Coordinated DC voltage control strategy for the receiving end hybrid LCC-VSC system

The receiving end hybrid line commutated converter (LCC) — voltage source converter (VSC) system has the obstacle that when an AC fault occurs at the sending LCC side, the DC voltage of the rectifier side will fall, resulting in the power transmission reduction of the whole system. Particularly, when the rectifier side DC voltage falls below the inverter side DC voltage, the power transmission is interrupted, posing a huge threat to the stability of the power grid at both the sending and receiving ends. To tackle these problems, a coordinated DC voltage control strategy for the receiving end is proposed. By controlling the DC voltage of the receiving VSC, the proposed method can improve the DC system power transmission capability when voltage drops at the sending side. The proposed control strategy is simple but practical, which is good for the stable and secure operation of both the sending and the receiving power system.


Introduction
The receiving end hybrid line commutated converter (LCC)-voltage source converter (VSC) system consists of three parts: the rectifier side adopts LCC, the high voltage valve of the receiving end adopts LCC, and the low voltage valve of the receiving end adopts VSC.This structure has both the advantages of LCC and VSC [1][2].In practical engineering applications, thyristors of LCC have a higher tolerance level to voltage and current than IGBTs of VSC, so the connection method of connecting multi-terminal VSCs in parallel and then in series with LCC can be adopted [3][4].
The receiving end hybrid LCC-VSC system has certain requirements for coordinating the two converter control systems.Liu et al. [3] and Xu et al. [5] introduced the hybrid topology, connection, control mode, and operating characteristics under various transient operating conditions of the LCC-VSC transmission system.Li et al. [6] comparatively analyzed the AC fault response characteristics of different hybrid DC transmission systems.Xue et al. [7] analyzed the voltage drop fault characteristics and low-voltage ride-through strategies for hybrid DC transmission systems.Chang et al. [8] conducted simulation studies on the response of the receiving end hybrid LCC-VSC system under different types of faults.However, it did not further analyze the sending-end fault and its impact on the system's operating characteristics.When the voltage of the sending side LCC-connected AC system drops too much due to a fault, the power transmission will be interrupted.Hiremath and Moger [9] proposed a maximum modulation ratio-based strategy for the hybrid LCC-VSC system.
The above studies mainly focus on the pole-to-pole DC transmission system.However, there are new problems to be considered for the receiving end hybrid LCC-VSC system: 1) To reduce the possibility of phase change failure of the converter, the LCC is usually equipped with a voltage dependent current order limiter (VDCOL) [10][11].Therefore, it is necessary to consider the impact of VDCOL on the operating characteristics of the system under different fault conditions and the coordination with the VSC control of the low-voltage valve group.2) The low-voltage valve adopts a multi-terminal parallel VSC structure, and the power distribution of the VSC should be considered.
Therefore, this paper focuses on the issue of voltage drop on the sending AC network of the receiving end hybrid LCC-VSC system, which greatly limits the active power transmission of the HVDC transmission system.Firstly, the specific impact of different degrees of voltage drop at the sending side on the active power transmission capability is studied.Secondly, a coordinated DC voltage control strategy is discussed to improve the power transmission capability of the hybrid HVDC transmission system by reducing the DC voltage of the VSC of the low-voltage valve group.Besides, the paralleled multi-VSCs adopt an equal loading rate-based master-slave control, which ensures the average distribution of active power and achieves the stable operation of voltage reduction in the receiving end hybrid LCC-VSC system.

Topology
The topology of the receiving end hybrid DC transmission system is shown in Figure 1.This HVDC system adopts a bipolar symmetrical structure.For example, the LCC at the rectifier side consists of two sets of 12-pulsation converters connected in series, the LCC at the inverter side of the high-voltage manifold adopts a set of 12-pulsation converters, and the low-voltage valve group consists of three sets of HB-MMCs which are connected in parallel.Receiving end hybrid HVDC system topology.

Basic control strategy
Figure 2 shows the basic control strategies of the LCC-VSC system.A fixed DC current mode with minimum and maximum trigger angle control controls the rectifier side LCC.The inverter LCC adopts a fixed arc quenching angle control mode.In addition, the inverter LCC is equipped with a fixed current control mode and a voltage dependent current order limiter (VDCOL) as a backup control strategy.Moreover, a current error control function (CEC) is used to realize the smooth switching between the two control modes [12].VSC converter stations mostly use direct current vector control technology based on dq-axis decoupling.The commonly used control methods for multi-terminal VSC systems are the master-slave control method, voltage margin control method, and sag control method [13].The master-slave control method can guarantee a fixed DC voltage output from the converter, so it is widely used in practical applications [14].

Operational characteristics of the reactive-end hybrid LCC-VSC
From Figure 3, for the rectifier side, the AB segment represents the traditional minimum trigger angle control of the rectifier LCC, the BC segment represents the fixed DC current control of the rectifier LCC, and the CDE segment represents the VDCOL link of the rectifier LCC.From Figure 3, for the rectifier side, the AB segment represents the traditional minimum trigger angle control of the rectifier LCC, the BC segment represents the fixed DC current control the rectifier LCC, and the CDE segment represents the VDCOL link of the rectifier LCC.For the inverter side, the OJ segment represents the fixed arc quenching angle control of the inverter LCC, the OF segment represents the CEC of the inverter LCC, the FG segment is the fixed DC current control link of the inverter LCC, and the GHI segment is the VDCOL link of the inverter side LCC.

