Calculating additional Increase in Current due to Harmonics in the capacitor bank 2

[adi 3291]

I am going through some of the NEPSI webinars and online resources, it was very helpful content on the harmonic filter and capacitor banks. Thank you for the resources.

Our wind turbines experienced an abnormal amount of turbine faults and after investigating we identified that the faults were caused by high levels of harmonics while the capacitor banks were in service. After harmonic study analysis, identified resonant point at or near 5th harmonics and have high individual harmonic distortions at the 5th, 7th, and/or 11th harmonics. So sized and installed a single tuned bandpass filter tuned to the 5th harmonic on one of our capacitor banks. See attached for reference. Each capacitor bank is 17.5MVAR, rated @34.5KV and we have 4 of them on each bus.

After installing the filter also, we frequently saw blown fuses on capacitor banks. So what is the methodology to calculate an additional increase in current due to harmonics other than 5th in these capacitor banks, so we can size the fuses and disconnect switches correctly?

Each capacitor bank is 17.5MVAR, rated @34.5KV and we have 4 of them on each bus.

 

[Parchie]

@Lagudu Aditya (Electrical),
As you install filter capacitors, the voltage across your filter capacitors increases depending on the absolute difference between your system inductance and your added capacitance. It is advisable to choose a capacitor with a higher voltage rating than your system voltage. For example, I have a 480V system and after computing the voltage across my capacitors (sum of the product of each harmonic current at each order X respective capacitances), the results told me the voltage across the capacitor was 600V, Please check your capacitor voltage rating.

 

[electricuwe]

The comment of Parchie is correct. The voltage at the capacitors is a concern and will lead to an increased failure rate on the long run. However the reason why the fuses are tripping is most likely different.

The filter is tuned close to the 5th harmonic. So if there are X% 5th harmonic in the voltage, there will be a multiple of X% harmonic in the filter current and this is most likey the reason for the fuses to trip.

I'm surprised about the content of 5th harmonic in your system. Do you understand what equipment causes this ? Maybe tackling this at the source might be more effective than filtering.

 

[beyond86]

Do you have records current\voltages when fuses blow?
Maybe the reason can be in resonant of filter and your network at another frequency.

 

[adi 3291]

I think the fuses are blowing, as they been continuously running around 90% of their rating. It is causing a thermal stress. I want to know, how to manually calculate harmonic currents in the filter bank.

 

[dfdt]

Capacitive reactance is inversely proportional to the frequency hence Xc@250 hz will be 1/5 of Xc@50. Additional current due to prevailing fifth harmonic voltage on the network will therefore be V@250 divided by Xc@250

 

[adi 3291]

@dfdt, I can calculate Xc@250. But how do I get the V@250 in order to calculate fifth harmonic current.

 

[electricuwe]

Harmonics are usually injected into the system by current sources. Hence V at 250 Hz will be strongly dependent on the characteristic of your filter.

Furthermore consider, taking 250 Hz current out of the system, is the purpose of such a filter.

I'd expect that a fuse for such filter (and as well inductors and capacitors) are rated approx. at 200% of the 50 Hz nominal current.

The equipment you describe is in the million $ range at it is really surprising that it was obviously installed without appropriate calculations.

 

[adi 3291]

@electricuwe I think the general rule of thumb for the main fuses on the capacitor banks is 1.35*rated current of cap bank. Whereas the fuse sizing on the capacitors is 2* Capacitor fundamental current.

We have sized our main Cap bank fuse as 1.35 * nominal current. Our cap bank is 17.5MVAR, 34.5KV L-L, so I rated is 293A. The total RMS current with harmonics is coming as 330A.

Our fuse is sized for 400A (> 1.35* I rated). And we still experiencing frequent blown main fuses. What other factors we need to look into on the main fuses to prevent from blowing out?

 

[bacon4life]

I had a cap bank where we eventually surmised that individual capacitor fuses were failing due to excess heating from adjacent components. Eventually a crew cleaned and/or replaced every fuse holder and retorqued all of the bolts on the buswork. A diagnostic option is to use a thermal imager to verify that the fuse holder and buswork are running cooler than fuses.

Are the fuses blowing only on the filter bank? Or on all 4 capacitor banks? If the 3 non-filter capacitor banks are still tuned close to the 5th harmonic, adding an additional shunt filter might not due much to reduce harmonic flows through the existing capacitors.

 

[jghrist]

The total rms current flowing in the capacitors is the square root of the sum of the squares of the individual harmonic currents. These can either be measured or determined by a harmonic analysis of the system. The total rms voltage across the capacitors is the square root of the sum of the squares of the individual harmonic voltages, which can be calculated from the current times the reactance at each harmonic. You may have to increase the voltage rating of the capacitors.

 

[adi 3291]

@bacon4life Our capacitor bank fuses were not blowing, the problem is with the main power fuses. It is not only limited to filter bank fuses, it is random with other capacitor bank main fuses.

Can we tune any non-filter cap bank to 5th harmonic?

 

[LionelHutz]

Can we tune any non-filter cap bank to 5th harmonic?

