the influence of passive filters, the harmonic content of the voltage on the grid side is relatively low, mainly consisting of 12k ± 1 characteristic harmonics such as the 11th, 13th, 23rd, and 25th. The highest content of 11th characteristic harmonics in voltage harmonics is only 0.111%, and the highest content of 25th characteristic harmonics in current harmonics is only 0.241%. However, since the LCC valve-side current is not filtered before being fed into the AC transformer, the harmonic current content of the transformer is relatively high. The 5th harmonic with the highest content in transformer current harmonics reaches 17.102%.

Meanwhile, the SLCC-HVDC system is capable of filtering harmonic currents during the non-commutation periods of the grid-side current. Although it cannot alter the trend of the grid-side current during the commutation period, the reduced commutation time results in lower harmonics. The harmonic levels of the grid-side current are reduced by nearly 14.6% and maintained below 2.5% for all orders. Furthermore, as illustrated in Figure 6c, only a small amount of the harmonic current is able to flow into the transformer, which is beneficial for reducing the noise and enhancing the stable operation capability of the transformer.

Thus, it is worth noting that SLCC is proposed to be oriented to retrofit AC transmission lines, which provide feasible opportunities for a large capacity and short distance in a local power gird. Basic control performance and covering area are two important aspects to consider. There are four remarkable advantages of symmetrical unipolar SLCC:

(1)

No need to configure a passive AC filter device, and the area of the converter station is significantly reduced.

(2)

The harmonic current flowing through the converter is very small, which is beneficial for reducing the vibration and noise of the converter.

(3)

The reactive power injected into PCC can be flexibly controlled, which can effectively prevent LCC CFs and continuous CFs, and reduce the dependence of the converter station on the AC system.

(4)

The operation mode is more flexible, can achieve unipolar operation, and change the operation mode without additional input from other filters.

Notably, though SLCC benefits from such technical challenges, more economic investment compared to LCC and VSC and the further application at a higher voltage level or longer distance need to be further investigated. This paper focuses on its actual application condition in the Jiangsu Power Grid, China; we pay more attention to the technical challenges in its control and fault interaction with other LCC lines.

3. A Characteristic Analysis of the Line-Touching Fault of Double-Circuit HVDC Lines on the Same Tower

We note that the aim of this paper is to find out the influence mechanism of unique line touching between different types of an HVDC transmission system. This is because, in an actual project, each type of HVDC operates independently and is far away from other HVDC lines. Accordingly, the influences of other complicating factors like the climate and environment are neglected. This is with the configuration of double-circuit LCC-SLCC HVDC on the same tower, when a line-touching fault occurs. There are four situations to be considered: (1) positive (LCC)–positive (SLCC), (2) negative (LCC)–negative (SLCC), (3) positive (LCC)–negative (SLCC), (4) negative (LCC)–positive (SLCC). And (1) and (2) are same-polarity line-touching faults, and (3) and (4) are different-polarity DC line-touching faults, which will be theoretically analyzed in this section.

3.1. Transient Characteristics of DC Line-Touching Fault

Same-Polarity DC Line-Touching Fault

We consider a line-touching fault between the same polarity on double circuits sharing the same tower in Figure 1. The equivalent circuit of the DC side of the system can be depicted with three circuits as shown in Figure 7. Loop “1” and Loop “2” denote the circuit for LCC and SLCC under normal operation; Loop 3 describes the short circuit under the line-touching fault. And i and v indicate the DC current and DC voltage, respectively. The subscripts “R” and “I” mean the rectifier and the inverter; “n” and “p” represent the negative pole and positive pole. L_d is the reactance of DC lines, R_ds represents the equivalent resistance of Loop 3, and i_s1, i_s2, and i_s3 are the currents in each loop.

Since the rectifiers are responsible for current control, which belong to the conventional loops and are unaffected by the fault loop, the currents in Loop 1 and Loop 2 remain at their rated values, I_dN. The expressions for the currents in each loop are given by Equations (1)–(3).

$$i_{\mathrm{s}1}=i_{\mathrm{Rp}1}=I_{\mathrm{dN}}$$

(1)

$$i_{\mathrm{s}2}=i_{\mathrm{Rp}2}=I_{\mathrm{dN}}$$

(2)

$$i_{\mathrm{s}3}={\displaystyle \frac{1}{2L_{\mathrm{d}}}}{\displaystyle {\int }_{t_0}^{t_1}\left(v_{1\mathrm{p}1}-v_{1\mathrm{p}2}-R_{\mathrm{ds}}{i}_{\mathrm{s}3}\right)\mathrm{d}t}$$

(3)

Due to the small difference between the fault line voltages v_1p1 and v_1p2, the transient response component in Equation (3) can be neglected, and thereby it can be simplified to

$$i_{\mathrm{s}3}={\displaystyle \frac{\left(v_{1\mathrm{p}1}-v_{1\mathrm{p}2}\right)}{R_{\mathrm{ds}}}}$$

(4)

From Equations (1) to (4), it can be inferred that during a same-polarity line-touching fault, the currents in all loops except the fault loop remain at their rated values, while the fault loop current deviates from its rated value. The magnitude and direction of this deviation depend on the difference in fault line voltages and the distance between the fault grounding point and the receiving end. Specifically, the farther the distance, the smaller the fault loop current. Since the voltage levels of the same-polarity lines are identical, there is no significant change in line voltage during a line-touching fault.

