Non-unit Protection Scheme Research Articles

A self-adaptation non-unit protection scheme for MMC-HVDC grids based on the estimated fault resistance

The existing ultra-fast non-unit protection methods for modular multilevel converter (MMC) HVDC grids based on the protection boundary have shortcomings, such as a lack of adaptive ability and difficulties in threshold setting. The key point is that it is hard to obtain the status of the fault point, which makes the method that uses a fixed threshold value difficult to adapt to various fault conditions. In this paper, the fault voltage which is composed of many sub-components is analyzed based on the built equivalent circuit of MMC-HVDC grids after DC line faults. A fault resistance estimation method is proposed by utilizing the ratio of the second module maximum to the initial module maximum after filtering by a multi-resolution morphological gradient algorithm, and the approximate difference in propagation distance is obtained by using the time difference and the change directions of the initial two surges to correct the influence of distance. A self-adaptation non-unit protection scheme is proposed based on the estimated fault resistance, which consists of the start-up unit, identification of fault types, fault resistance estimation, and self-adaptation main protection. A four-terminal ring-topology MMC-HVDC grid built in PSCAD/EMTDC demonstrates the higher performance of the proposed scheme under various fault conditions.

A Non-unit transient travelling wave protection scheme for multi-terminal HVDC system

The rapid and accurate dc line protection scheme is of great value in isolating dc line faults and improving the reliability of multi-terminal HVDC systems. Nowadays, the traveling wave (TW) protection is widely used as the primary protection in HVDC transmission systems. However, the traditional TW protection has the problems of poor resistance to high impedance faults and the setting value depends on simulation. For the hybrid three-terminal LCC/MMC HVDC system, this paper provides a fast non-unit protection scheme. Firstly, the time-domain expressions of the backward line-mode voltage TW after different line faults occur are derived respectively. Based on the expressions, a non-unit protection scheme depending on the difference value of the line-mode backward TW is proposed. Finally, the scheme is verified by simulation. The method only needs single-ended data to realize the rapid dc line protection without converter station communication which has a clear theoretical calculation method in setting values and a strong robustness to high-resistance fault.

A non-unit transmission line protection scheme for MMC-HVDC grids based on a novel distance criterion

Ultra-fast, selective, and reliable protection is significant for MMC-HVDC grids. The methods utilizing the characteristics of fault current limiting reactor (FCLR) are unsatisfactory to detect high resistance faults. In this paper, a novel distance criterion is proposed to form the non-unit protection scheme. The line-zero-mode fault equivalent circuit is built to analyze the characteristics of fault traveling waves at the inductive terminal considering the influence of MMC and the refraction and reflection at the fault point. A multi-resolution morphological gradient algorithm (MMGA) is employed to extract the geometric features of the fault traveling waves. The magnitude of the first modulus maxima of the line-mode fault voltage after MMGA constitutes the start-up criterion. The estimated velocity and the change direction of the line-mode components are adopted to identify if the fault happens in-region. The 4-terminal ±500 kV MMC-HVDC grid built in PSCAD/EMTDC demonstrates that the proposed scheme can accurately identify various faults. The sensitivity analysis shows that the proposed scheme is almost immune to the size of FCLR, the equivalent parameters of MMC, the noise environment, and the DCCB breaking process. The proposed scheme has low requirements for sampling frequency and can be utilized to identify the temporary fault.

Non-unit protection for long-distance LCC-HVDC transmission lines based on waveform characteristics of voltage travelling waves

Currently, the voltage-derivative-based protection (VDBP) method has been applied in High-Voltage Direct Current (HVDC) lines as primary protection. But it has the problem of low reliability to high-resistance faults, and even more difficult or fail when applying to long distance lines. For such circumstances, current differential protection will play backup protection role with slow operation speed. Therefore, HVDC transmission lines require high reliable and high-speed protection scheme. In this paper, the complex frequency domain expressions of the line-mode fault component voltage (LFCV) at the line end as well as its waveform characteristics for internal and external faults are derived respectively. Based on the theoretical analysis of the LFCV at the line end, a novel non-unit protection scheme utilizing the waveform similarity coefficient is proposed. A ± 800 kV HVDC system built in PSCAD/EMTDC is used to verify the feasibility and validity of the proposed protection method. Comprehensive simulation results show that the proposed scheme can accurately identify internal and external faults, including high resistance faults (600 Ω) and long-distance faults (over 2000 km).

