Saman Babaei

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.

Convertible Static Compensator (CSC) performance under system fault

Convertible Static Compensator (CSC) which is a versatile FACTS technology has been installed on Marcy 345 kV substation and increases power transfer capability, operational functionality, and power flow controllability of the New York Power Authority (NYPA) transmission system. This paper presents a detailed description of CSC inverter operation and PSCAD simulation results of the CSC when it is operating as a STATCOM. The STATCOM steady state and dynamic operation simulation results under different conditions including unbalanced condition and faults is discussed. For fault condition analysis, attention is focused to Double L-G (LLG) fault at three different locations, one very close and two others electrically far from STATCOM terminals. This paper also demonstrates the effectiveness of the emergency PWM (EPWM) strategy a solution for limiting the poles current and preventing STATCOM over current tripping during fault conditions.

Dual Angle Control for Line-Frequency-Switched Static Synchronous Compensators Under System Faults

Voltage-sourced converter (VSC)-based static synchronous compensators (STATCOMs) are used for voltage regulation in transmission and distribution systems. Unlike PWM-controlled STATCOMs, angle-controlled STATCOMs are switched at line frequency to limit the system losses. In recent years, angle-controlled STATCOMs have been deployed by utilities for the purpose of transmission system voltage regulation, voltage stability improvement, and increasing operational functionality. Despite the superior feature on voltage waveform quality and efficiency, the practical angle-controlled STATCOMs suffer from the over-current (and trips) and possible saturation of the interfacing transformers caused by negative sequence current during unbalanced conditions and faults in the utility. This paper specifically proposes a control structure to improve the angle-controlled STATCOMs performance under unbalanced conditions and faults. The main improvement is a substantial decrease in the negative sequence current and dc-link voltage oscillations under power system faults by the proposed control. This eliminates the need to redesign the STACOM power components to operate under fault current and dc-link voltage oscillations. The proposed control structure is designed based on adding appropriate oscillations to the conventional angle-controller output that is the control angle by which the VSC voltage vector leads/lags the line voltage vector. Since this control structure uses two angles for controlling the VSC output voltage, it is called dual angle control (DAC). PSCAD/EMTDC and experimental results verify the validity of the proposed control structure under unbalanced system conditions and faults. The experiments were conducted on a transient network analyzer, a unique hardware-based flexible ac transmission system simulator which was designed to study system faults and transients for a 2 × 100 MVA STATCOM field installation.

High-Power VSC-Based Simultaneous Positive- and Negative-Sequence Voltage Regulator

A voltage-source converter-based static synchronous compensator (STATCOM) is used in transmission and distribution systems for the purpose of voltage regulation and reactive power compensation. In the transmission level, angle-controlled STATCOMs are of primary interest due to their high efficiency and excellent waveform quality. There is a considerable number of angle-controlled STATCOMs installed in the different utilities in the U.S. Despite the high efficiency and good voltage quality, this type of STATCOM shows poor performance under ac system faults. They are usually tripped under severe unbalanced condition and system faults to protect the switches from huge negative-sequence current flow. This paper provides a solution to improve the transmission-level STATCOM performance under power system faults. The proposed solution is based on adding a single-phase inverter in series with the converter dc bus. One specific controller is designed which provides the capability of simultaneously controlling positive- and negative-sequence voltages. The results are supported by detailed simulation studies on the New York Power Authority STATCOM model using PSCAD/EMTDC.

Design considerations in development of Active Mobile Substations

While conventional mobile substations are used to bypass the whole substation in case of loss or maintenance of power transformers, Active Mobile Substations (AMS) can be used in normal conditions as a power router and contingencies as a recovery transformer. The AMS is a mobile substation with integrated power electronics and by controlling its throughput power can be connected across different transformers of the grid. The AMS is expected to be at least 20MVA with 230kV and 69kV outputs. This paper proposes transmission-level active mobile substations that provide back-up in case of power transformer failure or forced reduced operation scenarios in addition to power flow control for seasonal renewable energy transmission. These functions altogether have been aggregated not only because of the technical merits but also to address the economic concerns regarding the cost of the power electronics for transmission applications. In this paper, design considerations in development of the AMS will be provided in terms of power electronics building blocks, converter system control and its effects, and required supervisory control. Throughout the paper, theoretical analyses and relevant results are presented.