Mathematical modeling
Due to system symmetry, the equivalent mathematical model of the system is established by taking the positive or negative pole of the HVDC system as an example, and the output DC voltage dr U of the rectifier LCC is: U is: where i E is the RMS AC line voltage of the inverter LCC;  is the trigger overrun angle of the inverter LCC; ' c L is the phase inductance of the inverter side LCC.i R is the equivalent phase change resistance.The output DC voltage of the inverter-side low-voltage valve bank VSC diL U is: where v U is the MMC net-side phase voltage amplitude; m is the modulation ratio, which is generally rated at about 0.85 for half-bridge MMCs, with a maximum value of 1.Using the above conditions, the DC current of the system at steady state d I can be calculated as: ( ) From Formulas (1)-(5), it can be seen that a drop in the rectifier LCC grid-side AC voltage due to a fault will result in a drop in the line voltage RMS value of E , and then diH U will decrease, which will further lead to a decrease in d I , and if the drop in E is too large, d I will decrease to 0 and eventually make the active power sent from the sending end dr P also become 0, which ultimately leads to the system stopping the transmission of the active power.

Voltage coordination control method
As shown in Figure 4, an adaptive voltage coordination control method is designed in this section, which is capable of automatically adjusting the DC voltage command of the inverter VSC when the AC voltage at the rectifier LCC drops to improve the active power transfer capability in the lowvoltage state at the sending end.The control is responsible for providing the DC voltage reference value Udcrefi_L to the outer loop controller of the MMC master of the low-voltage valve group, in which Idc_MMC is the DC outlet current of the MMC master (standardized value), Idcref is taken from the DC current command of the LCC controller of the high-voltage valve group (standardized value), and Udcmin is the minimum value of the PI output voltage command.The modulation ratio limits the half-bridge MMC, which cannot be greater than 1, so the output DC voltage value has a lower limit.When the modulation ratio m=1, the MMC output DC voltage is the minimum for the converter station AC grid-side phase voltage amplitude Ui_L twice.Udcrefi_L2 is the rated value of the voltage command of the LV manifold MMC outer-loop controller.
When the AC voltage of the rectifier LCC drops, the three-terminal parallel VSCs with traditional master-slave control method cannot guarantee that the DC current of each group of MMCs can still maintain the proportionality of 1:1:1 under the new operation state, i.e., it cannot evenly distribute the active power of the three-terminal MMC stations of the low-voltage valve group, which is determined by the working principle of the traditional master-slave control.To solve the problem, we introduce the master-slave control based on the equal load factor proposed by Wang et al. [15], and its control logic is shown in Figure 5: Pi in Figure 5 is the active power transferred between each VSC station and the AC network side, and Si is the maximum capacity of each VSC station.With this control method, the active command of the slave station is no longer a fixed value.Still, it is calculated according to the active power of each station and the capacity of each converter station.Therefore, this control method can adjust the active power reference value according to the change of real-time power, which not only retains the advantages of the traditional master-slave control method that can control the DC voltage but also can carry out a reasonable power allocation according to the operation status of the system.The average distribution of power in the parallel VSCs ensures the average distribution of DC current in the three-terminal VSC so that the DC current of the VSC at each end and the DC current of the LCC of the high-voltage valve group is constant at the ratio of 1:3, which ensures that the adaptive voltage coordinated control method can operate normally on the receiving-end mixed-connection type of DC power transmission system.
When the receiving end hybrid DC transmission system recovers from the low-voltage operation state, the rectifier side LCC recovers the current control capability, Idc is restored to 1.0 pu, the VSC voltage reference value is changed back to the rated DC voltage, and the system is restored to the rated operation state.In summary, the proposed coordinated method can switch between the rated and buck operation states.

Simulation verification
As shown in Figure 1, the DC voltage of the LCC-VSC system under rated operation is ±800 kV, and the rated DC current is 5 kA.Main parameters are listed in Table 1.In this case, modular multilevel converter (MMC) topology is adopted to adapt to the high voltage level.
The comparison simulation results of the hybrid LCC-VSC system with and without the proposed method are shown in Figure 6.If the line voltage of the AC side of the LCC at the sending end drops to 42.8%, the power transmission will be interrupted without the proposed method.In contrast, it can be seen that under the precondition that the line voltage RMS of the rectifier side LCC falls to the same extent, the inverter-side low-voltage valve group VSC station put into the coordinated control link can reduce its output DC voltage after the fault, which in turn boosts the DC line current and restores the DC system's ability to transfer active power to some extent.Table 1.Parameters of receiving end hybrid HVDC system.

Conclusion
In this paper, the following conclusions have been obtained from the study of the receiving end hybrid LCC-VSC system: (1) This paper analyzes the effects of different voltage drops on the operating characteristics.
(2) A coordinated DC voltage control strategy for the receiving end hybrid LCC-VSC system is proposed, which can improve the active power transmission capability when voltage drops at the sending side.Besides, an equal loading rate-based master-slave control mode is applied to the three-terminal parallel MMC of the low-voltage valve group.

Figure 2 .
Figure 2. The control strategy diagram of the LCC-VSC system.

Figure 3 .
Figure 3. Operating characteristic of receiving end hybrid HVDC system.

E
is the RMS AC line voltage of the rectifier LCC;  is the trigger angle of the rectifier LCC; d I is the DC line current; c L is the phase inductance of the rectifier LCC.r R is the equivalent phase change resistance.The output DC voltage of the inverter LCC diH

dR
is the DC line resistance.The active power delivered by the rectifier side LCC dr P is:

Figure 4 .
Figure 4. Block diagram of adaptive voltage coordinated control.

Figure 5 .
Figure 5.The control strategy diagram of equal loading rate based master-slave method.

Figure 6 .
Figure 6.The comparison simulation results with and without the proposed method under the same fault at the sending end.