Under the basic operating condition, the committed capacity of renewable and thermal power units near the DC sending-end was modified while maintaining constant total generation output. Three scenarios with renewable energy penetration levels of 40%, 50%, and 60% at the DC sending-end were established. A bipolar lockout fault was applied at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 under these conditions are shown in Figure 5.

The simulation results demonstrate that the transient overvoltage at the sending-end bus caused by the DC blocking fault increases significantly with the rise in renewable energy penetration level.

When transient overvoltage occurs at the DC sending-end converter bus, a large number of renewable energy sources in the nearby collection area will enter a high-voltage ride-through (HVRT) state. Therefore, different HVRT control parameters result in varying reactive power responses from renewable energy during the fault, consequently affecting the transient overvoltage levels at nearby buses.

The reactive current control strategy during high-voltage ride-through (HVRT) for renewable energy often employs the specified current control method. The control formula for the reactive current component I _qHV is shown in Equation 2: I qHV = K 1 q _ HV V Hin − V t + K 2 q _ HV I q 0 (2)

Where K _{1q_HV} is the HVRT reactive current calculation coefficient 1, K _{2q_HV} is the HVRT reactive current calculation coefficient 2, V _Hin is the HVRT entry threshold voltage, V _t is the terminal voltage magnitude, and I _q0 is the initial reactive current. Since the initial reactive current I _q0 is small and close to zero, the influence of K _{2q_HV} on the voltage during HVRT can be neglected. Therefore, the analysis primarily focuses on the impact of the calculation coefficient 1 K _{1q_HV} on the overvoltage.

Under the base operating condition, the first HVRT reactive current calculation coefficient, K _{1q_HV}, was configured to values of 1, 1.5, and 2 respectively. A bipolar lockout fault was applied to the DC system at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 6.

The simulation results show that the higher the renewable energy HVRT reactive current calculation coefficient 1 K _{1q_HV}, the lower the overvoltage at the system’s sending-end bus.

Based on the base operating condition, the DC transmitted power was reduced while the sending-end filter banks were adjusted. Scenarios with DC transmitted power levels of 10,000 MW, 9,000 MW, and 8,000 MW were established. A bipolar lockout fault was applied at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 7.

The simulation results show that the higher the DC transmitted power, the more severe the transient overvoltage at both the sending-end converter bus and the nearby renewable generator terminal buses during a fault.

Synchronous condensers, as a common type of dynamic reactive power compensation equipment, can enhance system voltage stability and are widely used in DC sending-end systems. Under the base operating condition, two additional comparative cases were established by configuring one and two synchronous condensers, respectively, each with a rated capacity of 300 MVA, at the DC sending-end converter bus B19. A bipolar lockout fault was applied at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 8.

The simulation results demonstrate that the installation of synchronous condensers can mitigate transient overvoltage during bipolar lockout faults. Furthermore, a higher total configured capacity of synchronous condensers results in lower transient overvoltage levels at the DC sending-end converter bus. In this paper, “SC” is the abbreviation for Synchronous Condenser.

In summary, a higher renewable energy penetration level, a smaller HVRT reactive current coefficient 1 K _{1q_HV}, a greater DC transmitted power, along with a lower configured capacity of synchronous condensers in the DC sending-end system will collectively lead to more severe transient overvoltage at the sending-end bus caused by DC block faults.

Based on the base operating condition and while maintaining constant total system power, three operational scenarios were configured by modifying the renewable energy penetration level at the DC sending-end to 40%, 50%, and 60% respectively. The system was subsequently set to experience a DC commutation failure fault at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 10A.

The simulation results indicate that as the renewable energy penetration level increases during a DC commutation failure fault, the severity of the transient low voltage at the sending-end converter bus decreases, while the degree of transient over-voltage remains largely unchanged. This suggests that the renewable energy penetration level does not have a significant impact on a single DC commutation failure event.

Under the base operating condition, the first renewable energy HVRT reactive current calculation coefficient, K _{1q_HV}, was configured to values of 0, 0.5, and 1 respectively. A DC commutation failure fault was applied to the system at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 10B.

