Proportional resonant controller based circulating power setup for thermal testing of multilevel inverter

A regenerative or circulating-power method is presented in this paper for heat run test on the legs of a three-level neutral point clamped (NPC) inverter. This test ensures that only losses are drawn from the dc supply, while rated power is circulated between the two legs, thus minimising wastage of energy. A proportional-resonant (PR) controller based current control scheme is proposed here for the circulating power test setup in NPC inverter. Simulation and experimental results are presented to validate the controller design at various operating conditions. Results of thermal test on the inverter legs are presented at two different operating conditions.

Physical Layer Data Fusion Via Distributed Co-Phasing With General Signal Constellations

This paper studies a pilot-assisted physical layer data fusion technique known as Distributed Co-Phasing (DCP). In this two-phase scheme, the sensors first estimate the channel to the fusion center (FC) using pilots sent by the latter; and then they simultaneously transmit their common data by pre-rotating them by the estimated channel phase, thereby achieving physical layer data fusion. First, by analyzing the symmetric mutual information of the system, it is shown that the use of higher order constellations (HOC) can improve the throughput of DCP compared to the binary signaling considered heretofore. Using an HOC in the DCP setting requires the estimation of the composite DCP channel at the FC for data decoding. To this end, two blind algorithms are proposed: 1) power method, and 2) modified K-means algorithm. The latter algorithm is shown to be computationally efficient and converges significantly faster than the conventional K-means algorithm. Analytical expressions for the probability of error are derived, and it is found that even at moderate to low SNRs, the modified K-means algorithm achieves a probability of error comparable to that achievable with a perfect channel estimate at the FC, while requiring no pilot symbols to be transmitted from the sensor nodes. Also, the problem of signal corruption due to imperfect DCP is investigated, and constellation shaping to minimize the probability of signal corruption is proposed and analyzed. The analysis is validated, and the promising performance of DCP for energy-efficient physical layer data fusion is illustrated, using Monte Carlo simulations.

A method of tracking the peak power points for a variable speed wind energy conversion system

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-omega curve correspond to dP/domega = 0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a dc motor fed from a commercial dc drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.

Apparatus and method for heat-run test on high-power PWM converters with low energy expenditure

Before installation, a voltage source converter is usually subjected to heat-run test to verify its thermal design and performance under load. For heat-run test, the converter needs to be operated at rated voltage and rated current for a substantial length of time. Hence, such tests consume huge amount of energy in case of high-power converters. Also, the capacities of the source and loads available in the research and development (R&D) centre or the production facility could be inadequate to conduct such tests. This paper proposes a method to conduct heat-run tests on high-power, pulse width modulated (PWM) converters with low energy consumption. The experimental set-up consists of the converter under test and another converter (of similar or higher rating), both connected in parallel on the ac side and open on the dc side. Vector-control or synchronous reference frame control is employed to control the converters such that one draws certain amount of reactive power and the other supplies the same; only the system losses are drawn from the mains. The performance of the controller is validated through simulation and experiments. Experimental results, pertaining to heat-run tests on a high-power PWM converter, are presented at power levels of 25 kVA to 150 kVA.

Thermal test method for high power three-phase grid-connected inverters

Semiconductor device junction temperatures are maintained within datasheet specified limits to avoid failure in power converters. Burn-in tests are used to ensure this. In inverters, thermal time constants can be large and burn-in tests are required to be performed over long durations of time. At higher power levels, besides increased production cost, the testing requires sources and loads that can handle high power. In this study, a novel method to test a high power three-phase grid-connected inverter is proposed. The method eliminates the need for high power sources and loads. Only energy corresponding to the losses is consumed. The test is done by circulating rated current within the three legs of the inverter. All the phase legs being loaded, the method can be used to test the inverter in both cases of a common or independent cooling arrangement for the inverter phase legs. Further, the method can be used with different inverter configurations - three- or four-wire and for different pulse width modulation (PWM) techniques. The method has been experimentally validated on a 24 kVA inverter for a four-wire configuration that uses sine-triangle PWM and a three-wire configuration that uses conventional space vector PWM.

