Electrical Grid Control
A Simplified Explanation of How Grids are Managed.
The electrical grid operates in real time, requiring a precise, second-by-second balance between supply and demand. If this delicate equilibrium is disrupted, blackouts or even a complete grid failure can occur within minutes.
Generation Classifications
Electrical generators are categorized into three types:
Baseload: Generators: in this category operate at a constant output set by the operator, independent of fluctuations in grid demand.
Variable: Variable Renewable Energy (VRE) generators are unpredictable, functioning independently of both grid demand and other generators. See Wind and Solar - Part-Time Generators for more information.
Dispatchable: To ensure grid stability and meet all demand, dispatchable generators adjust their output in real time to closely match grid requirements, maintaining the critical balance between supply and demand.
Grid Main Operating Parameters
There are two main operating parameters that all generators must comply with on a second-to-second basis to prevent black-outs or complete grid failure.
Voltage
A grid generator typically outputs voltage in the range of thousands of volts, with large power plants generating electricity at voltages between 13,800 and 26,000 volts, which is then stepped up to even higher voltages for long-distance transmission on the power grid.
Frequency
Grid frequency reflects the balance between electricity supply and demand and must be regulated within strict limits on a second-by-second basis.
The standard frequency for electric power systems is typically 50 Hz or 60 Hz, depending on the region, and is maintained within a narrow range of a few hundred millihertz.
In a 50 Hz system, such as the UK’s, the frequency must remain between 49.5 Hz and 50.5 Hz. If it deviates beyond this range, automated protective devices will disconnect sections of the grid, leading to blackouts or, in extreme cases, a complete grid failure.
Other secondary factors, such as reactive power and short-circuit protection, also play a role but are beyond the scope of this discussion.
Figure 2 illustrates the grid frequency of the UK system over a 24-hour period.
Grid Control
Frequency acts as an indicator of the balance between electricity supply and demand, serving as the primary controlled variable for all generators.
When demand exceeds supply, frequency decreases.
When supply exceeds demand, frequency increases.
Baseload
Baseload refers to the constant output (in megawatts, MW) that certain generators are set to provide to meet ongoing grid demand.
Each generator’s output can be adjusted to align with grid requirements.
Operating at a steady output, baseload generators enhance grid stability by enabling dispatchable generators to operate within their control range.
Typically, several dispatchable generators are run in baseload mode to maintain grid stability.
Note: Historically, baseload was linked to generators slow to respond to demand changes, a misconception rooted in early coal-based systems, which were once the dominant power source. Modern usage of the term focuses on consistent output rather than response time.
Variable
Wind, solar, and other variable sources like tidal are not controllable and operate independently of demand or other generators. See Wind and Solar - Part-Time Generators for more information.
If their combined output causes supply to exceed demand, generation can be reduced by curtailing (shutting in) these sources.
Output can only be increased by bringing previously curtailed generators back into service, as their production cannot otherwise be actively ramped up.
Dispatchable
Dispatchable generators are the most important generator type. They are CRITICAL to maintaining grid reliability.
They are the only generators capable of adjusting output to match demand, ensuring an exact balance with total supply.
Failure to maintain this balance can lead to blackouts or even complete grid failure within minutes.
They depend on baseload generators to ensure they are operated within their designed control range.
Energy Storage
Due to the high variability and unpredictability of variable renewable energy (VRE) sources like wind and solar, grid energy storage is becoming increasingly important.
Advantages:
Storage can function in either baseload or dispatchable modes, providing flexibility.
It enables VRE generation to be stored and shifted to periods of higher demand.
Disadvantages:
High costs: Both capital expenditure (Capex) and full cost of energy (FCOE) are significant. See Wind/Solar - Grid Based Energy Storage Explained for more information.
Efficiency losses: Typically, 10% to 20% of the energy received is lost during storage and retrieval.
Seasonal demand: Storage capacity needs fluctuate, often leaving storage facilities underutilized.
Limited capacity: Positioned downstream of solar panels or wind turbines, storage is constrained by their output. In contrast, coal and natural gas benefit from abundant, low-cost, and nearly unlimited upstream fuel storage.
Interconnectors
Interconnectors enable the following capabilities:
Import Energy: When a grid’s generation falls short of demand, interconnectors allow additional energy to be imported. This can also occur when another grid offers cheaper power, often due to surplus wind or solar generation that would otherwise face curtailment.
Export Energy: When wind or solar generation exceeds local needs, surplus energy can be transmitted to another grid to prevent curtailment, typically at a reduced price.
While energy imports usually serve as baseload, some High Voltage Direct Current (HVDC) systems offer a degree of dispatchable flexibility.
The Role of Inertia
Grid inertia is the energy stored in rotating generators and is CRITICAL to maintaining grid stability. Without it, blackouts or even total grid failure would occur within minutes.
How does grid inertia work?
Stores kinetic energy: The rotating masses of large generators store kinetic energy.
Balances supply and demand: This stored energy helps balance supply and demand fluctuations.
Slows frequency changes: Inertia slows down changes to frequency, which helps reduce the impact of imbalances.
Prevents blackouts: Grid inertia helps avoid blackouts and keep the lights on.
Why is grid inertia important?
Withstands disturbances
High inertia means the system can better withstand sudden disturbances, such as a generator tripping or a sudden surge in demand.
Provides time for corrective actions
Frequent changes in a grid with high inertia give grid operators time to respond and take corrective actions.
Available Inertia from Different Generators
Source: Michael E. Webber (@MichaelEWebber) - Link
