With the yearly increment of renewable sources for electricity generation and the world government’s incentives towards them, the displacement of synchronous generators with inverter-based sources on the electric grid is a plausible reality that can occur.
Nevertheless, as probable as it could be, here the current electric power system network faces a few problems with this scenery.
Let’s explore how the utility grid works, what inertia means to the grid control and how does solar penetration affects the electric network system.
How does Frequency Regulation work?
The electric grid is designed to function in a steady-state where it can deliver safe and consistent power to every load connected, despite any disturbance (big load changes, partial generation, transmission, or distribution failures), this is the standard for reliable electricity service.
In order to do so, the utility operators need to control the power flow throughout the grid (real and reactive power). Towards this idea, the operators constantly monitor 3 variables that determine the grid state, voltages, angles and frequency.
- Voltage: Refers to the voltage level at every point of the grid. The operator has to monitor the voltage level from the biggest generation point to the last load attached to the service.
- Angles: As the energy is transmitted in alternating current (AC), all voltages and currents come with an angle reference. In particular, the relation in-between angles of the grid determine the flow of active power.
- Frequency: As expected, the whole AC transmission needs to be made at the same frequency. For example, 60 Hz on the US, or 50 Hz on Europe.
By monitoring and controlling these variables, the operator is able to diagnose and regulate all the grid system, allowing preventive or corrective actions in case of any possible problem.
The present utility grid was designed and still operates on the premise that energy sources work mainly with synchronous or asynchronous generators or rotating generators altogether.
As rotating machines are the heart of the grid, the operators control them to regulate all 3 variables.
The rotating speed of any electrical rotating equipment connected to the grid is determined by the grid frequency and many system protections are tripped by the deviations on the system’s frequency. Currently, the grid frequency is determined by the biggest rotating generators of the grid.
Frequency disturbances are generated mainly by the imbalance between the supply of power and the end-user demand. As so, the generators can react to these differences delivering or consuming energy thanks to the inertia of their mechanical rotating parts.
While the grid requires it, and up to a certain point, a generator can increase or decrease the speed of its rotor to deliver energy, guaranteeing the frequency control of the system.
Though this process is just transitional, it offers to the utility grid operator the required time to take further corrective actions on the load change.
This is called primary frequency response (PFR) and acts as the frequency regulation of the system. This is a property that solar power lacks in case of a disturbance on the grid.
Solar Generation Paradigm and Frequency Regulation
In the last few years, the increasing penetration of solar and wind energy generation has resembled some problems to the grid.
As an example, the state of California has found a phenomenon commonly referred to as “The Solar Duck Curve”, in which a graph of power production throughout the day shows the timing imbalance of peak demand and renewable energy production.
Apart from this, as more and more solar and wind power plants are introduced to the grid, the problem extends to the inability of these renewable sources to respond to grid needs in case of a failure or disturbance. Basically, the lack of inertia that rotating generators provides to the grid.
The current procedure for a solar or wind power plant in case of a frequency or voltage deviation on the grid is to completely isolate itself (IEEE 1547). In the case of a total blackout, this procedure is called “Anti-islanding” protection, as it prevents the plant from energizing the grid when there could be a failure in the system.
This doesn’t mean that the inverters used on renewable plants are not capable of frequency control. But in order to do so, these plants will have to operate in a state where they cannot deliver their maximum capacity.
On the other hand, many propose battery storage as a solution, as they can handle energy fluctuations rather fast but at this time this technology is still expensive for massive storage and massification.
Currently, there are two types of categories for inverters used to connect to the grid, grid-following, and grid-forming inverters.
Grid-Following and Grid-Forming Inverters.
At present all PV inverters connected to the grid operate as grid-following (GFL) sources, they regulate their power output by measuring the angle of the grid voltage in a phase-lock loop (PPL). Therefore, they just follow the grid angle/frequency and do not actively control their frequency output.
Alternatively, grid-forming (GFM) sources continuously control their output frequency and voltage, such as rotating generators do.
GFM inverters are mostly used in microgrids, as they actively regulate their output based on measured real and reactive power values. They must be capable of operating as parallel voltage sources with very good load sharing capability while maintaining a stable AC output voltage and frequency with varying loads.
More research needs to be conducted in order to use GFM inverters as a replacement of the grid inertia of rotating generators. But literature suggests that it can be a viable option for the transitioning into a grid of GFM inverters and rotating generators parallel operation.
At the time of writing, solar power plants lack the ability to respond to frequency deviations on the grid, increasing the reliance on synchronous and asynchronous generators inertia to sustain the grid frequency.
The increasing renewable generation penetration on the energy grid market is a reality that needs to be addressed, hence, GFM inverters in parallel operation with rotating generators are a viable option for transitioning into an inverter-dominated grid.
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