Tarik Abdulahovic

Dynamic Performance Comparison of Synchronous Condenser and SVC

In this paper, a comparison of the dynamic performance between a conventional synchronous condenser, a superconducting synchronous condenser, and a static Var compensator (SVC) is made in a grid setup by simulating different cases that affect the performance of reactive power compensation. The results show that the SVC injects more reactive power and has a better dynamic performance during faults that cause a moderate or minor voltage drop on its terminals, such as single-phase to ground faults in weak grids. The synchronous condensers, on the other hand, bring the voltage to the nominal value quicker and show a better dynamic performance for severe faults such as three phase to ground faults in stiff grids. The superconducting synchronous condenser injects up to 45% more reactive power compared to the conventional synchronous condenser during a nearby three phase to ground fault.

An FPGA implementation of a voltage-oriented controlled three-phase PWM boost rectifier

Voltage-oriented control (VOC) of a three-phase PWM rectifier is one of the widely used structures in active converters. Design, simulation results, an FPGA implementation, and practical results of a VOC based three-phase PWM boost rectifier are presented and explained in this paper. National Instruments CompactRIO programmable automation controller platform is utilized in this application. The developed software and implemented hardware based on this controller are presented for this application.

Comparative Study of a Multi-MW High-Power Density DC Transformer With an Optimized High-Frequency Magnetics in All-DC Offshore Wind Farm

In this paper, the design of a 1/30 kV, 10 MW modular isolated dc-dc converter is presented. The design and optimization of a high-frequency transformer as the key part of such a converter is addressed. Efficiency curves for different semiconductors and frequencies are presented in order to find an optimum frequency-enabling adequate transformer volume reduction with a suitable compromise with the converter efficiency. It was found that for this voltage level and size, it is no point to go above 6 kHz from the transformer perspective since the isolation requirement that leads to that the size is not reduced much more for even higher frequencies. At 5 kHz, the efficiency of the transformer having a nanocrystalline core reached 99.7% while the power density was about 22 kW/l. For the entire dc-dc converter, an efficiency of 98.5% was reached at 5 kHz switching frequency using SiC metal-oxide semiconductor field-effect transistors as switching elements while the efficiency when using insulated-gate bipolar transistors reached 97.2% at the same frequency.

Vacuum Circuit-Breaker Parameter Calculation and Modelling for Power System Transient Studies

In this paper, a black-box vacuum circuit-breaker model is developed and its parameters are calculated. The developed model is deterministic and does not include the stochastic behavior of arc interruption. The phenomena, such as the current chopping, high-frequency current quenching capability, and rate of rise of dielectric strength are treated as deterministic and calculated using mean values. The vacuum breaker model is verified using a laboratory test setup, and simulation results show good agreement with the obtained measurements. The withstand voltage of the breaker during appearance of the voltage restrikes shows good agreement with the measurements. Moreover, the magnitude and repetitiveness of the high-frequency transients obtained in simulations are within a 10% margin when compared with measurements.

Simulation accuracy of the built‐in PSCAD and an owner‐defined synchronous machine model

Purpose - The purpose of this paper is to investigate the influence of time steps, integration methods, and saturation modeling on the accuracy of the synchronous machine model. This model is compared with the PSCAD built-in synchronous machine model in order to compare the accuracy of one of the most used synchronous machine models in a commercially available software versus a well-documented and widely accepted state-space synchronous machine model. Design/methodology/approach - In the paper, a synchronous condenser with the saturation phenomenon is modeled using state-space equations in the rotating dq-reference frame and is implemented both in Matlab/Simulink and PSCAD. Integration methods of up to the fifth order are implemented for increased accuracy. The saturation modeling includes modeling of the saturation in both d- and q-axis. A steady-state and dynamic performance comparison towards the built-in PSCAD synchronous machine model is performed. The saturation modeling does not include the saturation of the leakage fluxes. Findings - When the forward Euler method is used, in order to obtain less than 5 percent error, the time step should not exceed 5 its. The third-order Runge-Kutta method is the preferred choice and it provides desired accuracy when the time step is equal or smaller than 1,000 its. The built-in PSCAD model satisfies the error criteria for time steps smaller than 300 its. A small discrepancy of 2 percent is found during the steady-state test. Originality/value - The paper presents the performance of the higher order integration methods in an EMTP-type software environment where the trapezoidal integration method is most often used. It provides a good guide for building an owner-defined model. A comparison of a dynamic performance between the publicly documented state-space and a synchronous machine models commonly used for power system transient studies is presented.

