An 'Intuitive Understanding' of Electrical Harmonics: A Conversation

Most times when posting to LinkedIn or other media platforms, I focus on a particular technical topic relative to the subject of electrical harmonics… I center my discussions about the relationships between the major electrical components that create the harmonic conditions or specific strategies to mitigate the harmonics present; without discussing the fundamental relationships. Understanding harmonic condition is impossible without a basic knowledge of the relationships associated with the topic. The following is meant as a conversation to begin the process of developing this 'Intuitive Understanding'.

Often, it is assumed that the relationship between Ohm’s Law and Harmonics is understood, but that is a poor assumption. Ohm’s law is a basic and fundamental principal, beautifully simple and easy to understand. The best definition and the easiest to understand for me was published in Wikipedia: “Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: I=V/R, where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms.”

Now taking that principal and modifying it a bit from a harmonic context… try this: Harmonics current when injected into or drawn through a system impedance, will create a voltage distortion. The level of voltage distortion created, and the harmonic frequencies of that distortion is a function of the harmonic spectrum and magnitude of the current harmonic and the level of system impedance within the circuit, Vh = Ih x Zh... or any variation of this basic equation.

The Ohm’s law interpretation when it comes to harmonics and a intuitive understanding of harmonics should start to take shape. From this, a few qualified basic relationships will start to develop.

System Impedance, Short circuit Ratio, and Current Harmonic Limits:

System Impedance is the cumulative total of all the impedances between the load, and the Source, including the Source impedance. The system impedance will include all the impedances of the cables, secondary series mounted devices, such as transformers and inductors. If the system is Utility feed, this includes the Utility impedance and if Generator feed, the Unsaturated Sub-transient Reactance of the Generator (X”d unsat.). In essence, just like when trying to calculate short circuit… these impedances must be known and understood in order to understand the relative strength or weakness of a source.

From a harmonic prospective, the relative strength or weakness of a source is expressed as the Short Circuit Ratio of the circuit, and defined as the ratio of the short circuit current at the designated point where the analysis is being done, divided by all the load currents feed from that point. So, based on the circuit, the Short Circuit ratio can change within the circuit, based on all the cumulative impedances up to that point, and all the current loads being feed from that point. Here is an example: Available SC at the point where the system is being evaluated… 20kA, Total Connected Load secondary of that point: 1000A, Short Circuit Ratio: 20,000A/1000A = 20. If the available short circuit is 20 kA, but the Total Connected Load secondary of that point: 100A, Short Circuit Ratio: 20,000A/100A = 200.

IEEE-519 uses this ratio when evaluating the pass/fail criteria of a measured or calculated harmonic current profile. See below Table 2 from IEEE519-2014, which is the latest version in effect. As the Short Circuit Ratio increases the allowed Itdd – Current Total Demand Distortion increases. This is a result of the understanding that the ultimate Voltage Distortion created by the Current Harmonic injected into the system impedance will be lowered as the relative stiffness of the source increases. Within IEEE, they also place individual current harmonic limits within specific frequency ranges, which is a topic that should be explored separately. From the table, a SC ratio of 20 or less would designate a 5% Itdd limit, and as the short circuit ratio increases the Itdd limit also increases.

Current Harmonics Vary Based on How Stiff the Source Is:

The next concept might be a bit harder to intuitively understand, so bear with me through the discussion… When we calculate current harmonics, the magnitude of a measured and calculated current harmonic is a function of how stiff the source is… i.e. how well the source can supply the current as the load demands it. A non-linear load draws current in pulses, as the rectifier fires the SCR’s, Diodes, or even IGBT’s used to create the DC bus voltage, or even when directly converting from AC to AC with a Matrix style technology. If the source is stiff, i.e. has a high short circuit ratio, the source is able to provide the current easily, but if the source is weak, i.e. a low short circuit ratio, the current is drawn out over a longer time period, lowering the calculated and measured current harmonics numeric value (Itdd and/or Ithd). See example below, where the load structures are kept constant, but the SC Ratio is changed, with the resulting changes in the measured Total Harmonic Current Distortion Measurements. The example of the Stiff Source is a Utility feed and the Weak Source is a small generator. Keep this relationship in mind when we examine the role of short circuit ratio, current harmonic and associated voltage distortion.

