VFD multidrives: Grid harmonics

VFD multidrives: Grid harmonic performance

This is the continuation of our short series on benefits of VFD multidrives [1]. We have already explained improved energy efficiency [2] and reduced footprint [3]. Today’s article focuses on grid harmonic performance and how a multidrive solution has the edge over individual single drives. Let’s explore the potential in reducing harmonics: multidrive harmonic footprint versus grid harmonics of multiple single drives.

Introduction

Network harmonics are of increased concern as most of the grids are gradually getting weaker. it is related to the transition from concentrated power generation (large thermal power plants) towards smaller decentralized power generation (mainly associated with renewable energy sources). VFDs are well-known source of harmonics. Although the modern VFD topologies have improved their waveforms both on grid and motor side there is still a potential issue with power quality.

At this place we do not repeat all the basics around network harmonics [4, 5], harmonic mitigation methods [6] or harmonic filters [7] (please refer to previously published articles). Instead, we will have a look at how a multidrive solution performs compared to individual single drives.

Explanation on reduced multidrive harmonic footprint

Increased pulse number

In a multidrive solution it is much easier to achieve higher rectifier pulse number (economically and technically) and minimize the harmonic impact of VFD on the grid power quality.

First of all, in a multidrive there is just one rectifier. For example, it is much more economical to utilize a single 24-pulse rectifier on a VFD multidrive rather than installing several single motor VFDs with each having a 24-pulse rectifier.

It is obvious that the more individual single drives there are the more costly it is to realize them all with high pulse number (cost of rectifier itself plus cost of transformer).

Yes, one could argue that the single motor drives could use only 12-pulse rectifiers with phase displacement of the individual transformers to achieve higher pulse number as total reaction. That is a known method. However, it has some limitations:

— Transformers need to be phase shifted against each other.

— Harmonic cancellation performs well when the drives are operated with exact same load. The larger the difference in loading the less effective the “quasi” method is.

— Principle of virtual increase of pulse number works when all considered drives are in operation (and share the same load as per previous point). Whenever one of the drives is not in operation, the harmonic cancellation does not really work.

Multidrive with a 24-pulse rectifier has typical 24-pulse spectrum on the grid side also when the motors have different loads or when some of the motors are not in operation.

Example: Grid harmonics of single drives versus multidrive harmonic footprint

We will illustrate it on a small example. For a project XY two identical variable speed motors are required, each with rated shaft power of 1.4 MW. The grid is 11 kV with short circuit power of 150 MVA.

Variant A are two 12-pulse single drives with phase shifted transformers to build a quasi 24-pulse system. Variant B is a multidrive common for both motors using a 24-pulse rectifier. In both variants we consider 10% of non-characteristic harmonics as per recommendation from IEC standard. Therefore, we see small portion of 5th and 7th harmonics.

Figures 3a and 3b show the current and voltage harmonics when both 12-pulse phase shifted single drives operate at full load or when a 24-pulse multidrive operates also at full load. The spectrum is the same, i.e. ideal cancellation of 11th and 13th harmonics is assumed.

Now let’s look how the situation changes when first motor is operated at 100% load while the second operates at 60% load only.

Figures 3c and 3d depict the current and voltage harmonics of the 2×12-pulse single drives. As the load of each drive is different, the 11th and 13t harmonics are only partially cancelled and become visible in the spectrum.

In contrast, the multidrive configuration keeps its 24-pulse behavior towards the grid despite of different loading of the motors – see figures 3e and 3f.

The difference between 3c/d and 3e/f is not very dramatic, but we can notice better harmonic performance of a multidrive compared to single drives arranged into a quasi system. The difference becomes more pronounced when the number of drives increases. Harmonic footprint of a multidrive is less than equivalent harmonic footprint of several single drives.

Connection to a stronger grid

One of the well-known methods to reduce harmonic distortion is connecting the VFD to a higher grid voltage level.

Having just one common rectifier it is economically more feasible to connect it to higher grid voltage. Larger industrial plants might have several voltage levels to be used. Let’s use 11 kV and 33 kV for argument sake. In situation with a number of single motor drives it becomes expensive to connect each drive to 33 kV. The input transformers become more costly and so does the switchgear. However, in case of a multidrive with one rectifier and like just one input transformer it is not a significant burden. Moreover, you will need just one 33 kV switchgear.

Multidrives significantly reduce the additional investment cost for connecting the VFD to higher voltage level. The higher the voltage level the stronger the grid. Therefore, VFD has less impact on the grid and causes less harmonic distortion.

Reduced power exchange with the grid

One key feature of a multidrive is the power exchange through the dc link which is common for multiple inverters. The principle was briefly explained in [2] with regard to loss reduction and improvement of efficiency. In fact, the concept helps for harmonics as well.

Considering a regenerative application one single motor VFD is driving the machine (motor mode, i.e. drawing power from the grid) while other VFD might operate in power regeneration mode (generator mode, i.e. feeding power into the grid). The total consumed power is only the difference of the two. However, this is not quite true for harmonics. We cannot assume that one VFD produces the harmonics and the other one consumes them. The harmonics will propagate into the grid and other loads depending on the impedance of individual paths. From this perspective it is much better having a multidrive where the power exchange happens in the common dc link and is decoupled from the grid.

Conclusion on multidrive harmonic footprint

VFD multidrives offer additional options to reduce grid harmonic footprint when compared to individual single motor drives. Higher rectifier pulse number is easier to realize, especially from economical standpoint. It is easier to connect to a higher grid voltage level, if available (again mainly economical aspects). The multidrive solution performs marginally better compared to single drives arranged in quasi systems (e.g. 24-pulse VFD multidrive versus 2 x 12-pulse single drives with +/- 7.5° phase shifting) in case the motor load is not equal. Finally, the largest advantage is in regenerative applications. Multidrive exchanges this power internally through the dc link without involving the grid. Individual drives need to exchange the power through the grid and this exchange is associated with certain harmonic pollution. Overall, multidrive harmonic footprint is lower than total harmonic distortion of equivalent individual drives.