Iranian Journal of science and Technology (B: Engineering)
Volume:30 Issue: 6, December 2006

In this paper, a new algorithm based on processing differential current harmonics is proposed for digital differential protection of power transformers. This algorithm has been developed by considering different behaviors of second harmonic components of the differential currents under fault and inrush current conditions. In the new method, a criterion function is defined in terms of the time variation of the second harmonic rising rate during instants after the occurring disturbance. By evaluating the signs of the criterion function for the three phases, the internal faults can be accurately recognized from inrush current conditions in less than a half-cycle after the occurrence of a disturbance; this is one advantage of the method. Another advantage of the proposed method is that the fault detection algorithm does not depend on the selection of thresholds. A suitable performance of this method will be demonstrated by the simulation of different faults and switching conditions on a power transformer. For this purpose, a small part of the Iran power system involving a power transformer and the transmission lines on both sides of the transformer has been considered. To include effective factors on differential current components, the elements of this power system have been precisely modeled in PSCAD/EMTDC.

Partial discharges (PD) are recognized as the main cause of the inner insulation deterioration process in power transformers. Therefore, the optimum inner insulation design is one of the challenges a transformer designer is faced with. Transformer strength, especially during transient conditions, is a criterion for transformer insulation designers. This challenge has made designers initiate and employ other types of winding, for example, rather than ordinary layer and disc windings employ the multiple-α windings. Multiple-α windings have a more complicated structure and are comprised of various parts with different physical structures and electrical characteristics. Typical partial discharge signals cover a wide frequency range from DC up to hundreds of MHz and different frequency components propagate through the winding depending upon the winding structure in different modes. Partial discharge propagation in single-α winding is more predictable compared to multiple-α winding. A 66 kV / 25 MVA interleaved winding, which has 19 fully interleaved discs, plays the role of a single-α winding. When this main winding is connected to the tap winding with a different structure and magnitude response, a multiple-α winding is constructed. Two terminal current signals are detected by the application of two home-made high frequency current transformers (HF-CT). The signals were amplified and fed into a 500 MHz digital storage oscilloscope. Home-made sensors are designed to provide maximum sensitivity in the desired frequency range. In order to evaluate the partial discharge signal accurately, a method for selecting the optimal wavelet is introduced to reduce the noise effects. This method is based on the capability of the chosen mother wavelet for generating coefficients with maximal values. The wavelet based de-noising method proposed can be employed in extracting the PD pulses from the measured signal successfully to provide enhanced information and further infer the original site of the PD pulse through the capacitive ratio method..

After a disturbance occurrence, fast damping of power oscillations is essential to reduce the risk of instability and thus increase the power transfer capacity of transmission systems. Recently, the Static Series Synchronous Compensator (SSSC) has justified its ability to improve power oscillation damping. Due to substantial interactions among the SSSC damping control loops and the power system variables, the use of conventional Single-Input Single-Output (SISO) control approaches results in a poor damping performance. In this paper, these interactions are taken into consideration, and a power system equipped with an SSSC is modeled as a multivariable system. The impact of the SSSC’s dc voltage dynamic is also considered in the modeling. The power system equipped with an SSSC is multivariable with effective interactions among its variables. Traditional SISO control design techniques do not take into consideration these interactions and therefore cannot provide adequate damping over a wide range of operating conditions. Based on this developed multivariable modeling, a multivariable controller is proposed to improve power oscillation damping while keeping the dc-link voltage regulated. Simulation results verify the validity of the proposed modeling and control and show that the proposed approach can successfully damp out power system oscillations. Further simulation results show that the proposed controller provides a superior performance and a better robustness when compared to conventional SISO controllers.

Abstract– This paper addresses a new concept for determining the loss quota of any transaction and any participant entity in a multilateral open access power system. This new concept, which is based on the incremental transmission loss allocation (ITLA) technique, is proposed for application on multilateral open access power systems. This method considers trader entities that have no analogy in the electrical network and classic ITLA for which load flow study could not be applied. Using Optimal Power Flow (OPF) in the proposed method overcomes the problem of dependency of the final decision to the slack bus selection, which is the worst disadvantage of the ITLA method. The proposed approach is tested on IEEE RTS 24-bus network with satisfactory results.

This paper presents a new method to analyze the bidding strategies of Generating Companies (GENCOs) with regard to demand elasticity. It is assumed that the available information of each GENCO about its opponents is incomplete and only the minimum and maximum generation levels of their opponents, as well as their fuel type, are known. In the proposed methodology, GENCOs prepare their strategic bids according to a Supply Function Equilibrium (SFE) model. GENCOs will change their bidding strategies until Nash equilibrium points are obtained. The general Algebraic Modeling System (GAMS) has been used to solve the maximization modules using the MINOS optimization software with non Linear Programming (NLP).

