Ravi Seethapathy

Nighttime Application of PV Solar Farm as STATCOM to Regulate Grid Voltage

This letter presents a novel concept of utilizing photovoltaic (PV) solar farm (SF) as a flexible ac transmission systems controller-static synchronous compensator, to regulate the point of common coupling voltage during nighttime when the SF is not producing any active power. This concept, although general, is presented for the scenario of a distribution feeder, which has both PV solar and wind farms connected to it. The proposed control will enable increased connections of renewable energy sources in the grid. A MATLAB/Simulink-based simulation study is presented under variable wind power generation and fault condition to validate the proposed concept.

A Two Ways Communication-Based Distributed Control for Voltage Regulation in Smart Distribution Feeders

Smart grid initiative is based on several pillars among which integrating a wide variety of distributed generation (DG) is of particular importance. The connection of a large number of DG units among loads may result in a severe voltage regulation problem and the utility-side voltage regulators might no longer be able to use conventional control techniques. In addition, smart grid should provide new digital technologies such as monitoring, automatic control, and two way communication facilities to improve the overall performance of the network. These technologies have been applied in this paper to construct a distributed control that has the capability to provide proper voltage regulation in smart distribution feeders. The functions of each controller have been defined according to the concept of intelligent agents and the characteristics of the individual DG unit as well as utility regulators. To verify the effectiveness and robustness of the proposed control structure, a real time simulation model has been proposed. The simulation results show that distributed control structure has the capability to mitigate the interference between DG facilities and utility voltage regulators.

Seamless Formation and Robust Control of Distributed Generation Microgrids via Direct Voltage Control and Optimized Dynamic Power Sharing

Seamless formation and robust control of distributed generation microgrids are essential requirements to facilitate powerful and flexible control infrastructure in future smart power grids. Motivated by this objective, this paper presents a control structure for microgrid converters based on direct-voltage control and optimized dynamic power sharing. The salient features of the proposed scheme are 1) minimum switching actions between grid-connected and isolated microgrids systems to minimize internal microgrid formation disturbances; 2) active damping control performance in the converter control voltage vector to effectively reject both voltage magnitude disturbances and power angle swings associated with mode transition and load disturbances; and 3) high bandwidth direct voltage control loop in both grid-connected and isolated microgrid modes to improve the dynamic response and disturbance rejection performance. Theoretical analysis and comparative experimental results are presented to validate the effectiveness of the proposed control scheme.

Impact of wind turbine generators on network resonance and harmonic distortion

Distributed generators (DGs) are being increasingly connected at medium voltage level distribution networks. It is likely that different types of DGs, i.e. wind turbine generators (WTGs), photovoltaic solar systems, etc, may be connected on the same feeder. Most of the modern DGs are equipped with power electronic converters at their terminals, which act as sources of harmonic injection into the network. Any distribution feeder system will have its own network resonant frequencies. In this paper, the impact of WTG connections on resonant modes of system impedance and total harmonic distortion in the network are discussed in detail. First, a representative study system is devised where the impact of different elements of power system on system resonance frequencies is analyzed. Then the influence of system background harmonics on harmonic distortion is shown for the study system as well as for one of the actual Hydro One distribution feeder systems. It is shown that several scenarios are possible when these resonant frequencies align with harmonic frequencies that are likely to be injected by other power electronic based system equipment including DGs, thus causing unacceptable total harmonic distortion. The understanding will be useful to utilities for avoiding network resonance based harmonic amplification in distribution systems where wind farms and/or inverter based generation systems are integrated.

Evaluation of Two Anti-Islanding Schemes for a Radial Distribution System Equipped With Self-Excited Induction Generator Wind Turbines

Local distribution companies (LDCs) require disconnection of distributed generation (DG) as soon as disturbance events occur to prevent sustainable islanding and damaging DGs and other equipments. In this paper, real-time laboratory testings were carried out to evaluate existing anti-islanding detection schemes. A comprehensive analysis and a testing process are proposed to assess the selection of a fast and reliable scheme for a practical radial distribution system equipped with a group of self-excited induction generator wind turbines (SEIG). Two anti-islanding schemes (passive local detection and communication based) were examined on various commercial relays to disconnect SEIG-DG in case of a main breaker opening due to a fault. The impact of existing recloser timer characteristics on the selection of the proper scheme is illustrated. The selected scheme was verified to disconnect SEIG-DGs in case of a manual main breaker opening in a proper time to protect public safety. A dedicated fiber optic connection and radio frequency communication-based schemes were evaluated and compared with passive local detection scheme performance. Various scenarios of SEIG-DG penetration levels were simulated, and different available commercial relay functions were tested. The obtained results, recommendations, and economic evaluations for choosing a proper scheme are reported and discussed.

