Peter, Yes, grid impedance is another variable, and one that we can not easily control. The impedance looking into the grid can be high due to loose connections or undersized conductors or both. In some cases we've had to have the utility out to check their pole top and meter connections. >From my experience, you should be OK with 100 mohms or less grid impedance (ignoring inductance and capacitance here), and will probably be in trouble over 150 mohms with a larger residential system if the utility voltage wanders on the high end of ANSI-A. I think the highest we've seen is 250 mohms on a mixed overhead and buried service.
Jeff Irish Hudson Valley Clean Energy -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter Parrish Sent: Monday, August 17, 2009 2:00 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] utility line voltage issues One thing I have read during this thread, is the voltage rise at the inverter output required to "sell to the grid". I have heard all sorts of numbers from "less than a volt RMS" to "2-3 volts". As an engineer, I would expect the number to depend on the AC current being exported and the impedance of the grid seen from the distribution panel. Let's say we want to export 2,400 W-ac at 240 V-ac, or 10 A-ac, and the dynamic impedance of the grid is 0.1 ohm. The voltage rise needed to push that 10 A onto the grid is 1 V-ac (i.e. 241 V-ac). I am going to do a test on our system today, monitoring the L1-L2 voltage at the inverter breaker and cycle the PV system on and off. Should be interesting! One other point: since IEEE929 and IEEE1547 require the inverter to shut down within 16 ms (one full period of 60 Hz) -- correct? -- what is the typical averaging time (Tavg) for a typical RMS voltmeter (the "M=mean" in RMS)? If the voltage excursion occurs between 0.16mS and a fraction of Tavg, then the RMS will miss the event. Seems to me, we have to have a peak reading voltmeter or something equivalent. - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bill Brooks Sent: Monday, August 17, 2009 9:05 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] utility line voltage issues William and others, The standard requirement in IEEE929 and IEEE1547 that is tested in UL1741 is +10%/-12%. Admittedly, some utilities, particularly in more rural areas, may get close to these limits at times. Just for a little history lesson that very few people know about, this happens to be a very hard fought and won battle. Initially when IEEE929 (the original PV interconnection document) was being written, utilities wanted inverters to trip at +/- 5%. Realizing that this would be a death blow to the PV industry, several of us worked on the justification for why that did not make sense. We finally won when a key utility engineer got on our side and the proper limits went into place. The standard also recognizes that special locations, like island grids (Hawaii) and other remote grid areas may need even wider ranges to handle typical fluctuations. Widening voltage windows of the inverter requires utility permission. The voltage limits we have now are good, but most people don't understand the rest of the story. As was mentioned in the post, most PV inverters do not trip at their actually limits, they typically trip a few percent inside those limits (+8%/-10%). This is due to the difficulty of measuring ac voltage accurately, and the penalty within UL1741, the test standard, if you fail one of these limit tests. Since there is no penalty in the standard for tripping early, but a large penalty for tripping late, inverter manufacturers constrain their limits so that they trip within the limits every time, even if their transducers are at the maximum offset allowed. That is why it is so important for installers to try to stay within 1% voltage drop on the ac side. Since the utilities are allowed to go to the limits of +/-5% routinely (ANSI Range B), and outside that range for "short" periods of time (a few hours is short compared to 8760 hours), a 3% voltage drop will cause inverter tripping that is not the utility's fault--it is the installers fault. I'm not defending the utilities, but having responded to dozens of utility voltage complaints, I can say that 80-90% of the complaints were the installer's fault in having too high a voltage drop in their inverter output circuits. The legitimate issue is that PV inverters are one of the only things on the residential utility system that reacts to bad voltage. We are now placing tens of thousands of voltage sensors on the utility grid, and they will find bad places. The key is to be part of the solution, not part of the problem--keep your ac voltage drop low. For every voltage drop dollar you spend on the dc side of your design, you should put $2 toward the ac side voltage drop--it is at least twice as important because it may determine whether or not the system runs. Bill. Bill Brooks, PE Brooks Engineering 873 Kells Circle Vacaville, CA 95688 Office and Mobile: 707-332-0761 Office Fax: 707-451-7739 email: b...@brooksolar.com www.BrookSolar.com -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Joel Davidson Sent: Sunday, August 16, 2009 1:48 PM To: RE-wrenches Subject: Re: [RE-wrenches] utility line voltage issues Correction: 4 KV ----- Original Message ----- From: "Joel Davidson" <joel.david...@sbcglobal.net> To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Sent: Sunday, August 16, 2009 12:08 PM Subject: Re: [RE-wrenches] utility line voltage issues > Hello William, > > SCE has some 4kVA long, skinny feeders that get voltage sag during high > usage periods (August afternoons air conditioning and December nights xmas > lights and heating). High grid voltage can occur in some areas when SCE > increases the voltage to compensate for voltage sag. High grid voltage can > also occur when utility and/or customer wires are undersized. > > The CPUC, not lawyers, tell electric utilities to fix grid voltage > problems. You need to file a complaint with specific information to the > CPUC. With no PV system on and using an rms meter, measure the voltages. > Record the times and in what neighborhoods are you measure high or low > voltage. Then file a written complaint to the CPUC (contact info on the > back of an electric bill). The CPUC accept the complaint, investigate, and > tell SCE to correct the problem(s). > > If the grid voltage is within acceptable range and the grid-tie inverter > is still shutting down, then leave the inverter off and measure grid > voltage at the inverter AC in. If voltage is high, pull the grid-tie > breaker and measure grid voltage. It should be within range. If not, then > the wiring from the grid-tie breaker may be undersized or the inverter > input voltage setting may be out of range. > > Joel Davidson > > ----- Original Message ----- > From: "William Korthof" <wkort...@eesolar.com> > To: <re-wrenches@lists.re-wrenches.org> > Sent: Sunday, August 16, 2009 10:06 AM > Subject: [RE-wrenches] utility line voltage issues > > >> I'm beginning to wonder if the allowed voltage range for grid-tie >> inverters (+/-10%) is too >> sensitive in some networks and contributes more harm than benefit. This >> is close to home. >> >> Generally, the utility voltage at my house is in the mid 120's--- around >> 125 vac per phase. >> But at times, the voltage goes up higher---two weeks ago I saw 129 to >> 130V per phase. >> That voltage was high enough to put all of the inverters that I checked >> offline for much >> of the day. >> >> So I've had to call in "voltage trouble" complaints to the utility at >> least a dozen times >> over the past 5 years in response to seeing inverters offline and line >> voltage about >> 8% above nominal. _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