Furthermore, it can be analyzed that both DC line protections will detect a certain differential current of the two lines i_dp1 and i_dp2 in the faulty pole, as indicated in Equation (7). This differential current provides an indication of the fault condition and is crucial for fault detection and isolation mechanisms.

$$i_{\mathrm{dp}1}=i_{\mathrm{dp}2}=i_{\mathrm{s}3}=I_{\mathrm{dN}}-{\displaystyle \frac{\left(v_{1\mathrm{p}1}-v_{1\mathrm{p}2}\right)}{R_{\mathrm{ds}}}}$$

(5)

A simulation model based on the system in Figure 1 is established. Then, the validation of the same-polarity line-touching fault is conducted, and the results are presented in Figure 8 and Figure 9. We simulate a positive-to-positive line-touching fault; the currents in Loop 1 and Loop 2 remain at rated levels, while i_s3 in Loop 3 is approximately 0.7 kA, equivalent to the differential current between the two ends of the line. The voltages of the faulted lines remain the same, exhibiting no significant change during the line-touching fault. It can be concluded that only the positive current of the inverter of Line 2 deviated slightly from the rated value, and the two HVDC circuits remained in the CCC mode. And there is a certain longitudinal difference current between the two DC circuits.

3.2. Different-Polarity DC Line-Touching Fault

Regarding the different-polarity DC line-touching fault, we take positive and negative line-touching faults into consideration; the equivalent circuit is shown in Figure 10.

In the initial stage of the fault, the expressions for the currents in each loop are given by Equations (6)–(8).

$$i_{\mathrm{s}1}=i_{\mathrm{Rp}1}$$

(6)

$$i_{\mathrm{s}2}=i_{\mathrm{Rp}2}$$

(7)

$$i_{\mathrm{s}3}={\displaystyle \frac{1}{2L_{\mathrm{d}}}}{\displaystyle {\int }_{t_0}^{t_1}\left(v_{\mathrm{In}1}+v_{\mathrm{Ip}2}-i_{\mathrm{s}3}{R}_{\mathrm{ds}}\right)\mathrm{d}t}$$

(8)

Since the sum of the voltages across the two faulted lines is significantly greater than the voltage drop caused by the equivalent resistance of the loops, the equivalent resistance in Equation (10) can be neglected, thereby simplifying Equation (8) to

$$i_{\mathrm{s}3}={\displaystyle \frac{1}{2L_{\mathrm{d}}}}{\displaystyle {\int }_{t_0}^{t_1}\left(v_{\mathrm{In}1}+v_{\mathrm{Ip}2}\right)\mathrm{d}t}={\displaystyle \frac{1}{L_{\mathrm{d}}}}{\displaystyle {\int }_{t_0}^{t_1}{V}_{\mathrm{dN}}\mathrm{d}t}$$

(9)

Due to the large fluctuations in the line voltage and current during a positive-to-negative line-touching fault, the transient response process involves multiple stages. During the line-touching fault, the fault loop current i_s3 increases under the action of the two series converters, causing the currents in the two faulted lines to continue to decrease until they reach zero, while the current in the non-faulted line increases. At this time, the two inverters on the receiving side operate in the discontinuous current mode, leading to a drop in the DC voltage.

Since the CCC of both DC transmission lines uses the negative polarity current as the input to the controller, and due to the different polarities of the faulted lines, the response of their controllers during the fault will also differ. The equivalent circuit and development process under this scenario are shown in Figure 11 and Figure 12.

Line 1 first detects a drop in the negative current, and the inverter control enters the constant current control mode. After a slight rebound in the current, the VDCOL control detects that the negative voltage drop begins to take effect. To limit the fault current, it will increase the lead angle to reduce its own DC voltage until the current control is completed when v_Ip2 = −v_In1 = −V_dN. Therefore, during the occurrence of the line-touching fault, the voltage polarity and transmission power of Line 1 will reverse. On the other hand, although the rectifier side of Line 1 still operates in a conventional manner, the faulted pole’s inverter-side line current, influenced by the fault current of Loop 3, will also decrease. Consequently, the differential current between the two ends of the faulted line increases.

