**4. Test System**

The two different strategies will be compared through simulations of a test system on PSCAD/EMTDC. The designed test system is shown in Figure 4. Both the AC system at the sending end and the receiving end are connected to a modified two-zone four-generator system [22]. The main parameters of the four-machine system are listed in Table 1. The generator and the control system parameters, as well as the network structure and parameters remain consistent with the original system. Only the load and output of the generator are changed. The DC grid is in the monopole-grounded return operation mode. The model of all DC lines is 4 × LGJ-720. The parameters of DC lines in the simulation are resistance 0.009735 Ω/km, inductance 0.8489 × 10−<sup>3</sup> H/km, and capacitance 0.01367 × 10−<sup>6</sup> F/km. The parameters of the four MMCs based on HBSMs are shown in Table 2. The control mode is shown in Table 3. Table 4 gives the initial state of the two AC systems. Among them, GA1–GA4 and GB1–GB4 are the generators of the two AC systems A and B, respectively. LA7, LA9 and LB7, LB9 are the loads connected to the No. 7 and No. 9 buses. CA7, CA9 and CB7, CB9 are the reactive power compensators.

**Figure 4.** Structure of the designed test system.


**Table 1.** Main parameters of the four-generator alternating current (AC) system.

**Table 2.** Main parameters of multi-level modular converters (MMCs) in the test system.


**Table 3.** Control mode and reference value of MMCs.



**Table 4.** Initial state of the generators, loads, and reactive power compensators in the AC systems.

#### **5. Performances of the Two Strategies**
