Ali Hooshyar

Distance Protection of Lines Connected to Induction Generator-Based Wind Farms During Balanced Faults

Wind farms (WFs) are increasingly integrated with high-voltage (HV) grids, for which distance relaying is normally the protection of choice. This paper reveals some serious defects of distance protection for the lines connected to induction generator (IG)-based WFs during balanced faults. It is shown that for the squirrel cage IG (SCIG) WFs, distance protection becomes insecure, while for the doubly fed IG (DFIG) WFs, the relay performance is utterly unreliable, due to operating scenarios that are unique to such WFs, and are unfamiliar to the existing relaying practices. The detected failures can easily result in unnecessary WF tripping, thus jeopardizing the objectives pursued by the new grid codes that oblige WFs to remain connected to the grid during disturbances. Moreover, a novel modified permissive overreach transfer trip (POTT) scheme along with a fault current classification technique is proposed to address these problems for both types of IG-based WFs, and the accurate nondelayed protection of a distance relay over the entire line length is restored. A comprehensive performance evaluation confirms the findings of this paper and validates the efficacy of the proposed solution for all operating conditions. Results are particularly promising for the DFIG-based WFs with nonzero crowbar resistance, which is the most likely situation confronted by distance relays.

CT Saturation Detection Based on Waveshape Properties of Current Difference Functions

Derivative-based approaches have been widely used for current-transformer (CT) saturation detection. Previous approaches dealt with the magnitude of current derivatives. This paper, however, proposes a novel CT saturation detection algorithm that is subject to the waveshape properties of current derivatives. Two indices are introduced to extract certain key features of a current waveshape using its first two difference functions. Combining these two indices, CT saturation is rapidly and reliably detected. This method operates correctly irrespective of CT parameters, fault current characteristics, and power system conditions. Derivative-based techniques are suspected to be noise sensitive. Meanwhile, appropriate measures are taken, and the robustness of this method against noise is guaranteed. Extensive analysis of various real and simulated currents confirms that this algorithm can meet the rigorous speed and accuracy standards of industrial relays.

Hybrid Passive-Overcurrent Relay for Detection of Faults in Low-Voltage DC Grids

Detection of high-resistance faults on meshed low-voltage dc grids poses a challenge, as such faults have very low fault current magnitudes. This paper proposes a hybrid passive-overcurrent relay to overcome this problem. The proposed relay consists of one current and one voltage transducer, as well as two passive elements: 1) an inductor; and 2) a capacitor. For bolted and relatively low-resistance faults, the relay uses a simple overcurrent function to detect the resultant high fault current magnitudes within 2 ms. On the other hand, for relatively high-resistance faults, a real-time discrete wavelet transform is used to detect the voltage transients generated by the relay passive elements in less than 5 ms. Furthermore, the proposed relay is inherently capable of identifying the type of fault. The proposed approach relies on local-bus measurements to detect and classify various types of faults with resistance up to 200 ohms. Analytical modeling proves that the proposed approach is system independent. Testing the hybrid passive-overcurrent relay on a ±750 V meshed TN-S dc grid reveals that the proposed relay is fast, sensitive, and selective under various conditions.

Three-Phase Fault Direction Identification for Distribution Systems With DFIG-Based Wind DG

Distributed generation (DG) integration necessitates upgrading some distribution system overcurrent relays to directional ones to offer selective protection. The directional feature is conventionally achieved by phase angle comparison between phasors of the fault current and a polarizing quantity, normally a voltage signal. Doubly fed induction generator (DFIG)-based wind turbines constitute an appreciable portion of todays DG power. This paper unveils that conventional directional elements malfunction during three-phase short-circuits when a distribution system incorporates DFIG-based wind DG. The maloperation is due to the exclusive fault behavior of DFIGs, which affects the existing relaying practices. The paper also proposes a fault current classification technique that replaces the conventional directional element during problematic conditions and provides accurate fault direction quickly based on waveshape properties of the current. An extensive performance evaluation using PSCAD/EMTDC simulation of the IEEE 34 bus system corroborates the effectiveness of the proposed method. Results are exceptionally encouraging in the case of resistive crowbar circuits for DFIGs, which is the typical scenario in practice.

