Yonghe Liu

Frequency-to-voltage conversion using OTA

In recent years Operational Transconductance Amplifier (OTA) based integrated circuits, filters and analog circuits have been widely investigated. The usefulness of OTAs over conventional OP-Amps in the design of active circuits are well documented. This paper presents a new sinusoidal frequency-to-voltage converter which can linearly convert low voltage signals which is not possible with commercial converter ICs such as LM2917. This converter is based on an OTA based differentiator and an OTA based half wave rectifier.

Simulation study of conventional and hybrid HVDC rectifier based on CIGRÉ benchmark model using PLL-less synchronisation scheme

The objective of this paper is to compare and assess the performance of the conventional and hybrid high voltage direct current (HVDC) rectifiers based on the CIGRE benchmark model for both static and dynamic conditions with the phase lock loop (PLL) control replaced by PLL-less control. Traditional HVDC rectifier uses a 12-pulse thyristor bridge whereas the hybrid HVDC rectifier is based on one 6-pulse thyristor bridge and one 6-pulse diode bridge. Simulations are carried out in Matlab/Simulink. Some of the simulation results show that the hybrid option doesnt look too attractive in spite of the cost savings it offers.

DC Ripple Reinjection: A Review

This paper presents a comprehensive review of power quality improvements made by applying the concept of DC ripple reinjection in current source series connected AC-DC rectifier for HVDC transmission. Recent advances in applying the reinjection concept, their suitability and selection is discussed. The main aim is to present a state of the art on the DC ripple reinjection technique over the years.

An Investigation into Thyristor-based Seven-level Multi-level Current Reinjection Current Source Converter

Abstract This article presents a comprehensive analysis of the thyristor based seven-level multi-level current reinjection current source converter. The seven-level multi-level current reinjection current source converter increases the pulse number from 12 to 72, thereby producing high-quality AC current and DC voltage waveforms, thus lowering harmonics on both sides. The ability to switch thyristors with a negative firing angle using auxiliary bridge reinjection insulated-gate bipolar transistors enables the converter to have a leading power factor. This article also details the design of the firing angle controller for the seven-level multi-level current reinjection current source converter, which enables the converter to have a negative firing angle. A wide range of PSCAD/EMTDC simulation results are presented to evaluate the performance of this converter, especially the effect of RC snubbers across the reinjection insulated-gate bipolar transistors.

Comparative Evaluation of m-level Thyristor based MLCR CSC

Abstract In this paper a comparative evaluation study is carried out for a 3-level, 5-level, 7-level thyristor based multi-level current reinjection (MLCR)current source converters (CSC) in terms of line current total harmonic distortion (THD) obtained, reinjection transformer requirements, reinjection switch ratings, reinjection control circuitry complexities. MLCR concept adds self commutation capability to thyristors, produces high quality line current waveforms and provides reactive power compensation in CSCs. Multi-level conversion is applied to generate the required optimum injection current which, when reinjected to the ac side reduces the line current THD. Extensive PSCAD/EMTDC simulations are presented for the comparing the different m-level reinjection schemes.

Comparative study between gate firing units for CIGRE benchmark HVDC rectifier

This paper presents a comparative simulation study between the DQO gate firing unit (GFU) and a proposed GFU for high voltage direct current (HVDC) system, using the CIGRE benchmark model in the Matlab/Simulink environment. The proposed GFU does not use traditional phase lock loop (PLL) and offers structural simplicity, fast response and immunity to ac system voltage unbalance, harmonics, voltage sag, frequency jump and phase jump etc. Since there is no loop filter, no tuning issues are involved.

6-pulse controlled rectifier synchronisation method

The 6-pulse controlled rectifier circuit is very well known. For proper operation of the 6-pulse rectifier, it is necessary to synchronise the firing signals with the mains voltage, else the operation of the rectifier is affected. A firing angle controller, without using any phase lock loop (PLL), is described here. This scheme employs the dq-reference frame to estimate the magnitude and phase angle of the fundamental component of the mains voltage. This method offers structural simplicity and immunity to mains voltage unbalance, harmonics and voltage sag etc.

Experimental Evaluation on the Influence of RCD Snubbers in a 3-Level Thyristor Based MLCR CSC

Multilevel current reinjection (MLCR) concept provides self-commutation capability to thyristors, enabling thyristor based current source converters (CSC) to operate under negative firing angle. It also lowers the input current harmonic distortion. This is achieved by using an auxiliary reinjection bridge. Extensive experimental results are presented in this paper to analyse the performance of the 3-level MLCR CSC for different snubber components across the reinjection bridge. The trade-off in the choice of the snubber circuit is illustrated, with its influence on the AC side line current and DC side output voltage of the 3-level MLCR CSC.

Firing thyristors with negative firing angle using multilevel current reinjection concept: an experimental evaluation

Thyristor based multilevel current reinjection (MLCR) current source converter (CSC) provides self-commutation capability to thyristors. It also has high quality line current waveform by accurately shaping the DC bus current using an auxiliary reinjection bridge. However the theoretical and PSCAD/ETDC simulation analysis do not take into account the inevitable stray capacitances and inductances which may influence the thyristor turn-on/off and the simulation switching model may not represent the switching characteristics of the main bridge thyristors fully or accurately. Questions have been raised about the ability to achieve self-commutation with thyristors and the possibility of operating thyristors with negative firing angles. Therefore, it is necessary to experimentally verify whether neglecting the real-world artefacts actually impedes the performance of thyristor based MLCR CSC. The experimental results presented in this paper prove that the auxiliary reinjection circuit can force the thyristor to commutate independently from their respective line-to-line voltages and thyristor converter can operate with a negative firing angle.

Mains Frequency Counter Using Phase Lock Loop

Phase-Lock-Loops (PLL) are critical components for control of power electronic converters that interface to the electrical power system and are also used in variety of other applications such as communication etc. This paper presents a phase-lock-loop based mains frequency counter with resolution up to three decimal places. Phase lock loop design equations and response to frequency variation is presented. The hardware is very simple, low cost, and designed using off-the-shelf components.