What is the time constant of a dc link?
Introduction
Most commercially used variable frequency drives (VFD) belong to the category of indirect frequency converters. The AC power of a fix frequency (usually 50 or 60 Hz) is first rectified and then the DC signal is inverted into AC power of variable (adjustable) frequency. Thus, the frequency conversion happens in two stages: AC/DC and DC/AC. This functional principle requires an intermediate dc link. The character of the inverter is determined by the type of dc link. In this regard we differentiate two main groups of VFDs:
- Current source inverters (CSI)
- Voltage source inverters (VSI)
CSI drives have an inductor (coil) in the dc link providing the character of a current source. VSI drives use capacitors in the dc link giving them the character of a voltage source.
Since VSI technology dominates among modern power electronic drives, this article provides practical considerations about the dc link of VSI drives.
Role of dc link
Intermediate dc link (also called ‘dc bus’) is an integral part of most VFDs. An exception are direct frequency converters that represent a clear minority in the drive landscape. Most VFD work on principle of indirect frequency conversion. The first stage is a rectifier converting the input AC power into a DC quantity. Then comes the dc link, a subject of this article. The next stage is the inverter converting the DC power into AC quantity of adjustable frequency. In this scheme the dc link is an integral part of the VFD. Voltage source inverters have dc link consisting of capacitor banks (usually in some kind of series-parallel arrangement).
DC link shall fulfil several functions. These are:
- Decoupling of the rectifier and inverter (i.e. decoupling of grid and motor side)
- Intermediate energy storage (balancing input and output power)
DC link as a big battery?
Many people imagine the dc link as a large battery that can provide energy for a considerable period of time. However, this picture is not quite correct. Although the energy stored in dc link is not small, it is still quite limited with relation to the rated output power. We talk about time constant of the dc link. That is because the ratio of energy and power has the dimension of time.
Dimensioning of a dc link
The dimensioning of dc link is a compromise between several requirements. Some of the design criteria are:
- Ripple of the dc voltage
- Amount of stored energy / time constant of dc link
- Short-circuit consideration
- Charging and discharging time
- Physical size of capacitors
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Time constant of a dc link
Finally, we are going to answer the main question of this article: what is a typical time constant of a dc link? The exact value depends on several factors mentioned in previous paragraphs. It was explained that the sizing of the dc link is a compromise of multiple requirements. It also depends on the VFD topology and manufacturer’s choice. Nevertheless, some indication can be given. An example of a 5 MW drive is mentioned in [1] with time constant of DC link of xx ms. Typical range of the DC link time constant is between … ms and … ms. Such range applies for the nominal dc voltage. Since the energy stored in a capacitor is quadratically proportional to the capacitor voltage, larger deviation from the nominal voltage has a significant impact on the time constant.
Cell based drives tend to have a DC link time constant in the middle/upper range. Neutral point clamped (NPC) inverters are designed with a time constant in lower/middle range.
Regardless of the exact value, it is obvious that the time constant of a DC link is very short (might be less than 1 period of the power frequency!). Thus, the VFD must act very fast in case of a grid fault. That is a challenge since in normal operation the control shall be rather smooth and soft while in case of a grid disturbance a rapid reaction is required.
Most VFDs have a standardized design. They are available in several frame sizes. Normally, the time constant of a DC link is the same (or very similar) for all frame sizes of specific VFD model as they follow the same design rules. The capacity of the dc link grows proportionally with the inverter rated active power.
Summary
Most modern VFDs are voltage source inverters with a dc link. The role of the dc link is to provide a decoupling function in between the two conversion stages AC/DC and DC/AC and to balance the instantaneous difference between input and output power. Due to practical aspects, the energy stored in the dc link is limited. Instead of absolute value of stored energy a more interesting quantity may be the time constant of a dc link, i.e. the ratio between rated stored energy and rated output power. Typical range of a time constant of dc link is some … ms.
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References
[1] Wymann, P.Jörg, Power loss ride-through in a variable speed drive system, paper AM-07, PCIC Europe 2014
