Peter, The excursion time depends on how high the excursion is. There is a longer grace period for being just above or below the limits to allow for inverters to ride through normal transients. However, beyond that are some very short timeframes for excessive transients and large excursions. There is a table in 1547 that covers this. This is the table that is used for UL1741 testing. The "instant trip" rates are typically 2 cycles to correspond to fast operating relays. An inverter can trip as fast as you want it to. The question is how much information you accumulate before you realize you are in a situation where a trip is required.
Every manufacturer is required to pass the voltage tests, but there is no requirement to ride through the full grace period and then trip. This causes some inverters to overreact to voltage excursions and transients that the standard intended to allow inverters to ride through. In that case, the inverter manufacturer would be to blame for being overly sensitive. Lots of blame to go around if you want to play the blame game. A grid-connected inverter is regulating output power by measuring line voltage and determining how much current to inject for a given voltage value. Looking at the trend of the last few voltage cycles, it predicts what the next voltage cycle voltage will look like (magnitude) and injects the appropriate amount of current that is available with power from the PV array. For example, a PV array has enough power to produce an ac output of 4,000 Watts (meaning it has 4,200Watts dc on an efficient inverter). The ac voltage at the inverter terminals was 245Vac for the past few cycles, so the inverter produces a perfect current waveform of 16.33A ac (I = P/V = 4,000W/245V = 16.33A). Now let's say that on the next cycle all of a sudden the voltage drops to 50%. The inverter would normally try to double the current, so to prevent damage to the transistors, the inverter limits the current and will either stop producing current shut down immediately. You mention grid impedance, but the inverter may not know the actual grid impedance until it has produced its first few cycles. The voltage will rise and the inverter must compensate--up to the upper voltage limit of the inverter (+8% above nominal). Bill. -----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 11:00 AM 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 _______________________________________________ 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
