Sayan Acharya

Control of modular dual active bridge DC/DC converter for photovoltaic integration

The DC transmission system provides a cost effective solution for long distance power transmission compared to the AC transmission system. Hence, this has increased the emphasis on the development of the DC transmission system. Development of power converter with modular structure has now made it possible to achieve higher voltage and power level. This opens the possibility for further development of a multi-terminal DC grid. Now once the DC grid system has been formed, it is also important to include more renewable energy sources directly to the DC grid. Therefore, a power conversion stage is required to condition the available power from a source to the grid. This paper shows the operation and control of such a kind of converter system which integrates the solar cell to the DC grid directly. The paper mainly focuses on control of the series connected DAB that have been integrated to HVDC power network. In order to deliver power in HVDC system, the total number of DABs must be high enough to achieve the DC link voltage. The control in that case must be a combination of current and voltage control. In order to validate the proposed control, complete system has been implemented on Opal-RT™ and hardware in the loop (HIL) using external controller has also been implemented to show the system operation.

Dynamic performance evaluation of hybrid multi-terminal HVAC/HVDC grid

The multi-terminal DC grid can be integrated to the existing meshed AC grid system to provide back-up in case of transmission line failure and enhance power transmission capacity and flexibility in existing ac grids. In addition to that, Power oscillations can also be damped effectively through modulation of both active and reactive power of a voltage source converter (VSC) based multi-terminal DC grid. In this paper, the ability of the multi-terminal DC grid to effectively damp the power oscillation in an interconnected AC grid has been investigated. Also, VSC based MTDC transmission systems are vulnerable to DC side fault. This paper demonstrates a control method of a dc fault resilient voltage source converter that has ultra-fast electronic isolation capability following dc fault which can be protected against dc fault. To verify the control structure, the dynamic performance of the integrated multi-terminal DC grid in a reduced order three-bus AC equivalent power system is investigated through hardware-in-the-loop testing. Controller hardware-in-the-loop simulation of the embedded multi-terminal DC grid in a meshed AC power system is performed by Real Time Digital Simulator (RTDS), and RTDS results are presented to verify the control structure.

Comparison of DC fault current limiting capability of various modular structured multilevel converters within a multi-terminal DC grid

With the development of Modular structured Voltage Source Converters (VSC), Multi-Terminal DC (MTDC) transmission systems have now become a feasible solution to transmit power at high voltage which greatly improves the electric power transmission system. The MTDC grid has lower capital costs and lower losses than an equivalent AC transmission system. Thus for long distance power transmission MTDC grid becomes a very attractive solution. Since the MTDC network is now built based on VSCs, it automatically offers better quality of transmitted power along with more flexibility in power transmission over the conventional current source converters. However, VSC based MTDC transmission systems are vulnerable to DC side fault and expensive DC circuit breakers are required to protect them against DC fault. This paper compares the DC short circuit fault response of different modular multi-level converters (MMC) inside a MTDC system. For the comparison purpose two different kind of MMC topologies have been considered namely, Modular Multi-level Converter (MMC) with High Frequency DC/DC Isolation Stage and MMC with full bridge sub modules. The paper analyzes the fault current limiting capabilities of each of the converters. PSCAD simulation is also done to prove the relevance of the analysis.

Operation of hybrid multi-terminal DC system under normal and DC fault operating conditions

Recently, multi-terminal DC (MTDC) system has received more attention in the power transmission areas. Development of modular structured power converter topologies has now enabled the power converter technology to attain high voltage high power ratings. Compared to current source converter technology, voltage source converters have several benefits including higher power quality, independent control of active and reactive power etc. This paper focuses on a unique MTDC system consisting of terminals with different converter topologies especially considering the fact that each of the terminals may be manufactured by different vendors. In this particular configuration, the MTDC system consists of four terminals namely two advanced modular multi-level converter with high frequency isolation, one standard modular multi-level converter (MMC) with half bridge sub modules and the fourth terminal is modular DC-DC converter which integrates PV along with a Battery energy storage system with the DC grid directly. This paper presents a system level study of hybrid MTDC System. Also the DC fault contingency case has been explored thoroughly. An algorithm has been proposed to prevent the system damage. All the cases have been demonstrated with the PSCAD simulation results. To show the system practically works in real time, the system is also evaluated in a unique real time platform, consisting of interconnected RTDS and OPAL RT systems.

