Houlei Gao

A design scheme of line current differential protection based on IEC61850

Current differential relay plays a very important role in transmission line protection because of its absolute selectivity for isolating the failure equipment. Nowadays, in digital substation, lots of new technologies (such as non-conventional instrument transformers, process bus, time synchronization, embedded Ethernet and information modeling following IEC 61850) bring huge challenge for relay design and application. This paper reviews traditional technical solutions to the transmission line current differential protection, addressing its basic principle, typical architecture, differential criterion and communication requirement. Then a new design scheme of line current differential protection is discussed in detail. Network time synchronization protocol - IEEE 1588 and process bus communications based on IEC 61850-9-2 are introduced into the new scheme. The communication between substations adopts directly optical fiber link or wide area Ethernet networks. Consequently, the protections in different substations receive synchronous sampled value of both sides though its own merging unit is just like communication in the same substation. Information exchange models (Logic Devices and Logic Nodes) in the transmission line protection are also established. There are many advantages behind this scheme, one significant being to increase the interoperability and availability of protective relay.

Algorithm for ultra high speed travelling wave protection with accurate fault location

Basing fault generated current travelling wave, a novel algorithm implementing ultra high speed protection and fault location for transmission lines is proposed. By applying wavelet transform to the current travelling wave signals measured at two terminals of a protected line, the corresponding modal maxima of the two current signals are obtained. The algorithm uses the polarity relationship between two initial modal maxima to determine whether the fault is internal or not. Simultaneously, the fault location can be easily calculated according to the interval between the two modal maxima. The factors influencing on travelling wave protection and fault location are analyzed and some measures are presented. Extensive simulation results based on EMTDC demonstrate that the algorithm is able to achieve double functions of ultra high speed protection and highly accurate location, which is just required by EHV/UHV transmission systems.

Realization scheme of directional unit protection based on travelling wave integral

A set of scheme of directional unit protection based on travelling wave integral is designed. For main components in this scheme, such as fault phase selection component, fault direction discrimination component, lightning stroke and circuit breaker operation verification, etc., the theoretical analysis and criterion construction and simulation test are all carried out. Simulation data demonstrate that the fault conditions hardly have any influence on the performance of proposed discrimination principle for fault direction; the identification criterion for different signals can correctly distinguish fault signals from various disturbance signals and fault phase selection criterion can precisely choose fault phase. This protection technique has clear principle and simple construction, which is promising to be used in practical power engineering.

A novel directional protection based on transient energy for HVDC line

Using the characteristic difference of transient energy, a transient energy based directional unit protection is proposed. Assuming the positive direction of current is from DC busbar to DC line, for an internal line fault, the detected transient energies on both ends of the line are all negative, which denote the positive direction fault. When an external fault occurs, the transient energy on one end is positive, which denotes a negative direction fault, but the transient energy on opposite end is negative, which means that the fault direction is positive. Through comparing fault directions on both ends, the internal or external DC line fault can be identified. Besides, according to the magnitude difference of transient energy, the faulty line can also be discriminated for bipolar line. Finally, this paper constructs a ±800kV DC transmission system model, and simulation results verify the validity and feasibility of the proposed method.

Frequency spectrum characteristic analysis of single-phase ground fault in a Petersen-coil grounded system

This paper theoretically analyzes the frequency spectrum characteristics of the single-phase ground fault in a Petersen-coil grounded distribution network. Different equivalent circuits are analyzed for faults with low transition resistances and with high transition resistances, separately. The analysis concludes that the system makes a transition from an under-damped state to a damping state, and finally to another under-damped state with the increase of transition resistances. A distribution network model is built with PSCAD/EMTDC. Simulations are performed for different fault locations, different fault initial angles, different transition resistances and different cable proportions. The analysis in time domain and frequency domain is performed. Results show that the use of cables enables the main frequency of the fault current move towards lower frequency. Meanwhile, the amplitude of the fault current obviously increases due to the high capacitance to ground of cables. The transition resistance makes the biggest influence on the characteristic frequency band. Fault location mainly affects the zero sequence current main frequency amplitude. The further the fault location is away from the Bus, the little the amplitude of high-frequency component is. The transient capacitive current component increases, the transient inductive current component decreases and the high-frequency component amplitude increases with the increase of fault initial angles.

