Impact of imbedded wind generation on distribution systems
Impact of imbedded wind generation on distribution systems
(OP)
Even though wind farms are becoming common almost everywhere, they are mostly large scale connecting to HV transmission lines and appear much less common as imbedded distributed generation (DG) connecting to medium voltage distribution lines. Such distribution scenarios appear not to be well researched yet and available references such as IEEE P-1547 not particularly informative.
So, I'm working on a 10MW wind DG facility project connecting to a rural radial distribution line 44kV. The line originates from a 230/44kV substation. The 44kV line is about 40km long and the DG facility about 25km from the substation. There are residential loads about 5-10km downstream from the DG facility.
The DG facility is served by a switching station with a breaker and transfer trip (TT) scheme which interacts with upstream utility protections located at the substation and also a line recloser, which is about 10km upstream.
The teleprotection scheme uses a radio link. Upon detecting a fault, the utility will trip its own interrupting device (breaker or recloser) and also send a TT signal to the DG facility protection. Once TT signal is received and the DG facility breaker is tripped, a Distributed Generation End Open (DGEO) signal will be sent back to the utility, which enables their auto-recloser to attempt to restore the line.
This seems to work; however, the problem is the islanding condition which occurs when the utility interrupter is already tripped while the TT signal is still being processed before it trips the DG breaker, which may take about 100ms.
During this time consumers on the line will continue to be fed from the islanded DG, while the fault may or may not be isolated. Islanding condition may also occur when the utility interrupter is inadvertently opened and no TT signal is sent, line conductor broken far upstream, etc.
The specific problem is that according to the WT supplier, if an islanding condition occurs, the overvoltage which lasts several cycles may be over 2.0 pu before the turbine controller reacts and readjusts inverter output. Apparently, utility consumers still left on the line may be severely affected by a large overvoltage surge.
The WT supplier also states that such islanding condition leading to severe overvoltage problem should somehow be cleared within half of cycle which is technically impossible, unless the TT scheme is such that utility protections trip the DG facility first and then trip utility breaker or recloser.
This, however, is not acceptable to the utility as the tripping time for their instantaneous protections would probably reach some 200ms total, by the time TT signal is received at DG end, DG breaker open, and DGEO signal sent back.
Would anyone be able to provide some comments please.
So, I'm working on a 10MW wind DG facility project connecting to a rural radial distribution line 44kV. The line originates from a 230/44kV substation. The 44kV line is about 40km long and the DG facility about 25km from the substation. There are residential loads about 5-10km downstream from the DG facility.
The DG facility is served by a switching station with a breaker and transfer trip (TT) scheme which interacts with upstream utility protections located at the substation and also a line recloser, which is about 10km upstream.
The teleprotection scheme uses a radio link. Upon detecting a fault, the utility will trip its own interrupting device (breaker or recloser) and also send a TT signal to the DG facility protection. Once TT signal is received and the DG facility breaker is tripped, a Distributed Generation End Open (DGEO) signal will be sent back to the utility, which enables their auto-recloser to attempt to restore the line.
This seems to work; however, the problem is the islanding condition which occurs when the utility interrupter is already tripped while the TT signal is still being processed before it trips the DG breaker, which may take about 100ms.
During this time consumers on the line will continue to be fed from the islanded DG, while the fault may or may not be isolated. Islanding condition may also occur when the utility interrupter is inadvertently opened and no TT signal is sent, line conductor broken far upstream, etc.
The specific problem is that according to the WT supplier, if an islanding condition occurs, the overvoltage which lasts several cycles may be over 2.0 pu before the turbine controller reacts and readjusts inverter output. Apparently, utility consumers still left on the line may be severely affected by a large overvoltage surge.
The WT supplier also states that such islanding condition leading to severe overvoltage problem should somehow be cleared within half of cycle which is technically impossible, unless the TT scheme is such that utility protections trip the DG facility first and then trip utility breaker or recloser.
This, however, is not acceptable to the utility as the tripping time for their instantaneous protections would probably reach some 200ms total, by the time TT signal is received at DG end, DG breaker open, and DGEO signal sent back.
Would anyone be able to provide some comments please.






RE: Impact of imbedded wind generation on distribution systems
RE: Impact of imbedded wind generation on distribution systems
Second, how on earth do you get a 2pu overvoltage? The inverters can adjust their output in a fraction of a cycle if they see that they are causing a voltage rise.
RE: Impact of imbedded wind generation on distribution systems
There is also the problem with inverse energy flow. The normal distribution grid takes care of voltage drop by having taps set for normal load. If load goes to zero, the voltage increases. And more the further away from the infeed.
Now, if you connect wind turbines to the far ends and reverse energy flow, there will be unexpectedly large voltage swings. The grid has to be fundamentally different when you start connecting generators to it on a large scale. We have had the problem already but politicians are slow to understand and encourage wind turbines also where they do not belong.
