VFD compliance with voltage and current harmonic limits
National and international standards provide guidelines and limits for individual harmonics and total harmonic distortion. But how to assess the situation when the VFD meets the limits for voltage harmonics while current harmonic limits are exceeded? This blog post is a short polemic on that topic.
Introduction
Power electronic consumers constitute non-linear loads that often inject certain portion of harmonics. Although modern power electronics made a large progress to minimize harmonic distortion, there is still some portion of harmonics injected into the grid. Moreover, there are many older power electronic devices in use that emit more harmonics.
As the power electronics quickly emerged in the second half of 20th century, standards regarding electromagnetic compatibility and harmonic control had been developed.
Terms and Definitions
PCC … Point of Common Coupling
Interface point in the grid between the system owner or operator and user
TDD … Total Demand Distortion
Harmonic distortion of demand load current
THD … Total Harmonic Distortion
Harmonic distortion of the voltage signal
Harmonic standards
National, international or corporate standards are used to control the harmonic distortion. Most widely used global standards regarding harmonics are IEEE and IEC standards. Understanding the “philosophy” behind the harmonic limits is useful to assess the project-specific calculations, especially cases where the limits are marginally exceeded.
IEEE 519
IEEE Standard for Harmonic Control in Electric Power Systems
Scope of IEEE 519:
IEEE 519 is a standard for harmonic control. Currently valid edition is from the year 2022 (IEEE Std 519 2022). As mentioned in previous publications, the standard is aimed for utility grids. Nevertheless, many users apply it for industrial grids as well.
The standard provides limits for voltages and currents. The voltage limits change with the nominal grid voltage (several ranges defined). The higher the grid voltage the lower the percentage limits. Current harmonic limits depend on the ratio of the short circuit current at the PCC (ISC) to the load demand current (IL). For high ratio ISC/IL more current harmonics are allowed and vice versa. This is an important observation. It shows that the voltage harmonic distortion is the main concern. Current harmonics shall also be limited, but it is a secondary problem. If the grid is strong (high ISC) or the load is relatively small (low IL), or both, then fairly high current harmonics are allowed (up to 10% TDD). In fact, in the introduction to the standard it is mentioned that “Harmonic voltage distortion limits are provided to reduce the potential negative effects on user and system equipment. Maintaining harmonic voltages below these levels necessitates that all users limit their harmonic current
emissions to reasonable values determined in an equitable manner based on the inherent ownership stake each user has in the supply system.”
Above wording clearly points out that the aim is to limit the grid voltage distortion and current limits are meansto achieve it.
IEC 61000-2-4
IEC 61000-2-4: Electromagnetic compatibility (EMC) – Part 2-4: Environment – Compatibility levels in industrial plants for low-frequency conducted disturbances
IEC 61000-2-4 defines three classes 1, 2 and 3.
Class 1 is the most stringent one (lowest levels) and applies for sensitive environment such as laboratories, hospitals etc. Class 2 targets normal industrial network and is the most used class. Finally, class 3 is foreseen for heavy industry with larger harmonic pollution.
Latest edition from July 2024 introduces new classes 2a and 2b and relaxes the limits for odd harmonics.
Scope of IEC 61000-2-4:
“This part of IEC 61000 is concerned with conducted disturbances in the frequency range from 0 kHz to 9 kHz. It gives numerical compatibility levels for industrial and non-public power distribution systems at nominal volt ages up to 35 kV and a nominal frequency of 50 Hz or 60 Hz.
Power supply systems on ships, aircraft, offshore platforms and railways are not included.
The compatibility levels specified in this standard apply at the in-plant point of coupling. At the power input terminals of equipment receiving its supply from the above systems, the severity levels of the disturbances can, for the most part, be taken to be the same as the levels at the in-plant point of coupling. In some situations this is not so, particularly in the case of a long feeder dedicated to the supply of a particular load, or in the case
of a disturbance generated or amplified within the installation of which the equipment forms a part. Compatibility levels are specified for electromagnetic disturbances of the types which can be expected at any in-plant point of coupling (IPC) within industrial plants or other non-public networks…”
IEC 61000-3-6
IEC 61000-3-6: Electromagnetic compatibility (EMC) – Part 3-6: Limits – Assessment of emission limits for the connection of distorting installations to MV, HV and EHV power systems
Scope of IEC 61000-3-6:
“This Technical Report, which is informative in its nature, provides guidance on principles which can be used as the basis for determining the requirements for the connection of distorting installations to MV, HV and EHV public power systems (LV installations are covered in other IEC documents). For the purposes of this report, a distorting installation means an installation (which may be a load or a generator) that produces harmonics and/or interharmonics. The primary objective is to provide guidance to system operators or owners on engineering practices, which will facilitate the provision of adequate service quality for all connected customers. In addressing installations, this document is not intended to replace equipment standards for emission limits.”
The planning levels introduced in this standard are naturally lower than the compatibility levels, adding extra margin (uncertainties in design stage) or provision for harmonics from other sources.
