How to understand IEEE 519-2014?
IEEE 519 is one of the worldwide most common standards related to harmonics and power quality. Yet, the standard is not always correctly interpreted and sometimes even misused. In this article we bring up few points that will help us how to understand IEEE 519.
As the title says, IEEE 519 is a guide for harmonic control. The standard was originally developed for utilities. However, users often apply it for industrial networks as well. We will discuss what to take care of and how not to misuse the standard.
Purpose of IEEE 519
IEEE 519 is a recommended practice for harmonic control. In other words, the recommended limits are suggestions and shall not be considered as binding in all cases. The purpose of the standard is two-fold:
- to establish goals for the design of electrical systems
- to address steady state limitations
Note that IEEE 519 is a standard taking the system approach. It is not primarily meant to provide limits for equipment.
Responsibilities
The philosophy is to limit the harmonic current distortion of individual consumers which leads to acceptable values of voltage distortion on the utility side / point of common coupling (PCC). Therefore, controlling the voltage distortion is the main concern of the IEEE 519. Simply said, one neighbour in the electric grid shall not affect the other neighbour. The responsibility of the power consumer is to limit the current harmonic distortion while the responsibility of the utility is to provide consumers with voltage waveform close to a sine wave.
Why do we actually need standard related to harmonic distortion?
Harmonics pollute the electric grid that shall ideally contain only sinusoidal voltage of the fundamental frequency (called power frequency – mostly 50 Hz or 60 Hz). Yet, the harmonics are frequency components other than the fundamental frequency. Most harmonics are so called characteristic harmonics that usually are odd integer multiples of the fundamental frequency.
Real grid is never perfectly sinusoidal due to the presence of non-linear components (consumers but sometimes also producers). Therefore, each device shall have certain minimum immunity against distorted waveform. However, if the distortion is too high, issues are likely to occur.
Effects of harmonics
Harmonics cause additional power losses in the electric grid and consequently increased heating of electric components. They also contribute to pulsating torque of electric motors. Harmonics are known to increase voltage stress of capacitors. Excessive harmonic distortion may cause disturbances or even malfunction.
History of IEEE 519
The IEEE 519TM standard was so far issued in three editions. The first publication originates from 1981 and was issued jointly by ANSI and IEEE standard organizations. Next edition is from 1992. This one was in use for over 20 years. The most recent edition is from 2014. Compared to 1992 issue it had been significantly shortened. However, the basic rules and limits remained practically unchanged. It was mainly the theoretical introduction and explanation that was reduced.
IEEE 519TM editions published until present:
- IEEE 519-1981
- IEEE 519-1992
- IEEE 519-2014
Differences between IEEE 519-1992 and IEEE 519-2014
Understanding the key terms
Although the IEEE 519TM is used since many years there is still certain confusion how to apply specific parts of the standard in real life.
Voltage and current distortion
People sometimes talk about harmonics and harmonic distortion without specifying if they refer to voltage harmonics or current harmonics. Indeed, it makes a significant difference. Most practical power grids with reasonable short circuit power have low source impedance. Especially for small and medium power VFDs the grid can tolerate relatively high current harmonics without causing considerable voltage distortion. Although the current distortion of the VFD can exceed several percent the voltage distortion in the grid remains low.
Absolute and percentage values
To express the harmonic distortion either absolute or percentage values can be used. The percentage values refer to the fundamental component and are easy to quantify. Specially to describe voltage distortion the percentage values are straight forward. When talking about harmonic currents more care is needed to correctly interpret the percentage values. It may not be obvious what is the reference current used for scaling.
Limit values
Limit values given in IEEE 519TM are recommendations and as such should not be considered binding. They shall allow certain flexibility to be agreed between the utility and the consumer. This part is often forgotten and users sometimes strictly require an absolute compliance with the standard.
THD vs TDD
Total harmonic distortion (THD) and total demand distortion (TDD). While THD refers to the fundamental current component (I1), TDD uses the maximum demand load current (IL) at the PCC. Since IL is normally higher than I1, TDD indicates lower percentage value for the same harmonics.
ISC/IL ratio
This ratio is used when addressing the recommended limits of current harmonics (individual as well as TDD). Small ISC/IL ratio means weaker grid and therefore stricter limits apply. On the other hand, large ISC/IL ratio indicates a stiff grid or a small (insignificant) load and the limits can therefore be relaxed.
Short time harmonic considerations
How to handle short time harmonic injection, e.g. in soft starter applications?
Modified limits for higher pulse number
As described in chapter 5.5 of [1], it is allowed to use specific multipliers for characteristic harmonics of a rectifier with higher pulse order (pulse number) p. That is very useful for medium voltage VFDs as they usually have either a multi-pulse diode rectifier or an active front end type of rectifier.
Point of common coupling (PCC)
PCC – Point of Common Coupling is the interface point to the utility grid. Therefore, PCC is the point where the harmonic limits for individual harmonics and for total harmonic distortion typically apply. Please note that many users apply IEEE 519TM also for point of connection (POC) or internal point of coupling (IPC). While the user can do so, it was not the original intention of the authors of IEEE 519TM. Applying the limits of the IEEE 519TM within the industrial grid may pose strict limits on harmonics that are difficult to meet although the harmonic distortion at the PCC actually complies with the standard.
Conclusion
The aim of this post was to provide a little guide how to correctly interpret and consequently apply IEEE 519 standard. Due to limited length we were not able to cover all points in detail. A more comprehensive article is available for our premium subscribers.
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References
[1] IEEE Std 519TM-2014: Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, IEEE Power and Energy Society, New York, March 2014
[2] Network harmonics (series), https://mb-drive-services.com/category/net-harmonics/
[3] NEPLAN – Software for power system analysis, https://www.neplan.ch/
[4] T.M.Blooming, D.J. Carnovale, Application of IEEE STD 519-1992 Harmonic Limits
