Mahsa Ghapandar Kashani

SVC and STATCOM application in Electric Arc Furnace efficiency improvement

Electric Arc Furnaces (EAF) are high power industrial loads which cause power quality problems at all voltage levels due to their unbalanced and nonlinear characteristics. The rapid, stochastic large swings in real and reactive power required by the arc furnace causes voltage drops, rapid voltage variation and distortion across the ac supply network. These voltage drops and fluctuations not only have negative impact on the power system quality and other loads, but also have an effect on the arc furnace operation, power output and efficiency. Hence, some sort of reactive compensation is required to limit the voltage disturbances injected by arc furnace into the electric power system. In this paper, an accurate electric arc furnace model, whose parameters have been set according to a 80 MVA actual arc furnace, is studied. A Static VAR Compensator (SVC) is simulated in PSCad and Real Time Digital Simulation (RTDS)/RSCAD platform for the purpose of comparison of voltage regulation at EAF bus. It is shown that the SVC mitigates the reactive power fluctuations in addition to providing the fundamental reactive power, and regulates the Point of Common Coupling (PCC) bus voltage precisely during the arc furnace operation. To verify the PSCad simulation results and make a comparison, a real time simulation study based on Real Time Digital Simulation (RTDS)/RSCAD platform has been performed in this case. On the other hand, a 80 MVA static synchronous compensator (STATCOM) is simulated in PSCad. It is illustrated that the SVC is inherently limited in its ability to respond rapidly to the fluctuating arc furnace load. It is found that the transient performance of the EAF voltage in case which equipped with the STATCOM is better than the case equipped with SVC. It is also demonstrated that although the voltage regulation by the SVC compensates a portion of the reactive power fluctuation, it is completely unable to supply any portion of the fluctuating real power drawn by the arc furnace, while the STATCOM can supply those components of active and reactive power fluctuation. The STATCOM will not normally have a source of real power connected to its DC terminals. It is therefore unable to supply sustained real power or real power fluctuations. With suitable choice of DC capacitor, however, it is capable of supplying in large part the fluctuating real power requirement of the furnace.

Variable interleaving technique for photovoltaic cascaded DC-DC converters

This paper introduces a variable interleaving technique for photovoltaic cascaded DC-DC converters. A series rather than parallel connection of converters allows higher switch utilization and lower rating of components; however, they suffer from nonhomogeneous irradiations condition. Under partial shading conditions, the input power of all PV panel will not be the same and as a result the output voltage of each converter will not be identical. This causes system to operate in an asymmetric condition, in which, the conventional interleaving techniques are not capable of eliminating the DC link output voltage variations. In this work, an interleaving algorithm is reported on cascaded DC/DC converters under asymmetric condition to minimize the DC link output voltage variations. The effectiveness of this algorithm for cascaded DC-DC converters has been validated by simulation and Hardware-In-the-Loop tests.

Comparative study of DC circuit breakers using realtime simulations

One of the main limitations in using high voltage and medium voltage DC grids is the issues related to the current-limiting devices and circuit breakers. These devices should be able to handle large currents in the normal operation condition and block high DC currents in a few microseconds in the case of DC faults. Moreover, they are required to dissipate huge amounts of energy stored in the transmission line inductances. In this paper, four configurations of DC circuit breakers from three general groups are evaluated and the results are compared in terms of the time required by the breakers to interrupt the current, maximum DC breaking current, rated voltage, efficiency and current state of development. The performance of DC circuit breakers is evaluated when a line to ground DC fault happens in a 9 module/2 terminal DC system. Simulation results were obtained using Real Time Digital Simulator (RTDS) and PSCAD.

A novel control approach for protection of multi-terminal VSC based HVDC transmission system against DC faults

Overall performance of the voltage source converters (VSCs) has improved during the recent years. Improvement of the VSCs along with the attractive features of the VSC based high voltage direct current (HVDC) transmission systems over the thyristor based HVDC transmission systems make it possible to build multiterminal (MT) VSC HVDC transmission systems. However, the VSCs are vulnerable against dc side faults and a method needs to be employed to extinguish the dc fault current. In this paper, three different configurations of solid state dc circuit breakers (CB) for protection purposes are studied. Moreover, a new control method to protect the VSCs against the dc side fault is proposed, the new method makes it possible to extinguish the dc fault current with the existing ac breakers on the ac side or with the lower rating solid state (SS) DCCBs. The performance of the SS DCCBs and the proposed method are studied using Real Time Digital Simulator (RTDS).

Control of modular dual active bridge DC/DC converter for photovoltaic integration

The DC transmission system provides a cost effective solution for long distance power transmission compared to the AC transmission system. Hence, this has increased the emphasis on the development of the DC transmission system. Development of power converter with modular structure has now made it possible to achieve higher voltage and power level. This opens the possibility for further development of a multi-terminal DC grid. Now once the DC grid system has been formed, it is also important to include more renewable energy sources directly to the DC grid. Therefore, a power conversion stage is required to condition the available power from a source to the grid. This paper shows the operation and control of such a kind of converter system which integrates the solar cell to the DC grid directly. The paper mainly focuses on control of the series connected DAB that have been integrated to HVDC power network. In order to deliver power in HVDC system, the total number of DABs must be high enough to achieve the DC link voltage. The control in that case must be a combination of current and voltage control. In order to validate the proposed control, complete system has been implemented on Opal-RT™ and hardware in the loop (HIL) using external controller has also been implemented to show the system operation.

