How strong is a strong grid?
When evaluating the impact of a VFD on the electric grid, particularly the harmonic distortion, we often talk about “strong grid” or “weak grid”. These terms may be a bit vague or at least relative. In this post we try to put the grid strength in a perspective. How strong is actually a strong grid?
Short circuit current and power
The grid strength may be expressed using a short circuit current (Isc or Ik”) or a short circuit power (Ssc or Sk”). Both values are obviously coupled together through the grid nominal voltage. For high ratio (i.e. high grid short circuit capacity or low VFD power) the impact on the grid is smaller and consequently higher harmonic limits (in %) are allowed.
Alternatively (or additionally), the grid may also be characterized by its short circuit impedance. When both X and R are given we can get a simplified impedance characteristic for an inductive network. Sometimes, the resistance R may be neglected. Then we talk about purely inductive network.
The relationship is rather straight forward. Strong grid is a grid with high short circuit current and low grid impedance. In contrast, a weak grid has low short circuit current rating and therefore higher grid impedance.
Figure 1 depicts the impedance of a linear (purely inductive [1]) grid at 11 kV level. Blue line represents a grid with short circuit capacity of 50 MVA which may be considered as “weak grid”. Orange line plots the impedance of a grid with short circuit capacity of 250 MVA which may be considered as a fairly “strong grid”.
Strong grid is desirable to minimize the impact of a VFD on the power quality of the grid, especially the harmonic voltage distortion. The relationship is probably quite intuitive. If a VFD is connected to a very strong grid it will hardly be able to cause a notable harmonic distortion. However, if the exact same VFD is connected to a weak network the distortion will be larger. A physical explanation is equally simple when imagining the VFD as a source of current harmonics. Harmonic current of certain ampacity flowing through a grid impedance causes certain voltage distortion. For low impedance the distortion is small and for higher impedance the distortion is larger.
Grid strength and harmonic distortion
The grid strength impacts both current and voltage harmonic distortion. For simplicity we consider a case without any grid resonance interacting with VFD harmonics. In such case we can conclude following:
When increasing the grid short circuit power the current harmonics marginally increase while the voltage harmonics are reduced.
Remark 1:
It is important to note that the reduction of voltage distortion with increased short circuit capacity of the grid is much more significant than the relative small increase of current harmonics. In other words, strong grid is less distorted by the harmonics genereated by VFD.
Remark 2:
If there is a network resonance in frequency range where VFD produces harmonics then a general statement about grid short circuit power and harmonic distortion cannot be made anymore.
Grid strength and harmonic limits
Many national and international harmonic standards adapt the harmonic limits based on the ratio between grid short circuit power and VFD installed power using the justification as explained above.
For example, IEEE 519 defines sets of current harmonic limits for Isc/IL < 20, 20 < Isc/IL < 50, 50 < Isc/IL < 100, 100 < Isc/IL < 1000, and Isc/IL > 1000.
Austrian standard TOR D2 scales the permissible current harmonics with the square root of the short circuit power at PCC and installed power of the VFD.
Chinese standard GB/T 14549 provides harmonic current permissible values in Amps. However, these current limits are defined for a reference short circuit power (for example in 6 kV and 10 kV grids the reference short circuit power is 100 MVA). The project limits are then adjusted based on tabularized values multiplied with the ratio of actual grid short circuit power and reference short circuit power.
Ih = Sk1/Sk2 * Ihp
Ih … h order harmonic current permissible value when short circuit capacity is Sk1
Sk1 … Minimum short circuit capacity at the PCC
Sk2 … Reference short circuit capacity
Ihp … h order harmonic current permissible value from table in the standard
Grid strength and input power factor
For VFDs with diode rectifier (DFE) or line commutated rectifier the input power factor depends on the commutation reactance. Strong grid means lower reactance and therefore shorter commutation and somewhat higher input power factor. We shall remark that the dominant portion of the commutation reactance is given by the input isolation transformer. Hence the grid short circuit impedance usually does not have any significant impact on the input power factor of the VFD. However, for a “weak grid” the input power factor of a DFE drive will be slightly lower compared to the exact same VFD connected to a strong grid.
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How strong is a strong grid?
Let’s go back to the title of this post. How strong is actually a strong grid? The answer is: It depends…. From harmonic distortion point of view VFD manufacturers often state that their VFD complies with international standards regarding EMC and harmonics when the grid short circuit power is x-times larger than the VFD power. The multiplier x depends on the VFD topology. Generally, a fairly strong grid has the ratio Isc/IL > 20…25. Ratio > 50 indicates a strong grid. Modern medium voltage VFDs may have a very good harmonic performance even when the ratio is relatively small (e.g. Isc/IL = 12…15).
Summary
Grid strength in terms of short circuit capacity or short circuit impedance impacts the magnitude of harmonic distortion and marginally also the input power factor or voltage drop. When evaluating harmonic distortion of a VFD to be installed a ratio between grid short circuit capacity and VFD installed capacity is often used to assess the harmonic distortion. A ratio of short circuit power to installed power or short circuit current to rated current of the VFD are commonly used. Strong grid is generally less distorted. That is also reflected in current harmonic limits of several standards.
References
[1] What does a purely inductive network actually mean? https://mb-drive-services.com/what-does-purely-inductive-network-actually-mean/
[2] Harmonic mitigation methods, https://mb-drive-services.com/harmonic_mitigation_methods/
