from the The Start of Another Brouhaha? department...
The Physics of... Synchronizing Alternators
1 out of 2 members thought this post was helpful...
 Posted by Phil Corso on 14 May, 2010 - 5:10 pm
At the risk of fomenting brouhaha I've taken the liberty of addressing the challenge (?) by CSA. First my "claimer"... the following discussion is excerpted from the text* I used to teach an Electrical Technology course in Staten Island Community College, a Division of City University of New York! Copies of the pertinent Section(s) are available on request!

A) Fundamentals

Before paralleling two alternators (or an alternator and a bus) several requirements are necessary. Some of them were mentioned by other posters:

1. Their voltages must have the same waveform (sinusoidal.)

2. Their voltage waveforms must be exactly opposite in phase.

3. The product of their poles and speeds must be the same.

4. Their effective (rms) values of generated voltages should be closely matched

5. Their combined over-all alternator voltage and the prime-mover characteristic should be drooping (whether introduced naturally or electronically) with application of load.

6. Their rotational speeds (frequency), at the time they are switched together, should be closely matched.

7. For polyphase machines only, the phase-sequence must be the same as that of the bus.

In effect all of the requirements above (excluding #5) can be met with the single statement that "the polarities of the two sources of voltage must be equal, but opposite at all times." Also note the term "closely matched" in items #4 and #5. If the terms 'equal', 'identical', 'same', and 'zero' were used then Gurpreet's "floating generator" phenomenon would exist, but no current will flow and therefore synchronism cannot be accomplished.

B) The Mechanics of Synchronizing Single-Ph Alternators.

Consider alternator A, the running machine and alternator B the incoming machine. Their generated voltages are Ea and Eb, respectively. Finally, assume there is a means of "observing" the synchronization procedure. In the olden days lamps were used (please no arguments about whether the dark-method or light-method is superior!) Another observation method is to use a two-channel oscilloscope. A third method could consist of a single-channel scope synched to machine A. Today, of course, a synchroscope is used. But, for this discussion, consider two lamps, one across each contact of the switch or breaker connecting the two machines:

Case 1: Machine A's voltage is 220V, and its frequency is 60Hz, while machine B's voltage is 200V, and its frequency 59.5Hz. Then:
a) The maximum resultant voltage, Er, across each lamp is (Ea+Eb)/2 or 221V.
b) The minimum Er is (Ea-Eb)/2 or 1V.
c) The frequency is 60-59.5 or 0.5Hz.
d) The number of light pulsations = 0.5Hz x 60sec/min or 30 lamp-on pulsations per minute.

Case 2: Machine A's voltage is 220V, and its frequency is 60Hz, while machine B's voltage is 230V, and its frequency 61Hz. Then:
a) The maximum Er is (Ea+Eb)/2 or 225V.
b) The minimum Er is (Ea-Eb)/2 or 5V.
c) The frequency is 60-61 or 1.0Hz.
d) The number of light pulsations = 1.0Hz x 60sec/min or 60 lamp-on pulsations per minute.

C) Development of Synchronizing-Power

At the instant the switch is closed when the voltages are closely matched so that Er exists, a circulating or synchronizing-current, Is, will flow in both armatures, limited only by the synchronous-impedance, Zs, of the two machines (neglecting intervening impedances such as leads, bus bar, breakers, etc) in parallel. Because of the inductive nature of Zs, synchronizing-current lags, Er. The actual angle by which it lags is Theta = Arctan (Xs/Rs), where Xs and Rs are the synchronous-reactance and synchronous-resistance, respectively. Expressing it mathematically,

Is=(Ea-Eb)/(Zsa+Zsb)= Er/(Rsa+Rsb+j(Xsa+Xsb)

D) Effects of Synchronizing-Power

The synchronous-current, hence synchronizing-power, is circulating in the armatures of both machines. Assume that the excitation of B is such that Eb > Ea. Then, for B,

Psb = Eb x Is x Cos (Theta), while for A,

Psa = Ea x Is x Cos (180 - Theta) = - Psba.

Note that generator-action (+) is produced in machine B, but motor-action (-) is produced in machine A.

In closing, this discussion covers the very simplistic case of synchronizing like-machines, both of which are unloaded. If there is sufficient interest I will produce Part Deux, covering more advanced topics like the influence of Armature-Reaction and Load, and their effect on Torque- or Power-Angle!

