Power System Operation: Questions & Answers

[cnh1995]

I recently read a chapter on power system analysis in a book and I have some questions.
1) The three phase power at the secondary distribution level is unbalanced i.e. not equal for each phase but it is nearly balanced at the HV transmission level.
Is this true? Also, is the power balanced at the generator end? Basically, is there a difference in degree of balancing between transmission and distribution?

2) In the distribution network, there are a lot of power electronic and electronic loads which lead to distorting current waveforms and hence, harmonic content of the distribution network current is very high. Are these harmonics mitigated at the transmission level using filters? Is the transmission current almost sinusoidal? If no, then where is mitigation of the harmonics carried out?

Thanks a lot in advance!

 

[anorlunda]

Good questions.

Neither unbalances nor harmonics are 100% eliminated, but at the power plants, they are much less than at the distribution level.

The transmission system feeds many distribution circuits, the unbalances in those circuits tend to cancel each other. So the first answer is diversity. Distribution engineers worry about unbalances all the way up to the substation, as they assign small loads to phases in single phase branches.

Transmission lines can introduce their own imbalances because of asymmetric geometry. The phases are transposed at intervals to counter that. Look it up.

The reactive impedance is lines and transformers is higher for harmonics. $X=j\omega L$. For Nth harmonic X is N times larger. That is the main filter.

Edit: I forgot to mention that in extreme cases, harmonics are filtered using shunt capacitors. But that is rare.

 

[cnh1995]

Good questions.

Neither unbalances nor harmonics are 100% eliminated, but at the power plants, they are much less than at the distribution level.

The transmission system feeds many distribution circuits, the unbalances in those circuits tend to cancel each other. So the first answer is diversity. Distribution engineers worry about unbalances all the way up to the substation, as they assign small loads to phases in single phase branches.

Transmission lines can introduce their own imbalances because of asymmetric geometry. The phases are transposed at intervals to counter that. Look it up.

The reactive impedance is lines and transformers is higher for harmonics. $X=j\omega L$. For Nth harmonic X is N times larger. That is the main filter.

Edit: I forgot to mention that in extreme cases, harmonics are filtered using shunt capacitors. But that is rare.

Thanks a lot! This is very helpful. I'll read more about this and post again if anything is unclear.

 

[Aaron Crowl]

One of the ways they can mitigate harmonics is to use condensers. These are synchronous motors which draw little power. They act as "flywheels" for the power grid. The inertia of the rotor helps maintain a sinusoidal current. The excitation of the motor can be tuned to make power factor corrections as well.

 

[Windadct]

IMO - it really come from the averaging of many distribution level systems connected to one Transmission level feed. Not all ( none) of the Distribution level feeds would be imbalanced in the same way.

Sometimes large industrial users - that receive a large MV feed (distribution level by Definition) are forced by the utility to correct the imbalance, PF or harmonics, or they pay a hefty surcharge. I have a customer working with a battery factory with 30MW load - they have 2 36KV feeds - and 95% of their load are basic 6 pulse rectifiers (with the two transformer they "look" like a 12 pulse) - it is really a mess, and they pay a lot in surcharge AND additional Transformer losses (additional real power they pay for that yields no benefit).

 

[dlgoff]

A power line carrier is communication equipment that operates at radio-frequencies... to transmit information over electric power transmission lines.

Here's a schematic of a transmission line circuit. compliments of above link

 

[cnh1995]

The transmission system feeds many distribution circuits, the unbalances in those circuits tend to cancel each other. So the first answer is diversity. Distribution engineers worry about unbalances all the way up to the substation, as they assign small loads to phases in single phase branches.

@anorlunda, I understood your explanation about harmonics, but I am revising the symmetrical components theory for unbalanced three phase networks and I have a few questions.

If a transmission line is unbalanced, we can analyse the unbalances in terms of +ve, -ve and zero sequence components. But what is done after this analysis? I am not sure if there is any way to actively balance the lines. Is "diversity" or "careful distribution of single phase loads " the only way to minimize the unbalances?

Sorry if this sounds repetitive. I am asking because we had a good amount of stuff regarding voltage control, frequency control, pf improvement, performance analysis, faults and protection etc in my power system course, but nothing much about load balancing. Is it because it is too complicated for the bachelors level ? All we were asked to do was "assume balanced load" while doing analysis. So, how do power engineers deal with unbalances in the transmission network(other than diversifying the loads)? Is there any good book/material I can refer particularly for this?
Thanks in advance.

 

[anorlunda]

We have only limited ways to balance the phases at the transmission level.

Transposition of the phases is one way. That corrects only for the imbalances introduced by the transmission lines.

Per phase adjustable taps in transformers is possible. But I never heard of it used.

Insert L or C impedance in just one phase. I never heard of that being done either.

So to correct load imbalances, I believe that can be done only at the low level distribution level.

But there are others here at PF that might give a better answer. Wait a few days and see.

But the way, the methods off symmetrical components is primarily used for open/short cases or to trace harmonics.

 

[cnh1995]

Neither unbalances nor harmonics are 100% eliminated, but at the power plants, they are much less than at the distribution level.

Is it because of the ALFC and AVR systems? The power plant alternators, when synchronized with the grid, share active and reactive power in the correct proportion to maintain constant frequency and required grid voltage. This implies that if the grid (and the alternators) is stable, power plant alternator always sees a balanced load as AVR and ALFC take care of sharing the active and reactive power correctly. So, in steady state, power plant alternator can't see the unbalances in the transmission network unless there is a severe unsymmetrical fault.
Is this (hand-wavy) understanding correct?

 

[anorlunda]

Is it because of the ALFC and AVR systems? The power plant alternators, when synchronized with the grid, share active and reactive power in the correct proportion to maintain constant frequency and required grid voltage. This implies that if the grid (and the alternators) is stable, power plant alternator always sees a balanced load as AVR and ALFC take care of sharing the active and reactive power correctly. So, in steady state, power plant alternator can't see the unbalances in the transmission network unless there is a severe unsymmetrical fault.
Is this (hand-wavy) understanding correct?

Yes and no. Active/reactive has nothing to do with balanced/unbalanced, or with ALFC or AVR.

For practical purposes, assume that everything at the power plant level and ALFC level is fully balanced.

If there is a severe fault, it will be cleared before the AVR or ALFC can react to bad feedbacks.