vfd_article

Harmonic Mitigation Specifications for a Variable Frequency Drive

Written by Steve Rossiter, President/CEO

One must consider the effects of harmonics when working with motor controls.


Table of Contents

Background of Harmonics

Power system harmonics are associated with the operation of electronic equipment in the course of normal operation. Electrical engineers have applied specifications that address harmonic distortion issues associated with variable frequency drives (VFDs) in numerous ways. initially some thought that referencing iEEE 519 would be sufficient. However, many vendors responded that iEEE 519 is a system standard and not appropriate to reference in equipment specifications. This resulted in many vendors supplying equipment on projects at the lowest cost but with no consideration for the resulting system harmonic distortion. in some cases, electric utility companies have later sited iEEE 519 as a requirement and condition for customers (with harmonic generating equipment) to continue electrical service (due to power quality concerns that could negatively affect other customers).

in response to the growing concern regarding harmonics, some engineering firms have become very specific with their VFD specifications by requiring specific filter brands or technologies such as 12 or 18 pulse arrangements. With this approach however, it is often thought that follow-up verification and certification testing is not required. While immediate problems might not have been noticed, the result has been many installations that do not meet iEEE 519 recommendations. As an example, some specifications have required a VFD with a 12-pulse rectifier but are supplied without a phase shifting transformer that allows any harmonic mitigation at all. Also, filter manufacturer's test results are usually based on laboratory voltage conditions, which do not exist in most commercial electrical systems. The electrical system source impedance and loading conditions will have a significant effect on harmonic mitigation results.

iEEE 519 - Recommended Practices for Harmonic Control

iEEE 519-1981 was originated as a result of concerned US based utilities regarding their customer's harmonics affecting the grid. The original Recommended Practice was a limit of 3-5% voltage THD (total harmonic distortion) at the PCC (point of common coupling). General systems allowed 5% while special applications including hospitals and airports required a 3% voltage THD limitation. in 1992 the Recommended Practice was upgraded to also require total demand distortion (TDD) to be within the range of 5-20% (depending on the size of the installation). TDD is the total root-sum-square harmonic current distortion, in percentage of the maximum demand load current in a 15 or 30 minute window of time. The maximum limit is based on the ratio of iSC/iL, which is measurement of the customer's affect on the electrical distribution system. The higher the utility short circuit capability (iSC) or the smaller the customer load (iL) results in higher allowed limits. See the chart below:

Location iSC/iL iEEE Allowed TDD
<20 5.0
20 < 50 8.0
50 < 100 12.0
100 < 1000 15.0
> 1000 0.0

The PCC could be defined by the engineer at any point in the system (such as at an MCC bus) but it is usually understood to be the point where the customer connects to the utility (unless otherwise stated). Even though engineers are free to apply iEEE 519 at other points in the system to require equipment compliance, the standard is still intended to be a system specification. European iEC 61800 specifications, on the other hand, are equipment (rather than system), based.

Effects of Harmonics

According to iEEE 519, the equipment most susceptible to harmonics, includes communication and data processing equipment. "Most motor loads are relatively tolerant of harmonics". However, iEEE 519-1992 states further that, "Even in the case of the least susceptible equipment, harmonics can be harmful. in the case of an oven, for example, they can cause dielectric thermal or voltage stress, which causes premature aging of electrical insulation. A major effect of harmonic voltages and currents in rotating machinery (induction and synchronous) is increased heating due to iron and copper losses at the harmonic frequencies. The harmonic components thus affect the machine efficiency, and can also affect the torque developed". Harmonic distortion levels tend to increase over time in typical buildings and production facilities. This is the result of upgrades or the addition of electronic equipment such as computers, UPS systems, office equipment, motor drives, control systems etc. The effect of harmonic generating loads should be planned for in advance.

Types of Harmonic Mitigating Equipment

Early harmonic mitigating equipment consisted of shunt tuned filters which attract harmonic currents into a trap type filter. Later improvements consisted of installing a series reactor to help solve resonance issues. Voltage rise was often a problem with these type filters. 12-pulse VFDs were also an early implementation for harmonic reduction. When two VFDs of the same size and load exist, simple delta-delta (and delta-wye) transformers installed with the VFD can produce the same 12-pulse results. As harmonic mitigation became more prevalent, broadband filters and 18-pulse VFDs appeared on the market. Broadband filters are able to reduce harmonic levels down to 10-15% but are bulky and expensive. 18-pulse VFDs, while expensive, are able to achieve levels down to 3-5% if the phase shifting transformer is properly designed. in the last several years, new hybrid filters have reduced harmonic mitigation down to the 8-12% range (or lower).Hybrid filters have elements of both the "shunt tuned" and "broadband" filters. When these are optimized, they are not only economical, but can achieve results down to the 5-8% range (if optimized and properly integrated with a VFD system). it is also interesting to note that broadband filters have the best performance at light loads while hybrid filters offer the best performance at full load.

