Masood Hajian

Evaluation of Semiconductor Based Methods for Fault Isolation on High Voltage DC Grids

This paper investigates methods for dc fault current fast interruption in high-power dc networks. A four terminal 1.2 GW dc transmission grid is used as a test system. The study compares four semiconductor-based devices for dc fault isolation: series hybrid dc breaker, half bridge and full bridge dc chopper, and LCL thyristor converter. The study considers performance of devices, costs and losses, and also application with larger dc grids. A hybrid dc CB has lowest losses, but the component sizing crucially depends on the speed of fault detection. It is demonstrated that fast communication between various devices in the dc grid is mandatory but might be very challenging. On the other hand, dc choppers and LCL converter component sizing is not dependent on the speed of protection, and there is no need for communication across dc grids. Additionally, they offer the capability of voltage stepping and dc cable power regulation. The LCL converter provides inherent fault current interruption and needs no overrating for dc faults. It also gives better efficiency compared to dc choppers. The performance of these dc breakers is demonstrated using detailed transient PSCAD simulation for dc faults.

LCL VSC Converter for High-Power Applications

This paper studies an inductor-capacitor-inductor (LCL) voltage-source converter (VSC) ac/dc converter concept which can be employed as high-power static converter. The converter is designed to have fault current near or below the rated value under the dc-side short circuits. This is very important for applications with HVDC and, in particular, with high-power dc transmission networks. This converter is composed of an ac/dc insulated-gate bipolar transistor-based VSC converter and a passive LCL circuit. A transformer is not required since LCL circuit can achieve voltage stepping. The converter parameters are designed to have optimal response during the faults, good controllability, and to minimize converter losses. A detailed model is developed on the PSCAD platform for a 500-MW test system. The simulation confirms capability to independently control active and reactive power and demonstrates favorable fault responses. The transient fault current peaks are not significant and can be overcome with slight overrating.

DC Transmission Grid With Low-Speed Protection Using Mechanical DC Circuit Breakers

This paper introduces a dc transmission grid with fault-tolerant inductor-capacitor-inductor (LCL) voltage-source converters (VSCs) and using a slow protection system based on mechanical dc circuit breakers (CBs). LCL VSC inherently regulates dc fault current to levels that converters can sustain for prolonged periods which avoids insulated-gate bipolar transistor tripping and brings significant advantage to security and reliability aspects. Simple mechanical dc CBs are used at dc busbars and connecting points of each dc cable, in the same manner as it is normal practice used with ac transmission protection. The protection logic is based on differential methods which gives excellent selectivity and reliability. The fault clearing time is in the order of 30-60 ms which allows for reliable protection decision making. The simulation results obtained from a four-terminal dc grid modeled on the PSCAD platform confirm successful dc fault isolation and grid recovery for a range of severe dc fault scenarios.

30 kW, 200 V/900 V, Thyristor LCL DC/DC Converter Laboratory Prototype Design and Testing

This paper details design, development, and testing of a prototype 30 kW, 200 V/900 V resonant dc/dc converter. The converter achieves stepping ratio of 4.5 without internal ac transformer, and is capable of bidirectional dc power flow as well as fast dc power reversal. Very importantly, it has capability of dc fault isolation on either side, which is of extreme importance for high-power dc applications. The design and selection of the individual passive and active components are presented. A 30-kW test setup including one 900 V voltage source converter (VSC), one 200 V VSC, and one 200 V current source converter (CSC) are designed and built to provide a test rig for dc/dc converter. The analytical studies of efficiency are compared with the converter measured efficiency. The prototype shows overall efficiency of around 92% at full power and weight of around 32 kg, which is promising for scaling to high-power applications. The converter operation at full power in step up mode as well as step down mode, and fast power reversal when connected to CSC on its low voltage side, are demonstrated by experimental results. Severe pole-to-pole dc faults are applied on 900 V and 200 V dc terminals and the inherent dc fault isolation capability is confirmed.

