**6. Conclusions**

To address the factory test of high power STATCOM, a phase full-power testing method is present in this paper. By changing the port connection, three-phase STATCOM was reconstructed into a structure that two phases are in parallel, and then in series with the third-phase, and then connected to two phases of the rated voltage grid. Then, by rationally matching the three-phase voltage and current, the parallel two phases can run a reactive current hedging under both their rated voltage and rated current. The difficulty with this method is in maintaining the active balance of three-phase converters under this special structure. By mathematical modeling and theoretical analysis, it can be concluded that, the output voltage of the series phase converter cannot be simply set to the algebraic difference of the rated line voltage and phase voltage. Moreover, the reactive current components of the two parallel phases cannot be exactly offset, otherwise three phase convertors cannot maintain their active power balance. A specific combination of three-phase voltage and current is designed to maintain the steady operation of the system, and a corresponding control system is designed. Since the circuit configuration of the proposed testing system is different from the conventional STATCOM, this paper proposes a novel soft power-on method for this structure. The testing system was verified by a simulation in Matlab/Simulink. The simulation results show that, the proposed system can run stably, and two phases can operates at both, the rated phase voltage, and rated current, while the grid current is about 3% of the rated current. As a result, the three-phase STATCOM can experience a phase full-power test, phase-by-phase.

The proposed method has a certain similarity with the common method, where the two singe-phase convertors are in parallel and then connected to the single-phase grid, since it also employs two convertors to perform reactive power hedging. But the novelty of this method is that it uses the third-phase convertor of STATCOM to compensate for the voltage difference between the power supply voltage and the required phase voltage, so that the power voltage does not need to be directly equal to the required voltage for STATCOM testing. As a result, the existing power grid can be used to directly provide the test power, thereby avoiding the additional platform hardware and its cost. The main contribution of this paper is to clearly explain the constraint among three-phase voltage, current, and active power under such special structure, and solve the problem of charging and active power balance of three phase converters, and design a control system to realize it. This makes the aforementioned idea achievable.

Compared with other methods for the testing of STATCOM, the most prominent feature of this method is that its test platform only needs one tested STATCOM and a small capacity rated voltage grid, meaning no extra hardware is required and a lower test cost. Therefore, it is suitable for the whole-machine test of high-power STATCOM before its leaving the factory.

**Author Contributions:** Conceptualization, Q.H.; formal analysis, Q.H.; data curation, X.M.; writing-review and editing, Q.H. and Y.T.; software Y.Z. and S.Z.; project administration, B.L.

**Funding:** This research was partly funded by the science and technology project of State Grid Electric Corporation gran<sup>t</sup> number 5216A016000P, and partly funded by the science and technology project of State Grid Hunan Electric Company Limited gran<sup>t</sup> number 5216AF17000A.

**Conflicts of Interest:** The authors declare no conflict of interest.