Optimum Evaluation of Reactive Power Requirements in Power Distribution-Systems

This paper presents three methodologies for determining optimum locations and magnitudes of reactive power compensation in power distribution systems. Method I and Method II are suitable for complex distribution systems with a combination of both radial and ring-main feeders and having different voltage levels. Method III is suitable for low-tension single voltage level radial feeders. Method I is based on an iterative scheme with successive powerflow analyses, with formulation and solution of the optimization problem using linear programming. Method II and Method III are essentially based on the steady state performance of distribution systems. These methods are simple to implement and yield satisfactory results comparable with the results of Method I. The proposed methods have been applied to a few distribution systems, and results obtained for two typical systems are presented for illustration purposes.

A Current Source Inverter Fed Induction Motor Drive with Power Factor Angle Control Using Microprocessor

This paper describes a method of adjusting the stator power factor angle for the control of an induction motor fed from a current source inverter (CSI) based on the concept of space vectors (or park vectors). It is shown that under steady state, if the torque angle is kept constant over the entire operating range, it has the advantage of keeping the slip frequency constant. This can be utilized to dispose of the speed feedback and simplify the control scheme for the drive, such that the stator voltage integral zero crossings alone can be used as a feedback for deciding the triggering instants of the CSI thyristors under stable operation of the system. A closed-loop control strategy is developed for the drive based on this principle, using a microprocessor-based control system and is implemented on a laboratory prototype CSI fed induction motor drive.

An adaptive power-system stabiliser which cancels the negative damping torque of a synchronous generator

The paper presents a new criterion for designing a power-system stabiliser, which is that it should cancel the negative damping torque inherent in a synchronous generator and automatic voltage regulator. The method arises from analysis based on the properties of tensor invariance, but it is easily implemented, and leads to the design of an adaptive controller. Extensive computations and simulation have been performed, and laboratory tests have been conducted on a computer-controlled micromachine system. Results are presented illustrating the effectiveness of the adaptive stabiliser.

Evaluation of Charges for Power Transmission and Losses in Bilateral Power Contracts

The paper presents a method for transmission loss charge allocation in deregulated power systems based on Relative Electrical Distance (RED) concept. Based on RED between the generator and load nodes and the predefined bilateral power contracts, charge evaluation is carried out. Generally through some power exchange mechanism a set of bilateral contracts are determined that facilitate bilateral agreements between the generation and distribution entities. In this paper the possible charges incurred in meeting loads like generation charge, transmission charge and charge due to losses are evaluated. Case studies have been carried out on a few practical equivalent systems. Due to space limitation results for a sample 5 bus system are presented considering ideal load/generation power contracts and deviated load/generation power contracts. Extensive numerical testing indicates that the proposed allocation scheme produces loss allocations that are appropriate and that behave in a physically reasonable manner.

Theoretical Maximum Limits on Power-Handling Capacity of HVDC Cables

In the case of an ac cable, power transmission is limited by the length of the cable due to the capacitive reactive current component. It is well known that high-voltage direct current (HVDC) cables do not have such limitations. However, insulation-related thermal problems pose a limitation on the power capability of HVDC cables. The author presents a viable theoretical development, a logical extension to Whitehead's theory on thermal limitations of the insulation. The computation of the maximum power-carrying capability of HVDC cables subject to limits on the maximum operable temperature of the insulation is presented. The limitation on the power-carrying capability is closely associated with the electrothermal insulation failure. The effect of environmental interaction by way of external thermal resistance, an important aspect, is also considered in the formulations. The Lagrange multiplier method has been used to handle the ensuing optimization problem. The theory is based on an accepted theory of thermal breakdown in insulation and is an important and a coherent extension of great significance.

