Impact of smart transformer voltage and frequency support in a high renewable penetration system

Increasing penetration of power electronics interfaced generation decreases the stability of the system, due to the absence of the rotational inertia in their operation. Emulation of the inertia using converter controls in combination with storages can address this issue. However, this method relies on the use of large quantities of storage to compensate power during a transient power unbalance. Instead of increasing the supply, the smart transformer (ST), with a fast response, offers the possibility to dynamically regulate the demand. This paper investigates the use of an ST to dynamically control reactive power and demand to support voltage and frequency respectively in the grid. The demand is controlled dynamically to emulate inertia. From an analysis based on a 250 kVA, 10kV/400V LV distribution network, it is shown that a demand variation in the range of 6-10% can be achieved. These results are extended to a case study based on the entire all-Island Irish Transmission system which shows that widespread use of STs with these controls could potentially facilitate a 10% increase in wind penetration without the inclusion of any other storage

Neutral current reduction control for smart transformer under the imbalanced load in distribution system

The 13th IEEE Conference on Industrial Electronics and Applications (ICIEA 2018), Wuhan, China, 31 May-2 June 2018Imbalanced loads arouse neutral current looping in the distribution system, which increases power loss and results in neutral potential variation. Compared to the conventional power transformer, the smart transformer (ST) has advantages on the downstream voltage regulation. Thus, this paper proposes a voltage control strategy based on ST to reduce the LV grid neutral current according with EN 50160 imbalanced voltage standard. The proposed control has been validated in the Matlab/Simulink, and the system performance under the proposed control has been simulated under the imbalanced loading profile in a 400 kVA, 10 kV/400 V distribution network. The results prove the proposed control can practically reduce the neutral current.European Research CouncilScience Foundation IrelandEnergy Systems Integration Partnership Programme (ESIPP) Projec

Smart Transformer and Low Frequency Transformer Comparison on Power Delivery Characteristics in the Power System

AEIT 2018 International Annual Conference, Bari, Italy, 3-5 October 2018Smart transformer is a power electronics-based transformer, offering voltage regulation and DC connectivity. As a transformer, its basic function is still power delivery. Smart transformer with advanced controls can support MV gird voltage by absorbing/injecting reactive power while actively regulate the LV grid voltage. Due to the controllable voltage in both MV and LV side, the power delivery of smart transformer is flexible. This paper focuses on the power delivery characteristic of smart transformer and compares with the conventional low frequency transformer with the help of STACTOM at its primary side or on load tap changer at its secondary side, in the power system by means of maximum deliverable power and power-voltage curve analysis. The Simulink results validate that the smart transformer improves system voltage stability compared to the traditional low frequency transformer with load tap changer.European Commission - Seventh Framework Programme (FP7)Science Foundation IrelandEnergy Systems Integration Partnership Programme (ESIPP) Project funded by the Science Foundation Ireland (SFI) Strategic Partnership Programm

Harmonic stability of VSC connected Low Frequency AC offshore transmission with long HVAC cables

Low Frequency AC (LFAC) transmission has been proposed as an alternative to HVDC transmission for the integration of offshore wind. The LFAC offshore grid as a fully power electronic grid with a long HVAC cable provides significant challenges to harmonic stability. This paper presents an impedance based stability analysis to determine the stability of the power electronic offshore system across the harmonic frequency range. The stability analysis is introduced and applied to the LFAC system. The impact of different current and voltage control bandwidths and component sizes on the dynamic impedance of the converters is then examined and their impact on harmonic stability of the LFAC grid is determined. It is found that detailed knowledge of the control parameters and the ability to tune the bandwidths can mitigate significant harmonic instability with the presence of a long HVAC cable. Three phase simulations are then used to validated the impedance based stability technique.Science Foundation Irelan

Smart Transformer for the Provision of Coordinated Voltage and Frequency Support in the Grid

IECON 2018: 44th Annual Conference of the IEEE Industrial Electronics Society, Washington, United States of America, 20-23 October 2018Considering the increase in renewable generation and the consequent reduction in power system inertia, the Virtual Synchronous Machine (VSM) control method has been proposed to control power converters to emulate the inertia and other the characteristics of the synchronous machine. However, to achieve the function of VSM control, an extra energy base, typically storage, is required to connect to the controlled converter. In this work we investigate the application of the VSM control to the distribution system demand through the use of a VSM controlled smart transformer. Through control of the demand in this way, the demand itself can be used to emulate inertia and provide frequency support. This paper presents the details of the flexible demand control applied to a smart transformer supplying a low voltage distribution grid. The operation of the control is validated on scaled hardware using real time simulation with hardware in the loop. Simulations on a 400 kVA, 400 V distribution network are used to quantify the demand flexible. IEEE 39 bus is used to verify the benefit of the proposed control in terms of voltage and frequency in the power system.European Commission - Seventh Framework Programme (FP7)European Research CouncilScience Foundation IrelandEnergy Systems Integration Partnership Programme ESIPP Project funded by the Science Foundation Ireland (SFI

Energy scavenging for long-term deployable wireless sensor networks

The coming decade will see the rapid emergence of low cost, intelligent, wireless sensors and their widespread deployment throughout our environment. While wearable systems will operate over communications ranges of less than a meter, building management systems will operate with inter-node communications ranges of the order of meters to tens of meters and remote environmental monitoring systems will require communications systems and associated energy systems that will allow reliable operation over kilometers. Autonomous power should allow wireless sensor nodes to operate in a “deploy and forget” mode. The use of rechargeable battery technology is problematic due to battery lifetime issues related to node power budget, battery self-discharge, number of recharge cycles and long-term environmental impact. Duty cycling of wireless sensor nodes with long “SLEEP” times minimises energy usage. A case study of a multi-sensor, wireless, building management system operating using the Zigbee protocol demonstrates that, even with a 1 min cycle time for an 864 ms “ACTIVE” mode, the sensor module is already in SLEEP mode for almost 99% of the time. For a 20-min cycle time, the energy utilisation in SLEEP mode exceeds the ACTIVE mode energy by almost a factor of three and thus dominates the module energy utilisation thereby providing the ultimate limit to the power system lifetime. Energy harvesting techniques can deliver energy densities of 7.5 mW/cm2 from outdoor solar, 100 μW/cm2 from indoor lighting, 100 μW/cm3 from vibrational energy and 60 μW/cm2 from thermal energy typically found in a building environment. A truly autonomous, “deploy and forget” battery-less system can be achieved by scaling the energy harvesting system to provide all the system energy needs. In the building management case study discussed, for duty cycles of less than 0.07% (i.e. in ACTIVE mode for 0.864 s every 20 min), energy harvester device dimensions of approximately 2 cm on a side would be sufficient to supply the complete wireless sensor node energy. Key research challenges to be addressed to deliver future, remote, wireless, chemo-biosensing systems include the development of low cost, low-power sensors, miniaturised fluidic transport systems, anti-bio-fouling sensor surfaces, sensor calibration, reliable and robust system packaging, as well as associated energy delivery systems and energy budget management