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Load Synchronization

We are discussing grid synchronization in class. Hand waving did not get me far enough into the subject and I am left wondering ... How does it work? Is there a requirement to be in lock-step on each and every cycle of the 60 Hz? If there is only a requirement to be in step on average, wouldn't this be like joining the two circuits of 240V household wiring? The two 120V lines are 180 deg out of phase so ...zap! Surely the Ontario grid is not lined up that exactly. Do they have local active and passive loads to tune 700MWe output? Just how does it work anyway?

Brain stretch time.

I found this on the net. You really DO have to be precise and detecting the phase (or voltage) difference between the hot side of the generator and the hot side of the grid is the key. Once the breaker is closed, the synch is locked in unless you really get a load mismatch.

Cute. Here is the explanation I found on the net. It is not mathematical but it makes sense to me. Consider the grid and the nuclear power plant as two voltage sources in parallel. The grid source is already connected to the load. We want to connect the nuclear plant to that grid / load system.

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From: Neon John <johngdNOSPAM@bellsouth.net>
Subject: Re: Linking 2 generators
Date: Fri, 12 May 2000 11:40:18 EDT
Newsgroups: rec.outdoors.rv-travel

Two AC generators operated in parallel must be not only at the same speed but also the same phase to be paralleled. Doing so is not at all difficult but you do have to know what you're doing. If the neutrals are connected together and then the voltage between the hot legs of the two generators is measured (hot leg to hot leg and not hot to ground), the following conditions will be noted:

In phase: zero volts
180 degrees out of phase: the sum of the two voltages. Since the voltages must be the same, this will be 2X the nameplate voltage. The generators can only be connected together when the voltage between the hots is zero, e.g. in phase. This suggests a phasing method.

The standard power plant method of synchronizing two generators is to use a very expensive instrument called a synchroscope. This instrument indicates whether the incoming generator is faster, slower or in phase with the bus. Since the rotors of power plant generators weighs many tons, the phases must be extremely accurately matched or else the rotor will be forcibly yanked into phase, possibly wrecking the generator. For small units, we don't need to be so precise. We can use a pair of lamps.

What you'll need is a couple of lamps hooked in series and connected between the hot lead of the running generator and the hot lead of the newly cranked generator. You'll also need a switch of some sorts to parallel the units.

If the generators are 180 deg out of phase, the voltage across the lamps will be 2X the nameplate voltage (240 volts in the case of two 120 volt generators.) and the lamps will burn full brilliance. If the generators are nearly in phase, the lamps will be out because there will be no voltage on them. If one generator is faster than the other, the lamps will flicker on and off as the gens are in phase one moment and out the next.

The procedure is as follows. Start the second generator. The lights will be flickering or slowly coming on and off. Manipulate the throttle of the incoming generator which ever direction is necessary to slow the flickering. As the speeds become almost equal, the lamps will stay off for a long period of time and then slowly start lighting, slowly get fully bright and then slowly dim again. You want to manipulate the throttle until the lamps are off for as long as possible. You want to close the breaker when the voltage between the generator is the least. Since the lamps will go out before the voltage reaches zero, you'll want to mentally time the period between going out and coming back on again and close the switch about in the middle.

Once the breaker is closed, the generators are locked together. Indeed, you could close the throttle of one engine and the coupled generator will motor the engine at precisely the sync speed (3600 RPM for small gens). If the generators are just a little bit out of phase, then they will be yanked into phase as momentary heavy current flows between them. And if you close it out of phase, then you have a double voltage short circuit. Usually there is severe mechanical and electrical damage. (I heard and saw the results of a 50 MW diesel genset being synched 180 degrees out as the result of reversed leads on the synchroscope. Literally ripped the stator out of the foundation and twisted the shaft.)

<snip>

John