BLOG

Renewables and dispatchable power – the role of power system inertia and wattless megavars

Image: Shutterstock

Guest blog by David B Watson BSc C.Eng FIES MIET.

The public are generally aware that the output of key sources of renewable energy including wind and solar are variable and intermittent so they need back-up when supply cannot meet demand or indeed when renewables output collapses. Such back-up includes the running of quick-responding gas turbine generation and hydro power including pumped storage.

Superficially this exercise seems simple. However, there is considerable background serious and complex power systems science behind ensuring 24/7 security of supply in the face of these challenges.

System security risks are increasing steadily as we increase the amount of renewable generation on the grid. At the same time we are closing major coal, nuclear and older gas-powered stations and supplanting this large-scale dispatchable generation that historically provided all the system security characteristics such as inertia, frequency control, system strength (also known as Fault Level) and voltage control.

Two key elements among these characteristics are system inertia and, in support of voltage control, wattless power which is electrically measured in Megavars (millions of volt-amps reactive).

So what is reactive power and what are wattless Megavars?

We always require sufficient reactive power around the country to support transmission voltage as it does not travel well; if not the risk of near instantaneous voltage collapse escalates. Much, therefore, has to be locally provided.

Unless the reader is a trained power systems electrical engineer, the concept of reactive power is challenging to understand. The explanation of the theory quickly becomes mathematically impenetrable to the lay person involving, as it does, the mathematics of Complex Numbers which are based on the square root of -1. The topic is better summarised with analogies.

Starting with the dictionary definition – reactive power is the part of complex power that corresponds to storage and retrieval of energy rather than consumption. On an alternating current (AC) power system there are two kinds of power: real/active power that actually carries out work (measured in Megawatts); and reactive power that enables transformers to transform, generators to generate, motors to rotate and transmission lines to transmit (Megavars). Thus, reactive power is needed to keep the grid electric current flowing. It is the power that is not consumed as such but is reflected back into the grid (Volt x amps reactive, or Vars) as opposed to active power (Megawatts) which is the power consumed by the load. This reactive power function serves also to support the voltage in transmission lines.

Reactive power is created when the voltage in a system is applied across a device or ‘group’ of devices in a network which then causes the resultant current flow to be either in phase with, ahead, or behind the applied voltage depending on the characteristics of the device load. If the device is a resistor, such as a simple electric fire, the current will be in phase with the applied voltage, no reactive power is created and all the absorbed power, only kilowatts, goes into heat.

If the device is an inductive load, i.e contains coils, such as a motor, the current cycle will lag behind the applied voltage. Within a transmission line which will always exhibit both inductance and capacitance (see below), storing electrical energy along the length of the line, affects its power handling capabilities. The current cycle may lead or lag at various points in the grid system transmission lines, influencing the line voltage, which can be controlled by adjusting the network reactive power.

In motors the reactive power creates a magnetic field. This magnetic field provides the mechanical force that makes the motor rotate.  For the first quarter of the AC waveform reactive power is absorbed by the motor to create the magnetic field but in the next quarter of the waveform the magnetic field collapses and the energy is returned to the source. This is repeated over the next two quarters of the sine wave but with the opposite polarity meaning the reactive power absorbed twice is returned twice in a voltage sine wave cycle to the source and then repeats with the next cycle, (think of a spring returning the energy to source). Reactive power is therefore the power that circulates between the source and the load, hence reactive power is referred to as Wattless Power or alternatively ‘Wattless Megavars’.

Long cables and power lines introduce what are known as capacitive loads created by the parallel conducting lines because an electric field forms between them, through the air for example. One result of this is an increase in the voltage. Localised adjustment of reactive power is deployed to control this affect. Again, as with our motor, in each half cycle of supply voltage both the capacitor and inductor (coils) accept power and return it to the source meaning net reactive power consumption is zero, i.e. Wattless.

The well known, ‘Wheelbarrow Analogy’ should help readers understand the role of Reactive Power / Wattless Megavars in the transmission of real power Megawatts. (https://www.fossilconsulting.com/blog/power-plant-fundamentals/vars-explained-in-300-words/)

My transmission line analogy, below, is far from perfect but it helps us understand the basic role of reactive power/wattless megavars in supporting grid transmission system voltage and current.

Let’s imagine an Olympic gymnast progressing along a lengthy trampoline, i.e. transmission line, with the bed providing the reactive power to enable the voltage to stay up, and hence the current (gymnast) to travel along the line. If the gymnast encounters a localised inadequate level of transmission line reactive power i.e. a hole in the trampoline bed, the current (gymnast) and the voltage collapse almost instantaneously (in network reality this can be around a couple of seconds!) and the transmission line fails.

