Ahmed Eltom

Hardware Implementation of an Automatic Adaptive Centralized Underfrequency Load Shedding Scheme

The underfrequency load shedding (UFLS) mostly used in industry is a decentralized deterministic scheme designed to shed a prespecified amount of load after a predetermined time delay. It sheds the same amount of load from the same location irrespective of how fast the frequency drops and without consideration of the disturbance location or dip in bus voltage. Recent studies focused on adaptive UFLS, but these studies are still based on software simulation. This study presents an implementation of a real-time centralized adaptive UFLS scheme using industry-grade hardware. It estimates the amount of load to be shed based on the rate of frequency decline and distributes the load to be shed among the load buses based on the voltage dip at these buses. The UFLS in this study is implemented using a real-time digital simulator, phasor measurement units embedded in the relays, a global positioning system clock, and a synchrophasor vector processor. The load is modelled as a mixture of dynamic and static load. The implemented scheme restored the system frequency and voltage. The results emphasize its adaptability and suitability for implementation in industry.

Simultaneous solution of unit commitment and dispatch problems using artificial neural networks

Abstract In this paper a combination of Hopfield-Tank type and Chua-Lin type artificial neural networks is applied to solve simultaneously the unit commitment and the associated economic unit dispatch problems. The approach is based on imbedding the various economic and electrical constraints of the unit commitment and dispatch problems in a generalized energy function, and then defining the network dynamics in such a way that the generalized energy function is a Lyapunov function of the artificial neural network. The novel feature of the proposed approach is that the non-linear programming problem for unit dispatch and the combinatorial optimization problem for unit commitment are solved simultaneously by one network. The method is illustrated by an example.

Motor system energy efficiency in the nylon industry: A comparison of PWM and square wave inverters

Escalating energy prices and increasing environmental concerns have renewed interest in improving motor system efficiency during normal operating conditions and when the motor is fed from a polluted power system. In this study a laboratory study and a mathematical motor model are used to evaluate and compare energy consumption and performance of a 3 phase induction motor fed by a square wave and a sinusoidal width modulated (PWM) inverter.

Oil filled bushing secrets revealed

Bushings provide a point of interface such that electrical current can pass to and from an electrical apparatus. Much like bridges on the highway, bushings are very critical and are the weakest link on any high voltage electrical apparatus such as power transformers, circuit breakers or instrument transformers. The integrate design and high areas of stress of bushings makes electrical testing, visual inspection and infrared inspection a vital part of any maintenance program. This paper describes the two elements of stress on bushings and breaks down a bushing into first principle components. Next, this paper details how portable test equipment can be used to examine these basic components of a bushing and gives guidelines for acceptance criteria. By understanding the basic construction of a bushing and applying testing methodologies of power factor, resistance, infrared inspection, and dissolved gas-in-oil, a good test and maintenance program can be established.

A practical improvement to stator ground fault protection using negative sequence current

This paper discusses the phenomenon of zero sequence voltage coupling from the high-voltage system to the high-impedance grounded low-voltage bus for a synchronous generator and a simple improvement to accelerate stator ground fault protection (59G) using negative sequence current.

Transformer Load Tap Changer control using IEC 61850 GOOSE messaging

International Electrotechnical Commission (IEC) 61850 has many benefits including great flexibility and improved interoperability, and promises to be more widely implemented in the United States with time as is already the case in many other parts of the world. This study implements Load Tap Changer (LTC) control using the IEC 61850 standard in a laboratory environment. A load tap changing power transformer with 500 kV on the high voltage side and 169 kV on the low voltage side is assumed. The LTC is located on the high voltage side and allows for 17 tap positions. A programmable controller and a control unit are used to measure and process the voltage signal, and to initiate the LTC motor operation. Generic Object Oriented Substation Event (GOOSE) messages are used to facilitate all required communication. This study shows that LTC operation using IEC 61850 is reliable and brings with it all the benefits that the implementation of IEC 61850 has to offer. Above all, due to elimination of the majority of copper wiring, the proposed method is very flexible and can be implemented using a variety of different devices.

