A New Protection Principle of Hybrid Bipolar HVDC Line Based on Voltage Fault Component Rate of Change

At present, hybrid HVDC transmission has become a research hotspot. In this paper, the fault-added state characteristics of the hybrid bipolar HVDC transmission system with different faults on the DC transmission line are analyzed by connecting small inductors on both sides of the HVDC transmission line. A single-ended quantity protection method using voltage transient components is obtained. By analyzing the rate of change of voltage fault component, the transient energy component is used to calculate the instantaneous energy magnitude of the transient voltage component to distinguish the fault in the region. The energy function is constructed by the transient voltage of the positive and negative rectifiers to fault selection. The simulation results verify the correctness of the proposed protection principle and can achieve fault pole selection.


Introduction
In the protection of HVDC transmission lines, domestic and foreign scholars have carried out a lot of research work and obtained some meaningful research results. In [1], based on the characteristics of current waveforms at specific frequencies at both ends, a new protection scheme for DC transmission line longitudinal protection is proposed. Literature [2] proposed the possible influence of nonlinear line boundary on boundary protection, and the high-resistance fault may be invalid for traveling wave detection. Literature [3] analyzed the construction of time domain current differential protection under the distributed parameter model. In [4], several factors affecting the traveling wave characteristics such as the distribution parameter fault location and transition resistance are analyzed. In [5], a method for protection of UHVDC transmission lines based on phase-mode transformation is studied. In [6], a protection strategy for high-voltage DC transmission lines using directional characteristics of voltage and current abrupt changes is proposed.
In this paper, the faults in the positive and negative regions, the faults in the negative region, the bipolar faults, etc. are analyzed. The fault additional state of the hybrid bipolar DC transmission system is analyzed. According to the difference between the rate of change of the voltage fault component in the fault outside the region, a new protection principle using the voltage fault component is proposed. And use the energy function of the transient current to achieve fault selection. Combined with the actual DC transmission project, due to the limitation of the converter of the twolevel flexible DC transmission system, a hybrid bipolar DC transmission system with positive +500kV and negative-200kV was built in the PSCAD electromagnetic transient simulation software, and MATLAB was used algorithm verification.

Topology of hybrid bipolar HVDC transmission system
The structure and fault setting of the hybrid bipolar HVDC transmission system are shown in Figure 1.  Table 1 gives the parameters for the Hybrid Bipolar-HVDC system.

Analysis of Failure Rate of Faulty Network Voltage Faults and Fault Criterion in and Out of Area
The fault additional state is shown in Figure 2. Therefore, the setting value of the instantaneous energy can be set. When S>S set , it is an intraregion fault, and when S<S set , it is an out-of-zone fault. The instantaneous energy setting S set is a threshold set according to the maximum instantaneous energy value under the obstacle outside the zone, S set =100.  (1) and (2):

Fault pole selection
In the formula, upr1 and unr1 are respectively the voltage fault component of the positive rectifier side and the voltage fault component of the negative rectifier side; E1 and E2 are the energy of the positive and negative poles respectively; N is the number of sampling points.
The energy at the fault pole is greater than the energy at the health pole, so the threshold value E set is set. When E1>E set , the positive pole fails; when E2>E set , the negative pole fails; when E1>E set and E2>E set , double pole faulty occurs. E set =1×10 6 .

Zone fault simulation verification
Due to the limitation of space, the fault simulation result in the area takes the fault f1 as an example. The simulation result of fault f1 is shown in Fig. 3.

Out-of-zone fault simulation verification
Due to the limitation of space, the simulation result of the out-of-zone fault takes the fault f3 as an example. The simulation result of fault f3 is shown in Fig.4.

In conclusion
Hybrid DC transmission has become a hot topic in current research. This paper establishes a model for hybrid bipolar HVDC transmission in PSCAD software, and analyzes the principle of relay protection of its line as follows. The simulation results show that the protection principle can quickly identify faults in the hybrid bipolar HVDC transmission system with high reliability.