Saurabh Tewari

A Statistical Model for Wind Power Forecast Error and its Application to the Estimation of Penalties in Liberalized Markets

The problem of accurately forecasting wind energy has garnered a great deal of attention in recent years. There are always some errors associated with any forecasting methodology. Although it is sometimes assumed that the forecast errors are Normally distributed, it is a special case arising from the geographical dispersion of wind resources, as shown in this paper. The distribution of the forecast error needs to be examined individually for every wind farm to determine the impact of this error on trading energy in electricity markets. This paper addresses the problem of modeling the distribution of the forecast errors associated with Persistence forecasts at the level of a single wind farm, and develops a novel, mixed distribution-based model to approximate the distribution of these errors. The model is then used to estimate the penalties for imperfectly forecast energy injections in the short-term markets. The results from the application of this model to trading are further used to assess the feasibility of energy storage in hedging against imperfect forecasts.

Value of NAS Energy Storage Toward Integrating Wind: Results From the Wind to Battery Project

Energy storage is a natural option toward adding the much-needed flexibility for integrating higher amounts of wind generation into the existing power system. This paper presents field results and analyses quantifying the ability and the value of Sodium Sulfur (NAS) battery energy storage toward shifting wind generation from off-peak to on-peak, limiting the ramp rate of wind farm output, and a strategy to integrate the aforementioned goals. The battery can also be considered on a system level independent of wind. It is argued that even in its capacity as a system resource independent of wind, the battery can aid integration of wind generation. Analyses quantifying the value of the battery in the energy and operating reserve markets are presented. Financially optimal locations to site the battery have also been identified. The presented research is a part of the “Wind to Battery” project led by Xcel Energy, a major U.S. utility.

Prediction of electric power consumption for commercial buildings

Currently many commercial buildings are not continuously monitored for energy consumption, especially small buildings which constitute 90% of all such buildings. However, readily available data from the electric meters can be used for monitoring and analyzing energy consumption. Efficient utilization of available historical data (from these meters) can potentially improve energy efficiency, help to identify common energy wasting problems, and, in the future, enable various Smart Grid programs, such as demand response, real-time pricing etc. This paper describes application of computational intelligence techniques for prediction of electric power consumption. The proposed approach combines regression and clustering methods, in order to improve the prediction accuracy of power consumption, as a function of time (of the day) and temperature, using real-life data from several commercial and government buildings. Empirical comparisons show that the proposed approach provides an improvement over the currently used bin-based method for modeling power consumption.

Three-level indirect matrix converter based open-end winding AC machine drive

A new space vector PWM technique for an indirect matrix converter based open-end winding drive is presented in this paper — The presented drive achieves the advantages of direct conversion, viz. 1) Machine phase voltages up to 1.5 times the grid voltage, 2) controllable grid power factor, and 3) zero instantaneous common mode voltage; and further achieves, 4) lower number of switching transitions and 5) simpler protection clamp circuit, owing to two-stage conversion. A detailed analysis of the PWM technique is presented along with the simulation results. Further, commutation strategies for the matrix converters of the presented drive have been developed and the simulation results are included.

A new sinusoidal input-output three-phase full-bridge direct power converter

A direct power converter based variable frequency drive with three-level input/output waveforms is presented. The proposed drive generates common-mode free output voltages while drawing sinusoidal input currents. Like the conventional matrix converter, the input and output quantities are controlled simultaneously with one controller. Unlike the conventional matrix converter, the circuit always provides a freewheeling path for the load and source currents, thus eliminating the clamp circuit. Further, zero voltage switching (ZVS) of the front-end is naturally achieved. Functional operation of the converter is demonstrated using a hardware prototype. The drive is evaluated for the output common-mode voltage as well as its performance to load and source transients. Extension of the presented drive to an open-end winding drive with enhanced voltage gain is also discussed in context of previous research.