Influence of zero sequence impedances of station auxiliary transformers on equipment performance under open-phase faults

Primary open-phase faults on station auxiliary transformers (SATs) are characterized by the presence of voltage unbalance at the auxiliary equipment level that could lead to protective device tripping, increased motor acceleration times, overheating or failure to start critical safety loads, particularly in the nuclear power industry. The transformer primary to ground zero sequence impedance has a profound impact on the voltage balance on the secondary (equipment) level but its effects not been fully analyzed in the context of nuclear power plant operation. This study investigates the influence of zero sequence impedance to ground as seen from the transformer primary terminals during an open-phase condition on the performance of nuclear plant auxiliary equipment, in particular the effect on large motor starting and running performance. Dynamic models for the motors were employed and adapted to the sequence network representing the overall system. The resulting system was simulated for a number of open-phase conditions using transformer characteristics representative of the SATs encountered in the nuclear power industry. It was revealed that a lower value of this impedance has a beneficial effect on motor acceleration time and voltage balance for both starting and steady state conditions. The influence of neighboring transformers with a path to ground was also investigated and found to serve towards further improvement of performance.

On-site low voltage determination of zero sequence impedances for station auxiliary transformers

Many station auxiliary transformers (SATs) in service today were not tested for complete zero sequence impedance data prior to installation. It is impractical to send them to a testing facility, and on-site testing using standard methods is difficult to perform. While it has been customary to approximate the zero sequence impedances from positive sequence data, the need for accurate zero sequence impedance data has been increasingly felt, particularly for analysis of an unbalanced power system such as during an open-phase fault. This paper proposes a new method to determine the primary-tertiary zero sequence short-circuit impedance for SATs by conducting simple measurements on the low voltage side. This eliminates the need to perform high voltage winding tests required by standard methods and simplifies on-site determination of missing zero sequence data. The new test method was performed on an 18 MVA SAT destined for service in the TVA Sequoyah Nuclear Plant. When compared to standard test methods, the resultant zero sequence impedance agreed within 0.4% using an exact model, and 1.7% using an approximate model.

Performance comparison between Dual-Blinder and Phasor-Based Out-of-Step detection functions using hardware-in-the loop simulation

Recent blackouts have shown the need for an innovative wide-area monitoring and control (WAMC) systems. With PMUs now embedded in protective relays, and the availability of fast processing technologies, many real-time wide-area protection and control schemes can be implemented. An Out-of-Step (OOS) event that occurs between two connected areas in the power system, can ultimately result in a total blackout, and in such cases the two areas should be separated to contain the damage. The separation is achieved via OOS detection and tripping functions that operate on appropriate transmission lines. One of the most commonly used functions to achieve OOS tripping is the Dual-Blinder function. In this paper, a Phasor-Based OOS function is compared to the Dual-Blinder function: the tripping times of these two functions are compared via multiple hardware-in-the loop simulations covering various disturbance scenarios. The implementation is carried out using a real-time digital simulator, Phasor Measurements Units (PMUs) embedded in the relays, a GPS clock, and a synchrophasor vector processor (SVP). Simulation results showed that the Phasor-Based function predicted the OOS event and responded faster in the cases where the system was poorly damped while the Dual-Blinder method has responded faster in the other cases. The paper concludes with suggestions for improved future implementation of the Phasor-Based OOS tripping function.

Fast bus protection using IEC 61850

International Electrotechnical Commission (IEC) 61850 is the latest technology that allows electric power utilities to build low cost digital substations. Because limited objective information is available, electric power utilities desire independent evaluations of IEC 61850 from end users. The objective of this study is to compare the IEC 61850 method to the hardwired method using actual hardware to find an optimal solution for the fast bus protection scheme. System disturbance clearing time, maintenance and construction costs, and interoperability between multivendor microprocessor-based relays were used as the criteria for this evaluation. This research measured the protection speed of the hardwired method and the IEC 61850 method for the fast bus protection scheme, utilizing relays from the same and mixed manufacturers. The results of the study show that IEC 61850 method is simple, cost efficient, and offers speed of operation comparable to the hardwired method.