Volume 12, Issue 2

This paper uses the fault direction information and the distance action information to design a wide area protection agent with high degree of failure tolerance. This protection agent locates the fault by comparing fault direction information. The concept of fault confirmation ring is proposed in this paper. The fault confirmation ring and the distance action information can effectively prevent the incorrect operation of the relay and the communication failure. Then, the division rules of cooperation zones is presented and the structure of protection agent is designed. Referring to the wide area protection algorithm, the dynamic cooperation mechanism is presented. Finally, through the simulation tests, the cooperative process and performance of protection agents are analyzed. Test results manifest that the proposed wide area protection agent with good fault tolerance capability.

This paper analyzes the performance of the electricity distribution system in Nepal. Weight restriction data envelopment analysis (DEA) technique has been used to quantify the relative operational efficiencies of the distribution centers (DCs) owned by Nepal Electricity Authority (NEA). The proposed model incorporates a wide range of variables that capture the essence of electricity distribution process. Decision maker’s opinion on the relevance and relative importance of the decision variables have been incorporated judiciously in the analysis in order to identify the possible improvement strategies for the DCs. This study also investigates the possibility of reorganization of the existing DCs inside the NEA’s distribution framework on the basis of geographical convenience. Sensitivity analysis is carried out in order to check the robustness of the results and to suggest the direction for improvements. Strengths and weaknesses of individual DCs are identified based on the results of the sensitivity analysis.

This paper studies the damping torque and synchronizing torque contributed by STATic Synchronous COMpensator (STATCOM), based on Pulse Width Modulation (PWM), in radial power system considering two different damping schemes: (1) the traditional damping scheme; and (2) a proposed damping scheme. Eigenvalue analysis is used to study the influence of the parameters of the Proportional and Integral (PI) of the dc voltage controller and ac voltage controller, real power, and reactive power as well as the damping controller gain of the STATCOM on the damping torque and synchronizing torque. Using a method developed in the literature, which depends on the time responses of the rotor speed, rotor angle and electrical torque, the damping torque and synchronizing torque coefficients are estimated and compared for different loading conditions and control parameters of STATCOM to confirm the results obtained from the eigenvalue analysis. Non-linear simulations of the STATCOM compensated radial power system using PSCAD are performed to validate the outcomes from the eigenvalue analysis and estimation of damping torque and synchronizing torque coefficients.

This paper proposes and tests strategies to accomplish high quality incomplete triangular factors (ILU) preconditioners for iteratively solve the load flow sublinear problem. The process begins constructing the preconditioner over the reordered Jacobian matrix calculated in the first Newton iteration. If the iterative process does not start due to high inaccuracies and/or numerical stability problems emerged during the preconditioner construction, the solution process is restarted and the Jacobian matrix calculated in the first Newton iteration is preprocessed through scaling, nonsymmetric and symmetric (reordered) permutations, leading to a new and better quality preconditioner. Numerical experiments considering two Brazilian power system configurations (2256/3515 buses) operating under adverse conditions (heavy loaded) corroborate the efficiency of such strategies over the preconditioner quality improving the robustness of the iterative solution process. A LU preconditioner and a direct solver for comparison purposes are also considered.

In this paper, a novel idea is presented to apply off-the-shelf commercial protective relays, widely used to protect induction motors against different electrical and thermal faults, to detect mechanical faults as a complementary capability. In other words, the available protective relays can be configured to protect induction motors against mechanical abnormalities in addition to their normal duty to detect electrical and thermal faults. To support the idea, the current and vibration signals are generated by sophisticated modeling of the motor for different faults and then the simulated signals are injected to a numerical relay simulator designed in full detail to resemble a commercial relay. It is shown that some mechanical faults like broken rotor bars and shorted stator windings can be diagnosed by the relay using the Motor Current Signature Analysis (MCSA). Other mechanical abnormalities harder to detect by MCSA would be diagnosed by using vibration analysis as a complementary tool to MCSA. In the latter case, an extra input (vibration) is needed to be delivered to the relay. Simulation results verify the presented analysis used for modeling healthy/faulty motor and the designed relay simulator.

This paper proposes an improved state-variable internal model based digital voltage controller suitable for three-phase PWM inverters with output transformer for medium and high power uninterruptible power supplies (UPS). The proposed controller is derived from the internal model principle using a state-variable approach in stationary alfa-beta frame. For the controller design an improved discrete-time model that uses the average of two samples in a switching period is proposed. In addition, this model takes into account the digital implementation time delay. With the proposed controller it is possible to obtain a high performance in both steady-state and load transients. Also, with the selected internal model, dc components resulting from the circuit implementation non-idealities are not amplified, avoiding transformer saturation. To validate the proposed digital controller and to demonstrate the steady-state and transient performance, experimental results from a 10kVA space vector modulated three-phase inverter, fully controlled by a DSP TMS320F241, are presented. In steady-state the UPS output voltages have very low total harmonic distortion (THD) for both balanced and unbalanced non-linear loads. The output voltage dynamic performance complies with the international Standard IEC 62040-3 Classification 1, which is the most severe limit making the three-phase PWM inverter suitable for most types of critical loads.

The digital control of three-level voltage source inverter fed high power high performance ac drives has recently become a popular in industrial applications. In order to control such drives, the pulse width modulation algorithm needs to be implemented in the controller. In this paper, synchronized symmetrical bus-clamping pulse width modulation strategies are presented. These strategies have some practical advantages such as reduced average switching frequency, easy digital implementation, reduced switching losses and improved output voltage quality compared to conventional space vector pulse width modulation strategies. The operation of three level inverter in linear region is extended to overmodulation region. The performance is analyzed in terms THD and fundamental output voltage waveforms and is compared with conventional space vector PWM strategies and found that switching losses can be minimized using bus-clamping strategy compared to conventional space vector strategy. The proposed method is implemented using Motorola Power PC 8240 processor and verified on a constant v/f induction motor drive fed from IGBT based inverter.