Fault Discrimination Algorithm for Busbar Differential Protection Relaying Using Partial Operating Current Characteristics

Differential protection is the unit protection system which is applied to protect a particular unit of power systems. Unit is known as zone in protection terminology which is equivalent to simple electrical node. In recent time, low impedance current differential protection schemes based on percentage restrained characteristics are widely used in power systems to protect busbar systems. The main application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. Researchers have suggested various solution of this problem; however, individually they are not sufficient to puzzle out all mis-operational scenarios. This thesis presents a new bus differential algorithm by defining alternative partial operating current characteristics of a differential protection zone and investigating its performance for all practical bus faults. Mathematical model of partial operating current and operating principle of the proposed bus differential relay are described in details. A CT saturation detection algorithm which includes fast and late CT saturation detection techniques is incorporated in relay design to increase the sensitivity of partial operating current based internal-external fault discriminator for high impedance internal faults. Performance of the proposed relay is validated by an extensive test considering all possible fault scenarios

Fault Discrimination Algorithm for Busbar Differential Protection Relaying Using Partial Operating Current Characteristics

Differential protection is the unit protection system which is applied to protect a particular unit of power systems. Unit is known as zone in protection terminology which is equivalent to simple electrical node. In recent time, low impedance current differential protection schemes based on percentage restrained characteristics are widely used in power systems to protect busbar systems. The main application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. Researchers have suggested various solution of this problem; however, individually they are not sufficient to puzzle out all mis-operational scenarios. This thesis presents a new bus differential algorithm by defining alternative partial operating current characteristics of a differential protection zone and investigating its performance for all practical bus faults. Mathematical model of partial operating current and operating principle of the proposed bus differential relay are described in details. A CT saturation detection algorithm which includes fast and late CT saturation detection techniques is incorporated in relay design to increase the sensitivity of partial operating current based internal-external fault discriminator for high impedance internal faults. Performance of the proposed relay is validated by an extensive test considering all possible fault scenarios

A microprocessor-based system for protecting busbars

Advancements in digital technology have led to the development of microprocessor-based relays. However, most of these relays use algorithms similar in principle to their electromechanical counterparts. Also, busbar protection using microprocessor-based relays has not received adequate attention unlike other power system components. Few algorithms proposed for protecting busbars lack inherent immunity to current transformer (ct) saturation. They achieve stability by using additional measures, such as, using special circuitry, multiple algorithms and changing the restraint factor, which are not likely to be effective during severe ct saturation. The impact of ct ratio-mismatch is countered by using percentage-bias characteristics that reduces the sensitivity of the relay. This thesis presents a new technique for protecting busbars. The technique uses positive-sequence and negative-sequence models of the power system in a fault-detection algorithm. While phase voltages and currents are used to detect faults, parameters of the power system are not used. Only the arguments of the positive-sequence and negative-sequence impedances computed by the relay are used to make trip decisions. The performance of the technique was investigated for a variety of operating conditions and for several busbar configurations. Data generated by empty simulations of model power systems were used in the investigations. The results verify that the proposed technique is able to distinguish faults in a busbar protection zone from those outside the zone correctly. Additionally, its stability during ct saturation, immunity to ct ratio-mismatch and applicability, without any modifications, to busbars of different configurations have been established. An analysis of the performance of the proposed technique during ct saturation and ratio-mismatch conditions is presented. The effect of various parameters, such as, presence of d.c. offset in the currents, mild and severe saturation of the cts, different sampling frequencies and the impact of the size of data-windows on the estimates of the current phasors have been included. The analysis indicates that the technique is stable during ct saturation and inherently immune to ct ratio-mismatch. The proposed technique was implemented using a general purpose relay hardware. The hardware and software constituents of the prototype, the procedure for testing these relays by using a playback simulator and selected test results are presented in this thesis

BEHAVIOR OF BUSBAR DIFFERENTIAL RELAYS WITH AIR-GAP CORE CURRENT TRANSFORMERS

Protective relays normally estimate the magnitude and phase angle of current. Since the level of current is normally too high to permit a direct connection to the power system, a Current Transformer (CT) is used to scale down the current value. A CT should faithfully replicate waveform of the primary current. Power utilities normally use air-gap core and solid-core CTs. Air-gap core CTs are expensive and hard to maintain but they are able to reproduce the primary current without becoming saturated unlike solid-core CTs. Nowadays relay manufacturers claim that no matter what type of CT is used in the power grid, protective relays can intelligently sense a saturated waveform produced by a CT, and facilitates a correct decision, based on a unique algorithm. Therefore, continued use of the air-gap core CTs is being questioned. To verify relay manufacturer’s claim, behavior of protective relays when subjected to solid-core and air-gap core CT output waveforms need to be analyzed. Therefore, a mathematical model for both CT types is needed. Output waveform of a solid-core CT is already simulated by IEEE Power System Relaying Committee. In this thesis, a mathematical model of an air-gap core CT is developed and simulated on the Excel platform. Output waveform of the proposed model is then verified using the IEEE PSRC CT Simulator. Then, two commercially available busbar differential relays with CT saturation detection logic were subjected to the output waveforms of solid-core and air- gap core CTs. After testing about hundred scenarios on each relay, it is concluded that first, there is no difference in the relays’ performances for internal faults. Besides, for external faults, an air-gap core CT renders the trip output less sensitive to the relay setting. Further, the relay might be considerably slow in operation for an evolving fault, if a solid-core CT is involved

Investigation into open phase faults on transmission circuits.

