Rajendra Naik

Analysis and control of phase-shifted series resonant converter operating in discontinuous mode

DC-DC series resonant converter operating in Discontinuous Conduction Mode (DCM) offers advantages in terms of low switching losses, high power density, and low control complexity. The low switching loss feature is lost when the converter operation shifts from DCM to Continuous Conduction Mode (CCM). This paper presents an in-depth analysis of such a converter operating in DCM and derives two boundary conditions under which the converter shifts to CCM. The fixed-frequency phase-shifted pulse width modulation is used to adjust the effective duty cycle and regulate the output voltage. Steady state analysis for above resonant frequency operation is also presented in this paper. The derived boundary conditions aid the design of the converter using which DCM operation and hence the soft switching feature is ensured at all operating points. Limits of these boundary conditions are verified using simulation and experimental results on a prototype converter.

Utility energy storage life degradation estimation method

Energy Storage installations have been used in electric utilities for a few decades now. The primary applications of energy storage in utilities include grid stabilization, back-up power and peak shaving, while other potential applications include arbitrage, reduction in renewable variability and frequency regulation. Selection of the appropriate energy storage system for each of the above applications depends on the power/energy ratio requirement, along with the cost, lifetime and other technical challenges. This paper presents a matrix that evaluates the state-of-art energy storage technologies for various utility applications. The characteristics of energy storage systems required for these applications and challenges inherent in the design of such systems are also summarized in this paper. A specific methodology used for analyzing and optimizing the size of the battery energy storage system for frequency regulation is discussed in detail. The same methodology can be extended to the selection of the most optimum technology for other utility applications.

Design approach for high power, medium voltage power conversion systems for wind turbines

In this paper, the design approach for the power conversion stage of typical medium voltage (3.3kV to 4.16kV), high power (6-10MW) multi-level converters like those used in offshore wind turbine systems is investigated and evaluated. The back-to-back AC-DC-AC multi-level power conversion stage for offshore wind turbines is usually driven by a Permanent Magnet Synchronous Generator (PMSG) for power ratings of the order of 6-10MW. The design approach, primarily relying on a comprehensive simulation model developed, aims to establish the power processing capability of the power conversion stage under the constraints of the thermal limits of the medium voltage power semiconductor devices, the power quality delivered to the grid (for power factor requirements of 0.9 to unity at the grid) and the overall efficiency. Based on the developed controls for the line-side and the generator-side converter, an instantaneous loss model of the semiconductor devices allows estimation of the instantaneous junction temperatures based on thermal models for the devices. Although the paper provides an overview of the different medium voltage power semiconductor devices, the analysis provided focusses primarily on the Integrated Gate Commutated Thyristor (IGCT). The maximum switching frequencies achievable at the power levels of 6-10MW is then determined based on the thermal constraints. Appropriate selection of the filter elements for the determined switching frequencies then enables quantification of the power quality and the overall system efficiency of the power conversion stage. The achievable switching frequencies at the different power levels of the line-side and the generator-side converters is finally established, along with the achievable power quality and efficiencies at the different operating points.

Applying expert systems and fuzzy logic for core selection for high frequency power transformers

A large number of factors go into core selection in the design of high frequency power transformers. An experto-fuzzy system called CoGSA (Core Geometry Selection Aid) has been developed to aid the magnetic designer in selection of a core geometry. The system is based on expert system and fuzzy logic techniques. The various decision components (power, cost, shielding, heating, power density etc.) are taken into account in the form of IF-THEN statements. The fuzzy logic is applied to take into consideration the uncertainty involved with the various factors.<<ETX>>

A novel capacitor voltage balancing method in modular multilevel converters

Modular multilevel converters (MMC) are used in the applications such as medium voltage multi-megawatt drives and in HVDC power transmission. The converter has several sub-modules connected in series that are switched appropriately to regulate the pole voltage. Besides regulation of output voltage, it is also essential to regulate the individual sub-module capacitor voltages. In this paper, a novel modulation scheme for an MMC converter based on carrier phase shifted PWM is proposed. The proposed modulation scheme balances the sub-module capacitor voltages without distortions in output voltage. The scheme combines the best features of the two well-known modulation approaches, namely, carrier-based modulation approach and sub-module insertion approach. Real-time simulation results modeling a single leg MMC converter using a Typhoon HIL system are presented to validate the performance of the proposed modulation scheme.

Cascaded H-bridge control for PV application

In this paper, application of a cascaded H-bridge (CHB) converter to connect strings of solar panels to a single-phase medium voltage AC grid is discussed. With solar insolation levels differing from one string to another, it is challenging to control a CHB converter in order to minimize the variations in the dc link voltages of each of the converter modules and hence the device voltage stress levels. To minimize the device voltage stresses, in this paper, a new Maximum Power Point Tracking (MPPT) method is proposed. The proposed method equalizes the power drawn from each of the solar strings connected to the CHB across a large geographical area (e.g. along a railway line) and avoids excess device stresses. The proposed MPPT method is verified using simulation results.

Control of a series input boost pre-regulator with unbalanced load

Conventional diode bridge rectifiers have poor input power quality due to high harmonic content in input current which leads to low power factor and high total harmonic distortion (THD). In order to extract maximum power from the supply it is advantageous to operate a rectifier at unity power factor with minimal harmonics. Therefore, UPF pre-regulators are used in modern rectifiers. When realizing pre-regulators for higher voltage application, a common strategy is to use multilevel structures which have the advantage of using lower voltage devices to implement higher voltage conversion stages. In this paper a series input boost power factor per-regulator is used to implement an AC-DC rectifier. As the number of series stages increases, the problem of un-equal loading at the output of each boost pre-regulator becomes an issue. A weighed average current mode control is proposed which maintains equal voltage at the output of the pre-regulator even under unbalanced loading conditions. The proposed control is verified using simulation and a scale down laboratory prototype.