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by Eliza Booth, Journalist

A reliable power system needs adequate levels of system strength and inertia, which to date have been provided by traditional synchronous generators. A lack of system strength or inertia within the power system brings with it an increased risk of system instability and supply interruptions. Here, we take a look at what inertia is, the role it plays in grid stability and the impact of increased renewable energy on the grid’s ability to balance supply and demand.

The role of inertia in electricity generation

Inertia is one of those aspects of energy supply that many people who aren’t in the industry are generally completely unaware of. However, for those in the energy business, inertia is an essential part of keeping the grid stable and ensuring that energy supply is able to keep up with demand.

In the simplest terms, inertia is one of the basics of physics, being that of Newton’s first law, which is often stated as ‘an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.’1. So, basically anything with moving parts has inertia, which brings us to the electricity grid.

Inertia in power plants refers to the energy that is stored in rotating generators or industrial motors. This stored energy helps to keep the rotating parts moving. Any power plant that has moving parts, like coal-fired and hydroelectric plants, has inertia, and this inertia plays a massive role in grid stability.

Let’s use a coal-fired power plant as an example. Coal is burned, which produces steam, which spins a turbine, which generates electricity.

The plant works with the help of a heavy, rotating mass that has inertia. If you suddenly stop burning coal, the turbine will keep spinning – until friction slows it down.

The stored energy from inertia is really valuable as a backup source of energy if a plant experiences a failure. The inertia can temporarily make up for the lost power, generally giving the mechanical systems that control most power plants several seconds to detect and respond to the failure.

Grid frequency, which is a measure of the balance of supply of electricity and demand, can suddenly drop if a large power plant or transmission tower fails. This is bad news as the electricity grid, and everything connected to it – power plants, powerlines and home appliances – are designed to work at a specific frequency: 50Hz.

As the electric grid constantly adapts to changes in how much energy people are using and how much energy is being generated, frequency fluctuates. The grid can handle these small, constant changes in frequency because the spinning parts in traditional power plants can rotate slightly faster or slower to help balance out supply and demand.

In this way, inertia acts as a shock absorber, meaning it can resist a drop in frequency and give the grid time to rebalance supply and demand. Thanks to the built-in inertia of hydro, coal and natural gas plants, and practices like load shedding – which involves disconnecting a portion of the customer load – grid frequency typically stays within a safe range.

But when the system deviates too far from 50Hz, things start to malfunction, leading to power outages and massive blackouts. Up until recently, inertia was often taken for granted.

However, the development and uptake of new energy production technologies that don’t have moving parts, like solar panels, is creating challenges for grid operators, and the industry is now having to look for solutions to this pressing issue.

The impact of renewables on grid inertia

The world is more conscious than ever about the impacts humans are having on the environment and climate. As such, we are continually making strides towards a carbon-neutral future and the uptake of clean energy options is on the rise.

More and more families and businesses are taking advantage of affordable and green energy production and storage options, with rooftop solar panels and batteries now common in communities across the country.

However, these new technologies, while fantastic for the environment, are creating new challenges for grid reliability, and operators are having to come up with new ways of maintaining stable frequency with declining amounts of conventional inertia.

Most conventional power plants turn the spinning energy of a turbine into electricity via a synchronous generator, which inherently produces alternating current (AC) electricity.

Solar PV and batteries produce direct current (DC) electricity, which must be converted into AC for use by the grid. This converter is known as an inverter, and conventional generators are increasingly being replaced with inverter-based variable-generation renewables.

As more of these inverter-based renewables are added to the mix and more traditional generators are switched off, the level of inertia available to grid operators is decreasing. So how do we overcome this to ensure the grid remains stable in case of an emergency?

Even though the amount of inertia is decreasing due to the uptake of renewables, these inverter-based resources actually have a counterbalancing effect. This is because of something called fast frequency response.

These inverter-based renewables reduce the amount of inertia available, but replace some of the mechanical processes of traditional generation with electronic sensors that can measure frequency and respond incredibly quickly – in fractions of a second in most cases.

Fast frequency response means that we don’t need to rely on the levels of inertia previously required to keep the grid stable and keep up with demand. In fact, one study from the National Renewable Energy Laboratory (NREL) in the US indicated that wind turbines can respond to changes in frequency ten times faster than traditional generators, and solar can respond up to 50 times faster2.

Modern wind turbines do not use synchronous generators and therefore do not provide inertia in the traditional sense. However, wind turbines do have kinetic energy in the rotating mass of the blades, shaft and generator that could be extracted to rapidly inject real power into the grid.

Some renewable technologies – including hydropower, geothermal or biomass – do use synchronous generators that can provide inertia.

It is worth noting that there are non-generation alternatives to provide inertia, the most common of which is the deployment of synchronous condensers, which are synchronous motors/generators that draw energy from the grid to maintain a spinning mass and provide power to the grid in the same manner as a synchronous generator.

Research shows that the combination of inertia and mechanical frequency response can, to a large extent, be replaced with electronic-based frequency response from inverter-based resources and fast response from loads, while maintaining system reliability.

Given these solutions, the reduction in inertia is no longer a barrier to the uptake of renewable energy generation, which is a win for the industry, customers and of course, the environment.

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