So how does system inertia protect electrical frequency?

When riding a bike or driving a car, inertia is the reason why the vehicle remains in motion initially when you stop pedalling or pressing the accelerator. Spinning objects like the wheels or a rotating electricity generator therefore have rotational inertia which is kinetic energy. Today in some of the latest electric vehicles, this inertia /kinetic energy is captured in regenerative braking systems which serve to improve the overall energy efficiency of the vehicle.

In today’s grid, electrical supply frequency is determined by how fast conventional generators spin. In the UK, they spin at 3000 rpm viz 50 cycles/sec known as 50 Hertz in electrical engineering parlance.

The grid acquires inertia from both the generators and the large rotating motors running within it. When a large power plant fails or there is a system disturbance from a generator fault, this inertia can temporarily make up for the power lost. It is available for a few seconds which is typically enough time for the protection systems that control most power plants to respond to the failure before the generators slow down, causing serious supply frequency changes and/or system failure. Sudden loss of load rather than loss of generation has the opposite effect and frequency will potentially rise above 50 Hz; both effects are, not least, domestically hazardous. This is why in the UK for example, National Grid is required to control frequency within 49.5 – 50.5 Hz.

Wind turbines are not directly connected to the grid so do not directly contribute rotational inertia/kinetic energy. As conventional thermal driven generators are replaced with inverter-based resources including wind, solar and battery storage, we decrease the amount of rotational inertia that is available to the grid. However, these same resources can also reduce the amount of inertia/stored kinetic energy actually needed to maintain frequency (see below).

Control of Frequency and its Rate of Change (ROCOF) under system disturbances and its reliance on system inertia/ stored kinetic energy is analogous to the two following figures.

Sketches from an AEMO talk on the Australian South West Interconnected System (SWIS) (unable to locate on the internet).

Imagine high system inertia between the generator supply and the load, such as we had before the renewables era, being analogous to a very large tank of water (Figure 3). If either suffers a disturbance, the Rate of Change of Frequency (ROCOF), or the water level representing the system frequency, reacts slowly as it is little disturbed allowing the grid protection to recover frequency stability.

As renewables contribute increasingly to grid generation the available inertia/kinetic energy has been reduced significantly for the same load (Figure 4) causing a quicker movement in water level i.e. frequency change, resulting from the much less available system inertia; ROCOF significantly speeds up when there are grid disturbances which is now a more common circumstance within the UK grid. Indeed recently, the UK has been changing existing ROCOF protection across almost the entire country, at huge cost, to cope with this factor.

Ongoing serious research and development into maintaining grid stability as the system inertia decreases due to increasing renewables is underway in many countries including into ‘grid forming’ inverters. This R&D needs to continue at pace to meet and stay ahead of the increasing challenges.

Hydro back-up in Scotland

In Scotland, annual output from Jan/Dec 2020 of hydro, including pumped storage, was 10.5% of demand but from July 2022 to June 2023 it was down to 6.8%. This is partly due to an increase in Scotland’s total generation output via the start-up of new renewables. But, I suspect, this 35% reduction in measured Megawatts power output was partly attributable to quick responding hydro stations, such as Foyers, being increasingly run to output only reactive Megavars, not active Megawatts, in order to keep the weakening north of Scotland grid voltage afloat.

Summary

Electrical power systems are immensely more complex than simply installing Megawatts of renewables generation to match Megawatts of load demand. For example, the UK is having to introduce huge and expensive catch-up changes and related upgrades to protect the grid system as a result of an unco-ordinated rush to renewables. The costs are over and above the oft-quoted Contracts for Difference strike prices for renewable generation and will largely, and probably totally, be met via our bills.

Looking forward to 2025, the UK’s National Grid predict that new tools to forecast and manage the growing dynamic nature of an increasingly renewables dominated grid towards 2035 are “crucial”.

 

(This blog has been developed and expanded from an initial piece by the author recently published by The Herald newspaper.)

David B Watson is a Chartered electrical engineer who was in the energy business for 35 years 30 of which within Foster Wheeler Energy Ltd where before retirement he was Manager of Projects of their Glasgow Operations.

David is a Fellow of the Institution of Engineers in Scotland and a Corporate Member of the UK Institution of Engineering and Technology and has been widely published within the UK on power systems issues.