Reality and theory differ sometimes.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Impact of imbedded wind generation on distribution systems
Rafiq Bulsara
http://www.srengineersct.com
RE: Impact of imbedded wind generation on distribution systems
I'm just quoting what the WT manufacturer indicated. If they used a coservative number, then I'm wrong. Actually, they didn't say 2pu, but potentially exceeding 2pu, which is worse...
As for the cost of studies, in this renewable craziness, where the show is mostly driven by accountants and lawyers, I haven't heard that anyone has done any such serious study for connecting to a distribution system, especially not one which did cost several hundred $.
"The inverters can adjust their output in a fraction of a cycle if they see that they are causing a voltage rise."
May be so. But can that really be generalized to any sort of inverter or converter, regardless what technology and topology it employees and regardless of collector system configuration, number of units running, nature of the line load etc.?
RE: Impact of imbedded wind generation on distribution systems
RE: Impact of imbedded wind generation on distribution systems
I've attached a paper which you may find of interest.
Regards
Marmite
RE: Impact of imbedded wind generation on distribution systems
The minimum load is not greater than the maximum generation.
As for the DG protections, it is not only the inter-tripping scheme, but we are also using a multifunction relay which has many features including change of voltage vector.
However, none of the protections + breaker opening time (which is 5 cycles in the 60Hz world) is quick enough to detect islanding and open the DG breaker in no time, which seems to be the recommendation of the WT manufacturer (1/2 cycle is practically no time...).
The only solution seems to be that islanding never happens, i.e., that utility never trips before the DG facility does. Which in theory would be possible (excluding incidents such as open conductor), but in that case utility instantaneous protections at the remote end would likely mis-coordinate.
The only interrupting device I'm aware of which exists on the marker and which may be sufficiently quick is ABB IS limiter. But I'm not sure if it could be set to detect and operate on a rapid voltage rise, as it is an extremly fact current limiting device used in applications where breakers or fuses are too slow.
As Gunnar indicated, distribution systems don't seem to be designed to easily accommodate dispersed generation of substantial size (similar or larger in size than the line minimum load).
RE: Impact of imbedded wind generation on distribution systems
RE: Impact of imbedded wind generation on distribution systems
As various technologies connect to the network it will be impossible to adapt the network to solve all their problems. There are a number of solutions on the generation side...might cost money but they should solve it.
Daniel
RE: Impact of imbedded wind generation on distribution systems
RE: Impact of imbedded wind generation on distribution systems
Rafiq Bulsara
http://www.srengineersct.com
RE: Impact of imbedded wind generation on distribution systems
Rafiq Bulsara
http://www.srengineersct.com
RE: Impact of imbedded wind generation on distribution systems
RE: Impact of imbedded wind generation on distribution systems
Can`t resist adding this here:
How does the WT supplier get a result of more than 2 PU voltage rise on load rejection? Is that some peak waveform value from the inverter output or an actual RMS amount? In a conventional synchronous generator with the best AVR there would be only a couple percent rise, but here we are dealing with some very fancy software based system that is proprietary and they wont tell you about it. Likely the WT agent doesent know himself, only what the factory gurus say over the phone.
Depending on the contract and your authority in the situation, they should be refused a connection until more is known, and the WT supplier has put in place some fast acting OV protection at their own expense, and properly type tested also.
rasevskii
RE: Impact of imbedded wind generation on distribution systems
WT manufacturer indicated that overvoltage of 2pu would be RMS lasting several cycles. They didn't elaborate much as why it may occur. Seemed to have something to do with Wind Turbine Ride Through which will attempt to boost the voltage, but if the fault is cleared by the action of utility remote protections only, and depending if the generation is higher than load the overvoltage may occur.
The utility has fairly comprehensive guidelines for the DG connections, and elaborate islanding as well, but seems that the case of dynamic (overvoltage or any other) conditions, when the utility is lost and the DG continues to island, are not considered in details.
RE: Impact of imbedded wind generation on distribution systems
Don't know where other posters are reporting from, but in the jurisdiction where I live things seem to be a bit different - the electricity grid is becoming more and more impacted by decisions made by politicians, accountant and lawyers, and there is an obvious pressure leading to compromised situations like this one.
On the other side, private investors building renewable DG facilities definitely don't chose equipment based on the best performance under various dynamic conditions (and supported by comprehensive studies) but rather price and availability.
Sometimes, the best wind turbine is one which you can get on time to meet your in-service date (which is in the contract and there are financial consequences in case of a delay).
Similarly, large WT manufactures, and there is only few out there, do not appear particularly accommodative, they all have enough business for now, so their logic seems to be more of: take it or leave it...
At the same time some larger utilities are overwhelmed with renewable applications and may not know or notice everything when it comes to assessment and their own studies they do for such connection.
Consultants, in most cases, also don't seem to know everything about anything when it comes to various power converter technology connecting to medium voltage lines, but even if they did know everything, somebody usually needs to pay them to do comprehensive studies which in some cases simply doesn't happen.