Instantaneous fault current limiter for PWM-controlled Voltage Source Converters

The PWM-controlled Voltage Source Converters (VSCs) are commonly used in industrial and utility applications. In spite of superior features of fast voltage regulation and stable DC-link voltage, PWM-controlled VSCs have the major drawback of being sensitive to the grid disturbances especially the unbalanced conditions and system faults. Unbalanced input voltage generates large negative sequence current flow into the converter which results in oscillations with twice the line frequency on the DC-link voltage. This negative sequence current flow might damage the semiconductor switches. Beside the negative sequence voltage, the input voltage distorted with other harmonics also causes converter performance deterioration by producing harmonics on the DC-link voltage. This paper presents an alternative solution to improve the PWM-controlled VSC performance under unbalanced conditions and system faults and also under distorted input voltage condition caused by other harmonics rather than the negative sequence voltage. This solution is based on direct calculation of the negative sequence (or other harmonics) reference voltage without using any current regulator. This elimination of the current regulator makes the proposed controller very fast and robust. The effectiveness of this solution has been validated by simulation and Hardware-In-the-Loop test.

Autonomous Inverter Voltage Regulation in a Low Voltage Distribution Network

Inverter voltage control techniques, including Volt-Watt and Volt-VAR, have been developed to support higher penetration integration of photovoltaic (PV) generation. These techniques typically focus on voltage regulation as measured at the point of common coupling (PCC). Implementing voltage control with distributed inverters within a low voltage network is challenging due to voltage rise between the PCC and the electrical connection point (ECP). This paper proposes a voltage correction and control method for distributed PV microinverters in a low voltage network by utilizing readily available data measurements, i.e., voltage and power at the ECP of inverters. It is shown that this method could reduce unnecessary PV microinverter tripping and power curtailment while supporting voltage control schemes at the PCC. Test results are provided from simulation-only scenarios and a power-hardware-in-the-loop test platform.

Design consideration of volt-VAR controllers in distribution systems with multiple PV inverters

Advanced control techniques such as Volt-VAR Control (VVC) are required for integration of multiple distributed renewable energy, such as Photovoltaic (PV) resources, on an electric distribution system. However, undesired interactions have been observed among these Volt-VAR controlled PV inverters which leads to oscillation and instability of the system. In this paper, an analytical approach to study the stability of local voltage control in high PV penetrated distribution systems with advanced Volt-VAR control functions is employed. The transient of inverter Volt-VAR Control interactions and dynamics of the interconnected feed-back loops in the distribution circuits are investigated. It is shown analytically that the Grid impedances, droop slope, PI controller parameters, response time and delay time in the VVC are the main factors affecting the dynamic response of the system, and the absence of a standard selection criteria for inverter and controller parameters under different Grid impedances results in undesired potential interactions among the PV inverters and distribution power system.

Operation of hybrid multi-terminal DC system under normal and DC fault operating conditions

Recently, multi-terminal DC (MTDC) system has received more attention in the power transmission areas. Development of modular structured power converter topologies has now enabled the power converter technology to attain high voltage high power ratings. Compared to current source converter technology, voltage source converters have several benefits including higher power quality, independent control of active and reactive power etc. This paper focuses on a unique MTDC system consisting of terminals with different converter topologies especially considering the fact that each of the terminals may be manufactured by different vendors. In this particular configuration, the MTDC system consists of four terminals namely two advanced modular multi-level converter with high frequency isolation, one standard modular multi-level converter (MMC) with half bridge sub modules and the fourth terminal is modular DC-DC converter which integrates PV along with a Battery energy storage system with the DC grid directly. This paper presents a system level study of hybrid MTDC System. Also the DC fault contingency case has been explored thoroughly. An algorithm has been proposed to prevent the system damage. All the cases have been demonstrated with the PSCAD simulation results. To show the system practically works in real time, the system is also evaluated in a unique real time platform, consisting of interconnected RTDS and OPAL RT systems.

Design considerations and test setup assessment for power hardware in the loop testing

Power Hardware in the Loop (PHIL) simulations have recently been developed as a substitution for traditional methods of testing and analyzing real and physical electric power apparatus, especially power electronic based devices. However, due to some inherent characteristics of the PHIL test setup — such as use of power amplifiers, software/hardware interfaces, and real time simulations — there are certain operating constraints and implementation challenges that need to be considered and incorporated in the design. This work provides an overview of design and implementation of a PHIL platform for electrical power system testing. A frequency-domain stability analysis and a time-domain accuracy assessment of several PHIL test setups using different approaches have been presented. PHIL simulation and experimental results have been provided to demonstrate the effectiveness and functionality of each approach.