The simulation results demonstrate that the transient overvoltage at the sending-end bus progressively decreases as the renewable energy HVRT reactive current calculation coefficient 1 K _{1q_HV} increases.

The reactive current control strategy during low-voltage ride-through (LVRT) for renewable energy often employs the specified current control method. The formula for the reactive current component I _qLV during LVRT is shown in Equation 3. I qLV = K 1 q _ LV V Lin − V t + K 2 q _ LV I q 0 (3)

In the formula, K _{1q_LV} is the LVRT reactive current calculation coefficient 1, K _{2q_LV} is the LVRT reactive current calculation coefficient 2, and V _Lin is the LVRT entry threshold voltage. Since the initial reactive current I _q0 during steady-state operation of renewable energy is small and essentially zero, adjusting the LVRT reactive current calculation coefficient 2 K _{2q_LV} has a negligible impact on the voltage during LVRT. Therefore, the analysis primarily focuses on the effect of the calculation coefficient K _{1q_LV} on the low-voltage response.

Under the base operating condition, the first LVRT reactive current calculation coefficient, K _{1q_LV}, was set to 0.5, 1, and 1.5, respectively. A DC commutation failure fault was applied to the system at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 10C.

The simulation results indicate that as the renewable energy LVRT reactive current calculation coefficient 1 K _{1q_LV} increases, the low-voltage level at the sending-end bus caused by commutation failure becomes less severe, while the overvoltage severity increases.

Based on the base operating condition and while maintaining constant total power delivery through both AC and DC transmission channels, scenarios with DC transmitted power levels of 10,000 MW, 9,000 MW, and 8,000 MW were established. A DC commutation failure fault was applied to the system at t = 1.0 s. The voltage response characteristics at the sending-end converter bus B19 are shown in Figure 10D.

The simulation results indicate that as the DC transmitted power increases, the transient low voltage at the sending-end converter bus during the fault decreases, while the transient overvoltage increases. Specifically, at DC transmitted power levels of 10,000 MW, 9,000 MW, and 8,000 MW, the transient low voltages caused by a single commutation failure are 0.662 p.u., 0.676 p.u., and 0.692 p.u., respectively, while the corresponding transient overvoltage values are 1.314 p.u., 1.266 p.u., and 1.223 p.u.

Based on the base operating condition, the DC sending-end converter bus B19 was configured with zero, one, and two synchronous condensers respectively, each with a rated capacity of 300 MVA. A DC commutation failure fault was applied to the system at t = 1.0 s. The voltage response characteristics at the DC sending-end converter bus B19 are shown in Figure 10E.

The simulation results demonstrate that configuring synchronous condensers can improve the transient low voltage and suppress the transient overvoltage during DC commutation failure events.

In summary, a higher renewable energy penetration level at the DC sending-end results in less severe transient low voltage during commutation failure events; a smaller HVRT reactive current calculation coefficient 1, K _{1q_HV}, leads to higher transient overvoltage; a smaller LVRT reactive current calculation coefficient 1, K _{1q_LV}, reduces both transient low voltage and overvoltage; while greater DC transmitted power and lower synchronous condenser capacity exacerbate both transient low voltage and overvoltage conditions.

Based on the base operating condition, the committed thermal power capacity connected to Bus B34 was adjusted to 100 MW, 200 MW, and 300 MW, respectively. A three-phase short-circuit fault was applied at the 220 kV load bus under Node B34 at t = 1.0 s. The voltage waveforms at the renewable energy terminal buses under Bus B34 are shown in Figure 12A.

The simulation results indicate that a higher committed capacity of thermal power near the renewable energy generation area supplies more reactive power during voltage dips, making the system less prone to delayed voltage recovery.

Under the base operating condition, the proportion of unstable induction motors in the load was adjusted to 20%, 30%, and 40%, respectively. A three-phase short-circuit fault was applied at the 220 kV load bus under Node B34 at t = 1.0 s. The voltage waveforms at the renewable energy terminal buses under Bus B34 are shown in Figure 12B.

