A differential protection technique for multi-terminal HVDC

Highlights

• A three-terminal HVDC system is modeled for DC fault study.

• DWT is used to analyze DC faults.

• A differential protection technique based on the DWT decomposition of fault currents at both line ends is proposed.

• Simulation results for different types of faults is reported.

Abstract

This paper presents a technique for differential protection of Multi-terminal High Voltage Direct Current (MTHVDC) transmission lines. The proposed technique depends on the electrical current data at both ends of each line section. Discrete Wavelet Transform (DWT) is used to detect DC faults as well as filter out the high frequency transients superimposed on the current signals. An operating signal and a restraining signal are used in this technique to discriminate between internal faults and external faults through their ratio in each section of the MTHVDC. The operating and restraining signals depend on energy contents of the de-noised current signals at both ends of each line section. MTHVDC modeling and relay design are carried out in the MATLAB environment. The results demonstrate the high reliability of the proposed relay in the zonal protection of MTHVDC.

Introduction

Voltage source Converter (VSC)-HVDC installations are promising power transmission methods for transmission of bulk power over long distances. The deployment of offshore wind power generation is expected to contribute a major portion of the world's future energy portfolio due to the expected depletion of fossil fuels along with the climate changes introduced by CO₂ emissions. Offshore wind power generates a large amount of power that can be collected and transmitted to different onshore grids via HVDC systems. Different wind farms can be ultimately interconnected to multiple grids by means of multi-terminal HVDC (MTHVDC). Moreover, MTHVDC enables the transmission of high power levels, which can contribute to the realization of the future super grids such as the European super grid [1], [2], [3].

Many research articles discuss the protection of a two-terminal HVDC system such as the research reported in [1], [4], [5], [6], [7], [8]. However, these methods are suitable only for two-terminal HVDC systems. Some of these methods cannot be extended to MTHVDC while others have not been tested for operation in MTHVDC. Generally, the design of protection systems for MTHVDC grids is considered in its early stages. Limited research has tackled the protection problems of MTHVDC [9], [10], [11], [12], [13], [14].

The research carried out in [9] has presented a thorough discussion concerning the DC-fault current and voltage waveform shapes, but it did not present a complete protection scheme for MTHVDC systems. The employment of circuit breakers (CBs) on the AC side of the converter for protection of MTHVDC was introduced in [10]. In case of faults inside the DC grid, all CBs on the AC sides of all terminals isolate the DC grid, then the faulted section is identified by means of a handshaking method [10]. The faulted section is isolated and the DC grid is restored afterwards. This method avoids using DC CBs, however, it takes a long time for isolation, fault zone identification, and system restoration. Moreover, the MTHVDC system is completely shut down unnecessarily. Alternatively, the use of DC CBs is now recommended to avoid unnecessary shutdown of healthy DC lines, isolate the fault quickly, and increase the DC system reliability [2], [3], [9], [11], [12], [15]. The research relevant to DC CBs is promising and several architectures of DC CBs have been developed in literature [2], [3], [9], [11], [12], [15]. With proper design of a protective relay, the DC CBs can isolate the faulted section quickly without the need to trip the whole DC system.

Yang et al. simulated the DC faults in MTHVDC including wind farms in [11] and proposed a distance protection scheme in [12] to be used in MTHVDC. However, this method did not address large systems with a high fault resistance. The largest rate of change of current is taken as a discrimination feature to identify the faulty line in [13]. However, it depends on system topology, where in some DC fault cases, the healthy line may have a larger rate of change of fault current. Moreover, this method avoids the deployment of DC CB; therefore, it takes a relatively long time for isolation. The work done in [14] presents three criteria to identify the faulted cables in meshed MTHVDC systems. These criteria are voltage Wavelet coefficients, current Wavelet coefficients, and voltage derivative and magnitude. Nevertheless, the authors did not consider the non-zero resistance fault case. Moreover, the proposed method depends on the initial change of voltage and current Wavelet coefficients, which may be confusing to the relay especially during the initial transient period of the fault. Furthermore, a combination between overhead lines and underground cables is not considered in this paper, which is the existing case in practical systems.

The proposed protection algorithm in this paper presents a differential protection method for protection of MTHVDC with different configurations of DC sections (overhead lines and underground cables). The method depends on calculating an energy index for the de-noised versions of positive and negative pole currents at both ends of a line to discriminate between internal and external faults. Pole to ground, pole to pole, and pole to pole to ground faults are simulated to test the proposed method. Various fault resistances were simulated to prove the reliability of the proposed method in low resistance and high resistance faults. The paper is organized as follows. Section 2 presents the MTHVDC model used to test the protection algorithm. Section 3 introduces the main concept of the proposed method. Protection functions and the design steps of the proposed algorithm are explained in Section 4. Section 5 presents the simulation results, and finally, Section 6 concludes the work presented in this paper.