Harmonic Analyis 1

[HamidEle]

Hi, Guys,
As per IEEE519, Isc is maximum short circuit current available at PCC. But the source short circuit current we based on to perform the harmonic analysis should be minimum short circuit level, which represents the worst case senario. Is it correct. This is as per IEEE-519 new revision. another question is, is the short circuit half cycle value or 1.5-4 cyle or steady state value? It is so clear in IEEE 519.
Appreciate it if somebody can shed some lights.

 

[Desrod2]

Using the max SCC gives you the minimum impedance of the supply network therefore the max harmonics currents injected into it from which you compute the current THD. At minimum SCC, you have the max stress on your mitigation devices (filters), lower harmonics injections but max vltage THD. When performing harmonics studies and filter design, we used both cases and check for both parallel and series resonance.

 

[HamidEle]

Thanks, Desrods2,
In our case, all the LV VFDs come with input filters. As per the vender design, all the filters are tuned filter. Not sure we need to check the resonance or not in the system. Our system is very strong.

 

[krisys]

Dear HamidEle,

The short circuit impedance shall be the continuous one. As per IEC 60909-0, it shall be the steady-state short-circuit current (Ik, the r.m.s. value of the short-circuit current which remains after the decay of the transient phenomena). Because, the harmonic current injection to the system is generally a steady state phenomena.

 

[HamidEle]

Thanks. Looks like it is very common to see higher mignitude of high order harmonic exceeding IEEE 519 limit, such as 37th,41th order. Don't see any adverse impact on the system.

 

[HamidEle]

How dose harmonc phase angle play roles in the harmonic analyis? It is always difficult to get harmonic phase angle from the vendors. What would be the best approch top come up with the realistic result?

 

[krisys]

Consider phase angle of all the harmonic source same which would give a most conservative result.

The value of angle doesn't matter as long as the angle is same for all the harmonic sources.

 

[jghrist]

The IEEE-519 Isc value is used to reflect the stiffness of the system. A stiff system can absorb more harmonic current distortion without having voltage distortion that adversely affects other customers. Using a larger value for Isc allows higher current distortion and thus is unconservative.

 

[krisys]

jghrist,

Agreed, your point is right on its own. But Hamid's problem is with the phase angles of the harmonic currents injected from various sources and my response was for that.

Imagine, when all the harmonic (say fifth harmonics)current injected have the same phase angle, that means the current wave would rise and fall together. When all the waves are rising and lowering together can cause higher distortion. Same as the vector sum of in phase components is higher than the out of phase components.

Now, say the injected harmonic components starts at different values. Thus the vector sum of these harmonic components is relatively lower due to their phase shifts.

Thus the severity of in phase components is much higher than the out of phase. This makes the same phase angle more conservative.
Hope I am making it clear.