Voltage Distortion Will Vary Based on How Stiff the Source Is:

The higher the Short Circuit Ratio, i.e. the greater the available short circuit or the smaller the associated demand load, the lower the voltage distortion created by any injected current harmonic. This should be easy to understand since we now will introduce source regulation into the discussion. Here again, Wikipedia does an excellent job of defining regulation…

“In electrical engineering, particularly power engineering, voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. The term may refer to a passive property that results in more or less voltage drop under various load conditions, or to the active intervention with devices for the specific purpose of adjusting voltage.”

So as a high current harmonic is drawn from the source, you will have a distortion of the voltage… the stiffer the source, the greater the short circuit ratio, the tighter the source regulation, the lower the associated voltage distortion from the non-linear load current draw. In the case of harmonics, we are actually looking at the voltage regulation capability of the source based on an instantaneous change to the harmonic current draw and it's instantaneous impact to the source voltage, versus a longer period event, like a motor start or introduction of a large linear load package. The graphic below will highlight this and is based on the same circuit as the discussion of current harmonic and short circuit ratio. As you can witness, the stiffer the source is, the less of an impact the current harmonic will have on distorting the source voltage, and for a weaker system the greater the impact on the source voltage.

Which is worse, Current Harmonic or Voltage Distortion?

As we can see they go hand in hand. But Voltage Distortion can have consequences within the entire system, since the distorted voltage then feeds all the loads within the circuit; whereas current harmonic tend to flow directly from the load to the source, and not move into parallel circuit structures. The following points should help you understand the consequences of both criteria:

* Current Harmonics increases the total current drawn within a circuit, increasing heating within upstream cables, transformers, and the source itself. Total efficiency is lowered due to primary system losses and the increase of secondary eddy losses within all the upstream system components. The current harmonic also creates harmonic kVAR (Harmonic Reactive Power Consumption) which is a significant contributing factor to poor ‘Total’ power factor (pf) seen within the system… versus just the displacement power factor (dpf) we were all taught about in school. The Utility measures Total Power Factor and uses this to calculate power factor correction surcharges. If you wish to explore the topic of ‘Total’ Power Factor, I have other discussion, which may be of interest.

* Voltage distortion is created within the entire circuit including parallel circuits to the current harmonic load structure. This means we are now feeding distorted voltage to all the loads within the circuit, which can increase the current harmonic profile of existing non-linear loads, interfere with existing harmonic mitigation equipment and even trigger harmonic current draw characteristics from linear load devices… the old saying, “garbage in, garbage out”. So by feeding a regular inductive load like a lighting circuit or cross the line motor with a distorted voltage, these non-harmonic loads will now draw current in a harmonic “non-linear” fashion. This then makes them a contributing factor to the overall harmonic mitigation challenge. Also, the resulting Voltage Distortion is reflected back into the Utility grid, which then can and will compromise other users further upstream or downstream of your position within that distribution grid.

* Harmonic mitigation equipment effectiveness can be compromised due to a source/background voltage distortion, as well as system voltage imbalance. There have been a number of published studies that highlight that Multipulse Drives harmonic performance can be significantly compromised with as little as 2% source voltage distortion and the same level of source voltage imbalance. So, the greater the overall voltage distortion within the circuit, the less effective most harmonic mitigations strategies and equipment perform within that application. There are also studies that have been published establishing that Source Background Vd and Voltage Imbalance can compromise the effectiveness of Active Harmonic Solutions such as Active Front End Drives and Active Harmonic Filters. Any harmonic solution selected for an application must be qualified as to their ability to withstand and perform to specification based on these source voltage conditions. IEEE519-2014 allows for up to 8% Vthd, where IEEE519-1992 recommended a 5% Vthd level. So, by specification of the IEEE519-1992 recommendation of 5% Vthd, you are more proactive in controlling source background Vd that can compromise the effectiveness of your design.

* “Chicken Or the EGG” question – As can be seen, it is easy to get into a circular argument where the current harmonic creates voltage distortion, which then creates more current harmonic… etc. but keep this one relationship in mind… without current harmonics being created within the circuit, you will not have a resulting voltage distortion. Ultimately we have to resolve the Voltage Distortion to assure the overall health of the electrical distribution system, by controlling the injection of current harmonics into the source impedance.

Harmonic Mitigation Strategy Development.

