Harmful effect on power systems from using big VFDs or tons of computers at home

[ICanHas]

An old small industrial VFD or new bare-minimum built CHINA MADE VFD eBay devoid of all but the essential parts can have a power factor of 0.5 to 0.6 with a total harmonic distortion of 100-150%.

The power company provides medium voltage power to a distribution transformer in your subdivision to provide the low voltage power. A subdivision with 10 houses with average power usage level will have something like 34.45kW of demand. This is computed by 7.03kW demand x 10 homes x 0.49 coincidence factor) Such a subdivision maybe fed from a 50kVA transformer. The short circuit rating on this type of transformer is about 4,000A wth a 5.2%Z rating.

When some DIYer installs a surplus industrial/eBay China made 3 hp VFD without harmonics filtration out of ignorance or selfishness, it wreaks havoc on the local power system and cause this to happen to the entire subdivision:



http://www2.schneider-electric.com/m..._solutions.pdf

The losses in the power company transformer also increases substantially which means the power taken from primary side is greater than it is with a 3kW non-harmonic load. The additional loss contributes to increased "carbon footprint" and energy consumption which is anti-eco.

You can expect an input power of 3kW after motor and drive losses at full output. The highest 15 minute average is used for demand in the power industry and since you can anticipate it can operate more than 15 minutes at a time at full load, 3kW will be used for demand.

VFDs and computer power supplies contain power electronics that produce e EXTREME amounts of harmonic currents. The problem is that power system for subdivision of 10 homes is not designed to see 3kW of sustained harmonic load. CFLs do not escalate to this level of problems unless you switch on 230 13W CFLs all at once and leave them on for well over 15 minutes in a subdivision of 10 homes. Yeah, that ain't gonna happen in real life.

A large, straight rectifier power supplies with straight rectifier input is common in internet general purpose CHINA MADE VFDs.

Why is it a problem when someone use power polluting loads that is not even meant to be used at home like a 3hp VFD or run ten computers to do "coin mining" mining and do dumb stuff like "omg I'll save 40W between mah ten computers if I bypass the PFC".

A 0.5PF 3hp VFD will have an input characteristics of
240V
input amps: 25A RMS, 6kVA total
PF 0.5
60Hz components: 12.5A, 3kW
THD of 100% fundamental: 12.5A, 3kVA
3rd harmonic is probably in the order of 2.4kVA
PSC used in air conditioners produce no harmonics and power factor is close to 1.0.

That one customer is drawing 3kVA of harmonic demand on the subdivision transformer at the point of power coupling. 3kVA/34.45kVA =8.7% TOTAL DEMAND DISTORTION (TDD)
IEEE 519-1992 recommendation is not more than 5%.

Computers, ECM, X13 and such from all the other homes will add up to about 2kVA for a combined total of 5kVA or 14.5% TDD leading to situation that breaks the camel's back.

100 CFLs turned on at the same time across 30 houses would not be an issue, because, the demand is small RELATIVE to the total demand and transformer size that would be used to feed 30 houses. The effect of you alone 3kVA of harmonic current depends on the size of the subdivision transformer on which you're connected.

The attached PDF tells you the effect of when proportion of harmonics relative to total demand and transformer size is excessive. This is an example of large portion of electronic battery charger being used at the same time by one customer. High proportion of harmonic producing load reduces efficiency of power company transformers and reduce efficiency of induction motors used by other customers by distorting the voltage waveform. Both of these have negative environmental impact.

This is why harmonic loads often cause a problem with generators. The harmonic producing dirty load is HUGE relative to the capacity of your home generator which is much smaller than the neighborhood distribution transformer. When you have to "oversize" generator to work with harmonic loads, you get a first hand experience of concept of wasted capacity.

Until recently, harmonic loads relative to total demand was negligible in residential power systems. Increased use of CFLs and harmonic producing embedded systems type appliances is changing this.

A 3 hp VFD in an industrial facility or an apartment complex with a 400kW demand fed from a 1,000kVA transformer is not a problem, because it's small in proportion to the local power system.