Modeling and fault simulation of active distribution network based on RTDS

Active distribution networks (ADN) connect many distributed generators (DG), and compared with traditional distribution network, running state and fault characteristics of ADN change greatly, especially the ADN with connection of inverter-based DG (IBDG). To analyze and test the protection applicability in ADN, a model of ADN with IBDG is established based on RTDS in this paper, including primary system of distribution network, photovoltaic (PV) generation and control strategy for grid-connected inverter. Based on this model, both the characteristics of normal and fault condition are simulated and analyzed, and then compared with traditional distribution network. Meanwhile, based on the ADN model platform, a close-loop test for the protection device we developed is introduced briefly.

Performance testing of complete digital relays based on ATP-EMTP and IEC61850-9-2

The complete digital relays are used increasing widely in recent years, where the traditional analog, hardwired and interface have been replaced by non-conventional instrument transformers and the digital communication link (process bus) based on IEC 61850-9-2 standard. It is shown that these new techniques will make a strong impact on relay performance testing. General methods of testing protective relays will be reviewed, including static characteristics testing, dynamic simulation and real time digital simulator (RTDS). Approaches for testing IEC 61850 compatible relays based on RTDS offer significant advantages when compared with other methods. However, the RTDS simulator is designed with powerful software and expensive hardware, which is not convenient enough to test complete digital relays. This paper presents the design of new relay performance testing scheme, especially for complete digital relays. At first, power system model is established in ATP-EMTP and fault transient data in accordance with COMTRADE will be obtained by offline simulation. Subsequently, transient data are converted to IEC 61850-9-2 format through the static relay testing instrument. Finally, the complete digital relays directly receive the fault sampled value by process bus and Ethernet switcher, and relays will act or not. The new testing scheme will be applied more efficiently to researching the complete digital relays, as the scale of testing is out of the limitation of simulators hardware. A brief example by the new testing system is presented at the end of the paper.

Disturbance identification method for lightning wave invading grid

Lightning stroke on line may lead to mal-operation of relay protection, possibly invades substation, and further results in over-voltage of system. A S-transform based method for monitoring and identifying lightning inrush in substations is proposed in this paper. By monitoring busbar voltage signals in real-time, the waveform characteristics of transient voltage signals in both time domain and frequency domain are extracted using S-transform technique. According to the amplitude difference between main harmonic and fundamental frequency of voltages, a discrimination criterion for general fault, lightning stroke with and without fault is established. PSCAD simulation results have tested the validity and correctness of this proposed method.

A Fast Non-Unit Protection Method Based on S-Transform for HVDC Line

Abstract To overcome the problems existing in the main protection of HVDC transmission lines, such as low reliability and poor ability of resisting high ground resistance fault, this paper proposes a novel non-unit protection method based on S transform. Analysis shows the high and low frequency components of the transient voltage signals suffer from different degrees of attenuation when they pass through the HVDC line boundary. Therefore, according to this characteristic, the voltage signals of specific frequency bands are extracted accurately using S transform, which be used to identify the internal from external faults. By comparing the high and low frequency components generated by the lightning disturbance with those generated by the faults, S transform based energy ratio criterion to identify lightning disturbance is constructed. In addition, for a bipolar HVDC transmission system, a method using the 0 Hz energy difference of S transform of the voltage transient signals is proposed to identify the faulted pole. Simulation results verify the feasibility and validity of the proposed protection method.

Development of a Substation-Area Backup Protective Relay for Smart Substation

A substation-area protective relay is proposed to overcome the inherent defects of the traditional backup protection system. The relay performs local backup protective functions for the whole substation and its connected transmission lines. The setting coordination of backup protections will be simplified by this relay with a shorter fixed time-delay. The algorithm of locating the faulted component employs the current differential principle with a self-adaptive restraint characteristic. The current sampled values are obtained by process-level networks inside the smart substation and by wide-area Ethernet between adjacent substations. The relay also works with the breaker states and the tripping generic object oriented substation event (GOOSE) from primary protections. A prototype based on the proposed scheme and IEC61850 standards has been developed by digital signal processor / field programmable gate array. The performance of the prototype was evaluated through the complete digital testing system using real time digital simulator / gigabit transceiver network and a communication simulator. Tests were carried out under different typical scenarios, such as fault transition resistances, inrush currents, current transformer saturations, and abnormal communications.