A waveform-similarity-based protection scheme for the VSC-HVDC transmission lines

The protection for the voltage source converter based high voltage direct current (VSC-HVDC) transmission lines must detect the faulty DC lines and faulty poles within a few milliseconds, because the fault current rises quickly. The traditional protections are easily influenced by fault resistance and signal noise. What is worse, the double-pole-to-ground (DPG) fault with its positive-pole fault resistance unequal to negative-pole fault resistance cannot be detected accurately. To solve the problems, a non-unit protection scheme based on the waveform similarity and wavelet transform modulus maximum (WTMM) theories for the VSC-HVDC lines is proposed in this paper. A four-terminal VSC-HVDC power network is modeled in PSCAD to verify the protection. Extensive simulations of various fault situations prove that the protection scheme can detect high-fault-resistance, DPG, and remote faults. Moreover, it is insensitive to the sampling frequency, signal noise, smoothing reactor, and start delay. Furthermore, it can meet the rapidity requirement. Therefore, the proposed protection can identify the DC faults accurately, which minimizes the disturbance to power grids and improves system security and stability.

Threshold-free localized scheme for DC fault identification in multiterminal HVDC systems

In the future grid scenario, the continuous operation of large-capacity remote power transmission over an augmented DC network is imperative for implementing a reliable multi-terminal high voltage direct current (MT-HVDC) grid or, alternatively, an MTDC grid. A fast and selective DC fault identification method with improved reliability eventually enhances the operational continuity and flexibility of the MTDC grid. Based on the V/I ratio of a current limiting reactor, this paper proposes a new non-unit protection scheme for DC fault identification without a threshold value setting. The DC fault is precisely identified in a span of three sampling intervals by the V/I ratios of four sampling instants. The selectivity of the proposed scheme is verified in a four-terminal MTDC grid. Simulation results demonstrate that the effective detection, discrimination, and location of a DC fault strictly satisfies the speed requirement (<1ms) with a location error of less than 1%. Moreover, the relevant sensitivity analyses reveal that the proposed scheme remains invariant for a wide range of fault locations and resistances and perform efficiently, even with a lower sampling frequency. In addition, transient events apart from DC faults fail to trigger the protection, indicating the high robustness of the proposed scheme.

Non‐unit protection scheme for HVDC transmission lines based on energy of voltage difference

This paper presents a non-unit protection scheme for LCC-HVDC transmission lines based on the energy of voltage difference. The change in the line average voltage is used to detect the fault, whilst faulted pole selection criterion is calculated by using the ratio of the average voltage change at positive and negative poles. The protection criterion calculated as the energy of second-order difference of the DC line voltage determines whether the fault is internal or external. Numerous simulation and field data test are performed to validate effectiveness of the proposed scheme under a vast variety of fault conditions. The tests include different types of internal and external faults, sensitivity to transferred power variation, sampling frequency, length of data window, change of smoothing reactor, effect of lightning interference, change of the line length, change of the operation mode, and effect of noise. The scheme is a single-end protection method and does not require sophisticated computations.

A novel non‐unit protection scheme for HVDC transmission lines based on multi‐resolution morphology gradient

This paper presents a novel non-unit protection scheme for the protection of LCC-HVDC transmission lines. The fault-generated travelling waves at the faulty line ends are filtered by a filter–reactor unit. The filtered current waveforms are then processed by a multi-resolution morphological gradient and are used by a fast non-unit protection scheme for LCC-HVDC transmission lines. The proposed method does not require communication links between transmission line ends and uses only current signals with a low sampling rate of 10 kHz. In addition, the proposed scheme is computationally efficient, making it suitable for practical applications. The performance of the proposed protection scheme is validated by comprehensive simulation studies as well as field data on a bipolar LCC-HVDC system. Both simulation and field data test results verify the accurate performance of the proposed method for different internal fault conditions, including high fault resistances up to 1000 Ω. Furthermore, the proposed scheme is robust against external DC and AC faults, change of sampling frequency and data window length, non-ideal faults, change of power flow, change of DC filter parameters and smoothing reactor, change of operation mode, and measurement noise.

Modulated Low Fault-Energy Protection Scheme for DC Smart Grids

DC smart grids enabled by the integration of advanced power electronic converters (PECs) can ease the integration and control of distributed renewable energy resources, electric vehicles, and energy storage systems. However, these highly flexible power systems introduce many challenges when considering the design of reliable, plug-and-play protection that does not rely on dedicated communications infrastructure for device coordination. One particularly difficult challenge is the management of dc-side filter capacitor discharge during short-circuit faults where the large peak fault-current produced can permanently damage exposed semiconductor components within the converter. One solution is to ensure that the trip-time of dc protection devices is sufficiently rapid (sub-millisecond) to guarantee that fault-current is blocked prior to reaching destructive magnitudes. However, such high-speed protection devices do not offer much margin for effective selectivity with downstream devices due to the narrow time window of operation. Accordingly, this paper proposes a non-unit protection scheme for future large-scale dc smart grid applications that increases this time-window of operation to enable improved selectivity whilst retaining a lower level of energy dissipated in the fault. Reliable protection coordination is demonstrated on a dc radial network and is realized using conventional millisecond trip-time devices, and a single solid-state microsecond trip-time device.

