What is difference between THD and TDD?
While power electronics have created considerable energy savings, they can distort the quality of electrical power delivered through the building they serve, reducing the effectiveness and life span of the power electronics contained within that building.
THD is described as the instantaneous value of the ratio of the root-mean-square (rms) of the harmonic content to the rms value of the fundamental quality, as expressed as a percentage of the fundamental. This can apply to either current or voltage.
TDD uses a similar formula as THD, except that the values used for current or voltage are based on a sliding average seen over a 15- or 30-minute window of time, and so gives a less biased picture of the overall harmonic problem.
Power Qaulity Problems in HVAC systems
HVAC systems are increasingly being equipped with Variable Speed Drives (VSDs), and are now commonly used with air handlers, pumps, chillers, and tower fans.Here we focuses on the largest share of power electronic loads in today’s buildings, variable frequency drives (VFDs), which are a form of power electronics used to vary the speed of the motor they serve. These drives offer precise control and consume significantly less energy but have a drawback; due to their construction, they draw current from the electrical supply in non-linear pulses instead of linear sine waves. This causes harmonics in the system; if left unaddressed, harmonics will result in increased heating of the equipment and conductors, equipment operational problems including the VSDs themselves, and torque pulsations in motors. All of which lead to inefficiency, damage and even system failure.
Harmonics can also affect other building elements. The electrical distribution supplying VSDs is often shared by other sensitive electronic systems like computers, communications networks and lighting. Protecting the system from distortions doesn’t just protect HVAC systems, it protects everything.
The resulting harmonics in the building voltage can have several detrimental effects on connected electrical equipment:
Advanced Active Harmonic Filters for HVAC systems
Three phase Harmonic filters reduce distortion by diverting harmonic currents in low-impedance paths. Harmonic filters are capacitive at the fundamental frequency, so they are also used to produce the reactive power required by converters and for power factor correction.
YTPQC-AHF Active Harmonic Filters are the best Harmonic Filters for harmonics compensation, reactive power compensation and imbalance compensation.
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