Frequency control and short term volatility
Short term volatility is what will crash the grid
What is frequency control and why is it important in our energy systems
Energy supply needs to be available exactly when the consumers need it. Matching supplies with demand is the key responsibility of all actors in the energy supply chain. In the current hardware of electricity systems, the supply-demand-balance has another very important implication for reliability. The balance is necessary for maintaining the system’s frequency, which needs to be stable each second of the day for the system to function properly. When the frequency becomes volatile, this results in system-level instability that can crash the grid and cause blackouts.
Frequency became a basic variable in our electricity systems with alternating current (AC), the form that supplies electricity to most end-users and everyday appliances today. When synchronised to the AC energy system, the electric motors that produce power have to run at the same speed (within limits) as the whole electricity grid’s frequency in order to deliver power to it. Power sources - for example, solar cells - that generate direct current (DC) can be linked to AC energy systems with special inverters.
Power systems have operated at a wide range of frequencies historically, but today’s electricity grids can be divided to two groups: 60 Hz in the United States, Canada, Latin American and Asian countries: and 50 Hz in the United Kingdom, China and elsewhere in the world (Japan uses both frequencies).
This synchronised frequency is what allowed expansion of the interconnected energy supplies in society. The interconnected electricity grid of Continental Europe, Australia’s National Electricity Market, or most other national systems would not function in the same manner without having a single stable frequency. One frequency enables energy exchanges between electricity utilities and even different countries in a single interconnected system.
But by relying on this synchronised operation of key components, controlling the frequency and keeping it within limits becomes an essential requirement for modern energy systems just as having energy when and where we need it, or energy security. The obligation of frequency control typically lies with the big actors of the energy industry: national high-voltage operators such as UK’s National Grid, regional transmission organisations in the US, or the Australian Energy Market Operator (AEMO).
The history of frequency control shows how closely it was linked with a centrally controlled system. Initially, system frequency was monitored by central human controllers. When the frequency fell, these controllers asked power plants to increase the flow of energy to the system, for example by fueling boilers in thermal plants or increasing water flows in hydropower installations - and the opposite if the frequency rose. Other generators in the energy system and even some energy users can participate in frequency control via special frequency reserve markets.
In today’s energy systems, frequency control also uses systematic automation and specialised control components. The Faraday Exchanger is such a power flow control device which maintains target voltage, frequency, and power factor.
What is short term volatility and why is it more destructive than intermittence?
In a synchronised energy system, any unanticipated volatility in either energy supply or demand will affect the grid frequency rapidly. If electricity generation increases unexpectedly, this will increase the system’s frequency. If it decreases, the frequency will decrease. If demand increases, it decreases frequency, and vice versa. These issues need to be handled by proactive frequency control.
Renewables such as wind power and photovoltaics have changed frequency control. First, because these resources are intermittent, they cause short-term balancing issues that has direct bearing on frequency. Second, these renewables do not function in the same way as conventional frequency control services. Sun and wind resources cannot be turned on or off to adjust the system frequency, like water flowing through a hydro installation. This separates them from so-called dispatchable power sources - notably hydro reserves and natural gas plants - whose output can be adjusted rapidly to participate in frequency control. As the energy mix changes, the feasibility of utility-scale photovoltaics as a frequency response is an area of current interest and research.
The current power system can handle small frequency deviations - for instance, the UK’s National Grid aims at plus and minus 1% from the 50 Hz standard. Yet, the system as such and its demands were not built to deal with frequency deviations. Non-standard frequencies can harm both transformers and appliances in people’s homes. Too many frequency events can also be highly destructive for electricity generators, as the Australian system in 2016 demonstrated.
Case study example - Australia
On 28 September 2016, South Australia experienced a state-wide electricity blackout. Altogether, about 850 000 customers were affected. As the AEMO’s final report highlights, the initial trigger for these events were tornadoes that damaged several, partly distant transmission lines almost simultaneously. The damaged transmission lines were disconnected from the grid which caused a string of anticipated voltage dips. However, wind farms in the region had a special protection setting for reducing their power if they suffered from too many voltage dips during a short period. The power from the wind farms decreased appropriately. Next, an interconnector to Australia’s national electricity market became increasingly strained and subsequently went offline. As a result, the South Australian electricity system became “islanded” from the national system. The generation and the connected load were substantively out of balance and the frequency of the islanded system could no longer be maintained. This led to losing all supply to the grid and collapse of its frequency, and the blackout. All of this had happened only 2 minutes after the initial voltage dips.
The Australian case study shows important conclusions for frequency volatility. Firstly, the wind turbines were not damaged by the high wind speed during the tornadoes nor even by the grid disturbances. The trigger was a safety setting that reacted to voltage dips, a part of power quality just as frequency. Secondly, the case shows how rapidly one problem with frequency - imbalance between supply and demand - can cascade in a synchronous system - that is to say, disconnecting all supplies and losing the frequency. Altogether, these risk events demonstrate how critical frequency control is in the underlying hardware of current energy systems. Yet it is also increasingly challenging with increasing deployment of more sustainable energy supplies, as AEMO sums it in the report:
As the generation mix continues to change across the national electricity market, it is no longer appropriate to rely solely on synchronous generators to provide essential non-energy system services (such as voltage control, frequency control, inertia, and system strength). Instead, additional means of procuring these services must be considered, from non-synchronous generators (where it is technically feasible), or from network or non-network services (such as demand response and synchronous condensers).
The Faraday Exchanger addresses short term volatility
The Faraday Exchanger is a bi-directional power flow device which manages the volatility amid increased renewable electricity generation. It resolves short-term balancing issues by modulating power flows to reduce their noise, dynamically managing the voltage and the frequency in the electricity grid in doing so. Each of these devices is autonomous, yet multiple of them can also be combined to become the Faraday Grid. There are several flexibility options being pursued in the current energy system, but the existing experiences have shown how complex and expensive frequency control has been in large energy systems. Seeking for new ways to organise this issue, rather than just more central control, is a formidable task for future technology solutions.