Masters Degree. University of KwaZulu-Natal Natal, Durban.Open phase faults are not commonly encountered on the South African power transmission systems. However, when these faults do occur it results in major power system interruptions and spurious tripping of healthy (circuits without faults) circuits to trip without giving any indication of the type or location of the fault. This results in lengthy restoration times to find the open phase fault without any fault detection devices. The intention of this study was to investigate open phase faults and assess the impact thereof. An investigation was conducted on an open phase fault on the bussection of the transmission high voltage yard at Koeberg Power Station. The investigation utilised the Koeberg network configuration, power system data, and protection settings in order to simulate the fault and validate the results with the simulation model. The simulation model was tested by simulating short circuit faults with the current feeder circuit protection scheme characteristics and settings. The results of the investigation confirmed that the current feeder protection schemes do not take open phase fault detection into account. The back-up earth-fault protection which is normally utilised to detect and trip for high resistance faults, did indeed detect and trip for open phase faults where the unbalance currents summation was above the minimum setting threshold of 300 A although the fault clearing times was extremely long. However, this was not the case for all instances. The feeder tripped due to zero sequence currents instead of negative sequence currents. In addition the impact of open phase faults was investigated on the Koeberg generator circuit to confirm that the generator would be protected against negative sequence currents and trip based on the generator protection philosophy, the coordinated and configure generator protection settings. The literature research comprised of the present feeder protection philosophies, a review of currently used feeder protection schemes, available new feeder protection schemes, technologies available or technologies that have the potential to detect an open phase fault. An evaluation of the currently used protection schemes and new protection schemes available was conducted. Considerations with respect to the protection scheme flexibility, adaptability with regards to coordination and configuration of the protection scheme in conjunction with the feeder protection philosophies, modifications and additions to the current feeder protection schemes were considered

Dual-Slope Percentage Bias Differential Relay (87) Protection Strategy for 11kV Underground Power Cable

The study focuses on the protection strategy for an underground power cable (transmission) for a medium voltage distribution network using dual slope percentage bias differential relay (ANSI 87). The underground cable is the bridge that connects the power supply from generator to distribution substations, it must be well protected from internal fault that may cause insulation breakdown and leads to electrical failure; while remain unaffected for external fault conditions. The fault simulation is done with PSCAD software, and the suitable protection settings for the applied differential relay is proposed. The study found that the relay is stable for external faults while allowing sensitive settings to pick up internal fault if the setting was carefully chosen

Controlled Switching of Reactive Loads and Commisioning Regimes

Switching is a vital task in any power system for ensuring its safe and reliable operation. Switching may be necessary for fault clearance, to ensure wider system stability and to prevent damage to plant. It is essential for isolation, to allow technicians to carry out maintenance tasks safely. Also, switching of reactive loads such as shunt capacitor banks and shunt reactors, is crucial for controlling system voltage. Switching of some loads however, may produce voltage transients and heavy transient inrush currents which can impact on wider system power quality, impact customers and cause damage or deterioration of the insulation of HV equipment. Therefore, it is important to provide some form of measure to control or mitigate transients caused by switching. The main control measures include: metal oxide surge arrestors, pre-insertion resistors, current limiting reactors and synchronised or controlled switching. Controlled switching is the favoured solution for frequently switched loads such as reactive plant, for economic benefits and as it reduces transients in the first instance. Controlled switching is defined as the use of electronic equipment to control the making or breaking of high voltage circuit breakers at pre-determined points on the system voltage and current waveforms. It has been implemented in Ireland for over 30 years for the energisation of shunt capacitor banks. Over the last two years, the benefits of controlled switching for different applications has become ever more apparent, with increased use such as switching of transmission shunt reactors and the energisation of large power transformers, particularly in remote areas of the network such as wind farm interfaces. The aim of this thesis is to provide a complete overview of the stages concerned in implementing controlled switching schemes, from examining the impacts of switching certain loads, to performing systems studies, up to site commissioning stage. The research in this thesis looks at both the theory and practice. It draws together the published work, manufacturers guidelines, international standards and simulation results, to give the total awareness of the issues involved in reactive load switching and commissioning regimes. The various solutions and strategies associated with controlled switching schemes are examined, to ensure that the best and most economical solution has being implemented. Several recent projects where controlled switching has been implemented for switching of transmission reactors and power transformers are also investigated