A Modified Heffron-Phillip's Model for The Design of Power System Stabilizers

In this paper a modified Heffron-Phillip's (K-constant) model is derived for the design of power system stabilizers. A knowledge of external system parameters, such as equivalent infinite bus voltage and external impedances or their equivalent estimated values is required for designing a conventional power system stabilizer. In the proposed method, information available at the secondary bus of the step-up transformer is used to set up a modified Heffron-Phillip's (ModHP) model. The PSS design based on this model utilizes signals available within the generating station. The efficacy of the proposed design technique and the performance of the stabilizer has been evaluated over a range of operating and system conditions. The simulation results have shown that the performance of the proposed stabilizer is comparable to that could be obtained by conventional design but without the need for the estimation and computation of external system parameters. The proposed design is thus well suited for practical applications to power system stabilization, including possibly the multi-machine applications where accurate system information is not readily available.

Negative sequence compensation within fundamental positive sequence reference frame for a stiff micro-grid generation in a wind power system using slip ring induction machine

An isolated wind power generation scheme using slip ring induction machine (SRIM) is proposed. The proposed scheme maintains constant load voltage and frequency irrespective of the wind speed or load variation. The power circuit consists of two back-to-back connected inverters with a common dc link, where one inverter is directly connected to the rotor side of SRIM and the other inverter is connected to the stator side of the SRIM through LC filter. Developing a negative sequence compensation method to ensure that, even under the presence of unbalanced load, the generator experiences almost balanced three-phase current and most of the unbalanced current is directed through the stator side converter is the focus here. The SRIM controller varies the speed of the generator with variation in the wind speed to extract maximum power. The difference of the generated power and the load power is either stored in or extracted from a battery bank, which is interfaced to the common dc link through a multiphase bidirectional fly-back dc-dc converter. The SRIM control scheme, maximum power point extraction algorithm and the fly-back converter topology are incorporated from available literature. The proposed scheme is both simulated and experimentally verified.

Application of artificial intelligence for the assessment of the status of power transformers

The Dissolved Gas Analysis (DGA) a non destructive test procedure, has been in vogue for a long time now, for assessing the status of power and related transformers in service. An early indication of likely internal faults that may exist in Transformers has been seen to be revealed, to a reasonable degree of accuracy by the DGA. The data acquisition and subsequent analysis needs an expert in the concerned area to accurately assess the condition of the equipment. Since the presence of the expert is not always guaranteed, it is incumbent on the part of the power utilities to requisition a well planned and reliable artificial expert system to replace, at least in part, an expert. This paper presents the application of Ordered Ant Mner (OAM) classifier for the prediction of involved fault. Secondly, the paper also attempts to estimate the remaining life of the power transformer as an extension to the elapsed life estimation method suggested in the literature.

Thermoelectric Power Under Strong Magnetic Field in Quantum Dots and Quantized Superlattices: Simplified Theory and Relative Comparison

In this paper, we study the thermoelectric power under strong magnetic field (TPSM) in quantum dots (QDs) of nonlinear optical, III-V, II-VI, GaP, Ge, Te, Graphite, PtSb2, zerogap, Lead Germanium Telluride, GaSb, stressed materials, Bismuth, IV-VI, II-V, Zinc and Cadmium diphosphides, Bi2Te3 and Antimony respectively. The TPSM in III-V, II-VI, IV-VI, HgTe/CdTe quantum well superlattices with graded interfaces and effective mass superlattices of the same materials together with the quantum dots of aforementioned superlattices have also been investigated in this context on the basis of respective carrier dispersion laws. It has been found that the TPSM for the said quantum dots oscillates with increasing thickness and decreases with increasing electron concentration in various manners and oscillates with film thickness, inverse quantizing magnetic field and impurity concentration for all types of superlattices with two entirely different signatures of quantization as appropriate in respective cases of the aforementioned quantized structures. The well known expression of the TPSM for wide-gap materials has been obtained as special case for our generalized analysis under certain limiting condition, and this compatibility is an indirect test of our generalized formalism. Besides, we have suggested the experimental method of determining the carrier contribution to elastic constants for nanostructured materials having arbitrary dispersion laws.