We have installed a 175kWp solar plant in a building. During weekends when the loads are less when there is a power cut and followed by trying to start the DG, the DG trips due to solar plant taking most of the load. I was informed like it was due to low PF. What is actually happening?

 

less power factor, that mean below 0.85 for example 0.75, 0.65 which is inductive load and DG feed ome motors etc... which may cause a trip on over voltage. less excitation

I advice to check DG Protection system and wiring

good luck

 

aksinfy,

What is the rated power output of the diesel generator?

What is the building load on the weekend at the time the generator trips?

When does the diesel generator trip?

How do you handle the issue when the diesel generator trips? Do you disconnect the solar panel output and re-start the diesel generator? Or do you just do something to reduce the building load so the solar panels can continue to power the building without the diesel generator?

ANY time a generator trips there should be some kind of alarm or indication to alert the operator what condition tripped the generator set. There are many different generator protective relays (sometimes multiple relays are combined into one or more relays) each of which look at different scenarios, including reverse power (where the generator would become a motor and spin the prime mover--the diesel in your case), and loss of excitation, and over-current, and over-voltage, and imbalanced current flows, and on and on and on.

Generally, even if these relays are connected to some kind of alarm panel or HMI they have "flags" or LEDs to indicate one of them has operated--which provide a clue as to what is happening or what is being detected (rightly or wrongly, as the case may be). And, generally, the "flags" have to be reset before the generator can be re-synchronized.... So, after resetting protective relays someone should be logging what relays were actuated to trip the turbine and you should be able to check the log to see what's actually tripping the generator.

It's not clear from your post if the diesel generator runs during the week, or only on the weekends when the power is cut from the utility. You seem to be saying that until recently the diesel generator could supply the building load on the weekend with no problems. But, that since the installation of the solar panels and the connection of the solar panel output with the building electrical system there is some problem. But we don't know enough about when the diesel generator trips on the weekends.

You also haven't told us anything about the load on the building on the weekend when there's no utility power. Is it lower than during the week? The same? Does it exceed the output of the solar panels, or is it just slightly more or less than the output of the solar panels, or is it much less than the output of the solar panels? I presume that in the past before there were solar panels that the diesel generator had no problem providing the power for the building when there was no utility power--but you haven't told us if that's the case or not.

Is there some way you can determine what the power factor of the building load is on the weekend before you try to start the diesel generator, or after the diesel generator starts?

When does the diesel generator trip--immediately after breaker closure, or some time after you try to start loading the diesel? Or some time during the day when the solar panel power is at or near maximum?

I believe that photo-voltaic solar panels do not produce any VArs--it's strictly watts. (They produce DC, which is converted to AC, and so there's little or no ability to produce VArs.) So, that would mean that when the building is separated from the grid and operating only on the solar panels that the power factor can be quite low--and it would depend on the equipment being powered if the power factor was leading or lagging. (In general, flourescent lighting is a leading power factor load, while air conditioning systems with all of their induction motors, represent a lagging power factor load, and computers and computer monitors and server installations can represent a leading power factor load.)

You really need to understand what function is tripping the generator--is it reverse power, under-excitation, over-excitation, over-voltage, etc. It's really important to understand what relay is operating to trip the unit, and then you can try to understand what's happening when there's no utility supply versus when there is a utility supply.

In thinking about this and writing this reply, here's my best guess as to what might be happening because there's a lot that's not known--so it's just a guess. Prior to the installation of the solar panels, the diesel generator was capable of powering the building when the utility power was cut--with no need to reduce building loads to prevent exceeding the output of the diesel generator. The building load is lower on the weekend than during the week, but maybe not that much lower. The building load on the weekend is somewhere close to the output of the solar panels (slightly higher or slightly lower). The power factor of the building load is not very good when it's being powered only by the solar panels--probably leading, and probably less than 0.8--but this is really just a SWAG (Scientific Wild-Arsed Guess) because we don't know anything about what the building is used for and what kind of equipment is powered-on during the weekend. I presume that the solar panels are capable of providing most, if not nearly all, of the building load on the weekend. I presume that the diesel generator is started in the morning, when the output from the solar panels is low, and as the day progresses and the output of the solar panels increases that the diesel generator must be unloaded to maintain the frequency of the building system (we don't know if the diesel governor is operating in Droop or Isochronous mode on the weekend). At some point, it's possible that the output of the solar panels exceeds the building load, at which point the generator would go into reverse power and trip. Most solar panel installations just produce as much power as they can--and it's not variable by an operator. So, if it the solar-produced power exceeds the building load when the diesel generator is also providing power to the building then the output of the solar panel will send the diesel generator into reverse power which will trip the unit.

Or, as the day heats up and the air conditioning and/or lighting loads increase the building power factor decreases and the diesel generator exciter ("AVR") can't keep up with the voltage demand and the unit trips on over- or under-excitation.

Again, the above is strict conjecture--there's too much you haven't told us about the situation and the building load (watts and power factor, and whether the power factor is leading or lagging), and when the generator trips--and most importantly, what alarm is being annunciated when the unit trips or what protective relay is operating when the unit trips. All of these things are very important to understand. It seems the diesel generator was previously capable of powering the building on the weekend when the utility power was cut--but, again, you didn't tell us that, and it would make a difference in our response.

Please write back with answers to most of the questions if you want more help with solving the problem. I know it seems like a lot of questions, but, it's information we need to have to be more helpful.