Accelerating the integration of renewables

With the demand for carbon-free electricity driving the global movement to add renewable energy to the grid, the question of how to integrate it into power systems most effectively is an important one. Researchers at UCT are ensuring that the university is at the cutting-edge of this field.

As demand grows to integrate renewable energy sources – most notably solar and wind – into power distribution systems, it becomes increasingly urgent to be able to design, analyse and operate systems in the presence of the uncertainties associated with these sources.

Analysing the flow of electric power in an interconnected system – known as load flow –allows engineers to determine the conditions of the system during normal, steady-state operation, such as voltage, line currents and power losses. However, the combination of random variability of customer demand and the intermittency of power production from renewable energy sources means many different scenarios are possible. This means we need a probabilistic approach – one based on probability – to accommodate these uncertainties.
 

“Although a great number of approaches have been developed, speed is usually associated with a loss of accuracy.”

Munyaradzi Justice Chihota, a doctoral candidate in UCT’s Department of Electrical Engineering, has developed an analytical tool for just this purpose. It examines the load flow of systems with high levels of renewable energy and offers faster, more accurate and more representative solutions than were previously possible.

Managing randomness

“The capacity of renewable energy doubled globally between 2007 and 2017,” says Chihota. “While the world recognises the benefits of adding energy from renewable sources to the grid, there are critical challenges.

“The biggest challenge is that power systems were not designed to cater for the additional distributed generation, which can create technical challenges, such as rises in voltage, thermal and equipment overloads, and an increase in faults.”

Because of the many unknowns associated with renewables – for example, the uncertainty of weather and therefore how much energy will be contributed by photovoltaic or wind supplies – the integration of these energy sources requires an extensive analysis of hundreds of possible scenarios.

“The most basic and renowned approach, called a Monte-Carlo Simulation, involves calculating likely system states by making up different scenarios and solving each one,” explains Chihota. “Solving the load flow for every combination of probable load and generation conditions is computationally expensive and slow, limiting the scale of possible investigations.

“Although a great number of approaches have been developed, speed is usually associated with a loss of accuracy.”
 

“The biggest challenge is that power systems were not designed to cater for the additional distributed generation, which can create technical challenges, such as rises in voltage, thermal and equipment overloads, and an increase in faults.”

Chihota’s research builds on collaborative work begun in the 1980s by Trevor Gaunt, now an emeritus professor in the Department of Electrical Engineering, and Dr Ron Herman, then at Stellenbosch and later a senior research officer in the UCT Department of Electrical Engineering.

Gaunt and Herman’s work led to the adoption of the Herman Beta approach as South Africa’s national standard for designing low-voltage electrification feeders, which transmit energy from central power stations to the distribution system. This had to be adapted when renewables landed.

“When we recognised that the nature of electricity distribution was changing, we incorporated a way to calculate the effects of distributed energy generation to the Herman Beta approach,” says Gaunt.

UCT master’s students Holiday Kadada and Emmanuel Namanya refined Gaunt and Herman’s load models and calculations. Their work was extended when Chihota began reviewing the basic assumptions that restricted its original application to low-voltage feeders.

A tool like no other

Chihota’s near-complete doctoral research, part of which he presented at the recent Power Systems Computation Conference (see box below), provides a tool for analysing probabilistic load flows. It is an algorithm that’s deployed to software for designing distribution networks, including low and medium voltage, and transmission networks.
 

Because of the many unknowns associated with renewables – for example, the uncertainty of weather and therefore how much energy will be contributed by photovoltaic or wind supplies – the integration of these energy sources requires an extensive analysis of hundreds of possible scenarios.

The tool has been developed to solve diverse feeder configurations – the branched power supply lines that feed connected customers – thus extending its application to a variety of types of power systems in all countries. It enables operators of distribution energy systems, network designers and distribution engineers to analyse systems with high levels of renewable energy penetration more quickly and accurately than was previously possible. The improved understanding of network performance under variable conditions also helps authorities to formulate policy and regulations for renewable energy integration.

“Municipalities that are trying to work out the limits of penetration for distributed generation, particularly from photovoltaics in residential and commercial sectors, have already shown interest in the tool,” says Chihota.

It is a rewarding project. “The relevance and even urgency of what I have been focusing on makes this work very exciting and my expertise worthwhile,” he concludes.

Gaunt underscores the significance of Chihota’s work. “The accurate transformation of customer loads and distributed generation to expected voltages and currents, as developed by Justice, is thousands of times faster than existing approaches and opens up the possibility of more complex studies,” he says.

UCT is well placed to make the most of these possibilities: “Five new students have registered to continue the work,” adds Gaunt.