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.

Series connected IGCT based three-level Neutral Point Clamped voltage source inverter pole for high power converters

This paper reports design and experimental verification of series connected 4.5kV, 4kA IGCT based high power three-level Neutral Point Clamped (NPC) inverter pole. A simplified EMTDC model of a three level NPC inverter pole for different switching transitions based on experimental results is presented which is used for designing of RC snubber circuit for the IGCT switches. Dynamic voltage sharing issues and advantages of series connected 4.5kV, 4kA IGCT for high power 12 MVA three-level NPC inverter pole are investigated. Experimental results are presented for two three level NPC inverter poles operated as H-bridge with three series connected 4.5kV, 4kA IGCT per switch.

A control structure for line-frequency-switched STATCOMs under system faults

Voltage-Sourced Converter (VSC) based Static Synchronous Compensator (STATCOM) is used for voltage regulation in transmission and distribution systems. Comparing with PWM STATCOMs, Angle-controlled STATCOMs are fired at line frequency to lower the system losses. In recent years, angle-controlled STATCOMs have been deployed by utility owners for the purpose of voltage regulation, voltage stability improvement and increasing operational functionality. Despite the superior feature on voltage waveform quality and efficiency, the practical angle-controlled STATCOMs suffer from the over-current (and trips) and possible saturation of the interfacing transformers caused by negative sequence current during unbalanced conditions and system faults. This paper specifically proposes a control structure to improve the angle-controlled STATCOMs performance under unbalanced conditions and system faults. The main improvement is to decrease the negative sequence current and DC-link voltage oscillations substantially under power line faults through the control and not the components design. PSCAD/EMTDC and experimental results verify the validity of the proposed control structure under unbalanced conditions and system faults.

Control of cascaded multi-level STATCOM using line voltage total harmonic distortion minimization technique

In this paper, a new switching strategy is proposed for a multi-level STATCOM system. An efficient approach in reducing the harmonic contents of the inverters output voltage is total harmonic distortion (THD) minimization. In multilevel inverters with fundamental frequency switching strategy (each switch turning on and off once per output cycle), the switching angles can be selected such that the output THD is minimized. In three phase multilevel inverters, the optimization algorithm is commonly applied to the phase voltage of the inverter. This results in the minimum THD in phase voltage, but not necessarily in the line to line minimum THD. In this paper, THD minimization process is directly applied to the line to line voltage of the inverter, and a new control strategy of multilevel STATCOM is proposed. The proposed method will be implemented, in RTDS and the closed loop operation of multi-level STATCOM will be explored.

Analysis of 48-pulse based STATCOM and UPFC performance under balanced and fault conditions

Convertible Static Compensator (CSC) which is a versatile Flexible AC Transmission System (FACTS) technology has been installed on Marcy 345 kV substation and increases power transfer capability, operational functionality, and power flow controllability of the New York Power Authority (NYPA) transmission system. This paper presents a comprehensive description of CSC inverter 48-pulse wave construction along with introduction of control strategy deployed in CSC when it is operating as a STATCOM and UPFC. STATCOM and UPFC steady state and dynamic operation PSCAD simulation results under different testing conditions including unbalanced condition and faults are presented. This paper also provides comprehensive analysis of the STATCOM response to negative sequence and harmonic voltage components on the transmission line during fault conditions. The amounts of DC-link capacitance that lead to maximum and minimum of fundamental negative sequence current on the tie line and 120 Hz oscillation on the DC-link voltage are calculated. Simulation results of STATCOM performance with different DC-link capacitance during fault condition will be shown. At the end of the paper, one exiting problem regarding to UPFC operation of NYPA CSC has been addressed and illustrated with precise simulation results.