Impact of High Penetration of Photovoltaics on Low Voltage Systems and Remedial Actions

Residential rooftop photovoltaics (PV) systems have great potential to supply part of the growing energy demand. However, its non-dispatchable, fluctuating, and intermittent characteristics may negatively impact the power quality and reliability (PQR) of low voltage (LV) distribution feeders. Large amounts of non-dispatchable PV sources, if integrated in a distributed way, can reverse the power flow in the feeder and lead to overvoltages. In diesel-based autonomous systems with high-penetration of PVs, the reduction in net load can significantly increase the wear and tear on the diesel genset(s). Therefore, connection of only a modest amount of PV is currently allowed at the LV level without a prior impact-assessment study. This thesis focuses on the detrimental impact of high penetration of PVs on LV systems and on the remedial actions that can be taken to increase PQR and the displacement of fossil fuels in diesel-based autonomous systems. Two scenarios are considered. First is the possibility of overvoltages in LV grid connected systems during periods of high PV generation and low load. The solutions used in medium-voltage feeders need to be revisited in light of the mostly resistive characteristics of LV feeders. An alternative is to use active power curtailment (APC) techniques. A droop-based APC approach, in which all inverters use the same droop coefficients, is analyzed. However, this strategy results in more APC in the PV inverters located near the end of the feeder than in the ones in the beginning. A new approach is proposed that allows equal sharing of the APC. A one-year simulation study assessed the overvoltage occurrences, the sharing of the burden for overvoltage prevention per house, and the total energy yield of the feeder using a benchmark based on typical parameters of Canadian LV feeders. The second scenario involves the impact of high-penetration PV systems in diesel-based autonomous LV systems, which are typical of remote communities. The use of APC of PV inverters is discussed, focusing on reducing the frequency variations and ensuring the diesel genset's minimum load, and can improve fuel consumption. This theoretical analysis is validated by simulations. Fuel consumption and yearly energy yields are estimated using statistical information about the load and solar irradiation

Impact of active power curtailment on overvoltage prevention and energy production of PV inverters connected to low voltage residential feeders

As non-controllable power sources, photovoltaics (PV) can create overvoltage in low voltage (LV) distribution feeders during periods of high generation and low load. This is usually prevented passively by limiting the penetration level of PV to very conservative values, even if the critical periods rarely occur. Alternatively, one can use active power curtailment (APC) techniques, reducing the amount of active power injected by the PV inverters, as the voltage at their buses increase above a certain value. In this way, it is possible to increase the installed PV capacity and energy yield while preventing overvoltage. This paper investigates a number of approaches for sizing and controlling the PV power generated by 12 net-zero energy houses equipped with large rooftop PV systems in a typical 240 V/75 kVA Canadian suburban radial distribution feeder. Simulations of a one year period with typical solar irradiance and load profiles are conducted with PSCAD to assess the performance of the different approaches in terms of overvoltage occurrence, sharing of the burden for overvoltage prevention per house and total energy yield of the residential PV feeder

Virtual Inertia: Current Trends and Future Directions

The modern power system is progressing from a synchronous machine-based system towards an inverter-dominated system, with large-scale penetration of renewable energy sources (RESs) like wind and photovoltaics. RES units today represent a major share of the generation, and the traditional approach of integrating them as grid following units can lead to frequency instability. Many researchers have pointed towards using inverters with virtual inertia control algorithms so that they appear as synchronous generators to the grid, maintaining and enhancing system stability. This paper presents a literature review of the current state-of-the-art of virtual inertia implementation techniques, and explores potential research directions and challenges. The major virtual inertia topologies are compared and classified. Through literature review and simulations of some selected topologies it has been shown that similar inertial response can be achieved by relating the parameters of these topologies through time constants and inertia constants, although the exact frequency dynamics may vary slightly. The suitability of a topology depends on system control architecture and desired level of detail in replication of the dynamics of synchronous generators. A discussion on the challenges and research directions points out several research needs, especially for systems level integration of virtual inertia systems

Overvoltage Prevention in Residential Feeders with High Penetration of Photovoltaics