A compact active filter to eliminate common-mode voltage in a SiC-based motor drive

This paper presents an active compensation device for common-mode (CM) voltage elimination in 3-phase space-vector pulse-width-modulated (SVPWM) inverters. The proposed device consists of a single-phase 2-level inverter (H-bridge) which supplies a compensating voltage to the inverter via a step-up common-mode transformer tied to all three phases at the output. The H-bridge active filter is supplied by a low voltage bus and switched several orders of magnitude faster than the inverter switching frequency. This device takes advantage of the direct knowledge of the switching pulses sent to the inverter to predict and generate the compensating voltage. A technique is employed to subtract the low frequency harmonics from the modulation of the H-bridge which allows for the size of the common-mode transformer to be reduced significantly. Small passive components are added to attenuate the active filters PWM frequency content and thus produce an effective compensating voltage. This paper will review existing common-mode voltage compensation techniques and demonstrate that the proposed method is a logical choice for certain drive applications. Design considerations are included to provide understanding and guidance for implementation of the device, as well as MATLAB/Simulink simulation results to demonstrate the operation of the active compensation device. Final validation is presented through experimental results from a hardware prototype.

Indirect matrix converter based open-end winding AC drives with zero common-mode voltage

Common-mode voltage (CMV) generated by semiconductor switching causes stray currents and mechanical failure in modern drive systems. Solutions employed to attenuate or isolate the common-mode voltage (CMV) require additional components, and may still fail to eliminate the detrimental effects. Matrix converter based open-end winding drives, when modulated using synchronous vectors, do not generate CMV to begin with. Additionally, these drives do not rely upon a large DC capacitor that is used in the state-of-the-art systems; and are therefore expected to be more compact and reliable. This paper will present prototypes of two distinct indirect matrix converter based open-end winding drives that eliminate output common-mode voltage, provide high voltage transfer ratio (up to 1.5), and allow input power factor control. These indirect drives have the additional advantages of clamp circuit elimination, lower voltage stress on the devices, naturally intelligent commutation, and natural low-voltage ride-through integration over their direct matrix converter counterpart. Experimental evidence of the voltage transfer ratio and input power factor control will be provided. Compared to 2-level and 3-level inverters, significant reduction in the CMV induced shaft voltage and ground currents will be shown. An optimal third-order grid filter applicable to all matrix converter based drives will also be discussed. This filter will be used with the presented drives to validate its superior performance.

Analytical evaluation of the peak-to-peak ripple current in the filter inductor for a space vector modulated grid-tied VSI

Two level voltage source inverters are extensively employed to integrate renewable energy sources and storage with the grid. The output voltage of the inverter, modulated with conventional space vector PWM, contains switching frequency components along with the desired grid frequency component. Usually a filter inductor or an LCL filter is used to remove these high frequency components resulting in low THD in the grid current. Either all, or a majority of the higher frequency components of the inverter output voltage appear across the inverter-side filter inductance, resulting in ripple current through it. This paper presents an analytical estimation of the peak-to-peak ripple current as a function of the modulation index, essential for the design of the filter. Simulation results confirm the analytical prediction of the ripple current.

Circuit-level characterization and loss modeling of SiC-based power electronic converters

This paper presents the design, characterization, and modeling of a power electronic converter based around Silicon Carbide (SiC) MOSFETs. A practical characterization procedure is proposed which takes a circuit-level approach, as opposed to a device-level approach, using only the power electronic circuit and no additional test circuitry. The converter circuit is general enough that it can represent a dc chopper circuit or an output phase of an inverter. The design of the converter, including the SiC-specific gate drive circuit, is described. The hardware setup was operated at frequencies up to 200 kHz and efficiencies up to approximately 99% were recorded. A model for predicting converter and driver losses at different load currents, dc bus voltages, and operating temperatures was constructed; the predictions from the model were in good agreement with the measurements.

The potential of sodium sulfur battery energy storage to enable further integration of wind

University of Minnesota M.S. thesis. February 2012. Major: Electrical Engineering. Advisor: Prof. Ned Mohan. 1 computer file (PDF); viii, 103 pages, appendix A.