I've got a 600 VAC, 75 HP VFD that I want to place line and load reactors in series with. Usually, I find charts like this that give me a horsepower table to follow as they are the easiest to use. However, today I googled sizing electrical reactors I've found sites have varying information and suggest different sizings.
I've never challenged suppliers on why these are sized the way they are. When I've purchased them it was simply by using the voltage and horsepower ratings of the VFD/motor but now I'm curious as to the rationale behind all these numbers.
With respect to the guidelines linked above and the following reactors: RL-08002, RL-08003.
- Why is 3% on the line side and 5% on the load side recommended?
- What does the 3% and 5% relate to?
- How is this percentage used to calculate the equivalent mH rating (0.4 and 0.7 mH)?
- If I wanted to calculate a required reactor size without the use of an equivalent horsepower table, what would be the proper starting point? How can I determine an unbiased value?
Best Answer
3% is recommended for the line side because a 3% voltage drop in addition to the "typical" voltage drop that would be seen without the reactors is estimated to be the maximum that is tolerable.
5% is recommended for the load side because that is the maximum that is estimated to be tolerable considering the combination of input line voltage drop, the capability for voltage adjustment by the modulation and the impact of reduced voltage on motor performance.
In both cases, it is assumed that any added impedance will provide significant benefit.
The percentage refers to the voltage drop as a percent of rated voltage at rated current due to the reactive impedance inserted.
Percentage = 100 x current x 2 x Pi x f x L / line-to-neutral voltage
You can use the equation above to calculate percentage based on current and voltage. The "bias" involved in the tables are estimates concerning what are the typical distribution system characteristics and what is typically tolerable.
Individual VFD manufacturers may "tune" their recommendations based on the input or DC link reactance built into the VFD, output dv/dt characteristics of the design and their own tolerance data.
Addendum
Is it good practice to install reactors for every VFD?
It is difficult to determine the need for harmonic current mitigation or the amount of mitigation provided by reactors without a detailed analysis of the facility power distribution system. Such an analysis would estimate the harmonic currents drawn by VFDs and the effect of those harmonics at points of common coupling with sensitive equipment of other utility customers.
Similarly, it is difficult to determine the need for dv/dt filtering at the VFD output and the effectiveness of reactors as dv/dt filters.
Experience may suggest to many engineers that the client’s money is better spent on reactors than on studying a power system that will probably change over time. However that comes down to advising the client where to place a bet.
Installing 18-pulse drives in sizes that are widely available as 6-pulse drives is a better bet but a larger one.
From the standpoint of the individual engineer, the best bet is to do what the supervising engineer says is good practice.
Voltage drop
Although the percent reactance refers to the percent of rated line to neutral voltage at rated current that is the voltage drop across the reactor, the voltage drop seen at the VFD input or at the motor is less because the impedance of the reactors is almost entirely reactive while the effective full-load impedances of the VFD and the motor are almost completely resistive.
References
Anyone who is dealing with the possibility of harmonic current in power distribution systems should read 519-2014 - IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. Also consider reading some of the many IEEE papers that discuss that standard and make recommendations related to it.
There are also a lot of IEEE papers dealing with motor insulation stress and partial discharge due to VFD waveforms. Also the related problem of motor bearing currents has been covered extensively by IEEE papers.