Harmonic Mitigation Cost

Like all technologies, initial costs for reducing harmonic distortion in power electronic equipment was high, sometimes doubling the cost of the equipment. As specifications required harmonic mitigation, costs were reduced. As an example, electronic fluorescent ballasts add little cost in the newer high power factor, low THD designs. Low harmonic distortion UPS systems now carry less than a 20% cost adder and some manufactures only offer the low THD designs. While future designed VFDs could reduce costs, current harmonic corrected or filtered VFDs presently result in a 20-300% cost adder depending on HP size, level of THD required, and the required options in the VFD package. See the cost comparison table below.

HP 10 30 100 300
Bare Bones Nema 1 VFD $1,025 $2,398 $6,400 $18,025
Typical VFD System with Circuit Breaker, Bypass & Optimized Reactor
(~30-40% THD)
$2,260 $3,823 $9,067 $26,474
VFD System with Typical Separate Mount Shunt Tuned Harmonic Filter or 12 Pulse Configuration (~15-20% THD) $3,791 $7,305 $14,692 $38,272
VFD System with Hybrid integrated
Harmonic Filter
(~7-10% THD)
$3,006 $4,818 $11,215 $33,366
VFD System with 18 Pulse Configuration w/ Phase Shift Transformer
(~4-6% THD)
N/A $11,582 $17,725 $40,913
VFD System with Active Rectifier or integrated Active Filter
(~3-4% THD)
$4,310 $8,619 $21,867 $58,524

it can be seen from the above table that standard shunt tuned filter arrangements and 12 pulse rectifiers with phase shift transformers have become outdated in general purpose VFDs. This is based on the cost premium and marginal THD mitigation improvement. To meet iEEE 519-1992, most facility specifications have required equipment limiting current THD to 8-15%. Using the most cost-effective harmonic mitigating methods usually results in an average of 20-30% cost addition over the typical packaged VFD system. installations with very large drives and a very high percentage of the total load being on VFDs sometimes requires a 5% current THD level which results in a cost addition of 50-100%. it should be noted from the table that small VFDs are not practical in 18-pulse configurations. Also, active rectifiers and filters are generally the most expensive solution today but it is believed will become very cost effective in the future as volumes increase. The chart below represents a general comparison of technologies as a percentage of cost increase above the typical base VFD system cost.

vfd filtering % increase cost comparison

Measurement Considerations

When a utility applies iEEE 519-1992 as a condition of service the PCC would typically be designated as the point at which the utility connects power to the customer. Harmonic distortion would be measured in terms of both voltage THD and current TDD (described above).

From iEEE 519-1992 and the utility perspective, it is less important what the THD % is when equipment is operating at reduced load as is typical with VFDs. This is important because most harmonic mitigating technologies are designed for maximum performance at full load. As an example, a 100HP VFD operating at full load might produce 43 amps (out of 124A for 35% THiD) of current distortion at full load with no filter and 9 amps (out of 124A for 7% THiD) with a filter.

At 80% speed this could change to 29 amps (out of 75A for 38% THiD) with no filter and 8 amps (out of 75A for 11% THiD) with a filter. Also, at the utility PCC, a building operating with the same 100HP filtered VFD on a fan would measure 7% THiD when running by itself which would change to less than 5% THiD when another 56 amps of linear load such as a compressor turned on.

At the same time, other non-linear loads such as computers, elevators and other electronic equipment will add distortion in the same fashion, which needs to be considered.

Therefore, it is important that an engineer, who specifies VFDs, consider the other loads in the building or facility and then specifies which levels of harmonic distortion will be acceptable in consideration of the added cost.

Recommendations

in view of the information presented to this point, it can be seen that in order for an engineer to comply with the Recommended Practice outlined in iEEE 519-1992, he must carefully consider facility loads and the equipment serving them. Older facilities that have been upgraded with electronic equipment, will likely be require additional filtering at some point. in new installations, which specify nonlinear equipment such as VFDs, filters or mitigating techniques can be included in the design to limit current distortion amps to a specific level or range to satisfy the overall objective. A general rule of thumb might be presented as follows:

Level 1 - No Harmonic Mitigation Equipment:

When a facility is comparatively large with a significant amount of linear load compared to nonlinear load, one or two small VFDs under 5 or 10 horsepower may not justify any specific harmonic filtering or mitigating technique. However, it is still recommended to add source impedance in the form of a simple AC or DC reactor. in addition to providing equipment protection, this will typically keep current distortion below 40%.

Level 2 - Standard Off-The-Shelf Harmonic Filtering or Mitigation Equipment:

Manufacturer's catalog equipment is typically available for harmonic mitigating levels of 12-20%. Facilities that have (or will be adding) a moderate amount of nonlinear load compared to linear load should make harmonic mitigating equipment a specific part of the VFD specification. Full load iTHD % Values (measured at the equipment) should be specified with the intention of meeting an iEEE 519-1992 TDD % level at the PCC. See example #1.

Level 3 - Extensive Designed or Highly Optimized Harmonic Mitigation VFDs or Equipment:

Highly optimized filters or 18-pulse VFD systems can achieve harmonic correction to allow the addition of no more than 3-5% iTHD at full load. A facility that has predominantly nonlinear loads may require specifications, which define a VFD that adds no more harmonic distortion than the same level required by iEEE 519-1992 at the utility PCC. See example number 3.

