Monolithic Bidirectional Switch Based on GaN Gate Injection Transistors

The paper deals with a bi-directional switch based on N-channel enhancement-mode GaN FET. The proposed device is a Gate Injection Transistor monolithic solution to reduce the volume of the switch with high current density and blocking voltage of 600V. It features a dual-gate control pin and two power terminal. In the paper, the main characteristics of the bi-directional switch and the performance in the four-quadrant of operation are examined and discussed. The device characteristics are compared with the traditional MOSFET and IGBT solutions. The gate driver design issues are considered to optimize the switching transient of the GaN-based switch. Finally, an experimental evaluation of the GaN FET as the bidirectional circuit breaker is carried out in an AC power supply system to validate the effectiveness of the proposed monolithic new device

Small-signal Analysis of Active Loads and Large-signal Analysis of Faults in Inverter Interfaced Microgrid Applications

Rectifiers and voltage regulators that have characteristics of constant power loads may form a significant percentage of a microgrid's total loads. The real part of the input impedance of a constant power load is negative and it may have control loop dynamics in a similar frequency range to the inverters that are supplying the microgrid. This thesis examines the interactions between an active constant power load and a microgrid for the impact on stability. Participation analysis of the eigenvalues that result from the model of the combined microgrid and active load identified that the low-frequency modes are associated with the voltage controller of the active rectifier and the droop-controllers of the inverters. The analysis also revealed that when the active load dc-voltage controller is designed with large gains, the voltage controller of the inverter becomes unstable but the low frequency modes associated with the droop controller of the inverter remain stable. The transient stability of a microgrid may require that the inverter-interfaced generation remain connected during a fault and return to normal power export once a fault is cleared. For an inverter to supply fault current, the controller of the inverter must current-limit the output and the fault strategy chosen must ensure that the current and voltage limiter do not latch-up and that the controller integrators do not wind-up. This thesis analyses different limiting and reset strategies and concluded that that it is not possible to successfully reset a limiter when using a reset signal from a closed-loop controller within the inverter. In a system where there are cascaded limiters, successful operation is obtained when the inner limit is a saturation limiter and the output limiter is a set-reset limiter. It was found that the transient stability of an inverter interfaced microgrid using a droop control algorithm is dependent on the current limiter and inductance of the network.Open Acces

Alternatives to SF6 in HV circuit breaker insulation

Sulphur hexafluoride(SF6) in a synthetic gas extensively used in the high voltage (HV) electricity transmission and distribution industry. The unique and unrivalled properties of SF6 gas that enable it to resist/recover from spark conduction have positioned it as the preferred HV arc interruption medium. Since its introduction in the 1960's HV circuit breakers utilising SF6 have provided less frequent maintenance requirements and reduced installation footprints compared to their Oil/Air predecessors. Unfortunately however, SF6 has an extremely high global warming potential (GWP), 22,800 times that of carbon dioxide. This high global warming potential combined with a long shelf life (3000 years of atmosphere) has seen the gas identified as one of the six most content contributors to climate change. Additionally, the decomposition by-products of SF6 used for arc extinguishment threaten the occupational health and safety of maintenance staff. A case study examining the use of SF6 in the region of Central Western NSW has been undertaken as a part of this dissertation. The Central Western NSW electricity grid was found to utilise 10,216k kg of in-service SF6 gas in its HV circuit breakers and associated apparatus. Additionally, SF6 insulated equipment was found to contribute a growing 70% market share of switchgear applications 66 kV and above. A government prescribe annual leakage rate of 0.89% of capacity combined with handling losses suggests the case study area's SF6 insulated equipment is responsible for 2,905 tonnes of C02 equivalent emissions per year. Alternatives, eliminating or reducing the use of SF6 insulation in HV circuit breakers and associate apparatus do exist and in some cases are fast becoming the more popular and cost effective option. Established solid-dielectric/vacuum and dry-air/vacuum circuit breakers are two such alternatives eliminating SF6 reliance in the 66 kV and below spectrums. Other advancements include non-conventional current transformers with digital outputs that are revolutionising traditional circuit breaker installation concepts. The 40 year dominance associated with SF6 insulated switchgear is placing utilities in vulnerable positions. With the ongoing replacement of obsolete oil designs expected to finalise in the next decade Australian circuit breakers 66 kV and above could be near 100% SF6 insulated by 2025. Despite a recent fall in national electricity demand, infrastructure upgrades and the diversifying renewable generation mix are supporting continual SF6 insulated equipment growth. Environment driven regulated SF6 price rises or application bans are of increasing concerns to network utilities. This dissertation provides reasonable and practical recommendations of pro-active technology implementations and trials that will help alleviate reliance on SF6. The proposals also seek to respect restricting network operating budgets recently implemented in an effort to curb rising electricity commodity prices whilst simultaneously offering environmental and OH&S beneficial alternatives

Loss allocation in a distribution system with distributed generation units

In Denmark, a large part of the electricity is produced by wind turbines and combined heat and power plants (CHPs). Most of them are connected to the network through distribution systems. This paper presents a new algorithm for allocation of the losses in a distribution system with distributed generation. The algorithm is based on a reduced impedance matrix of the network and current injections from loads and production units. With the algorithm, the effect of the covariance between production and consumption can be evaluated. To verify the theoretical results, a model of the distribution system in Brønderslev in Northern Jutland, including measurement data, has been studied

Analysis of the experimental spectral coherence in the Nysted Wind Farm

In this paper, it is analysed the coherence between wind speeds located in a horizontal plane corresponding to hub height of wind turbines in a large wind farm. The coherence is calculated through real data from Nysted Offshore Wind Farm. Concretely, the wind speed measured in the 72 Wind Turbines and in 2 of the meteorological masts during 9 months. The results are analysed in the scale of power fluctuations in large offshore wind farms. This analysis shows the needing of a new spectral coherence model.The work presented in this paper has been done in the research Project ”Power Fluctuations from large offshore wind farms” financed by the Danish Transmission System Operator Energinet.dk as PSO 2004 project number 6506. A. Vigueras-Rodr´ıguez is supported by the Spanish Ministerio de Educaci´on y Ciencia through the grant program “Becas FPU” and from the national research project “ENE2006-15422-C02-02

State-of-the-art of design and operation of power systems with large amounts of wind power, summary of IEA Wind collaboration

An international forum for exchange of knowledge of power system impacts of wind power has been formed under the IEA Implementing Agreement on Wind Energy. The task “Design and Operation of Power Systems with Large Amounts of Wind Power” is analysing existing case studies from different power systems.There are a multitude of studies made and ongoing related to cost of wind integration. However, the results are not easy to compare. This paper summarises the results from 15 case studies