GridStab News: Regular posts to demystify power system dynamics and stability

Topic of this week: System Strength

Did you say “System Strength”?

System strength traditionally reflects the amount of short-circuit power in the vicinity of a connection point. This is usually used as local indicator even if some grids are usually “stronger” (e.g., Continental Europe) than others (Australie, UK,…) on average.

Local indicators of system strength are based on what is called in the literature the Short-Circuit Ratio (SCR) which reports the short-circuit power to the rated power of the asset at the connection point. For instance, for a Short-circuit power of 3000 MVA at the connection point of a Wind Parks of 300 MVA, the SCR is 10.

Usually, the connection point will be considered as

• strong for SCR > 5

• weak for 3 < SCR < 5

• very weak for SCR < 3

High SCR reflects actually presence of Synchronous Generators (SGs) in the area, which are the principal providers of short-circuit power. Yet, from a stability point of view, their impact on stability is rather translated by their ability to provide voltage/frequency control instead of their ability to provide short-circuit power.

System Strength: Need for a new definition

With more and more PE converters (hence less and less SGs) and introduction of new technologies such as converters in Grid-Forming (GFM) control mode (see this article for more details on Grid-Forming) it is needed to review the definition of the system strength. Instead of the amount of short-circuit power, another indicator with a better physical meaning is the voltage (magnitude/angle) stiffness, i.e. the ability of the system to provide a stable AC voltage waveform. An illustration of this concept is shown in the Figure below, where SGs can form a voltage waveform contributing to the global voltage stability of the system, while most of the PE inverter (PEI) can’t. This situation is slightly changing with the apparition of GFM converters, which are also able to form the voltage of the grid and stabilize the Grid AC voltage waveform. This shows that the definition is also better adapted for system with GFM converters, which help to stabilize the AC voltage waveform but cannot provide large amount of short-circuit power.

In term of indicators, this new definition also forces the grid operators to review the screening indices they use to monitor the strength of their system. Still today, most of the indicator relies on the “short-circuit ratio” but these indices become less and less representative (as explained above). Hence it is important to introduce new indices able to capture the voltage stiffness of the grid at multiple locations. Voltage stiffness can be captured by voltage sensitivity indices that quantified how the voltage magnitude and angle vary in response to change of power injection.

Operational challenges in weak grids

Weak grids are facing multiple challenges due to this higher (resp. lower) voltage sensitivity (resp. voltage stiffness). This higher voltage sensitivity has a strong impact on the stability of the system and hence on its operation. New stability phenomena originate from this higher voltage sensitivity, combined with the control-based responses from large PE converters. This leads to what is called in the literature control interaction or resonance phenomena that basically are translated into poorly damped (or even undamped) oscillations of voltage, power,… in a given range of frequencies. Power System operators need to cope with these new challenges and be able to monitor the risk and to mitigate it at low cost (which is not always possible). The next article will discuss in more details the nature of these new stability phenomena.

How to maintain a sufficient level of system strength?

In order to cope with the evolution of power systems which are becoming weaker and weaker, multiple solutions exist at variable costs, some of them are reported below:

• Build new transmission lines, increase the meshing of the grid

  • It costs a lot and takes a lot of time due to permitting and often strong public opposition

• Connect dynamic compensators in weak network areas such as Synchronous Condensers (more information on Synchronous condenser can be found in this article) or E-STATCOM (i.e. STATCOM in GFM mode) able to stabilize the AC voltage waveform

  • It costs a lot with important footprint and takes some time before going into operation

• Change the grid code to enforce large converters to become GFM compliant which are more robust in weak grid conditions and can provide voltage and frequency support (see this article for more details)

  • Need a strong collaboration with the manufacturers

• Derive elaborated operational rules to maintain a sufficient system strength level

  • One solution is to keep a minimum amount of SGs online at all time (hence curtailing some RES production)

Other solutions exist in order to prevent the apparition of new stability phenomena in weak systems. This will be covered in more details in the next article.

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