Example #1

An engineer desires to add a 100HP VFD to an existing building with a 500A load measured at 4% TDD. He desires to maintain 8% TDD at the PCC to meet iEEE 519-1992. To achieve the 8% level he can allow 20 more amps of harmonic distortion to the 20 amps that already exists. With a 100HP full load of 124A this would be 16% iTHD and a filter would be specified appropriately.

Example #2

A new building is considered for construction with an estimated 2000A electrical load and a maximum of 5% iTHD to meet iEEE 519-1992. 5% of the load is assumed to be nonlinear at a 50% iTHD level in addition to (5) 15HP, (5) 60HP and (2) 75HP VFDs on HVAC equipment (682 amps). With 50 amps of existing nonlinear load, only 50 more amps of distortion can be allowed (approximately 7% of the full load of the VFDs). Appropriately, the VFDs would be specified with harmonic mitigating equipment to add no more than 7% iTHD at full load.

Example #3

A pump station is being designed which will have 50 amps of lighting and receptacles and the remainder of the load consists of (3) 500HP pump motors with VFDs. if iEEE 519-1992 requires 5% TDD at the utility PCC, it becomes obvious that each VFD must also be limited to approximately 5% iTHD.

Appendix A: Harmonic Mitigating VFD Specification

The following recommendations are given to specify VFDs to achieve the desired results concerning harmonic mitigation:

include a section in the front General section under "Work included" or "Description of Work".

VFD harmonic mitigation equipment shall be included, as part of the integrated VFD package to meet the THD levels required in the section titled "Harmonic Distortion Requirements".

include in "Quality Assurance" Section

iEEE Standard 519-1992 - Recommended Practices for Harmonic Control in Electrical Power Systems
(based on the engineer-identified levels stated in the "Harmonic Distortion Requirements" section).

include in "Submittals" Section

Submit sample input current waveforms that are to be expected. This should include examples from previous installations with similar integrated harmonic mitigating equipment VFD packages. The data submitted shall meet the levels required in the "Harmonic Distortion Requirements" section.

include in "Acceptable Manufacturers" Section

The following vendors/manufacturers have demonstrated the ability to meet the requirements of these specifications including the integrated harmonic mitigating equipment and commissioning requirements included herein: Energy Management Corporation - Utilizing Motor Drives international packaged VFD systems with optimized integrated harmonic filters. (See Appendix B) Vendors requesting approval by addendum must submit a point by point certification to these specifications at least 10 days prior to the bid. information must include proposed integrated harmonic mitigating equipment with sample waveforms from a minimum of three local installations, which have been in operation for a minimum of three years. Only vendors listed, or approved in writing by addendum, are approved to bid the project.

include in "Construction" Section

The VFD system and associated harmonic mitigating equipment shall be supplied as a complete, pre-integrated, stand-alone package produced by a single manufacturer regularly engaged in the production of same and maintains full system support responsibility. The VFD system manufacturer shall integrate all components and equipment required to meet these specification features and functions as a single UL (or equivalent) labeled system. Vendors supplying non-integrated equipment (or which require contractor mounting or wiring of separate components) is not allowed. Vendors supplying equipment, which is not warranted by a single manufacturer, is not allowed. Optional - Front door mounted, blown fuse indicators, shall be included for all phases for all fuses associated with harmonic filter capacitors.

include in "Harmonic Distortion" Section

iEEE 519-1992 - Harmonic Control in Electrical Power Systems shall be a requirement of this project. Harmonic filters (passive or active), phase multiplication devices, or any other components required to mitigate harmonic voltage THD to 5% (insert requirement 3% or 5%) and current THD to 8% (insert requirement 3-20%) maximum levels shall be an integral part of the VFD system. Compliance measurement shall be based on (insert one of the following) THD added (during VFD full load operation compared to across-the-line operation) at the VFD circuit breaker terminals or actual THD measurement at the VFD circuit breaker terminals during full load VFD operation. Designs which employ shunt tuned filters must be designed to prevent the importation of outside harmonics which could cause system resonance or filter failure. Calculations supporting the design, including a system harmonic flow analysis, must be provided as part of the submittal process for shunt tuned filters. Any filter designs which cause voltage rise at the VFD terminals must include documentation in compliance with the total system voltage variation of plus or minus 10%. Documentation of Power Quality compliance shall be part of the commissioning required by the VFD supplier. Actual job site measurement testing shall be conducted at full load and documented in the operation and maintenance manuals. Optional - Harmonic measuring equipment utilized for certification shall carry a current NiTS calibration certificate. The final test report shall be reviewed and compliance certification stamped by a licensed professional engineer (PE). Optional - Data (text and graphical) shall be supplied showing voltage and current waveforms, THD (or TDD) and individual harmonic spectrum analysis in compliance with the above standards.

include in "System Commissioning and Certification" Section

Conduct and document harmonic distortion compliance testing as identified in the "Harmonic Distortion" section of this specification.

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