A plausible reality that could occur is the displacement of synchronous generators with inverter-based sources on the electric grid, as evidenced by the annual increase in renewable sources for electricity generation and the world government’s incentives to do so.
Nonetheless, as likely as it may be, the current electric power system network is confronted with a number of challenges due to the surrounding landscape.
Let us investigate how the utility grid operates, what inertia means in terms of grid control, and how solar penetration affects the electric network system.
How does Frequency Regulation work?
A steady-state electric grid is one that can deliver safe and consistent power to every load connected, regardless of any disturbance (such as large load changes, partial generation, transmission, or distribution failures). This is the industry standard for delivering dependable electricity service.
In order to accomplish this, utility operators must maintain control over the flow of electricity throughout the grid (real and reactive power). In order to achieve this goal, the operators continuously monitor three variables that determine the state of the grid: voltages, angles, and frequency.
Voltage: This term refers to the level of voltage present at each point on the grid. The voltage level must be monitored by the operator from the most powerful generation point to the last load connected to the service network.
Angles: Because alternating current (AC) is used to transmit energy, all voltages and currents have an angle reference associated with them. Active power is determined by the relationship between angles on the grid, and this is particularly true for active power.
Frequency: As expected, the entire alternating current transmission must be carried out at the same frequency. For example, in the United States, 60 Hz is used, while in Europe, 50 Hz is used.
When these variables are being monitored and controlled, the operator is able to diagnose and regulate the entire grid system, allowing for the implementation of preventative or corrective actions in the event of a potential problem.
Because energy sources work primarily with synchronous or asynchronous generators or rotating generators altogether, the current utility grid was designed and continues to operate on the assumption that this is still the case.
In order to regulate all three variables, rotating machines are controlled by the operators, who are located at the heart of the grid.
The frequency of the grid determines the rotational speed of any electrical rotating equipment connected to the grid, and deviations from the system’s frequency cause many system protections to trip. At the moment, the frequency of the grid is determined by the size of the largest rotating generators on the grid.
Frequency disturbances are caused primarily by an imbalance between the supply of electricity and the demand for electricity from end users. As a result of the inertia of their mechanical rotating parts, the generators are able to react to these variations by delivering or consuming energy, respectively.
If necessary, and up to a certain point, a generator can increase or decrease the speed of its rotor to deliver energy, ensuring that the frequency control of the system is maintained.
While only transitional in nature, this process provides the necessary time for the utility grid operator to implement additional corrective measures in response to the load change.
This is referred to as the primary frequency response (PFR), and it is responsible for the frequency regulation of the entire system. This is a property that solar power does not have in the event of a power outage on the grid.
Solar Generation Paradigm and Frequency Regulation
The increasing penetration of solar and wind energy generation has resembled some grid problems in recent years, owing to the increasing penetration of solar and wind energy generation.
According to the state of California, a phenomenon known as “The Solar Duck Curve,” in which a graph of power production throughout the day reveals an imbalance between peak demand and renewable energy production at different times of the day, has been discovered.
Aside from that, as more and more solar and wind power plants are added to the grid, the problem becomes more complicated due to the inability of these renewable sources to respond to grid demands in the event of a failure or disturbance. Essentially, the lack of inertia that rotating generators provide to the grid is what causes it to fail.
A solar or wind power plant’s current response to a frequency or voltage deviation on the grid is to completely isolate itself from the rest of the grid and shut down (IEEE 1547). In the event of a total blackout, this procedure is referred to as “Anti-islanding” protection because it prevents the plant from re-energizing the grid if there is a potential failure in the electrical system.
This does not rule out the possibility that the inverters used in renewable energy plants are capable of frequency control. However, in order to accomplish this, these plants will have to operate in a state in which they are unable to deliver their full capacity.
On the other hand, many people advocate for battery storage as a solution because it can handle energy fluctuations quickly. However, at this time, battery storage technology is still prohibitively expensive for large-scale storage and massification.
Grid-following inverters and grid-forming inverters are the two types of inverters that are currently available for use in grid-connected applications.
Grid-Following and Grid-Forming Inverters
The majority of photovoltaic (PV) inverters connected to the grid currently operate as grid-following (GFL) sources, which means they regulate their power output by measuring the angle of the grid voltage in a phase-lock loop (PPL). Thus, they simply follow the grid angle/frequency and do not actively control the frequency output of their frequency generator.
Grid-forming (GFM) sources, on the other hand, are sources that continuously control their output frequency and voltage, much like rotating generators do.
Due to the fact that they actively regulate their output based on measured real and reactive power values, GFM inverters are most commonly found in microgrids. While operating as parallel voltage sources with excellent load sharing capability, they must be able to maintain a stable alternating current output voltage and frequency when operating with varying loads.
A lot more research needs to be done before GFM inverters can be used to replace the grid inertia created by rotating generators on the power grid. However, according to the literature, it may be a viable option for the transition to a grid of GFM inverters and rotating generators operating in parallel with one another.
Because solar power plants lack the ability to respond to frequency deviations on the grid, they are increasingly reliant on the inertia of synchronous and asynchronous generators to maintain the grid frequency, as of the time of writing.
It is a reality that must be addressed, and therefore, GFM inverters operating in parallel with rotating generators are a viable option for achieving the goal of a grid that is dominated by inverters.