Application of static synchronous series compensators in mitigating Ferranti effect

This paper discusses a novel application of the SSSC which is a VSC-based FACTS device connected in series with the transmission line. An unloaded transmission line experiences Ferranti effect, i.e. the unloaded end of the transmission line experiences a voltage rise, which increases in magnitude as the length of the line increases. The SSSC can inject a controllable voltage in quadrature with the line current. Since the transmission line current is also in quadrature with the line voltage in the unloaded condition, the SSSC can take advantage of this to reduce the line voltage magnitude by injecting a voltage in phase with it. To verify this effect, the system is implemented in PSCAD along with a two-level VSC based SSSC with slightly altered controls. Voltage reduction at the receiving end was achieved when the SSSC was put in operation.

Static and Dynamic Characterization of a 3.3 Kv, 45 A 4H-Sic MOSFET

Wide bandgap materials such as Silicon Carbide (SiC) has enabled the use of medium voltage unipolar devices like Metal-Oxide Field Effect Transistors (MOSFETs) and Junction Field Effect Transistors (JFETs), which can switch at much higher frequencies as compared to their silicon counterparts. It is therefore imperative to evaluate the performance of these medium voltage devices. In this paper, the static characterization and the switching performance of the new single die 3.3 kV, 45 A 4H-SiC MOSFET developed by Cree Inc are presented. The switching performance is measured through the conventional Double Pulse Test. Testing is done at a dc-link voltage of 1.5 kV for different values of current, and gate resistances.

Damping of power oscillations induced by photovoltaic plants using distributed series-connected FACTS devices

This paper demonstrates the capability of distributed series-connected Flexible AC Transmission Systems (FACTS) devices in damping power oscillations. Large power systems have resonant frequencies which result from the electro-mechanical power balance equations of synchronous generators connected to the power network. Transient events that affect power flow, like the loss of a transmission line, switching of loads, changes in renewable energy output can excite these resonant frequencies, referred to as modes, leading to power oscillations within the network. This paper proposes a power oscillation damping (POD) controller using multiple Static Series Synchronous Compensators (SSSC) connected in series on a single transmission line. The power oscillation frequencies in New York Power Authoritys (NYPA) three-bus power system network are identified using the Matrix Pencil method, and a controller is designed to block the most prominent frequencies from them. The controller is implemented in PSCAD to damp power oscillations in NYPAs network and its performance while damping power oscillations is recorded.

One switching cycle current control strategy for triple active bridge phase-shifted DC-DC converter

The paper presents two types of one cycle current control method for Triple Active Bridge(TAB) phase-shifted DC-DC converter integrating Renewable Energy Source(RES), Energy Storage System(ESS) and a output dc bus. The main objective of the current control methods is to control the transformer current in each cycle so that dc transients are eliminated during phase angle change from one cycle to the next cycle. In the proposed current control methods, the transformer currents are sampled within a switching cycle and the phase shift angles for the next switching cycle are generated based on sampled current values and current references. The discussed one cycle control methods also provide an inherent power decoupling feature for the three port phase shifted triple active bridge converter. Two different methods, (a) sampling and updating twice in a switching cycle and (b) sampling and updating once in a switching cycle, are explained in this paper. The current control methods are experimentally verified using digital implementation technique on a laboratory made hardware prototype.

Applications and characterization of four quadrant GaN switch

Bi-directional switches, also called four quadrant switches (FQS), are the basic building blocks in many power converter circuits, such as cyclo-converters, matrix converters etc. Conventional approaches to realize bi-directional switch involves combination of unidirectional controllable blocking device (IGBT or MOSFET) and diode. In this approach, current flows through multiple devices for any direction of current flow. This leads to higher conduction losses. Moreover, use of multiple devices increases system size. The die size and semiconductor losses can be reduced by realizing a bi-directional switch using a single die. Further improvement can be achieved by using Gallium Nitride (GaN) semiconductor. This paper discusses characterization of such a four quadrant GaN switch, made using a single die. Static characterization is performed, where the on-state resistances are obtained along with the output characteristics. A double pulse test setup has been built for characterizing FQSs and the experiments were performed to obtain the turn-on and turn-off switching energies.

Comparison of 1.7kV, 450A SiC-MOSFET and Si-IGBT based modular three phase power block

A three phase power block based on novel 1.7 kV/450 A SiC-MOSFET is designed and tested. To benchmark the performance of the power block, a through comparison is done with currently standardized 1.7 kV/450 A Si-IGBT based three phase power block. Key performance indices, including power rating curves at different switching frequencies and power factors; temperature ripple at different fundamental frequencies, are examined. It is shown that the SiC based power block has very promising potentials in various applications. Simulation and experiment results are provided to support the claims for SiC-MOSFET based power block.