The negative current of Line 2 experiences a temporary increase due to the influence of the fault loop current, and the constant current control comes into effect to reduce the current. As the positive voltage drops, the VDCOL control is triggered. However, since the negative voltage of Line 2 does not drop, after the converter regains continuous conduction, the voltage will gradually rise, exiting the VDCOL mode and eventually returning to normal voltage operation.

Simulations are conducted and the results are presented in Figure 13. From the results presented in Figure 13, it can be observed that after the occurrence of a positive-to-negative line-touching fault, the current and voltage at the inverter side of the first DC negative pole rapidly drop to zero. Following a slight recovery of the negative-pole current under current control, it immediately enters the low-voltage current-limiting mode. The negative-pole voltage continues to decrease, reaching V_dN. The voltage of the positive-pole converter shifts to −0.5V_dN, and the transmitted power reverses. Simultaneously, the maximum differential current detected at both ends is 4 kA.

From the simulation results presented in Figure 14, it can be seen that after the occurrence of a positive-to-negative line-touching fault, the positive-pole current and voltage of the second DC line rapidly drop to zero, while the negative-pole voltage does not experience a significant drop. Subsequently, the positive-pole valve of Line 2 resumes continuous conduction, ultimately operating under normal voltage and current conditions without experiencing power reversal.

3.3. Fault Characteristics for Line Touching Under Different DC Transmission Powers

Considering line-touching faults occurring between two lines on the same tower under different transmission powers, the equivalent circuit diagram on the DC side of the system remains the same as that for faults with the same transmission power, as illustrated in Figure 7.

A. 

Same-Pole Line touching

The transmission power of the conventional LCC DC line is set to 0.6 p.u., while that of the SLCC DC line is set to 1.0 p.u. The characteristics of a same-pole line-touching fault under these conditions are depicted in Figure 15 and Figure 16.

Based on the simulation waveforms, it is evident that during a same-polarity line-touching fault, the voltages of the two lines involved become equal, and the resulting fault loop current is relatively small. The rectifier station, operating with constant current control, remains unaffected by the fault loop, ensuring that both the voltage and current are maintained at their rated values.

Due to the DC power of LCC and SLCC being at 0.6 p.u. and 1.0 p.u., respectively, under the same voltage level, the rated current of the LCC is 0.6 times that of the SLCC. When a same-polarity line-touching fault occurs, there is a significant deviation in the current at the moment of fault initiation. Therefore, compared to faults with the same transmission power, line-to-line contact faults under different transmission powers result in a larger differential current.

B. 

Different-Polarity Line touching

Considering line-touching faults between two lines on the same tower transmitting different powers, the characteristics of such faults when they occur between lines of different polarities are depicted in Figure 17 and Figure 18.

From the simulations, it can be concluded that during a positive-to-negative line-touching fault under different transmission powers, the LCC (Line Commutated Converter), operating at 0.6 p.u., experiences a situation where the command value for constant current control on the inverter side is lower than the command value for low-voltage current limiting. Upon detecting a voltage drop, it directly enters the low-voltage current-limiting mode, resulting in a rapid voltage reversal that immediately reaches −v_dN.

When the SLCC operates at its rated state, it experiences a rapid recovery of the positive-pole voltage and current after they initially drop to zero following the fault. Without entering the low-voltage current-limiting mode, it directly returns to its rated state.

4. Protection Methods for Double-Circuit DC Line-Touching Faults on the Same Tower

4.1. Identification Methods for Conventional DC Line-Touching Faults

Based on the fault characteristic analysis presented above, during a line-touching fault, the DC side is divided into two conventional transmission loops and one fault loop. The current in the fault loop deviates in magnitude and direction due to differences in line voltages. Since the currents on the rectifier side and the inverter side belong to the conventional transmission loop and the fault loop, respectively, a differential protection system can be employed to detect line-touching faults in the system.

The longitudinal differential protection detects the differential current I_dl and I_{dl_os} at both ends of the line, and when the difference reaches the set value ${\Delta }_1$, the protection system sends out an action signal. The detection criterion is as follows:

$$\left|I_{dl}-I_{dl\_os}\right|>{\Delta }_1$$

(10)

During positive-to-negative line-touching faults, the DC voltage, in order to limit the fault current, reduces its own DC voltage and completes current control when the condition v_Ip2 = −v_In1 = −V_dN is met. Therefore, a differential undervoltage protection scheme can be employed to detect line-touching faults.

