The power output generated by wind farms (WFs) is integrated directly or indirectly into the grid through power electronic devices and the fault current is significantly different from that of conventional synchronous generators) and exhibits a limited amplitude, a controlled phase angle, rich harmonics, and strong nonlinearities. Therefore, the traditional ratio braking type differential protection may have low sensitivity or may even refuse to operate. In order to solve this problem, a pilot protection method based on Spearman's rank correlation coefficient is proposed in this article. When the system operates normally or an external fault occurs, a traversing current flows through the transmission line, so the current waveforms on both sides are the opposite. When an internal fault occurs, there are large differences in transient current waveforms on both sides. Spearman's rank correlation coefficient can be used to measure the degree of correlation between the waveforms on both sides to distinguish between faults inside and outside the zone. Compared with existing protection based on fault characteristics of WFs, the proposed method shows good performance when the power output of WFs is weak and the circuit breaker recloses with permanent failure. Both simulation results and field-testing data validate the proposed method.

Spearman Correlation-Based Pilot Protection for Transmission Line Connected to PMSGs and DFIGs

The wide application of power electronic component in power system with renewable energy sources has changed the fault characteristics of conventional power system, resulting in the performance degradation of conventional protection, and even the risk of maloperation and rejection. This brings challenges to the security and stability of the power grid. Therefore, in order to solve these problems, a novel principle of pilot protection based on structural similarity and square error method is proposed. The structural similarity criterion makes use of the difference of fault characteristics between renewable sources and synchronous generators to identify internal faults, while the square error auxiliary criterion is used to solve abnormal calculation of the existing similarity based protection. The detailed model of grid-connected renewable energy power plant is built in Real Time Digital Simulator (RTDS) and the proposed protection algorithm is solidified in the industrial protection device. The effectiveness of the proposed principle is verified by hardware-in-loop dynamic simulation experiments. Compared with conventional differential protection, the proposed protection shows excellent performance in speed and reliability during various faults. In addition, a field short-circuit test is carried out in a real wind farm to verify the effectiveness of the proposed protection scheme. It is proved that this proposed method has the prospect for industrial application.

A Novel Structural Similarity Based Pilot Protection for Renewable Power Transmission Line

Large-scale photovoltaic (PV) power plants connected to the grid leads to the complex calculations in modeling and simulation of the power system, especially in the electromagnetic transient progress. Common averaging methods are studied for modeling a grid-connected converter to simplify the calculation and balance accuracy and efficiency in the simulation. The piecewise technique is applied to the generalized state space averaging model of converters, which combine time segment with similar operating characteristic. In addition, a new piecewise generalized state space averaging (P-GSSA) model is derived and a multi time scale modeling is achieved for the grid-connected converters in PV systems. The model error and accuracy of the P-GSSA model are analyzed, the physical significance and the influence factors in the model error of P-GSSA model are discussed. The proposed P-GSSA model is flexible to be used in a PVs system simulation, and it can accurately reflect the steady-state and transient characteristics of a converter. The research shows that P-GSSA model improved the simulation efficiency over 20% by the index of consumed time, meanwhile, the model error analysis is under 5%. The analysis of the P-GSSA model is verified by simulating a DC/AC converter and a photovoltaic system.

Electromagnetic Transient Modeling and Simulation of Power Converters Based on a Piecewise Generalized State Space Averaging Method

This paper presents a reliable and fast index to detect the instant of arc extinction for adaptive single-pole automatic reclosing (ASPAR). The proposed method is a simple technique for ASPAR on shunt compensated transmission lines using the Hilbert–Huang Transform (HHT). The HHT method is a combination of the empirical mode decomposition (EMD) and the Hilbert transform (HT). The first intrinsic mode function (IMF1) decomposed by EMD, which contains high frequencies of the faulty phase voltage, was used to calculate the proposed index. HT calculates the first IMF spectrum in the time-frequency domain. The presented index is the sum of all frequency contents below 55 Hz, which remains very low until the fault clearance. The proposed method uses a global threshold level and therefore no adjustment is needed for different transmission systems. This method is effective for various system configurations including different fault locations, line loading, and various shunt reactor configurations, designs, compensation rates, and placement. The performance of the method was verified using 324 test cases simulated in electromagnetic transient program (EMTP) related to a 345 kV transmission line. For all the test cases, the algorithm successfully operated with an average reclosing time delay of 32 ms.

https://www.mdpi.com/1996-1073/13/20/5416/pdf

Adaptive Overhead Transmission Lines Auto-Reclosing Based on Hilbert–Huang Transform

Fault characteristics analysis normally provides a theoretical foundation for the protection design. Although conventional fault analysis of inverter-interfaced renewable energy sources (IIRESs) can characterize post-fault steady-state at the power frequency, it might not effectively present the fault transient. This can negatively affect the protection based on fault transient data within 20–30 ms after a grid fault. Thus, the fault transient of IIRESs needs to be fully investigated. Given this, the nonlinear effect of controller saturation is considered in this study. The domain of attraction (DOA) is first established to estimate the practical working state of the controller within the current loop. The transient fault current (TFC) analytical model is then proposed in accordance with the nonlinear control response of the controller under different working stages, which reveals a detailed transient transition of the fault current. Meanwhile, the proposed model is low-order and presents high applicability in different fault scenarios. Simulation results indicate that the TFC analytical accuracy obtained using the proposed method is improved by at most 50% relative to the accuracy reported in published studies. Field short-circuit test data are ultimately used to verify the performance of the proposed TFC analytical model. 

Transient Fault Current Analysis of IIRESs Considering Controller Saturation

Fault characteristics analysis is precondition of relay protection. Theoretical derivation of permanent magnet synchronous generator (PMSG) fault current is necessary. In accordance with the theory that grid-side converter of PMSG is a power balance system under control action, an electromagnetic transient model of PMSG was proposed. Based on it, the active and reactive current was deduced and then the expression of three-phase fault current was obtained and its components, frequency, and attenuation speed of every component and other characteristics were analysed comprehensively. Simulation on PSCAD and field fault recording data as practical analysis verifies that the expressions and the characteristics are correct. The results of this paper are not only important to relay protection, but also are references to transient calculation and transient modelling.

Analysis on current characteristics of PMSG under grid three-phase fault

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/joe.2018.0199

Three‐phase adaptive reclosure for transmission lines with shunt reactors using mode current oscillation frequencies

Scheme to improve the load characteristics in DC distribution networks based on optimal allocation of energy storage systems

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/joe.2017.0492

Dynamic modelling of the VSC‐HVDC for analytical studies

The invention discloses a three-phase adaptive reclosing method utilizing parallel reactor mode current frequency characteristics. After line fault tripping, different phase-mode transformation matrixes are selected according to fault phase selection results to convert a phase system to a mode system for analysis; the fault property discrimination start time is the time when a power frequency component can not be detected in a parallel reactor mode current; within the discrimination time, permanent faults and instant faults are distinguished according to parallel reactor 1-mode, 2-mode and 0-mode current oscillation frequency characteristics. The method is advantaged in that fault properties are identified before three-phase reclosing, reclosing is not carried out if a fault is determined to be the permanent fault, so secondary impact caused by reclosing the permanent fault to the system and equipment can be avoided.

Ting Wang

Papers

Analysis on current characteristics of PMSG under grid three-phase fault

Three‐phase adaptive reclosure for transmission lines with shunt reactors using mode current oscillation frequencies

Scheme to improve the load characteristics in DC distribution networks based on optimal allocation of energy storage systems

Dynamic modelling of the VSC‐HVDC for analytical studies

Three-phase adaptive reclosing method utilizing parallel reactor mode current frequency characteristics