SSR Alleviation by STATCOM in Induction-Generator-Based Wind Farm Connected to Series Compensated Line

In this paper, a static synchronous compensator (STATCOM) with a voltage controller is proposed to mitigate the potential of sub synchronous resonance (SSR) in a series compensated induction-generator (IG)-based wind farm. Detailed eigenvalue analysis is performed to demonstrate that IG effect SSR is successfully alleviated by STATCOM. The results are validated through electromagnetic transient simulation with PSCAD/EMTDC. The impacts of symmetrical fault at different locations and collector cables are investigated, and the effectiveness of the proposed STATCOM controller is illustrated. It is shown that a three-phase fault close to the wind farm may cause severe oscillations in the point of common coupling (PCC) voltage, electromagnetic torque, and shaft torque of the wind turbine generator. To examine this situation, an equivalent circuit analysis is presented, which predicts the band of resonant speeds within which the wind turbine becomes unstable. The study is extended to other commercially available IGs, which also show the potential for SSR even at realistic levels of series compensation levels, and the capability of the proposed STATCOM controller to obviate its occurrence.

Novel nighttime application of PV solar farms as STATCOM (PV-STATCOM)

Photovoltaic solar farms generate power only during daytime and remain completely idle during the nights. This paper presents a novel technology of utilizing Photovoltaic (PV) Solar Farms in the nighttime. New controls are developed for the solar farm inverters to operate as STATCOM - a Flexible AC Transmission System (FACTS) Controller, using the entire inverter capacity in the night for accomplishing various power system objectives, such as voltage regulation, improvement of power transfer capacity, load compensation, etc. During the daytime, the same objectives can be achieved to a substantial degree with the inverter capacity remaining after real power generation. This technology is termed PV-STATCOM. Two novel applications of solar farms operating as PV-STATCOM in realistic transmission and distribution systems are presented in this paper. These are : i) Improving power transfer capacity of transmission lines.; and ii) Increasing the connectivity of neighbouring wind farms.

Impact of distributed generation penetration on grid current harmonics considering non-linear loads

In this paper, the impact of large-scale penetration of distributed generation (DG) system in the presence of non-linear loads is addressed. The effect of different DG penetration levels on the grid side current harmonics is examined. It is found that current harmonics generated by downstream non-linear loads with a TDD (total demand distortion) that is compliant with IEEE Standards can lead to significantly high THD (total harmonic distortion) values at the grid side under certain loading scenarios. These low magnitude highly distorted currents may cause protection circuitry and grid-synchronizing circuitry malfunctioning, and may even cause a resonance condition with power factor correction capacitor on the network. This paper further presents a control application of Photovoltaic (PV) solar plant based DG inverter o mitigate the above harmonics problem. The increased harmonic level issue and the application of PV solar plant to mitigate such problem have been demonstrated both by MATLAB/ SIMULINK simulation studies and laboratory experimental results.

Grid voltage regulation utilizing storage batteries in PV solar — Wind plant based distributed generation system

In this paper, a novel night-time application of a photovoltaic (PV) solar power farm bidirectional inverter as a battery charger, is presented. It is shown that a combination of PV solar farm and storage batteries can be advantageous in a distribution system having wind farm in close vicinity to the solar farm. Under such a condition, the PV solar farm inverter can regulate the grid voltage (if allowed by the code) by charging/ discharging the batteries as well as provide better asset utilization and return on investments. The PV solar farm inverter is operated as a three-phase fully controlled rectifier to charge the batteries which draws sinusoidal currents at unity power factor operation. MATLAB/SIMULINK based simulation results are provided to show the effectiveness of the proposed indirect grid voltage regulation utilizing the PV solar farm and storage batteries.

Large-scale photovoltaic solar power integration in transmission and distribution networks

The province of Ontario in Canada has embarked on a major initiative to promote the grid interconnection of photovoltaic (PV) solar power systems. The Ontario Centres of Excellence, Centre of Energy, has recently approved a