GridStab News: Regular posts to demystify power system dynamics and stability
Voltage and frequency support: Difference between Grid-Following and Grid-Forming control
One of my previous article has already discussed why Grid-Forming (GFM) control is more robust than Grid-Following (GFL) in weak grid conditions (the concepts of weak grid/system strength will be discussed in more detail in an other article).
The present article will first discuss the difference between GFL and GFM from a voltage and frequency support point of view. Then it will explain what are the impacts on the electrical grid dynamics.
As a reminder, today, the majority of RES (e.g., Solar, Wind,…) are connected to the grid through power-electronic converters in GFL control mode. In addition, most of the Battery Energy Storage Systems (BESS) and HVDC converters (a future article will discuss in more details the HVDC systems) are also in GFL control mode. GFM control is an emerging and promising technology that is being developed by researchers and the industry to safely operate the energy transition while guaranteeing that electrical grids will continue to operate in a stable manner.
Please keep in mind that this article does not claim to be exhaustive and that much more details can be found in the literature.
Voltage Support in case of voltage dip
In the event of a short-circuit, both GFL and GFM converters are required to inject some reactive current to support the voltage (this article explains the impact of reactive power on voltage).
Both GFL and GFM converters can thus support the voltage in the event of a short-circuit. The main difference lies in the way it is implemented.
Grid-Following control
As already covered in a previous article, GFL converters rely on a PLL to track the voltage evolution at the point of connection and inject the appropriate amount of current. This “PLL” action comes with some delay. Therefore GFL converters cannot instantaneously change their current output in response to a voltage deviation.
Grid-Forming control
GFM converters do not rely on a PLL for synchronization. As per the fundamental definition, they are expected to behave as a voltage source behind an impedance and hence can instantaneously change their current injection in response to voltage deviation.
What is the impact on the electrical grid dynamics
At fault inception, a GFM converter instantaneously increases its current injection level while it takes some time (~[10; 30] ms) for a GFL converter to detect the voltage decrease through the PLL measurement.
At fault elimination, the voltage recovers and GFM converters current injection decreases instantaneous while it again takes some time for GFL converters because of the measurement delay.
In weak grid conditions, voltage sensitivity is high such that a change of current injection will have a significant impact on the voltage. The risk with a GFL converter is a post-fault over voltage. Indeed, while the voltage has recovered, a GFL converter will keep injecting current during a short amount of time since the voltage variation is detected with some delay. This can, if voltage sensitivity is high, lead to an over voltage above the protection limit of the converter and trigger its disconnection. This risk is avoided with GFM control since there is no measurement delay.
This is yet another example stressing that GFM control is more adapted to weak grid conditions.
Frequency Support in case of loss of generation
Advanced technical requirements request modern GFL converters to provide what is called “Fast Frequency Response” (FFR). While most of the existing technical requirements of GFM converters require the provision of what is called “synthetic” inertia. But what is then the difference between the FFR and the synthetic inertia provision?
Grid-Following control
While, by definition, FFR is a fast response of GFL converter in response to frequency variation, the latter still relies on the PLL measurements of the grid conditions at the point of connection. This means that there is an “unavoidable” delay pertaining to PLL action. Moreover, in most cases, FFR relies on the measurement of the Rate Of Change of Frequency (RoCoF), and quickly modulate its active power output to support the grid frequency. RoCoF is computed over a given time window which also leads to additional delay. The “length” of the time window is a trade-off between a sufficient measurement accuracy (it has to be sufficiently large to reduce measurement noise) and a small response delay.
Grid-Forming control
GFM converters do not rely on frequency measurement to provide synthetic inertia. The provision of synthetic inertia is an inherent active power response of GFM converters which can hence instantaneously respond to a frequency deviation.
What is the impact on the electrical grid dynamics
On the one hand, as FFR is not an instant response but a control function, it takes time to measure and identify signals. Thus, FFR provided by GFL converters cannot affect the initial RoCoF (more details on frequency dynamics are provided in this article). The impact on FFR on frequency dynamics is shown in the Figure below as an illustration. It is seen than FFR rather helps arresting frequency decline to avoid Under Frequency Load Shedding (UFLS) in low-inertia grids.
On the other hand, with their instantaneous active power response to frequency decrease, GFM converters are also capable to decrease the RoCoF and avoid additional stress on the system. This is why this response is referred to in the literature as “synthetic” inertia, as inertia is the only grid characteristic that has an impact on the RoCoF after the sudden loss of generation in the electrical system.
To keep in mind
Grid forming and grid following converters are both important technologies for integrating renewable energy sources into the power system. However, as the share of renewable energy sources increases in the grid, there is a need for more grid forming inverters to ensure the grid reliability and security.
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