A non-unit protection scheme for double circuit series capacitor compensated transmission lines

This paper presents a non-unit protection scheme for series capacitor compensated transmission lines (SCCTL) using discrete wavelet transform and k-nearest neighbor (k-NN) algorithm. All the protective relaying functions such as fault detection, fault classification, faulty phase identification and fault location estimation have been considered in this work. Such a comprehensive work providing all protective relaying functions for protection of double circuit SCCTL utilizing k-NN has not been reported so far. The signal processing and feature extraction are done using discrete wavelet transform due to its capability to differentiate between high and low frequency transient components. For fault detection and classification, only approximate wavelet coefficient of current signal up to level 1 has been used; while for k-NN location estimation, both voltage and current signals of the two circuits are decomposed up to level 3 have been used. Finally, the standard deviation of one cycle pre-fault and one cycle post-fault samples of the approximate wavelet coefficients are calculated to form the feature vector for the k-NN-based algorithm. The performance of the proposed technique is evaluated for large number of fault events with variation in fault type including inter-circuit faults, fault inception angle, fault location and fault resistance. The change in position of series capacitor and different degree of compensation has been discussed. The accuracy of the proposed k-NN-based fault detection and classification module is 100% for all the tested fault cases with a decision period of less than half cycle. The k-NN-based fault location scheme estimates the location of fault with ≤1% error for most of the tested fault cases, which is an exceptional attribute of the proposed scheme as compared with 10–15% error of conventional distance relaying scheme.

A Non-unit Protection Scheme for DC Microgrid Based on Local Measurements

This paper proposes a nonunit protection scheme for DC microgrids (DC MGs) utilizing only local measurements. This scheme is developed based on the natural characteristics of DC current and its first and second derivatives under fault transients. Since it is based on local measurements, the problems associated with the communication delay are avoided. The selected protection scheme detects and discriminates the faults within a few microseconds of its inception. In this paper, a method to calculate the thresholds, used for the protection scheme, is also discussed. The proposed scheme is validated on a ring-type DC MG architecture under different fault scenarios and tested through MATLAB/Simulink simulations.

A modular neuro-wavelet based non-unit protection scheme for zone identification and fault location in six-phase transmission line

To overcome the constraints on land availability, infrastructure and environmental problems, six-phase transmission lines have been proposed as a potential alternative to increase the power transfer capability of existing transmission lines without major modification in the existing structure of three-phase double-circuit system. The non-availability of a proper protection scheme due to large number of possible faults has been the prime reason behind the low popularity and acceptance of six-phase system. In this regard, the present work proposes a protection scheme for six-phase transmission line based on the hybridization of discrete wavelet transform and modular artificial neural network. The fault information (approximate coefficients) in the voltage and current signals is captured using discrete wavelet transform. The standard deviation of the coefficients of voltage and current signals in each phase is then computed and given as input to modular artificial neural network, which aims at identifying the faulty section/zone and estimate its location. Test results exhibit that the proposed scheme effectively discriminates the faulted section and estimates the fault location with maximum error of 0.675 %. It offers primary protection to the total line length and also provides remote backup protection for the adjacent reverse section of the line using data at relaying point only and thus avoids the need of a communication link.

Nonunit Protection of HVDC Grids With Inductive DC Cable Termination

This paper deals with nonunit protection of HVDC grids by proposing a set of parameters that characterizes the open protection zones together with an efficient method to determine the thresholds on these parameters. Selective HVDC grid protection schemes must detect and discriminate faults within the first milliseconds of the fault transient and consequently differ considerably from existing ac protection schemes. Due to the accompanying speed requirement, primary protection is expected to be based on open protection zones as communication delay impedes fast operation. In this paper, the principles of the nonunit protection scheme are developed based on reflection of a traveling wave at an inductive termination. Next, the method to obtain the protection scheme thresholds is elaborated. The method accurately calculates the thresholds for HVDC grids with an arbitrary topology. A sensitivity analysis of these thresholds toward grid and fault parameters demonstrates the applicability of the proposed protection scheme in cable-based HVDC grids with inductive cable termination. The results obtained with the reduced grid model are validated by comparison against simulations using a detailed model implemented in PSCAD.