Elmer Sorrentino

A hybrid genetic algorithm for optimal reactive power planning based upon successive linear programming

A hybrid methodology is presented for the solution of the problem of the optimal allocation of reactive power sources. The technique is based upon a modified genetic algorithm, which is applied at an upper level stage, and a successive linear program at a lower level stage. The objective is the minimization of the total cost associated to the installation of the new sources. The genetic algorithm is devoted to defining the location of the new reactive power sources, and therefore to handle the combinatorial nature of the fixed costs problem. At the lower level, the variable cost problem is solved by calculating the magnitude of the sources to be installed at the previously determined locations by means of a linear program iterated successively with a fast decoupled load flow. Results are presented for the application of the proposed methodology when applied to the Venezuelan electric network.

Typical expected values of the fault resistance in power systems

This article presents a range of possible values for the fault resistance in transmission power systems, considering six existing models for the arc resistance and a model for the grounding impedance of the towers. Resistance by possible additional objects in the path of the fault current was not considered. Known the short circuit level (without fault impedance), the fault resistance was calculated with the above mentioned models, for line-to-line and line-to-ground faults. This calculation was made for diverse nominal voltages and diverse short circuit levels for solid faults. The obtained range might be useful to improve the way of computing the settings for the corresponding protective devices.

Determination of Parameters of Zero-Sequence Equivalent Circuits for Three-Phase Three-Legged YNynd Transformers Based on Onsite Low-Voltage Tests

This paper proposes a new approach for obtaining parameters of the zero-sequence equivalent circuit of three-phase three-legged three-winding YNynd transformers (i.e., transformers with two wye windings and a third delta stabilizing winding). The methodology proposed in this paper includes a complete set of onsite field measurements using commercial equipment (traditionally used for onsite transformer leakage reactance tests). An easy-to-implement determination of the zero-sequence behavior of the YNynd transformer is achieved by means of separating the equivalent T-circuit into two different circuits, one of them to be used when the delta stabilizing winding is closed and the other when this is open. The effect of the resistive component of the zero-sequence impedances is also analyzed and quantified. Onsite measurements on a 45/16.05/10-kV 25-MVA power transformer and their comparison with high-voltage factory tests are presented to demonstrate and validate the proposed methodology.

Optimal-Probabilistic Method to Compute the Reach Settings of Distance Relays

This paper presents a novel method to optimize the settings of the resistive and reactive reaches of the zones of the distance relays. The method considers the probabilistic behavior of the variables that affect the apparent impedance seen by relays: prefault load flow, fault type, faulted line, distance up to the fault, fault resistance, and measurement errors. The optimization has been conceptually formulated as a multiobjective problem, with two objective functions: 1) minimize the probability of loss of sensitivity and 2) minimize the probability of loss of selectivity. To solve this problem, a preference function is defined, which is equal to the weighted sum of the two objective functions. A factor of weight represents the relative importance of selectivity regarding the sensitivity, and it is selected for each zone. This method was applied to a system with 7 busbars (3 of them with generation), where there are 18 distance relays with quadrilateral characteristic, and with an independent setting of the reactive and resistive reaches for the phase distance function and the ground distance function. The results obtained are compared with the results of other methods of adjustment.

Nontraditional Relay Curves for the Coordination of the Ground Overcurrent Function With Downstream Fuses

This paper describes the details for the application of some nontraditional relay curves for the coordination of the ground overcurrent function with downstream fuses. These curves have a segmented shape in order to achieve the best combination of sensitivity, speed, and selectivity. For this segmented shape, greater current does not necessarily imply a shorter time. These curves can be programmed in different ways, according to the available features in the commercial relays, and the expected behavior of two options is analyzed. Both options would have satisfactory behavior although they would have different dynamic behavior.

An approximate 2D method for computing the magnetizing zero-sequence impedances in 3-phase core-type transformers without tank

This paper describes an approximate 2D method for computing the magnetizing zero-sequence impedances in 3-phase core-type transformers without tank. These transformers do not have axial symmetry in their geometry, but a 2D model with axial symmetry is applied in order to obtain approximate results. The 3D models require more computational resources, and this fact justifies the search of an approximate method. The developed approximate method is based on the analysis of the main flux paths, and this method requires the application of approximation constants. These approximation constants are found by comparison between the results of 2D and 3D models, for units between 500kVA and 25 MVA. Experimental validation is included for one unit. The results indicate that the developed approximate method can be applied in order to compute these magnetizing zero-sequence impedances.

Limit Curves by Power System's Transient Stability for the Inverse-Time Overcurrent Relays

This paper presents the limit curves by transient stability for the inverse-time overcurrent relays. Two cases were analyzed: (1) faults in radial lines and (2) faults in lines which are fed from both ends. A novel time-current limit curve by stability was obtained for the radial case; this curve could be compared with the inverse-time overcurrent curve of the relay. Another limit curve by stability was obtained for lines which are fed from both ends; this curve represents the combination of the maximum time-dial settings at both line ends. The nonsimultaneous clearing for lines which are fed from both ends, the dynamic evolution of the fault current, and different prefault conditions were considered in the examples.

Two Improvements Related to Overcurrent Functions for Bus Protection in Distribution Systems

This paper shows two improvements related to the overcurrent functions (OCFs) for bus protection in distribution systems. The first topic is the use of two high-set definite-time OCF, in conjunction with the inverse-time OCF, for the main circuit breaker (MCB). A zone-interlocking signal can block one high-set definite-time OCF, while the other one is the fastest possible backup action. The second topic is the inclusion of a backup function at the MCB for cases of failure of a downstream relay, by using the OCF of this MCB to first trip the circuit breaker related to the corresponding downstream feeder. If the OCF of the MCB remains asserted, then this OCF trips the MCB after a suitable coordination time. Both improvements can be implemented by using the IEC-61850 standard.

Electromechanical Modeling of a Contactor with AC Coil

Abstract Two ways of modeling a contactor with an AC coil are shown in this article. Both models are based on the explicit definition of the inductances by lumped parameters, and the difference between them is the considering or not of the effect of the shading rings. The equations were formulated to obtain the contact operation time and to describe the contactor coil current when the coil is fed with different values of the AC source voltage. The comparison between theoretical and experimental results shows that the models are satisfactory in both cases. This demonstrates the possibility of obtaining accuracy without considering the shading rings. The main advantage of the inclusion of the effect of the shading rings is the obtaining of a qualitative description for the currents in the shaded rings.

Comparison of methods for measuring zero sequence impedances in 3-phase core-type transformers

This paper deals with two topics about the measurement of zero sequence impedances in 3-phase core-type transformers. The first topic is a comparison of 3 methods to perform the measurement in a 3-phase winding: a) parallel connection of windings; b) series connection of windings; and c) measurement of elements of the phase impedance matrix. A brief theoretical discussion about these methods is included. The second topic is a comparison of 2 methods for measuring the zero-sequence impedance for the Dyn connection: a) with all the delta branches short-circuited; and b) without short-circuiting the delta branches. The results for the Dyn connection are compared with the obtained ones for YNyn connection, by using the standardized method for this test (parallel connection of star windings). A discussion about the results is included.