Stability synthesis of power hardware-in-the-loop (PHIL) simulation

A virtual power system can be interfaced with a physical system to form a power hardware-in-the-loop (PHIL) simulation. In this scheme, the virtual system can be simulated in a fast parallel processor to provide near real-time outputs, which then can be interfaced to a physical hardware that is called the hardware under test (HuT). Stable operation of the entire system, while maintaining acceptable accuracy, is the main challenge of a PHIL simulation. In this paper, after an extended stability analysis for voltage and current type interfaces, some guidelines are provided to have a stable PHIL simulation. The presented analysis have been evaluated by performing several experimental tests using a Real Time Digital Simulator (RTDS™) and a voltage source converter (VSC). The practical test results are consistent with the proposed analysis.

Some of the design considerations in power generation from offshore wind farms

The global efforts to reduce carbon emissions from power generation have favoured renewable energy resources such as wind and solar in recent years. The generation of power from the renewable energy resources has become attractive because of various incentives provided by government policies supporting green power. Among the various available renewable energy resources, the power generation from wind has seen tremendous growth in the last decade. This article discusses various advantages of the upcoming offshore wind technology and associated considerations related to their construction. The conventional configuration of the offshore wind farm is based on the alternative current internal links. With the recent advances of improved commercialised converters, voltage source converters based high voltage direct current link for offshore wind farms is gaining popularity. The planning and construction phases of offshore wind farms, including related environmental issues, are discussed here.

Dynamic analysis and simulation of a VSC based Back-to-Back HVDC link

This paper presents the modeling and analysis of a voltage source converter (VSC) based back-to-back (BTB) HVDC link. The case study considers the response to changes in the active and reactive power and disturbance caused by single line to ground (SLG) fault. The controllers at each terminal are designed to inject a variable (magnitude and phase angle) sinusoidal, balanced set of voltages to regulate/control the active and reactive power. It is also possible to regulate the converter bus (AC) voltage by controlling the injected reactive power. The analysis is carried out using both d-q model (neglecting the harmonics in the output voltages of VSC) and three phase detailed model of VSC. While the eigenvalue analysis and controller design is based on the d-q model, the transient simulation considers both models.

Reduction of Torque Ripple in Induction Motor Drives Using an AdvancedHybrid PWM Technique

A voltage source inverter-fed induction motor produces a pulsating torque due to application of nonsinusoidal voltages. Torque pulsation is strongly influenced by the pulsewidth modulation (PWM) method employed. Conventional space vector PWM (CSVPWM) is known to result in less torque ripple than sine-triangle PWM. This paper aims at further reduction in the pulsating torque by employing advanced bus-clamping switching sequences, which apply an active vector twice in a subcycle. This paper proposes a hybrid PWM technique which employs such advanced bus-clamping sequences in conjunction with a conventional switching sequence. The proposed hybrid PWM technique is shown to reduce the torque ripple considerably over CSVPWM along with a marginal reduction in current ripple.

A Hardware Grid Simulator to Test Grid-Connected Inverter Systems.

Grid-connected systems when put to use at the site would experience scenarios like voltage sag, voltage swell, frequency deviations and unbalance which are common in the real world grid. When these systems are tested at laboratory, these scenarios do not exist and an almost stiff voltage source is what is usually seen. But, to qualify the grid-connected systems to operate at the site, it becomes essential to test them under the grid conditions mentioned earlier. The grid simulator is a hardware that can be programmed to generate some of the typical conditions experienced by the grid-connected systems at site. It is an inverter that is controlled to act like a voltage source in series with a grid impedance. The series grid impedance is emulated virtually within the inverter control rather than through physical components, thus avoiding the losses and the need for bulky reactive components. This paper describes the design of a grid simulator. Control implementation issues are highlighted in the experimental results.

Hybrid LCI/VSI Power Circuit-A Universal High-Power Converter Solution for Wound Field Synchronous Motor Drives

Load commutated inverter (LCI)-fed wound field synchronous motor drives are used for medium-voltage high-power drive applications. This drive suffers from drawbacks such as complex starting procedure, sixth harmonic torque pulsations, quasi square wave motor current, notches in the terminal voltages, etc. In this paper, a hybrid converter circuit, consisting of an LCI and a voltage source inverter (VSI), is proposed, which can be a universal high-power converter solution for wound field synchronous motor drives. The proposed circuit, with the addition of a current-controlled VSI, overcomes nearly all of the shortcomings present in the conventional LCI-based system besides providing many additional advantages. In the proposed drive, the motor voltage and current are always sinusoidal even with the LCI switching at the fundamental frequency. The performance of the drive is demonstrated with detailed experimental waveforms from a 15.8-hp salient pole wound field synchronous machine. Finally, a brief description of the control scheme used for the proposed circuit is given.

An Overmodulation Scheme for Vector Controlled Induction Motor Drives

This paper presents an algorithm for control of line side voltage of a voltage source inverter upto six-step mode. This is a modified version of an existing overmodulation algorithm. The modified algorithm maintains proportionality between the reference voltage and the output fundamental voltage, and also reduces the computational effort required for implementation, while resulting in a marginally higher harmonic distortion. An estimation method is proposed for calculation of lower order ripple current. This estimation method is applied to a sensorless vector controlled induction motor drive to improve the performance of the drive during overmodulation.