Development of a Flickermeter to Measure Non-Incandescent Lamps Flicker

The flickermeter described in International Electrotechnical Commission (IEC) Standard 61000-4-15 is widely used to quantify the voltage flicker level in practical applications. The IEC flickermeter generally performs well for incandescent lamps. However, it fails to measure the flicker of non-incandescent lamps caused by some ranges of interharmonics. This paper suggests modifying the existing IEC flickermeter by adding a new block to it in order to enable accurate measurement of the flicker caused by high-frequency interharmonics. A digital implementation of the proposed modified flickermeter is tested, showing encouraging results. The error of the modified flickermeter is very low compared to the original one, even for the worst conditions. The changes implemented in the flickermeter do not affect its regular performance measuring incandescent lamp flicker due to amplitude modulation, making it a more generic flickermeter version. An arc furnace simulated by PSCAD/EMTDC is used to check the performance of this flickermeter.

Addressing IEC Flickermeter Deficiencies by Digital Filtration Inside a Sliding Window

Although globally deployed for flicker assessment, the flickermeter presented by International Electrotechnical Commission (IEC) Standard 61000-4-15 has been proved to suffer from some deficiencies regarding voltage rectangular modulation and interharmonics. The latter results in flickermeter inability to accurately measure flicker for non-incandescent lamps, which are ubiquitous nowadays. The flickermeter inaccuracies tend to arise from its demodulator. So far, few solutions have been provided. Furthermore, the existing solutions deal with only the interharmonic case. This paper introduces a method to address IEC flickermeter deficiencies by developing a digital signal processing block that demodulates the voltage in a real-time manner for voltages that include interharmonics or are affected by rectangular modulation. The suggested demodulator implements discrete Fourier transform inside a sliding window. For rectangular modulation, this approach is used to demodulate the voltage. Meanwhile, for the interharmonics problem, this approach provides the interharmonic frequency. Using the measured frequency, least error squares technique is then employed inside another sliding window to find the interharmonics amplitude. On this basis, a modified flickermeter is devised. Performance of the modified flickermeter is evaluated and its effectiveness is verified using the results obtained from an experimental set-up. The contribution of this paper is not limited to IEC flickermeter modification; the proposed method can be employed for any application requiring interharmonic measurement.

Fault Type Classification in Microgrids Including Photovoltaic DGs

Protective devices of smart and fault-resilient microgrids are not expected to trip the healthy phases during unbalanced short-circuits. Thus, some utilities and relay manufacturers have started contemplating single- and double-pole tripping for distribution systems. Selective phase tripping demands fault type classification. This paper reveals that existing industrial methods misidentify the fault type in microgrids that include photovoltaic distributed generations (DGs). Due to interface similarities, this paper pertains to systems with type IV wind DGs as well. Two new classifiers proposed in this paper determine the fault type accurately for not only microgrids with photovoltaic DGs, but for any three-phase system. With low computational burden, they require only local information and operate successfully for high resistance faults. Furthermore, these techniques are not affected by the system imbalance and different DG power factors over disturbances.

A New Directional Element for Microgrid Protection

With protective relays exclusive for microgrid applications yet to be developed, microgrids are currently limited mostly to directional overcurrent relays (DOCRs) for protection. This paper unveils that the long-known small fault current of electronically coupled distributed generation (DG) units is merely one of several phenomena that DOCRs suffer from; the current magnitude in microgrids with DG units may even provide misleading indications of fault conditions. More important, commercial-grade torque- and impedance-based directional elements are shown to be adversely affected by the fault behavior of DG units. The misoperation may include incorrect fault direction identification or deactivation of directional elements by false detection of loss-of-voltage condition, ruling out existing impedance relays as workable solutions for microgrid protection as well. This paper also devises a new directional element that addresses these problems. The proposed element detects the direction of asymmetrical faults using the magnitude and angle of the superimposed negative-sequence impedance. For symmetrical faults, the proposed directional element uses the magnitude of superimposed positive-sequence impedance along with the positive-sequence current and torque angle. PSCAD/EMTDC simulation studies verify the performance of this new method.

Ultra-High-Speed Travelling-Wave-Based Protection Scheme for Medium-Voltage DC Microgrids

Fast detection of dc faults in medium-voltage dc (MVDC) microgrids poses a challenge as such faults can cause severe damage to voltage-sourced converters within few milliseconds. This paper proposes a new traveling-wave (TW)-based method to detect, classify, and locate different dc fault types in MVDC microgrids. Unlike the existing TW-based protection and fault location methods, the proposed technique: 1) utilizes only the first locally measured TW after the inception of a fault and 2) focuses on the waveshape properties and polarity of the TW, rather than its arrival time. Therefore, the proposed method is faster than the existing techniques, and also requires no form of communication. As a result, it can effectively operate as both primary and backup protection. The proposed method is robust against high-resistance faults, and has been tested for fault resistances of up to

Distance Protection of Lines Emanating From Full-Scale Converter-Interfaced Renewable Energy Power Plants—Part II: Solution Description and Evaluation

200~\boldsymbol {\Omega }