The "Carbon-Neutral" goal is the main driving force to build a new power system with renewable energy. Due to the advantages of low loss and no commutation failure, the voltage source converter (VSC) transmission based on modular multilevel converter (MMC) has been widely used in large-scale renewable energy grid-connected power transmission. When the DC line faults, the fault current increase rapidly, causing irreversible damage to power electronic power devices and even paralysis of the transmission system. It brings a serious crisis to the safe operation of the power grid. Therefore, fast and selective identification of DC line faults is one of the key technologies to be solved urgently. The protection research of the VSC-HVDC transmission system based on full-half hybrid sub-modular structure is of great significance to the large capacity transmission of renewable energy. Firstly, the bridge arm power calculation circuit is equivalent when the DC line is faulted, and the change characteristics of the bridge arm power under the DC line fault are analyzed. When the DC line faults, the bridge arm power increases. Then, the bridge arm power under DC fault is numerically analyzed, and the causes of errors are analyzed in detail. Finally, based on the power characteristics of the bridge arm, the protection scheme of the DC transmission line is proposed, the set-up element, the identification element of internal and external fault, and the faulty poles detection element are constructed, and the complete protection flow chart is given. A double-ended true bipolar full-half hybrid MMC-HVDC transmission system is built on the RTDS platform. The system adopts the overhead line, the transmission distance is 500 km, and the transmission line adopts the frequency-variable model, and the transmission capacity is 1 280 MW, the fault occurrence time is 3 s, the simulation step is 2 μs, the sampling step is 100 μs, the sampling frequency is 10 kHz, and the protection interruption time is 0.7 ms. Firstly, the protection effect under different fault types is verified. When an AC fault occurs, δ ＞0, it is judged as an external fault. When a DC fault occurs, δ ＜0, it is judged as an internal fault. When a positive fault occurs, S ≥ 0.9, when a negative fault occurs, S ≤1.1. When a pole-to-pole fault occurs, 0.9＜S＜1.1, which satisfies the pole detection criterion. The protection method proposed in this paper can correctly identify the fault and select the faulty poles under different fault types. Then the protection effect under different transition resistances and distance is verified. When the transition resistances are 0.01 Ω, 100 Ω, 300 Ω and 500 Ω, the fault distance are 20 km, 120 km, 300 km and 480 km, respectively. When a positive fault, a negative fault, and a pole-to-pole fault occurs, the internal/external fault identification criteria and pole selection criterion is satisfied. Furthermore, the anti-interference ability of the protection scheme is verified. And compared with the existing protection methods, the protection scheme has strong ability to response to the high transition resistance and noise interference. Based on the energy point, this paper analyzes the bridge arm power characteristics of the VSC -HVDC transmission system under DC line faults, proposes a protection scheme of this VSC -HVDC transmission system based on the bridge arm power characteristics. The conclusion can be drawn as follows: (1) When a DC line faults, the bridge arm capacitor discharges to the fault point on the DC side, and the fault current direction is the same as the current direction of the original system. Therefore, when the DC line faults, the bridge arm power increases. (2) The protection scheme is constructed by using the bridge arm power characteristics. The scheme is simple, and it can reliably identify the DC line fault. (3) Compared with the traditional time domain and frequency domain method, it is verified that the protection scheme proposed is highly reliable, selective, and sensitive, has strong ability to response to the high transition resistance and noise interference. © 2023 Chinese Machine Press. All rights reserved.