A Novel VSI- and CSI-Fed Active-Reactive Induction Motor Drive with Sinusoidal Voltages and Currents

Till date load-commutated inverter (LCI)-fed synchronous motor drive configuration is popular in high power applications (>10 MW). The leading power factor operation of synchronous motor by excitation control offers this simple and rugged drive structure. On the contrary, LCI-fed induction motor drive is absent as it always draws lagging power factor current. Therefore, complicated commutation circuit is required to switch off thyristors for a current source inverter (CSI)-driven induction motor. It poses the major hindrance to scale up the power rating of CSI-fed induction motor drive. Anew power topology for LCI-fed induction motor drive for medium-voltage drive application is proposed. A new induction machine (active-reactive induction machine) with two sets of three-phase winding is introduced as a drive motor. The proposed power configuration ensures sinusoidal voltage and current at the motor terminals. The total drive power is shared among a thyristor-based LCI, an insulated gate bipolar transistor (IGBT)-based two-level voltage source inverter (VSI), and a three-level VSI. The benefits of SCRs and IGBTs are explored in the proposed drive. Experimental results from a prototype drive verify the basic concepts of the drive.

Digital Simulation of Field-Oriented Control of Induction Motor

This paper describes the simulation of a control scheme using the principle of field orientation for the control of a voltage source inverter-fed induction motor. The control principle is explained, followed by an algorithm to simulate various components of the system in the digital computer. The dynamic response of the system for the load disturbance and set-point variations have been studied. Also, the results of the simulation showing the behavior of field coordinates for such disturbances are given.

Nearly constant switching frequency hysteresis current controller for general n-level inverter fed induction motor drive

A current-error space phasor based hysteresis controller with nearly constant switching frequency is proposed for a general n-level voltage source inverter fed three-phase induction motor drive. Like voltage-controlled space vector PWM (SVPWM), the proposed controller can precisely detect sub-sector changes and for switching it selects only the nearest switching voltage vectors using the information of the estimated fundamental stator voltages along α and β axes. It provides smooth transition between voltage levels, including operation in over modulation region. Due to adjacent switching amongst the nearest switching vectors forming a triangular sub-sector, in which tip of the fundamental stator voltage vector of the machine lies, switching loss is reduced while keeping the current-error space phasor within the varying parabolic boundary. Appropriate dimension and orientation of this parabolic boundary ensures similar switching frequency spectrum like constant switching frequency SVPWM-based induction motor (IM) drive. Inherent advantages of multi-level inverter and space phasor based current hysteresis controller are retained. The proposed controller is simulated as well as implemented on a 5-level inverter fed 7.5 kW open-end winding IM drive.

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.

Harmonic Analysis of DC-Link Capacitor Current in Sinusoidally Modulated Neutral-Point-Clamped Inverter

The voltage ripple and power loss in the DC-capacitor of a voltage source inverter depend on the harmonic currents flowing through the capacitor. This paper presents double Fourier series based harmonic analysis of DC capacitor current in a three-level neutral point clamped inverter, modulated with sine-triangle PWM. The analytical results are validated experimentally on a 5-kVA three-level inverter prototype. The results of the analysis are used for predicting the power loss in the DC capacitor.

A Quick-Simulation Tool for Induction Motor Drives Controlled Using Advanced Space-Vector-Based PWM Techniques

Space-vector-based pulse width modulation (PWM) for a voltage source inverter (VSI) offers flexibility in terms of different switching sequences. Numerical simulation is helpful to assess the performance of a PWM method before actual implementation. A quick-simulation tool to simulate a variety of space-vector-based PWM strategies for a two-level VSI-fed squirrel cage induction motor drive is presented. The simulator is developed using C and Python programming languages, and has a graphical user interface (GUI) also. The prime focus being PWM strategies, the simulator developed is 40 times faster than MATLAB in terms of the actual time taken for a simulation. Simulation and experimental results are presented on a 5-hp ac motor drive.

Study on Rotor Current Waveforms in an Inverter-fed Induction Motor Drive During Overmodulation

Overmodulation introduces low-order harmonics in the output voltage of a voltage source inverter. This paper presents the effects of low-order harmonics in the stator voltage on the rotor currents of an induction motor. Rotor current waveforms are presented for various operating zones in overmodulation, including six-step mode. Harmonic spectra of stator and rotor currents are compared in six-step mode of operation. Pulsating torque is evaluated at various depths of modulation during overmodulation.

Analysis of the modulation process in advanced bus-clamping PWM techniques

Advanced bus-clamping pulse width modulation (ABCPWM) techniques are advantageous in terms of line current distortion and inverter switching loss in voltage source inverter-fed applications. However, the PWM waveforms corresponding to these techniques are not amenable to carrier-based generation. The modulation process in ABCPWM methods is analyzed here from a “per-phase” perspective. It is shown that three sets of descendant modulating functions (or modified modulating functions) can be generated from the three-phase sinusoidal signals. Each set of the modified modulating functions can be used to produce the PWM waveform of a given phase in a computationally efficient manner. Theoretical results and experimental investigations on a 5hp motor drive are presented