Building HVACR systems rely increasingly on variable speed drives (VSDs) – known sometimes as variable frequency drives (VFDs) - to optimize the energy efficiency of their electric motors. However, it makes sense to pay attention to the significant impact that VSDs and other equipment can have on the network’s power quality. Here, we look at how ultra-low harmonic VSDs can improve the power quality in buildings by eliminating troublesome harmonics at their source.
Typical examples of low voltage VSD applications in HVAC systems include pumps (chilled water and heating circulators, booster sets), fans (supply, return, smoke extraction, cooling tower), and cooling compressors. These applications are typically run by VSDs as they can save a great deal of energy. HVACR systems are often designed for peak loads, yet most of the year they run at partial loads. VSDs control the speed of the motor to match the needs of the application and, compared to other methods for needs-based ventilation, VSDs are very energy efficient.
Power quality is an important aspect that should be considered from the early design stage of a building. This is because poor power quality affects the energy efficiency and can also result in potential disturbances to electrical systems and even premature failure of critical equipment. Although there are multiple factors that can influence the power quality of an electrical network, in this article we are focusing on the phenomena called power line harmonics.
Where do harmonics come from?
In an ideal situation a building’s alternating current (AC) power supply has a pure sinusoidal wave form with a frequency of either 50 or 60 Hertz (Hz), depending on the region of the world. In practice, this pure sine wave does not exist in a building’s power network as there are non-linear loads that create harmonics. These harmonics cause the sine wave to deviate.
VSDs are not the only equipment that create harmonics. They also result from other non-linear loads such as EC (electronically commutated) motors, LED or fluorescent lighting, computers, uninterruptible power supplies and basically every single type of modern electronic device. Direct on-line motors and light bulbs do not cause harmonics as they are linear loads.
Power quality is an important aspect that should be considered from the early design stage of a building
Harmonic disturbances should not be confused with radio frequency interference (RFI). Harmonics are multiples of the base frequency and are therefore relatively low in frequency, typically below 2,500 Hertz (Hz). In contrast RFI is usually above 150 kilohertz (kHz). RFI disturbances can be radiated and/or conducted. Harmonics are always conducted.
The impact of harmonics is measured as a percentage value known as the total harmonic distortion (THD). This is the ratio of the RMS (root mean square) harmonic content to the RMS value of the fundamental frequency. Where no voltage or current harmonics exist the THD is zero percent. As the level of harmonics increases, the THD value increases. THDi is the total harmonic distortion on current and THDu or THDv is the total harmonic distortion on voltage.
A single 4 kW drive even with 100 percent THDi will not trash the whole network. Rather, it is important to consider the cumulative effect of harmonic distortion at the point of common coupling (PCC) – the point where the building’s local network connects to the local utility distribution network.
Why are harmonics troublesome for HVAC installations?
An electrical network where there are high levels of harmonic distortion can experience a wide range of problems including:
- Premature failure and reduced lifespan of equipment, especially due to overheating of motors, transformers, cables, circuit breakers and fuses
- Reduced energy efficiency
- Nuisance tripping of breakers and fuses
- Unstable operation of backup generators
- Unstable operation of sensitive electronic devices
- Flickering lights
These issues can have a significant economic impact during all phases of the lifespan of the HVAC installation, especially in a critical facility such as a data center. Initially, there is the cost of either sizing equipment to handle harmonics or investing in harmonic mitigation. Then there is also the repair and replacement costs associated with premature equipment failure.
High levels of harmonics also result in additional heating of equipment, which means electricity that could be used to power equipment is lost as heat. Harmonics also affect the true power factor which has an overall impact on the power network’s quality and efficiency.
Approaches to handling harmonics
One approach to tackling the overheating created by the harmonic current is to oversize some of the key electrical infrastructure. For example, transformers and cables may be increased in size. Oversizing of backup generators is also a common way to mitigate some of the other challenges created by harmonics. An alternative approach is to mitigate the harmonics produced by VSDs.
High levels of harmonics also result in additional heating of equipment
A standard six-pulse drive does not feature harmonic mitigation. This type of drive is used because of its lower cost and small footprint. The exact current distortion varies according to the design, with typical values between 90 and 120 percent THDi.
Adding a DC choke or input line AC reactor to the standard drive increases the impedance. This decreases THDi levels to between 35 to 45 percent, which is recommended generally as the starting point. If calculations and surveys show the need for a further reduction in THDi then the drive should be upgraded to a better harmonic mitigation technology.
Passive filter solutions can be added on the supply (line) side of the drive. Their performance can be variable as some designs provide poor harmonic mitigation at partial loads, or when there is already existing voltage distortion on the plant’s power supply. Typically, passive filters result in a THDi between 5 and 10 percent, depending on the filter design.
Active harmonic filters work much like noise cancelling headphones. The filter measures the current distortion and then supplies a counter-waveform to cancel it out. This is an effective method for achieving THDi levels between 4 and 7 percent.
Another standalone solution is a multi-pulse package. But this occupies a large footprint – even a relatively small 23 A, 18-pulse package capable of reducing THDi to 5 to 6 percent is around the same size as a domestic refrigerator.
Active front end – ULH drives
In an active front end (AFE) drive, a rectifier based on insulated gate bipolar transistor (IGBT) technology enables it to draw nearly pure sinusoidal current. An LCL (inductor-capacitor-inductor) filter helps to remove the possible higher switching frequency noise caused by the IGBTs. The result is a THDi between 3 percent and 5 percent, hence this design is also known as a ULH (ultra-low-harmonic) drive.
ULH drives are the most compact solution that can achieve a THDi below 5 percent. They also have excellent harmonic performance at partial loads. And as a single, compact package, ULH drives are easy to install.
Power factor is also important
In addition to harmonics, it is also important to consider power factor (PF). This is a term that describes how effectively an electrical network uses the power it draws. True power factor considers both the displacement power factor (also known as cosφ) and distortion power factor (that is a function of the amount of harmonic current). In the very best case, a network will have a PF of unity.
In some cases, utilities might impose penalty charges on buildings with a poor power factor. Adding a standard VSD to a motor will improve its displacement power factor but add to the distortion. ULH drives mitigate harmonics which positively affects the true power factor. This drive also has the ability to compensate reactive power which improves the displacement power factor of the installation.
HVAC installations must handle harmonics
When designers and operators of HVAC systems appreciate the importance of harmonic mitigation then the improved reliability and longer equipment life soon shows a positive return on investment. A particularly elegant and cost-effective solution is to deploy ULH drives that mitigate harmonics at source, rather than the band-aid solution of external filters.