Certainly interesting. When I brought up the idea of a simple induction motor based system (driven by waterwheel) a year or so back I got a lot of flack from electricians (on this forum) saying that it wouldnt work, all very dangerous blah blah
Is this the thread?
http://www.navitron.org.uk/forum/index.php/topic,4438.0.htmlIf so, not one post in five pages said an induction motor based system would not work. What they did say was that your intention to meet the requirements of G83 by a DIY method, using off the shelf parts from RS would not work. Very dangerous? No. But did anyone say “very dangerous”? All electrical work is potentially dangerous and more so for those who are experimenting with little knowledge of the subject, worse even with the belief that they “know all about using induction motors as generators”. By all means further your electrical knowledge by experiment, it’s a fantastic way to learn, but when intending to connect generators to the mains consideration should be given to others using the network. I do however accept that using a small motor/generator with an appropriate size breaker/fuse link for experimental purposes is unlikely to cause any significant disturbance under fault conditions especially if you are the only one on the transformer.
Actually I knew it worked because I had already done it, unofficially but legally (its OK to connect to grid for short testing period as long as general G83 requirements are met.)
I would be interested to know where this can be found in Legislation or ER G83.
The G83 Annexes B to F gives the specific test requirements and methods for obtaining type certification and type verification. These do indeed require a mains connection for LoM and power factor test. It is also preferred that the SSEG is tested under normal conditions of operation using its intended energy source unless otherwise noted.
The fact is that ESQCR regulation 22 still applies. Namely the DNO must at the relevant point in time be notified of the connection. I am unaware of any exclusion for type testing.
OK, so l too would most likely say chocolate puff balls to this and give the local network the finger, but I doubt I would do the same to a magistrate. It was not difficult locating the cause of the fault Your Worship. His property was the only one in the area with the lights on.
In respect to your experiments;
November 03, 2009, 07:14:35 PM Before I installed the current proper system I experimented with a few lashups using a 1kw sp motor,a 2.5kw 3p and an 11.5kw 3p. (using one winding only on 3p) I tried synchronising on each at a little below sync speed, a little above, and dead on (as far as i could tell with a hand held tacho). In each case there was no significant bump or grind: they all settled down into motoring or generating mode semmingly instantly. I assumed that the grid would hardly notice the load.
The process you describe is NOT synchronising. The motors would have been drawing excitation VAr only when closed onto the grid. In this situation loss of grid will cause the motor to stop generating.
Using a three phase motor unbalanced will result in excessive heating. There are three connection methods that can be used to balance a three phase motor when generating single phase. 1. Steinmetz connection. 2. Smith connection. 3. Fukami connection. (Japanese not Italian) Perfect phase balance can be achieved by using capacitors (Phase converters). The generator operates as if it were connected to a three phase supply. Therefore efficiency is the same. As load changes the balance point changes so to restore balance capacitance has to change. In practice, it is the simplified Steinmetz connection that is utilised. The common name for this connection is C2C. This connection works well in both standalone and grid connected systems because it can achieve perfect phase balance over a wider range of impedance angle, so requires less capacitance change. This connection is most likely used with your water wheel.
You are quite correct in that a loss of mains will result in loss of generation from an induction motor, except for a condition where the load provides the generator excitation VAr. This is how your electric brake works in both of your generators if you consider the capacitors part of the load. Limiting condition being when the magnetising reactance is equal to the unsaturated magnetising reactance. Although this condition is highly unlikely to the point of insignificant in a domestic location, it is not impossible in a local network containing distributed generation and power factor correction equipment. For this reason loss of mains protection is required.
To assume that the grid would hardly notice the load is a little short sighted. When an unexcited induction generator running at synchronous speed is connected to the grid a magnetising inrush current occurs. You may well be unaware of this very short duration event on small machines unless you are using incandescent light connected near to the generator. This current can be several times load current. The argument of a DOL motor start seems credible; however there are standards that limit inrush current in all but name. Less than 16A phase, EN 61000-3-3. Over 16A and up to 75A. EN61000-3-11. To place products onto the market requires CE marking, these standards are part of showing compliance, so must be given consideration. What gain is there in allowing unnecessary disturbances from generation in addition to those we already have?
For an induction generator to meet the above standards some form of inrush current suppression must be adopted. Some methods which can achieve this are Soft starters, Variable resistors and Synchronisation. Your hydro systems use synchronisation. Excitation is provided by capacitors, this also has the benefit of providing power factor correction under load. G83 requires between 0.95 lead and 0.95 lag over the full operating range of the generator.
So, with the generator to mains breaker open and the excitation / power factor correction capacitors connected to the generator, the generator is run up to sync speed and excitation from residual magnetism and the capacitors provides rapid voltage rise to a reasonably stable level. While drifting into phase with the mains the breaker is closed. If you still believe that closing an excited induction generator out of phase onto the mains is without issue PLEASE go and try it! Give those breakers some excitement or fuse links a change.
In response to;
http://www.navitron.org.uk/forum/index.php/topic,8822.msg96453.html#msg96453When have I ever suggested that motors can be synchronised? My quote [Measure voltage on Gen side of contactor as the alternator comes up to speed and is excited by the fixed capacitance.] This is an asynchronous alternator, a self exciting rotating electrical machine with inertia, an all over bodily fault current stimulator. No longer a motor! Some machines such as Cranes, Lifts and Conveyers are capable of back feed during over run. It is important where any power factor correction capacitors are installed. If connected directly to the motor the risk of continued generation and out of sync re-closing of switch gear is real.
Putting aside our difference of opinion for a moment. Two hydro installations designed, installed and generating. Quite an achievement, well beyond the capability of most people. Well worthy of recognition.
