Power grid in a changing world
Part 1: Supply-Demand balance and Grid Frequency
Introduction
The power grid, often taken for granted, plays a critical role in our modern society. It serves as the silent conductor of electricity, ensuring that power is generated and delivered to meet the ever-growing demands of consumers.
In this series, we will explore the inner workings of the power grid, beginning with the concept of grid frequency and its pivotal role in maintaining the supply-demand balance.
Understanding these fundamental principles is essential as we prepare to address the challenges of a world increasingly reliant on renewable energy sources.
Quick refresher on AC generator principles
An alternative current (AC) generator is a device that converts a kinetic energy into electricity.
It consists of a coil of wire (a conductor) that is rotated in a magnetic field, inducing a current across the former. The direction of the current can be determined using Fleming’s Right Hand Rule, that we all know so well since high school …
The polarity of the induced electromagnetic field changes every half-rotation, producing an alternative current that switches its polarity in similar frequency. This is the basis on which each and every rotating electric generator works.
The frequency of the electrical output, measured in Hertz, is therefore equal to the conductor rotational speed in revolutions per second. When connecting to a power grid, all AC generators must operate at the same frequency for simultaneous generation. Therefore in the literature they are also referred as synchronous generators.
Finally, a major reason for selecting alternating current for power grid is that its periodic variation with time allows us to leverage transformers. These devices convert electrical power at any voltage and current level to high voltage and low current, enabling long-distance transmission.
Monitoring supply-demand balance with grid frequency
The most important principle of electricity is that it has to be consumed as soon as it is generated. The system operators have the crucial job of making sure that supply and demand always match. Any significant deviation from the equilibrium can have grave consequences for both supply and consumer side.
It turns out that the frequency metric presented in the previous section is the main measure used by operators to monitor and adjust the power grid.
Concretely, how is power consumption and grid frequency related ?
First, let’s remind that Power (P) = Current (I) x Voltage (V).
When power consumption increases, since voltage has to remain constant for devices to operate properly and safely, the other component in the formula has to increase: current.
When current in the armature winding of a generator increases, it strengthens the magnetic field in the armature, which goes the opposite direction of the magnetic field in the field winding that was used to induce the voltage.
Since the two magnetic fields oppose each other, they make the rotor slow down — not much, but enough that the prime mover (steam/water/gas/wind turbine that’s moving it) to consume more fuel to push harder. If the prime mover doesn’t respond to the increased load by spinning harder, the generator will slow down, which will cause a decrease in frequency and voltage.
50Hz have been adopted in Europe and Asia as the standard for power transmission and distribution. This means that all electronic devices in those regions are designed to operate at that frequency, with a tight margin of tolerance (+/- 10 mHz).
The beauty of this mechanism is that it is thus unnecessary to monitor all sources of supply and demand to work out whether there is any imbalance. Changes in grid frequency show this immediately, and also signal its gravity. This allows some control actions to be initiated locally based simply on grid frequency, which changes in a spatially uniform way across the entire AC grid regardless of where supply or demand might be changing.
Frequency control
When the frequency of a power system hits a critical range (typically between 47.5 and 51.5 Hz), a control strategy is activated to restore the supply-demand balance.
Primary Frequency Response
Primary frequency control acts as the initial line of defense, swiftly stabilising the frequency within seconds following an incident. It operates on a simple principle: local power control systems respond to low frequency by increasing inputs like steam, fuel, or water to boost power output and decrease power output in the case of high frequency. All power plants connected to the grid are obligated to provide this service, ensuring each unit has a dedicated reserve power to meet this regulation when needed.
However, there are exceptions to these requirements, especially concerning renewable energy sources. For instance, wind and solar plants are not expected to increase their output when they are already producing the maximum power available from wind and sunlight during low-frequency events.
Secondary Frequency Response
The secondary control’s primary goal is to restore the frequency to its nominal value. To achieve this, specific generators are authorized to execute secondary control using dedicated reserve power. The amount of reserve power varies according to each operator’s needs, typically as a percentage of the generator’s maximum power, with a predefined minimum value to ensure a reliable response regardless of each generator’s capacity.
Conclusion
The power grid, with its remarkable resilience, forms the backbone of our energy infrastructure. It ensures that the electricity we rely on continues to flow seamlessly to our homes, businesses, and industries.
As our world transitions towards a future heavily reliant on renewable energy sources, these principles become even more significant. The challenges of integrating large amounts of renewable energy, such as wind and solar, are intricately linked to the grid’s ability to adapt and maintain its supply-demand equilibrium.
