Reactive Power Compensation

[AGTB]

Hi everyone,

I've just been assigned a new project regarding Reactive Power Dynamic Compensation, which is kinda new for my employer in my country, as it is for me. I am going to be formally taught about it later on but I really want to get acquainted with it before training and all I know as of now is that it involves injecting and absorbing VAr from the power grid when needed by means thyristors controlling a set of reactor and capacitor banks as well as filtering the 5th and 7th harmonics (in this case). I would like to know why the 3rd harmonic (which is usually the most harmful for three phase grids) is not accounted for as well as any other helpful information about this technology. Any information would really help. Thanks in advance.

Cheers!


P.D. I don´t know if this will help but being a Mechatronics Engineer I am not too familiar with power transmission, I do know a bit more than the basics but I wouldn't call myself an expert so ABC types of answers are welcome too!

 

[chadwiseman]

I believe if you research the subject you'll find that the 3rd harmonic is cancelled out. It works out in the math due to the firing of the thyristors. I'm not 100% positive of this, but I think that's why. If you're not into the math, then don't worry about it.

 

[AGTB]

Hi 86Ranger,

The 3rd harmonic usually cancels out on mono-phasic grids, but in three-phase grids the harmonic series is not zeroed at the common point, therefore it isn't cancelled.

Cheers!

 

[ScottyUK]

AGTB,

Can you expand on

3rd harmonic usually cancels out on mono-phasic grids.

If this is what the rest of us know as a single phase circuit, then how does cancellation occur?


You might also look at how triplen harmonics behave when they reach a delta/star transformer. Conventional wisdom is that they are trapped by the delta and can not propagate into the upstream system.


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Start each day with a smile. Get it over with.

 

[AGTB]

Hi ScottyUK,

Yeah, I'm talking about single phase circuits, I'm sorry my english is getting rustier by the day.

About triplen harmonics, thats what I was referring to, on single phase circuits the harmonic is unable to propagate to the rest of the system and as it turns out, I was thinking of unbalanced three phase circuits in which the triplen harmonics are not trapped by the delta.

I am having a hard time understanding the different harmonics present on the system and how they are managed to improve the power factor. It is of my understanding that the spikes generated by the control system bring a huge number of harmonics with them, some of them are absorbed by the delta and higher orders by the set of filters we will design. There are more harmonics in the system obviously, those that come from the grid and alter the power factor of the overall system, they are filtered by the same means (then again that's my understanding, I may be wrong).

The harmonics issue is only one part of the actual problem, the SVC's main task is to compensate for the fluctuations of reactive power when big loads (i.e. factories) are switched on and off. For this the system uses phase angle modulation to set a bank of reactors and capacitors on and off, the closed loop control is also a mistery to me as I've seen several ways of doing it, including fuzzy logic. I would like to know if there is a standard way of designing the control system, or which is the most common way of doing it.

Thanks very much.

 

[Gestur]

This sounds like an SVC application for utility use.
The reasson for 5:th and 7:th harmonic filter is because
these harmonics are generated from the TCR(Thyristor Controled Reactor), also other harmonics are generated but 5, an 7 are generated with highest amplitude.

The amount of harmonics depends on the fire angle.

Check out

http://www.abb.com/facts

for more info about SVC applications.

/Gestur

 

[sitl]

SVC is usually made from capacitive banks and TCR (Thyristor Controlled Reactor). By changing voltage on the reactors using thyristors, you are able to control amount of reactive power consumed by reactors. Capacitive banks feed constant amount of reactive power to the grid and since the rest of consumers usually have variable reactive power consumption/generation, it is obvious why we need TCR. Since the thyristors are involved, in addition to harmonics generated by the consumers on the grid, you will have significant influence of TCR in THDi (total harmonic distortion of grid current); this means that TCR will generate significant 5th and 7th harmonic which must be absorbed by filter banks. Since the filter banks are usually conceived from reactor L and capacitor C (passive filters), instead using pure capacitors for reactive power compensation, you can calculate filter banks (as a series connection of L and C) to serve both as filters for higher harmonics and for reactive power compensation.

Third harmonic is usually canceled in TCR because TCR is connected in delta. It is always good engineering practice to devise grid (transformers connection, nonlinear consumers grouping etc.) to cancel out as much of third harmonic as possible.

Sometimes, even capacitor banks (or filter banks) are switched on/off by thyristors or IGBT’s because of very different working regimes in the rest of the grid (for example high consumption of reactive power and no consumption at all, all happening in 1-10 minutes period). Then, if you have control only through TCR, you can not maintain high power factor throughout all the regimes, hence you have to disconnect filter (capacitive) banks from time to time. This capacitor switching also inputs higher harmonics into the grid.