Genetic Programming (GP) is a powerful machine learning technique derived from genetic algorithms. We used GP to generate a mathematical function for image denoising based on statistical features derived from detail sub-bands of wavelet transform (WT). The function obtained from GP for image denoising is not dependent to any parameters as represented in other image denoising methods based on WT. Results of the proposed image denoising method is compared to the VisuShrink soft threshold image denoising method, both perceptually and in terms of Peak Signal to Noise Ratio (PSNR).

The performance of various estimators, such as maximum a posteriori (MAP), strongly depends on correctness of the proposed model for distribution of noise-free data. Therefore, the selection of a proper model for the distribution of wavelet coefficients is very important in wavelet based image denoising. This paper presents a new image denoising algorithm based on the modeling of wavelet coefficients in each subband with a mixture of Laplace random variables. Indeed, we design a MAP estimator which relies on mixture distributions. Using this relatively new statistical model we are better able to capture the heavy-tailed nature of wavelet coefficients. The simulation results show that our proposed technique achieves better performance than several published methods, both visually and in terms of root mean squared error (RMSE).

Formal verification of complex digital systems requires a mechanism for efficient representation and manipulation of arithmetic as well as random Boolean functions. Although the Taylor Expansion Diagram can be used effectively to represent arithmetic expressions at the vector level, it is not efficient in the use of memory for representing bit-level logic expressions. In this paper, we present modifications to TED that will improve its ability for logic representation while maintaining its robustness in arithmetic representation. Our experimental results show a 30% reduction in the number of nodes in some benchmarks.

We propose a novel algorithm called RCA_MC, in which we use the breadth first search method (BFS) in conjunction with edge contraction and connectivity properties of a given undirected graph to enumerate and scan all its minimal edge cutsets. It is known that the problem of enumerating all minimal edge cutsets of a given graph is #P-complete. In addition, we introduce the concepts of pivot vertex and absorbable clusters, and use them to develop our enhanced recursive contraction for scanning all mimimal edge cutsets, called ERCA_MC, of a given graph. Simulation results provide empirical evidence that the complexity of the ERCA_MC algorithm is linear per cutset.

Collocated CORBA objects that reside in the same address space can benefit from special local calls which can be performed without ORB intervention. This type of invocation can be particularly beneficial to fault detection mechanisms defined in FT-CORBA specifications. According to FT-CORBA, a group of objects, namely fault detectors, periodically monitor the status of replicated objects in the system to make sure that they are alive. In cases where a fault detector object is collocated with some of the objects that are monitored by this detector, direct invocations can improve the performance of fault detection mechanisms. All the known available methods for direct calls to collocated CORBA objects are flawed either with unnecessary pre-checks that are performed before each invocation, or with violation of the remote call semantics (like bypassing ORB and POA). In addition, as this paper shows, in some cases the default pre-checks that are performed before a call are not sufficient, and none of the available methods allow the applications to perform domain-dependent pre-checks or to only override the default ones. CMI (Configurable Method Invocation) is a new method that allows the pre-checks to be selected before each direct call in order to avoid investigating unnecessary conditions. Furthermore, it allows the developers to make application-dependent pre-checks or override the existing ones before each call. To achieve these two properties, we have changed the CORBA IDL compiler in such a way to generate a special code in addition to the code for collocation-safe stubs. This extra code permits the developer to manipulate the execution of pre-checks which must be performed before each call. Implementation results of our method show a 41% reduction of communication overheads in a fault detection mechanism compared to the standard approach. The possibility of checking a user-defined pre-check before each direct call is also shown.

Communication system recognition can be used in some civilian and military applications. The recognition of the system is done by inspecting the received signal properties like modulation type, carrier frequency, baud rate and so on. Therefore we need Automatic Modulation Recognition (AMR) in addition to carrier and baud rate estimation methods. In this paper we introduce a new AMR method based on time and spectral domain features of the received signal. A neural network is used as the classifier. A broad class of analog and digital modulations is considered. Baud rate and carrier frequency estimation is performed by existing methods referred to in this paper. Using this information the protocol used for signal transmission is detected.

Determination of the electric field in insulated cable will lead to an optimum design and a better selection of both conductor size and insulation thickness. A simple numerical method using the Charge Simulation Method (CSM) is used to calculate electric field stresses in high voltage cables. An image charge for each fictitious charge is considered in such a way that the potential of sheath is always kept at zero. The effect of cable sheath is considered and results of the calculation are shown.