Impacts of Improper Sizing of Harmonic Filter

Impacts of Improper Sizing of Harmonic filter

For those tasked with maintaining quality power at a commercial facility facing harmonics issues, the optimal approach involves the installation of a series of active filters. In the application of these filters, a meticulous examination of harmonic control requirements is crucial, and sizing the filter appropriately becomes pivotal in resolving the problem. It is unnecessary to address every amp of harmonic current originating from nonlinear loads, leading to significant savings in required filter size. 

Consideration of various factors is essential when determining the size of active filters, contingent upon the specific harmonic problem being addressed. Noteworthy challenges such as overloaded neutral conductors and adjustable speed drives pose threats to power quality, yet both can be effectively mitigated with properly sized active filters. 

Excessive neutral currents due to power supply loads 

Overloaded neutral conductors are probably the most common harmonic problem encountered in commercial facilities. The problem occurs when electronic power supply loads like PCs create third harmonic currents that add in the neutral, or zero sequence. If the entire load on a circuit consisted of this type of load, the rms neutral current could be as high as 173% of the rms phase current. The problem is aggravated because neutral conductors — especially in older buildings — are often sized for less current than the phase currents. 

Active filters are an effective way to correct harmonic components that cause the neutral current to be high, but how much of the harmonic current from the electronic power supplies need to be cancelled to avoid neutral current problems? It helps to focus on the third harmonic component. However, even with a control that corrects all the harmonics in proportion to their magnitude, you can realize significant savings by sizing the active filter just large enough to prevent problems.  

Required rating for the active filter in A/kVA of load for the following two scenarios: 

  • Compensation for the entire harmonic current in the nonlinear loads. 
  • Compensation for enough harmonic current so the neutral current is always less than the phase current. 

Applying the second scenario, we only need an active filter if the electronic load is more than 65% of the load on the circuit. Consider the following example. If you have a 100kVA circuit and the entire load is electronic, you could buy an active filter rated for 170A to cancel all the harmonics from the electronic loads (1.7A/kVA×100kVA). On the other hand, an active filter rated 60A would be sufficient to make sure the neutral current is less than the phase current. 

Harmonic distortion caused by adjustable speed drive loads. 

The other concern common in commercial facilities — as well as many industrial facilities — is the increased harmonic distortion levels created by Variable frequency drives (VFDs). Sometimes large UPS systems can have similar characteristics, but this example will focus on VFDs.  

Generally, VFDs improve efficiency and result in significant energy savings in HVAC motor applications. However, as these loads become a larger percentage of the total load in facilities, they can result in increased voltage distortion levels. 

Harmonics from AC VFDs are suitable for correction with active filters. These loads have a good displacement power factor and therefore don’t need the power factor correction inherently supplied with a passive filter.  

The voltage distortion that results from the VFD harmonic currents is a function of the system impedance and the amount of harmonic current. For a nonlinear load characteristic, the most important limit will almost always be the limit for the fifth harmonic. The limit for most realistic short circuit ratios is 8% of the average maximum demand load current. This tool aids in identifying the point at which VFD loads become a matter of concern in terms of penetration level. 

In sizing the active filter for this type of load, is it necessary to account for the entire harmonic content from the VFDs, or is it sufficient to correct only a portion of the harmonic current to avert problems? 

Required rating for the active filter in A/kVA of load for the following two scenarios: 

  • Compensation for the entire harmonic content in the VFD load. 
  • Compensation for enough harmonic current so the total load current complies with the limits in IEEE 519. 

Realizing cost savings is achievable through precise sizing of the active filter to align with harmonic control specifications, rather than accommodating the total harmonic content. When the entirety of the load comprises VFDs, targeting an active filter rating of about 0.5A/kVA for a 480V system is recommended. Adhering to IEEE 519 requirements allows for a more efficient approach at 0.4A/kVA, translating to a reduced compensation of 200A for a 500kVA load, as opposed to the initial 250A. This contrast is even more impactful when dealing with a load that incorporates adjustable speed drives only in part. 

The installation of a series of active filters facilitates enhanced control over power quality requirements in a commercial facility. To optimize savings, it is crucial to size the active filters appropriately. 

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