ECM's and Mirus Filters

[mshields]

Have only run into ECM AHU's a couple of times now. Both recent however and in both cases, they had a Mirus filter ahead of them This most recent example involved 4 7.5HP fans, with the entire assembly fed via a single Mirus filter. What my client is wondering about and I am too now that he mentioned it, is why are the added losses of what is essentially a dry type transformer with some capacitors, with losses associated with it, necessary? Are the harmonics reflected upstream by 4 7.5 ECM motors so bad as to warrant such isolation regardless of the size of the system they are on and/or the overall ratio of linear to non-linear loading? Does anyone know of an applications engineer out there who can discuss this in depth? Thanks,

 

[drktmplr12]

is it necessary for four 7.5 HP fans?

probably not in the strictest sense of the word, but the real answer is it depends. how many other non-linear loads does the facility have? how many induction motors does it have? What % of total load is non-linear? what size is the utility transformer?

IEEE 519 dictates the requirements for allowable harmonics any user of a utility can "inject" back onto the grid, relative to the demand load and service transformer. the point of measurement is defined as the point of common coupling (POCC) and typically exists on the bus where the utility connects to the owner's equipment. reading the standard would lead one to think they shouldn't care until the utility sent them a letter stating they are out of compliance. but the best approach is the be proactive and the supplier of the AHU has an interest in not being the straw that broke the camels back, as they do not have intimate knowledge of the system. they hedge by making sure their equipment contributes as little harmonic content as practical.

There are many ways to comply with IEEE 519. you could install a very large filter at the top of the power distribution system or you can distribute small filters at motor controllers and other large non-linear loads. alternatively several medium-sized ones at strategic locations in the power system. the important thing to note here is if you have many non-linear loads without any mitigation whatsoever, their effects are additive and, if significant, could become a ghost in the machine so to speak. the best way to know if you have a problem is to have someone study the system, ask questions, and measure the harmonics, suggesting remediation if needed.

all of this being said, they serve as cheap insurance/surge protection for the VFD(s) and any other electronics beyond the CPT. albeit, they are likely not as effective as a dedicated surge protective device.

 

[mshields]

is it necessary for four 7.5 HP fans?

probably not in the strictest sense of the word, but the real answer is it depends. how many other non-linear loads does the facility have? how many induction motors does it have? What % of total load is non-linear? what size is the utility transformer?

IEEE 519 dictates the requirements for allowable harmonics any user of a utility can "inject" back onto the grid, relative to the demand load and service transformer. the point of measurement is defined as the point of common coupling (POCC) and typically exists on the bus where the utility connects to the owner's equipment. reading the standard would lead one to think they shouldn't care until the utility sent them a letter stating they are out of compliance. but the best approach is the be proactive and the supplier of the AHU has an interest in not being the straw that broke the camels back, as they do not have intimate knowledge of the system. they hedge by making sure their equipment contributes as little harmonic content as practical.

There are many ways to comply with IEEE 519. you could install a very large filter at the top of the power distribution system or you can distribute small filters at motor controllers and other large non-linear loads. alternatively several medium-sized ones at strategic locations in the power system. the important thing to note here is if you have many non-linear loads without any mitigation whatsoever, their effects are additive and, if significant, could become a ghost in the machine so to speak. the best way to know if you have a problem is to have someone study the system, ask questions, and measure the harmonics, suggesting remediation if needed.

all of this being said, they serve as cheap insurance/surge protection for the VFD(s) and any other electronics beyond the CPT. albeit, they are likely not as effective as a dedicated surge protective device.

Thanks alot!

 

[Jraef]

Some engineers don't want to "bother" themselves with determining whether or not harmonic mitigation is necessary by calculating what it will be at the PoCC, so instead opt for doing mitigation at each and every non-linear load. I see a lot of specs for equipment stating that the PoCC is to be considered the line terminals of the equipment, even though that is NOT what it says in IEEE-519.

That said, if you mitigate harmonics at each contributor, the cumulative effect is that it will be mitigated at the PoCC, it's just probably the most expensive approach from the standpoint of the costs to the end user.