Excessive harmonic distortion of electrical supplies is a phenomenon that must be carefully guarded against. A harmonic frequency is simply a frequency that is a multiple of the fundamental frequency; for instance, a 250 Hz waveform superimposed on a 50 Hz network is the 5th harmonic. Harmonic currents are produced by non-linear loads such as fluorescent lights, computer power supplies and variable speed drives, and are seen as a voltage distortion when imposed on the supply.
The symptoms of harmonic voltage distortion can be severe and serious problems can result. Transformers may overheat, cables may get too hot and the insulation can break down. Motors may also overheat or become noisy. Capacitors can overheat or form tuned circuits that resonate. Electronic displays and lighting may flicker, circuit breakers can trip, fuses blow spuriously, computers fail and metering give false readings.
Variable speed drives are one source of harmonics and the many drives used in HVAC applications pose a challenge for the building services engineer seeking to provide a clean, reliable power supply.
Most office buildings contain many items sensitive to excessive harmonic distortion, such as computers, and these must be protected from the harmful effects.
Options for cutting harmonics
The options for cutting harmonics depend largely on whether the project is a new build or a refurbishment.
When installing variable speed drives to run a HVAC system in a new building, a solution can be designed that will produce zero harmonics. With a refurbishment, where a new drive system is needed, space may be a problem and this will affect the type of solution that can be applied. Also, with a refurbishment, the existing cabling may not be able to withstand the heating effect caused by the harmonic currents. For this situation, a solution needs to be found which will retain the existing transformer and keep the cable ratings unchanged.
Another consideration for HVAC applications is the size of the drives to be used. While smaller units take up less space, they produce more harmonic currents, which in some cases can reach 50 percent of their normal rated output
Larger drives have lower harmonic levels, but obviously the bigger the drive, the more usable floor space is taken up.
A number of solutions exist for keeping the harmonic currents from causing harm. One of the simplest is to segregate the drives from sensitive equipment, so that harmonics are not generated in their immediate vicinity.
A similar solution is to use drives that are phase shifted in relation to each other, so that any harmonics generated cancel each other out. The disadvantage with this method is that it requires a special transformer. These are physically large components, with associated losses, and the floor area may not be available to accommodate them, particularly in a refurbishment.
A better option for HVAC applications is to use a power quality filter on a standard drive. This is a compact item, which will occupy little valuable space.
Drives based on an Inverter Supply Unit or”active rectifier”, can also be used. These are regenerative drives, which can also use the braking torque of the motor to produce energy. These produce very low harmonic current levels.
New regulations
New regulations on harmonics, EA Engineering Recommendation G5/4, affect everybody connected to the public electricity supply. G5/4 limits the overall voltage distortion at each level in a network; the limits are applied at the Point of Common Coupling, or PCC, where the internal supply system meets the public network.
All consumers have an agreement to connect with their Network Operating Company, and part of any new agreement includes a requirement to meet G5/4. Failure to meet these conditions could, as a last resort, result in disconnection from the supply.
G5/4 defines three stages of assessment, which increase in complexity.
Stage 1
This concerns only 230V single phase and 400V three phase supplies.
Any equipment, or combination of equipment, under 16A aggregate input current, that meets the Harmonised European Standard EN 61000-3-2, may be connected without further assessment.
G5/4 also allows an aggregate total of 12 kVA of 6 pulse converters. Unfortunately, few installations have only single loads or total loads under 16 A.
When this is the case, the total harmonic currents from each item must be added together, ensuring that the total is less than the levels stipulated in Table 7 of the G5/4 document. `
If the connection is not acceptable at Stage 1 or is at medium voltage (6.6 kV to 22 kV), then it is possible to undertake a Stage 2 assessment.
Stage 2
If the total converter load is lower than 130 kVA of 6-pulse or 250 kVA of 12-pulse diode rectifiers, for a medium voltage PCC, then there is no need for further assessment.
For converter loads greater than this, Stage 2 connection may still be permissible, after an assessment to determine whether the existing voltage distortion and total harmonic currents introduced by the one consumer on the MV supply are within the limits set within Stage 2 of G5/4.
In this case, a measurement must be made of the existing distortion. This measurement should be based on a minimum of seven days to ensure that the peak distortion is recorded. G5/4 allows a background level of harmonic distortion that does not exceed the defined planning level for 95 percent of the time.
If, for 95 percent of the time, the value of existing distortion does not exceed 75 percent of the appropriate planning level, then the total harmonic currents defined in G5/4 can be introduced.
If the measured level is above 75% of the planning level, the voltage distortion due to the harmonic currents of the new installation needs to be calculated.
Should the resultant overall level or level of 5th harmonic remain within the planning levels, then connection may be agreed.
Stage 3
If the levels of harmonics exceed those for Stage 2, or if the point of common coupling is at 33 kV or above, then a different and substantially more complex procedure is called for. The supply utility, the consumer and the manufacturer need to agree upon the method to adopt.
It is vitally important that adequate and sufficient information is available at the outset of a project, as any delay in provision of information could be critical to implementation of a project. The calculations can be complex, and the main requirement is that the consumer, the supply utility and the drive supplier should work together to achieve the optimal solution.