The simulation results indicate that a higher proportion of unstable induction motors leads to increased reactive power absorption by the motor load after a three-phase short-circuit fault, resulting in lower voltage at the load bus, decreased voltage at the renewable energy terminal bus, and a higher likelihood of delayed voltage recovery.

Under the base operating condition, the first LVRT reactive current calculation coefficient, K _{1q_LV}, was set to 0.5, 1, and 1.5, respectively. A three-phase short-circuit fault was applied at the 220 kV load bus under Node B34 at t = 1.0 s. The voltage waveforms at the renewable energy terminal buses under Bus B34 are shown in Figure 12C.

The simulation results indicate that a larger LVRT reactive current calculation coefficient 1 K _{1q_LV} leads to a higher reactive current component during the fault, resulting in a less severe voltage dip. However, K _{1q_LV} has minimal impact on delayed voltage recovery.

Under the base operating condition, one and two synchronous condensers with a rated capacity of 50 MVA were configured at the 220 kV bus under Bus B34, forming two additional comparative cases. Subsequently, a three-phase short-circuit fault was applied at the 220 kV load bus under Node B34 at t = 1.0 s. The voltage waveforms at the renewable energy terminal buses under Bus B34 are shown in Figure 12D.

The simulation results demonstrate that the greater the configured capacity of synchronous condensers, the more reactive power is supplied during a voltage dip, making the system less susceptible to delayed voltage recovery.

In summary, the lower the committed thermal power capacity near the renewable energy area, the higher the proportion of unstable induction motors, and the less the configured capacity of synchronous condensers, the more severe the delayed voltage recovery phenomenon at the renewable energy terminal buses; while the LVRT reactive current calculation coefficient 1 K _{1q_LV} has minimal impact on delayed voltage recovery.

On the basis of the basic operating condition, while keeping the total system power unchanged, three scenarios are formed by adjusting the proportion of renewable energy at the DC sending end to 40%, 50%, and 60%, respectively. At t = 1.0 s, a three-phase permanent short-circuit fault occurs on one circuit of the double-circuit line between B09 and B10, near the B09 side. Subsequently, at t = 1.2 s, the double-circuit line is tripped. The voltage waveform of bus B04 after the AC fault is shown in Figure 13B.

Simulation results indicate that the greater the proportion of renewable energy generation at the DC sending end, the more severe the voltage collapse after fault clearance, and that the share of renewable energy generation significantly impacts voltage collapse.

On the basis of the basic operating condition, while keeping the total power of the AC and DC outgoing transmission channels unchanged, three scenarios are formed by adjusting the DC sending power to 10,000 MW, 9000 MW, and 8000 MW, respectively. At t = 1.0 s, a three-phase permanent short-circuit fault occurs on one circuit of the double-circuit line between B09 and B10, near the B09 side. Subsequently, at t = 1.2 s, the double-circuit line is tripped. The voltage waveform of bus B04 after the AC fault is shown in Figure 13C.

Simulation results indicate that the greater the DC sending power, the more severe the voltage collapse after fault clearance, and the DC sending power significantly impacts the voltage collapse.

On the basis of the basic operating condition, one and two synchronous condensers with a rated capacity of 50 MVA each are configured on the 220 kV bus under Bus B34, forming two new comparative scenarios. At t = 1.0 s, a three-phase permanent short-circuit fault occurs on one circuit of the double-circuit line between B09 and B10, near the B09 side. Subsequently, at t = 1.2 s, the double-circuit line is tripped. The voltage waveform of bus B04 after the AC fault is shown in Figure 13D.

Simulation results show that synchronous condensers provide voltage support after fault clearance. The more capacity of synchronous condensers configured, the less likely voltage collapse occurs after fault clearance.

In summary, a higher share of renewable energy generation, greater DC sending power, and a smaller capacity of synchronous condensers configured will make the renewable energy generator terminal voltage more susceptible to triggering voltage collapse after AC fault clearance.