Harmonic Mitigation Strategies are wide and varied in nature. Some are complicated and technically sophisticated and can be very expensive. There is no one right way to attack the challenge. In most of my work within the field, multiple strategies are used to achieve the goal. I have distributed a number of discussions on this subject… but I tend to try to keep the strategies in line with the fundamental principal discussed above, as well as, simple to allow for easy field deployment and service. Below is a quick summary:

* The goal will be to achieve the voltage distortion criteria as detailed with IEEE519, through correction of the current harmonic, whether or not it meets the current harmonic specified by the standard. To this end, I prefer to use the voltage distortion criteria as set forth with IEEE519-1992 to set the mitigation goals, i.e. 5% or less Vthd versus the anemic 8% requirements as adopted in the 2014 version. This allows for headroom within the harmonic modeling for conditions or loading structures and secondary impedance structures not recognized or understood, that may exist. Seldom do modeled results match ‘Real World’ results.

* All harmonic studies and strategies must have all potential/power sources accounted for within your strategy. Harmonic performance as previously discussed is significantly dependent on source impedance. A Utility tends to be a relatively stiff source, while a Generator source tends to be very weak. If a backup generator or a co-gen capability exists… you must model, and plan based on all these conditions. Results and strategies should be implemented on a worst case scenario. In the vast majority of my studies and field work, Generator Source systems require the greatest level of harmonic mitigation.

* Utilizing a qualified Harmonic Modeling Software that includes source background/source voltage distortion and can model the effects of system voltage imbalance is critical. Many harmonic solutions on the market are very susceptible to these two conditions. If a manufacturers modeling software does not take these two factors into consideration, then their results will not be accurate or predictive. In addition, they might not be including this parameter within their software since their product offering may not perform well with this qualifying condition. I use Mirus SOLV, a freeware package available from Mirus International, since it will allow you to inject both Source Voltage Distortion and System Voltage Imbalance into the analysis. For more info on the subject of the impact of Source/System Voltage Distortion and System Voltage Imbalance, consult my LinkedIn article dated August 11th, 2020.

* Partial and Staged Harmonic Mitigation is a foundation principal of a well-qualified and effective plan. I have written on this subject on LinkedIn before relative to retrofit harmonic mitigation deployment. Knowing harmonics are cumulative in nature and increases in voltage distortion will increase linear load sources of non-linear load currents… under a partial or staged implementation strategy, you can more effectively implement a effective and economical solution, by allowing you to stage qualified implementations via actual testing versus a less accurate modeling program. On the majority of the projects I have worked on, as the staged implementation was progressing, better than modeled results were actually witnessed during field testing. Allowing us to deploy the harmonic mitigation program more effectively and allow us to save money on the project. I have published on this topic previously, but should you like to discuss a particular project, please contact me directly.

* True cost including all components of Total Cost of Ownership must be evaluated when reviewing any harmonic solution. This is a subject all to itself. In essence, the energy efficiency of any harmonic solution must be anticipated when reviewing the mitigation strategy. I bring this subject up, since in many cases, Active Front End Drives and other active solutions are less energy efficient than a properly deployed 6 pulse drive with a low through impedance passive filter. In fact, multi-pulse drive solutions are also less efficient and effective in real world installations, due to the impedance structures associated with a phase shift magnetic strategy and real world source voltage distortion conditions. In some cases by as much as 2% to 8% energy consumption impacts can be seen. Even a 2% difference in energy efficiency can increase your TCO significantly in less than a years time period, and catastrophically effect the TCO over the life expectancy of the installation. The proper threshold for a drive/passive filter efficiency index is 96.5% or greater efficiency at full load and any passive filter shall be rated > 99.0% efficient as a standalone component.

* Within the next few weeks, Mirus International will be publishing a Technical Comparative, "Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front-End (AFE) Drives" which will be distributed through LinkedIn and other media. I was honored to have contributed to the technical review. TCO and other performance consideration will be discussed in detail, and may provide you with assistance in understanding the challenges associated with AFE technology. Other topics included in the review are the injection of Common Mode noise due to high frequency switching, Supraharmonics (High Frequency Harmonics), and other topics not normally recognized when evaluating an AFE Drive integration.

Summary:

As stated before, having a fundamental and intuitive understanding of harmonics and system conditions is critical to determining the right harmonic mitigation strategy. The above discussions were intended as a general relationship discussion. Should you wish to discuss any of these points above in further detail, please contact me. Modeling services and Partial/Staged Implementation assistance is available upon request.