The differential undervoltage protection should be able to accurately detect DC line-to-ground faults. When a DC line fault occurs, the DC voltage rapidly drops to a lower value at a high rate, prompting the differential detection component to act quickly. Simultaneously, it detects the DC undervoltage level to avoid the influence of disturbances during operation. The criterion for differential undervoltage protection typically consists of two components: differential and undervoltage protection criteria with action thresholds ${\Delta }_2$ and ${\Delta }_3$, which are specifically outlined as Equation (11). The protection configuration is as shown in

Table 3. DC Line Protection Configuration.

Protection	Criteria	Action Delay (ms)
Differential Undervoltage Protection	du/dt > 0.8l u > 0.25 p.u	20
Longitudinal Differential Protection	|Idl − Ios| > 500 A	200

.

$$\left\{\begin{array}{l}dv/dt>{\Delta }_2\\ v_{dl}<{\Delta }_3\end{array}\right.$$

(11)

To distinguish between two-circuit DC line-touching faults and inter-pole line-touching faults, the differential currents of both polar lines can be detected simultaneously. In the case of an inter-pole line-touching fault, differential currents can be detected in both polar lines, whereas in the case of a cross-line fault, only the faulty polar line exhibits a significant differential current. To further distinguish between a single-pole ground fault and other faults, the voltage drop criterion is used. When a single-pole ground fault occurs, the voltage of the faulty line drops to zero, while in the case of a line-touching fault, the voltage may reverse or return to its normal state. Therefore, the following current and voltage protection criteria can be utilized:

$$\left\{\begin{array}{l}\left|I_{R\_n1}-I_{I\_n1}\right|>{\Delta }_1 \mathrm{or} \left|I_{R\_p1}-I_{I\_p1}\right|>{\Delta }_2\\ \left|I_{R\_n2}-I_{I\_n2}\right|>{\Delta }_3 \mathrm{or} \left|I_{R\_p2}-I_{I\_p2}\right|>{\Delta }_4\end{array}\right.$$

(12)

$$\left\{\begin{array}{l}du/dt>{\Delta }_1\\ \Delta u>{\Delta }_2\\ u_{dl}<{\Delta }_3\end{array}\right.$$

(13)

The specific protection flow is illustrated in Figure 19. The protection action is triggered when both the current and voltage criteria are met.

4.2. Simulation Verification

The verification of the differential protection action is shown in Figure 20. During positive-to-positive line touching, the maximum differential current is 0.7 kA, and during a positive-to-negative line-touching fault, the maximum differential current reaches 3.2 kA. In both fault scenarios, the differential currents exceed the set value, and the protection can act reliably.

The verification of the voltage criterion protection is shown in Figure 21. When a positive-to-negative line-touching fault occurs, the maximum voltage variation in Line 1’s positive pole is 0.7 p.u., and the maximum rate of change in the voltage is 1.3 p.u./s, both of which exceed the protection set value, ensuring reliable protection action.

5. Conclusions

This paper investigates a form of slight LCC with SVG replacing a passive filter, and the detailed reactive power and harmonic filtering control methods are explored. Then, considering the practical situation of SLCC, line-touching faults in double-circuit LCC-SLCC HVDC transmission systems on the same tower are investigated, and applicable protection methods are proposed and verified. The main conclusions are as follows:

(1)

In an SLCC transmission system, the reactive power compensation is more flexible, with lower harmonic injection content and smaller covering space; the SVG compensation current is beneficial for the duration of the commutation process, thereby reducing the risk of commutation failure. Thus, it is suitable for the transformation of an AC line to a DC line in local power grids.

(2)

When two LCC and SLCC HVDC lines on the same tower experience a same-pole line-touching fault, the rectifier side remains in a normal circuit configuration and is not affected by the faulted circuit. The voltage and current remain near their rated values, while the current in the faulted circuit is influenced by the voltage difference between the lines, resulting in a small deviation in current amplitude.

(3)

In the case of a positive-to-negative line-touching fault, for a positive-pole DC line of the touching fault, the current rapidly drops to zero and enters the VDCOL mode and the voltage of the positive converter shifts to −V_dN. For a negative-pole DC line of the touching fault, the voltage shifts to 0.5V_dN. The current of the fault loop is limited to about 1.5 kA. Then, the negative-pole DC line operates asymmetrically, and the power transmission direction reverses.

(4)

Line-touching faults divide the line currents at the rectifier and inverter sides into two different loops, allowing for the detection of differential currents, which is suitable for longitudinal differential protection. Simultaneously, during positive-to-negative line-touching faults, to limit the fault current, the DC voltage is reduced to satisfy current control. Employing differential undervoltage protection can effectively detect the fault.

However, the application of SLCC is still at its initial stage in the real conversion of AC lines to DC projects. Further discussion on economic feasibility is needed for broader applications. The fault mechanism of different HVDC lines considering transmission distance, capacity (DC voltage level), and more types of HVDC (VSC-HVDC) as well as fault clearing and restoration methods is also challenging.