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Power quality
When assessing the limits for current and voltage harmonics it is important to keep in mind the definition of power quality. In essence, power quality is a voltage quality. According to [2]:
“Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform (which resembles a sine wave).”
Similarly, [3] defines power quality in following way:
“Power quality is the measurement of how close to perfect an electrical voltage is at any given time or point. High quality electrical voltage is a sine wave that measures exactly what is expected in both voltage and frequency. A high quality electrical source is one that can deliver all the electrical energy needed without any change in the voltage.”
Basically, the utility is responsible for providing supply voltage with adequate quality (voltage magnitude, frequency, waveform close to a sine wave with low harmonic distortion). On the other hand, the load currents depend on the character of the consumers. They are largely out of control of the utility.
Voltage and current limits: What is more important?
The question from the practice may be: What is more important to comply with, voltage or current harmonic limits?
In the author’s opinion the answer is clear: the voltage. For example, IEC 61000-3-6 explicitly defines only the limits for voltage harmonics. Emission limits for currents for backward calculated from voltage limits and (project-specific) grid impedance. Chapter 6.4.1 of IEC 61000-3-6 states:
“For converting emission limits from voltage into current limits, there are two ways to assess the harmonic system impedance depending on the size of the distorting installation and the system characteristics.”
The attachment A then provides envelope of the maximum expected impedance.
“Based on several site measurements, “worst case impedance curves” have been defined in some countries. If calculations using those empirical curves indicate that an installation can be connected (i.e. still meet the voltage emission limits at the point of evaluation), this may be done with minimum risk. However, if these calculations give results that indicate that the installation’s emission levels will exceed the voltage emission limits, a more refined approach should be used.”
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In short, the standard does not provide any generic limits for harmonic currents. Instead, the limits for currents shall be derived site-specific to ensure that with given grid impedance (site specific) and allowed current injection the voltage limits are not exceeded. It clearly shows that voltage distortion has priority over current distortion.
What does high harmonic emission mean?
When a utility alerts a customer about high harmonic emissions from the customer’s network, they are indeed concerned only with the ‘Voltage Distortion’ (%THDv). Main reason for this is, the utility is basically a constant voltage source, and when the customers connect to it, they charge their equipment to the available voltage and draw current based on their load as well as the available voltage waveform. Now, if the voltage waveform they receive is itself distorted, then the current they are drawing will be distorted too. This distortion only leads to increased RMS current, further causing unwanted heating and premature failure of customer’s equipment.
This will be perceived as the ‘dirty’ voltage from utility, whereas the main culprit is some neighboring customer who is producing the current harmonics, which in turn are causing distortion in utility voltage.
Worth to mention, the utility is also concerned for itself, these voltage harmonics when travel across their network, and the customers draw correspondingly distorted current, the conductors of utility too get overheated due to the extra current that is being drawn.
What about the current distortion then?
Nevertheless, although most harmonic standards impose limits on both current distortion (%TDDi) and voltage distortion (%THDv or %THDu), the main concern for the utilities is that you do not distort their voltage waveform. This is because although the harmonics source is current based, the main harmonics mal-effects to utility as well as nearby customers of that utility are caused by the voltage distortion.
There are cases where a proactive end user does a Power Quality study on their network. Most of the PQ meters can give an auto-generated report and this can be the results:
As one can see, the voltage harmonics are well within limits. The current harmonics distortion, at times, goes beyond limits. This will be recorded by PQ meters and reported as a non-compliance, and the user might be alerted, though the utility won’t be bothered in such cases.
These graphs can also be a result of ‘post-correction’ scenarios, wherein a harmonic filter is already connected to correct the harmonics, and the customer is falling within the local utility’s applied IEEE-519 limits. This might result in the user perceiving that the harmonic filter is not working correctly and that they still have to do something to respect the limits. Whereas, the filter is correctly working on the network and reducing the voltage distortion, which if not corrected would cause problems in the nearby network and alert the utility.
Summary
Electric power quality basically refers to voltage quality. Task of the utility is to provide voltage with adequate quality. In contrast, current waveform depends on the type of the load. Some standards, such as IEC 61000 3-6 explicitly define only the limits for voltage harmonics while current limits shall be calculated backwards considering the voltage limits and maximum expected grid impedance.
Current emission limits are defined in such way that for the given grid impedance the voltage distortion is not exceeded as long as current emissions do not exceed the limits. If the voltage distortion complies with the standard but actual current harmonics exceed the limits, then one shall think whether the current limits have eventually been set too conservative.
Acknowledgement
The author would like to thank Mr. Punit Pandya for sharing his rich professional experience in the field of power quality and harmonic measurements and co-authoring this article .
References
[1] How to understand the requirements of IEEE 519?, MB Drive Services, January 2022, available online, https://mb-drive-services.com/how-to-understand-the-requirements-of-ieee-519/
[2] Electric Power Quality, Wikipedia – The Free Encyclopedia, available online, https://en.wikipedia.org/wiki/Electric_power_quality
[3] What is power quality, Power Systems & Controls Inc., available online, https://pscpower.com/what-is-power-quality/
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