Overvoltages in low voltage (LV) feeders with high penetration of photovoltaics (PV) are usually prevented by limiting feeder\u27s PV capacity to very conservative values, even if the critical periods rarely occur. This paper discusses the possiblity of overvoltages in residental feeders with high penetration of PV, and the main factors that may lead to overvoltages. A state of the art review is performed regarding strategies that could be used to reduce the likelihood and magnitude of overvoltages, along with the main solutions for voltage control that can be applied in LV feeders with high penetration of PV

Data Center Load Forecast using Hidden Markov Models

The energy cost of data centers tantamount to their overall operational cost. A possible solution to this immense cost could be proper scheduling of the power resources. This can be achieved by forecasting the data center loads. However, highly variable nature of the data center loads makes it challenging to use the traditional methods of load forecasting. In this paper, a stochastic method based on Hidden Markov process is developed to model the data center load and is used for a day-ahead forecasting. This method is out-standing because of its flexibility in addressing the variable nature of the data center load. The utility of the model is illustrated using a dataset from National Renewable Energy Laboratory - Research Support Facility (NREL - RSF). Two models created based on the proposed method yielded Mean Absolute Percentage Errors (MAPE) of 1.49% and 3.89%

Impact of High PV Penetration on Voltage Profiles in Residential Neighborhoods

The objective of this paper is to provide an assessment on voltage profiles in residential neighborhoods in the presence of photovoltaic (PV) systems. The network was modeled in PSCAD using common feeder characteristics that Canadian system planners use in suburban residential regions. A simulation study was performed to investigate potential voltage rise issues in the networkupto11.25%totalPVpenetrationinthefeederandLVtransformer capacity penetration up to 75%. Results indicate that the PV penetration level should not adversely impact the voltage on the grid when the distributed PV resources do not exceed 2.5 kW perhouseholdonaverageonatypicaldistributiongrid.Moreover, the role of feeder impedance, feeder length, and the transformer short circuit resistance in the determination of the voltage rise is quantified

Impact of Active Power Curtailment of Wind Turbines Connected to Residential Feeders for Overvoltage Prevention

Overvoltage is a major limiting factor for the integration of distributed generation in distribution systems. Among various solutions to the overvoltage problem, active power curtailment is an attractive solution since only small adjustments in the controllers are necessary for implementing this technique. Furthermore, it is activated only when needed. In wind energy systems, this can be achieved by adjusting the pitch angle of a wind turbine. The objective of this paper is to develop an active power curtailment strategy for small wind turbines to prevent overvoltage by controlling the pitch angle of the turbine blades. The required blade pitch angle was calculated by using the voltage droop method. The developed control strategy was validated by a real time simulation of a rural feeder model developed using RT-Lab software. The feeder is based upon a North American rural distribution network consisting of 96 small wind turbines of 20 kW, a 30 MVA distribution substation with an on-load tapchanging transformer, a rural feeder with 288 houses, a 120 kV/25 kV LV transformer, and rural loads. The results show the effectiveness of the method in keeping the voltage within acceptable limits and a reduction in network losses

Load Management for Grid-Connection of Household Photovoltaic Systems without Net-Metering

Grid-tied systems have become the most common type of residential photovoltaic installation and self-consumption has become a key issue to improve cost effectiveness of these systems in the absence of feed-in tariffs and net-metering. The objective of this study was to analyze the effectiveness of load management and battery storage in increasing self-consumption in residential grid connected PV systems without net-metering. Home loads were divided into time manageable, storage capable and non-manageable loads. Shifting the time manageable and storage capable loads from evening hours to PV generation hours increased self-consumption by up to 50%. The addition of batteries to a grid tied PV system can increase self-consumption to 100%, but there was no net cost savings due to the additional cost of the batteries and power conditioning unit. Grid- tied PV systems with no net metering should be designed for daytime loads only to maximize cost effectiveness. A theoretical seasonal analysis shows that self-consumption through load management doubled in winter compared to summer. Load management can be an effective tool to make residential grid-tied PV systems economically beneficial to consumers as feed-in-tariffs are reduced and caps on net-metering are increased