No. If you're meaning de-tuning to help with the harmonic current the capacitors are seeing then the voltage applied to the capacitors tends to rise. It won't work if the caps aren't capable of handing this extra voltage. If you're talking about a trap, then both the current and voltage can rise and once again the capacitors need to be capable of handling this rise.

 

[electricuwe]

I agree to LionelHutz.

And I really wonder if a arrangement like yours can provide any benefit at all. The basic arrangments that I have seen before are:

- capacitors only --> only purpose is reactive power compensation, only feasible in systems with very low harmonic levels
- detuned capacitors (capacitors with inductors in series, but not tuned to or close to a specific harmonic) --> purpose is to provide compensation, but to protect the capacitors form excessive harmonic currents
- tuned filters (like the leftmost bank in your system) --> this is the only arrangement to reduce harmonic voltage levels significantly without harming components

Upgrading capacitor banks, either by a small inductor as protection or by a larger one to create a tuned filter, will always increase the voltage stress on the capacitor. In case of the tuned filter also the current will increase.

In your arrangement there is no clear prefernces where the 5th harmonic current can flow. That is the reason why you find fuse trips on both kind of banks.

Furthermore I tried to check what fuses you might use. At least in my region fuses for this current range (at this voltage) are very uncommon and typically breakers would be used (allowing much better coordination). Fuses at lower currents need an additional means to achieve tripping in case of overheating at currents only slighly above nominal current.

 

[Parchie]

@Lagudu Aditya (Electrical),
You will have to design your filter to be able to carry these additional harmonic currents plus you are to choose a cutoff frequency that is slightly lower than the target frequency, else you will have large currents in your filter equipment. Recommend about 90% of the target frequency (270Hz if you would choose the 5th harmonics).
Individual harmonic currents can be computed as the fundamental voltage divided by the difference of absolute difference of the reactances of your L and C component at that harmonic level. Keep in mind that in every harmonic level, both L and C are inversely proportional: The inductive reactance increases 2X while the capacitive reactance is halved at a 2nd harmonic, XL is increased 3X while XC is divided by 3 at harmonics = 3, and so on. . The formula for the harmonic current added at that level:

I[sub]h[/sub]=Vfund/(sqrt(3) x |X[sub]Lh[/sub]-X[sub]Ch[/sub]|)x harmonic voltage limit​

where h is the harmonic level and the harmonic voltage limit is the permissible voltage distortion for that harmonic level.​

As mentioned by others, the total rms current will be the square root of the sum of the squares of the individual harmonic currents and the fundamental current.

 

[LionelHutz]

What's the nameplate rating of the capacitor bank? 16.7MVAR @ 34.5kV is 37.2uF and with the reactor added it is effectively 17.5MVAR with the capacitors at 36.2KV, not including any voltage caused by harmonics.

Your 1-line is listing 17.5MVAR @ 34.5kV for all the capacitors which isn't true unless you also changed the capacitors when tuned it.

 

[adi 3291]

@ parchie, thanks you for the details. According to IEEE 519-2014, voltage distortion limit for each harmonic @35kV will same right, which is 3%? Is this the same value that will go in the formula. PLease comment.

 

[LionelHutz]

That's just a voltage distortion limit table though and does not necessarily reflect the reality at the site whatsoever.

 

[Parchie]

@Lagudu Aditya (Electrical),
IEEE 519 tabulated a straight 3% maximum voltage level for each individual harmonics at that voltage level. Those limits are made to give electrical equipment manufacturers how much harmonics they are allowed in designing their products. Those are consensus guidelines used by utilities and their customers. I would recommend you to commission a harmonic frequency sweep on your system and be guided as to how much a harmonic voltage distortion you've got at each harmonic level and use those data as your reference for your "limits". The lower the limit you set compared to the actual harmonics present, the higher the likelihood of your design failing.

 

[Parchie]

LionelHutz (Electrical)11 Jul 22 13:00
What's the nameplate rating of the capacitor bank? 16.7MVAR @ 34.5kV is 37.2uF and with the reactor added it is effectively 17.5MVAR with the capacitors at 36.2KV, not including any voltage caused by harmonics.

Your 1-line is listing 17.5MVAR @ 34.5kV for all the capacitors which isn't true unless you also changed the capacitors when tuned it.

@Lionel,
Please take note that the filter configuration is wye and 34.5kV voltage ratings are more than enough. The voltage across the capacitor will be just 34.5/(1.732 x (1-0.05))= 21kV and with only a 5th harmonic present, the voltage across the capacitor will only be 24kV per phase. That is of course, as you've mentioned the OP did change the values of the elements (L and C)to achieve the 17.5 MVAr rating.
My quick calcs, if the posted circuit diagram is true, (the reactor is a 30-ohm reactor = 79.6mH) tell me he'd be having a 4.42 uF capacitor to have a cutoff frequency of 270 Hz (4.5 harmonics). The total kVAr will be 732kVAr or a total of 2.2 MVAr! Definitely not 17.5 MVAr, IMO.