Voltage Stress in a Transformer Winding During Very Fast Transients Caused by Breaker Closing Event

Transformers connected at the medium-voltage level in cable grids, such as wind park collection grids and industrial grids, are exposed to the stress of very fast transients. These electric transients are mainly generated during breaker switching operations and the rise time of the transient voltage in such systems is much shorter compared to the rise times of transients generated in transmission systems at a high-voltage level. In this paper, the internal voltage stress is studied during very fast transients generated during transformer energization. Instead of using a breaker, the energizing tests are performed using a low-impedance pulse generator that can generate lightning impulse-shaped waveforms and voltage steps with rise times varying between 35 and 500 ns. Experiments show that during very fast transients with a 35-ns rise time and 1-p.u. magnitude, the interturn voltage exceeds the level obtained with a lightning impulse-shaped voltage waveform of 4.4 pu. Furthermore, during a specific switching scenario with delta-connected transformers, where the winding is excited from both ends, the same 1-p.u./35-ns voltage step generates an interturn voltage that exceeds the 1-p.u. level, which is more than 2.5 times higher voltage stress than during a lightning impulse test.

Improvement of admittance measurements for modeling of an ungrounded reactor/transformer winding

In this paper, a model of an ungrounded reactor winding is developed for transient voltage studies. The developed model is a black box model where the state-space model is developed using the directly measured admittance matrix of the reactor winding. Furthermore, an improved model is developed, where the accuracy of the admittance matrix measurement set in the low frequency band is obtained indirectly using an additional set of voltage ratio measurements. When the measured admittances are low, the accuracy of the diagonal admittance vectors in the low frequency band is low as well. However, the accuracy at low frequencies obtained using the indirect measurements is improved at nodes with the lowest admittances. In this paper, different approaches for indirect measurements are studied and the most accurate method is identified considering the physical structure of windings.

Transformers internal voltage stress during current interruption for different wind turbine layouts

The voltage transients generated during the breaker operations in cable systems, for instance, a wind park collection grid, can reach very low rise times. The rise times of these transients can be almost 50 times shorter than the rise time of a lightning pulse. Such transients can generate a very high voltage stress on the internal transformers insulation. In this paper, a test case is studied using verified models of different types of transformers and wind turbine layouts in order to account for typical wind turbine layouts found in modern wind farms. A critical switching scenario is chosen in order to provoke the highest possible voltage transients during a current interruption. Furthermore, internal overvoltages are estimated using model of a similarly sized winding. Simulations show that the magnitude of the voltage transients is higher than the basic lightning impulse insulation level (BIL) defined by present standards. Moreover, it is found that the rise time of the voltage surges is much shorter than the rise time of the lightning pulse. The shortest rise time of 40ns is obtained in a wind turbine layout where the wind turbine breaker is placed near the transformer. Due to very short rise times of the transients, very high internal overvoltages are estimated in dry-type transformer windings. These internal overvoltages are much higher than overvoltages recorded at the basic lightning impulse level. For a wind turbine layout where a breaker is placed in the bottom of a tower and a dry-type transformer in a nacelle, the highest turn-to-turn voltage of about 1.5pu is estimated. This is almost 4 times higher turn-to-turn voltage then the voltage obtained during the BIL test. In a wind turbine layout where a breaker is placed close to the transformer, the amplitude of the turn-to-turn voltages reached 1.8pu due to lower stray capacitances and thus a shorter rise time of voltage strikes.