Ratio between grid short-circuit power and VFD installed power
Successful integration of a variable frequency drive (VFD) into the electric grid shall follow certain rules. Moreover, the VFD shall comply with standards related to power quality and harmonics. In case of 4-quadrant operation (regenerative type of VFD), additional requirements typically apply1. The higher the VFD power, the more attention shall be paid. In order to define some rules of thumb and allow a first assessment (screening), the ratio between grid short-circuit power and installed power of a VFD is used. In the following paragraphs the meaning of the ratio is explained. Some guidance is provided for situations where the ratio is low.
1 The set of rules and requirements related to the behavior in the grid is described in so called grid code. Generating units must fulfil stricter requirements than systems that only act as consumers.
Ratio between grid short-circuit power and VFD rated power
VFD, as any other power electronic equipment, is a non-linear load that injects harmonics into the grid. It has its dynamics and depending on the topology and control algorithms may not act as a purely passive load. It means that engineers shall follow certain guidelines to ensure successful operation of the VFD without negative impact. The ratio between short-circuit power of the grid and installed power of the VFD is used as a factor to quantify the integration of a VFD based drive system into an electric grid.
Definition
Definition is rather simple: It is the ratio of grid short-circuit power at the point of connection and the VFD installed power.
The minimum grid short-circuit power is normally taken to cover the worst-case scenario. VFD installed power is similar to the rated power of the VFD. Installed power might be slightly higher due to the standardization of VFD sizes.
Range of the ratio
The ratio R has range from several hundred down to single digit value. High ratio means that either the grid short-circuit power is very high (strong grid) or the VFD installed power is small. It leads to a situation with minimum risk. R larger than 25…50 usually does not need any special care. In opposite, low R means that either the grid is weak (low short-circuit power) or the VFD power rating is high or a combination of both. When the ration drops below certain threshold, typically 10…15, the user shall generally be more careful. Extremely low R values require well-thought system design and eventually some risk mitigations.
Trend
Due to several factors, we can expect more VFD installations with low R in the future. The reasons are multifold. On one hand the penetration of the renewable energy sources reduces the short circuit capacity of the grid. Power electronics-based generation units have very limited overloadability and cannot provide the same level of short circuit power as the conventional large generators in thermal power plants. Industrial sites in remote areas are usually connected to weaker grids as well. At the same time, the VFD unit power is tendentially growing, at least in some applications, such as large compression trains. Thus, the ratio R is often small.
Effects of low ratio
Low R typically leads to an increased harmonic distortion in the grid (either because the grid is weak or because there is a large load producing harmonics as biproducts). The voltage at the point of connection may fluctuate stronger with the load. The grid faults may be more severe. Extremely low R may lead to system stability issues, especially in case of a high-dynamic drive system with strongly varying load.
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Rule of thumb
Below rule of thumb shall be understood as the first screening whether there is a potential risk or not.
a) R > 20…25 … No issues expected – straight forward integration
b) 10 < R < 20 … Harmonics may exceed the limits given by standards
c) R < 10 … Increased harmonic distortion, potential stability issue
Recommendations for cases with low ratio
To reduce the risk of adverse behaviour in drive installations with low R, some recommendations are provided.
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Summary
The ratio of grid short-circuit power and VFD installed power is a practical factor for first screening with regards to grid integration. High ratio normally means trouble-free system integration. As the ratio drops, more attention is needed. Moderate ratio may need better harmonic mitigation to ensure compliance with harmonic standards. Low ratio is even more challenging from power quality point of view. In addition, system stability may come into the picture. The risk can be minimizes by applying suitable measures, such as (but not limited to) those mentioned in this article.
References
[1] How do system parameters impact the grid harmonic distortion, MB Drive Services, November 2023, available online, https://mb-drive-services.com/how-do-system-parameters-impact-the-grid-harmonic-distortion/
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