Non-linear Direct Torque Control of Sensorless Induction Motor Drives with Parameter Identification and Capable for Very Low Speeds

Abstract The performance of sensorless induction motor drives is generally poor at extremely low speeds. The main recognized reasons are the limited accuracy of pulse-width modulation stator voltage acquisition, unwanted offset and drift components superimposed on the acquired measured signals, voltage distortions of induction motor caused by non-linear behavior of switching inverter, and finally, increased sensitivity against motor parameters mismatch. A sensorless induction drive capable of very low speed operation is presented in this article in which the principles of direct torque control and integrator backstepping non-linear control are combined to ensure high performance operation both in transient and steady state conditions. Stator flux vector is determined using a pure integrator in the stationary reference frame and is employed for rotor flux vector identification. An improved method is introduced for unavoidable offset and drift components identification and compensation. The average of stator phase voltages in each switching interval is estimated and employed in order to eliminate the AC voltage sensors. Feasible solutions are also proposed for on line stator and rotor resistances identification to ensure further improvement of drive performance at very low speeds. Effectiveness of the proposed control scheme is studied through simulation and experimental results.

Laboratory Demonstration of Closed-Loop 30 kW, 200 V/900 V IGBT-Based LCL DC/DC Converter

The inductor-capacitor-inductor (LCL) dc/dc converter has been extensively studied for high power and stepping ratio because of elimination of internal transformer, lower footprint/weight, higher efficiency, and most importantly providing dc fault isolation from both dc sides. This paper presents a two-channel, two-layer controller including two inner current loops, which is symmetrical for each bridge of LCL dc/dc. The real-time implementation of the control scheme and its performance under normal conditions and during transient dc faults at both sides are studied on a 30 kW 200 V/900 V 1.7 kHz prototype. The prototype development is presented in some depth. The experimental results show that the converter with closed-loop control operates well at full power and under fast power reversal. Further dc fault testing concludes that there is no need for blocking since the internal voltage and current variables are within the rated values. Detailed study of converter losses is performed and results show that full power efficiency is around 93.4%.

DC bus strength in DC grid protection

Fast detection and isolation of DC faults is recognized as the main challenge in developing DC transmission grids. Voltage source converter blocking must be prevented which can be very difficult given the large rate-of-rise of fault currents. The concept of multi-converter DC bus is introduced as a grid design approach aimed towards increasing robustness of DC grids to DC faults. Behavior of a multi-converter DC bus under DC fault conditions is analyzed using newly defined parameters such as peak fault current increase and bus voltage drop. Critical inductor size and critical fault neutralization time are studied and their implications discussed. The impact of converter rating on fault current sharing and voltage drop on a multi-converter bus is investigated. Theoretical studies are substantiated by simulation results using a 10-converter bus model in PSCAD.

DC transmission grid with low speed protection using mechanical DC circuit breakers

Summary form only given. This paper introduces a DC transmission grid with fault tolerant inductor-capacitor-inductor (LCL) voltage source converters (VSCs) and using slow protection system based on mechanical DC circuit breakers (CBs). LCL VSC inherently regulates DC fault current to levels that converter can sustain for prolonged periods which avoids IGBT tripping and brings significant advantage in security and reliability aspects. Simple mechanical DC CBs are used at DC bus bars and connecting points of each DC cable, in the same manner as it is normal practice used with AC transmission protection. The protection logic is based on differential methods which gives excellent selectivity and reliability. The fault clearing time is in the order of 30-60ms which allows for reliable protection decision making. The simulation results obtained from a four-terminal DC grid modeled on PSCAD platform confirm successful DC fault isolation and grid recovery for a range of severe DC fault scenarios.

fnLCL VSC converter for high power applications

Summary form only given. This paper studies a LCL (inductor-capacitor-inductor) VSC AC/DC converter concept which can be employed as high power static converter. The converter is designed to have fault current near or below the rated value under the DC side short circuits. This is very important for applications with HVDC and in particular with high power DC transmission networks. This converter is composed of an AC/DC IGBT-based VSC converter and a passive LCL circuit. A transformer is not required since LCL circuit can achieve voltage stepping. The converter parameters are designed to have optimal response during the faults, good controllability and to minimize converter losses. A detailed model is developed on PSCAD platform for a 500MW test system. The simulation confirms capability to independently control active and reactive power and demonstrates favorable fault responses. The transient fault current peaks are not significant and can be overcomed with slight overrating.