Sandeep Kolluri

Spatial repetitive controller for minimizing circulating harmonic currents in modular multilevel converters for variable frequency applications

In a modular multilevel converter (MMC), ripple component in the capacitor voltages of the submodules(SMs) results in second- and other even-order harmonics in the circulating currents. When, MMC is used for variable frequency applications like a front end converter in variable speed wind generator or as an inverter in a motor drive, the circulating current harmonic frequencies varies over a wide range because of the change in fundamental operating frequency. Traditional controllers like proportional resonant (PR) controller and time-domain based repetitive controllers have limited harmonic rejection capability at variable frequencies. This paper presents a control structure with spatial repetitive controller (SRC) to alleviate the circulating harmonic currents in MMC, when used for variable frequency applications. SRC has a good steady-state harmonic suppression capability and is independent of frequency. Submodule capacitor voltage balancing control strategy is also presented. Analytical equations governing the operation of a 3-phase MMC are derived and presented. MATLAB/Simulink model of a 3-phase, 9-level (Ln-Ln) MMC is developed to demonstrate the effectiveness of the developed submodule capacitor balancing and circulating harmonic current suppressing controller.

A new control scheme to improve load transient response of single phase PWM rectifier with Auxiliary Current Injection Circuit

This paper presents a new control scheme for a single-phase PWM rectifier with Auxiliary Current Injection Circuit (ACIC) to improve the load transient response and DC bus voltage controller bandwidth. The PWM rectifier and ACIC are controlled to mitigate the 2nd harmonic ripple in the DC bus under steady-state as well as the peak undershoot and overshoot during load transients. Analytical equations governing the operation of the single phase PWM rectifier with proposed ACIC controller are derived and presented. The proposed controller is designed and simulated using MATLAB Simulink and implemented on a 100 W test setup. The simulation and experimental results are presented to validate the performance of proposed controller.

A fault tolerant control approach for a three-stage cascaded multilevel solid state transformer

A solid state transformer (SST) is going to be a vital part of the future grid due to its attractive features such as step-up/step-down of grid voltage, provision for an intermediate DC bus to interface renewable energy source and energy storage, controlled bidirectional power flow between two girds etc. Even though the reliability of the SST is questioned due to high switching device count, redundant modules can be integrated into the SST architecture to ensure uninterrupted operation. Unlike the high short circuit current withstanding capability of the passive copper windings in a conventional iron-and-copper based transformer, an SST can fail in the event of any fault due to the limited over current rating of the switching devices. Several fault identification and isolation techniques for the power converters have been researched, but the power converter operation during the time frame between fault isolation and post fault restoration was neglected. In this paper, the grid current and sub-module capacitor voltage deviations are analysed during the time frame between fault isolation and post-fault restoration. A new control approach is proposed to improve the fault tolerance of the SST by identifying a short circuit fault and limit the short circuit current before a healthy module is brought into operation i.e., post-fault restoration. In the proposed method, the control is designed to switch from the conventional control to a fault tolerant control scheme in the event of fault. The proposed control method is validated using the PLECS real time (RT) hardware-in-loop (HIL) platform and results are presented.

Subsea power transmission cable modelling: Reactive power compensation and transient response studies

With the depletion of existing oil and gas reserves, there is an increasing demand for deep-water oil and gas production, which requires long distance power transmission and distribution to several subsea electrical loads. Subsea cables (power umbilicals) are one of the key components in subsea power transmission. Modelling of subsea cables is important to study their transient and steady-state behaviour. In this paper, the suitability of cable models such as Bergerons, frequency dependent and simple-models for transient study have been investigated and discussed. Simulation results comparing the performance of different cable models are presented. Models suitable for reactive power compensation study are also explored. Based on the specifications of the subsea cable a reactive power compensation study for two different cable lengths 50 km and 150 km is performed. Simulation results show that the HVAC transmission is not a preferred option for subsea applications, especially for long length (> 50 km) of the cable, hence HVDC transmission is a preferred choice for longer cable length (> 50 km).

Design of a novel APWM half-bridge DC-DC resonant converter with load-independent soft-switching and reduced circulating current

This paper presents a novel Asymmetrical-Pulse-Width-Modulated (APWM) half-bridge resonant converter with one additional active switching device at the secondary side of the transformer. The proposed converter features load-independent Zero-Voltage-Switching (ZVS) of both of the primary side switches with minimal magnetizing current. Empirical formulae are derived to design the resonant network and the high-frequency transformer systematically. The circulating current is minimized not only by operating the converter exactly at the resonant frequency, but also by reducing the magnetizing current. Moreover, the additional secondary-side switching device (which is a low-voltage and high-current rated MOSFET) achieves zero current turn-off at all load conditions. Thus the efficiency of the overall converter is significantly improved for wide load range. Simulation and experimental results are presented to validate the analysis and to prove the significance of the additional switching device at the secondary side of the transformer.