*Text: Electrical Machinery and Control, 1964
Author: Irving L. Kosow
Publisher: Prentice-Hall
LofC Card:64-22802

Regards, Phil Corso (cepsicon [at] aol [dot] com)

0 out of 1 members thought this post was helpful...
 Posted by Sunil Kumar on 9 March, 2012 - 3:52 am
Dear Phil,
> 2. Their voltage waveforms must be exactly opposite in phase.

As per my opinion we cannot parallel the machines while their polarity is opposite.

> single statement that "the polarities of the two sources of voltage must be
> equal, but opposite at all times."If the terms 'equal', 'identical', 'same', and 'zero' were
> used then Gurpreet's "floating generator" phenomenon would exist, but
> no current will flow and therefore synchronism cannot be accomplished.

Phil which current you are talking about? Of course there will not be any current within generators, we have to think about external circuit not within the closed loop!!!

if the generator voltages are in phase (not opposite) definitely there will not be any current flow within generators (circulating current). but if you connect some external load, certainly there will be.

If the voltages are exactly opposite in phase then there will be 100% (short circuit )circulating current.

This is my opinion, I expect your thoughts.

0 out of 1 members thought this post was helpful...
 Posted by Phil Corso on 9 March, 2012 - 10:23 am

2) You have confused 'polarity' with 'phase'!

3) If the two frequency's, hence rpm's, are exact and their voltages equal and in-phase, no synchronizing-power (or synchronizing-torque as some say, but torque x rpm is power) will exist.

4) The reason for having the 'incoming' alternator's frequency (rpm) higher (faster) is to insure, that upon CB closure, there will be slight phase-displacement, hence insuring a circulating-current will 'flow' between the two alternators! I suggest you obseve the plot of the resultant voltage before the CB is closed! (If anyone wants a copy, just ask)

5) The equation (apologies to PtPs that accidently come into contact with this thread) shows that, upon CB closure, the faster machine's power-angle will lead! Thus it becomes the generator of synchronizing-torque and the already running machine becomes the motor.

6) A reminder, the difference betwen the two Egp's is a vector difference, i.e., having magnitude and angle!

Regards, Phil

0 out of 1 members thought this post was helpful...
 Posted by sunil kumar on 9 March, 2012 - 1:50 pm
> Sunil... your post to this thread proves you are a true contributor to this forum.

Thanks Phil,

> 2) You have confused 'polarity' with 'phase'!

Polarity,Is it 180 degree displacement? OR anything else? please explain!! because normally in D.C we mention polarity like +ve -ve and in A.C Circuit we mention in phases or in phase angle.

> 3) If the two frequency's, hence rpm's, are exact and their voltages equal and
> in-phase, no synchronizing-power (or synchronizing-torque as some say, but
> torque x rpm is power) will exist.

I will try to explain you with one mechanical example;
let us assume that two identical engines are coupled (mechanical) together via single solid shaft, running at same speed and in same direction and there is no load connected to it. In that case what happens; they continue to run in parallel by their own, without sharing any load with each other.

Now let assume that some load connected to the shaft; in that case both engines will share the load equally since they are identical in capacity and running at same speed& in same direction.

In case if you increase speed of one engine, then that engine will try to take entire load and still if the speed is more than it will make other engine to run at similar speed and in that circumstances other engine will act like a (motor)load too.

Just compare this example with electrical sychronisation, you may get some clear picture.

As per my opinion what you are mentioned in item no.3 that concept applies only for induction motors not for generator.

While synchronising two generators which are not having load; more accurate speed and voltage means less circulating current, that is actually the operator tries to do.
This is just my opinion, I hope this explanation may help you upto some extent.

Regards
Sunil Kumar

1 out of 2 members thought this post was helpful...
 Posted by Phil Corso on 9 March, 2012 - 11:46 pm
Sunil... why do you insist on introducing a mechanical version of the operation of an electrical generator, and a 'load' in your explanation

I suggest re-read the 1st paragraph of the original post. It includes the definition of 'polarity'.

Phil Corso

 Posted by sunil kumar on 13 March, 2012 - 12:39 pm
> Sunil... why do you insist on introducing a mechanical version of the
> operation of an electrical generator, and a 'load' in your explanation

Phil

I thought it is easy to convey my thoughts to others by giving mechanical example, like some time we use water analogy to explain electricity.

As per my opinion when we synchronizing the machines they will not oppose each other. I say they will be in harmony. They take care of each other. if one gets weaker other will pull him upto its level or vice versa.

And also I feel that no circulating current is required to keep the machines under synchronized condition. There should not be any circulating current if the required parameters are exactly matching, but in practical it is not possible match them perfectly due to some error in sensing device, control devices etc.

Due these errors there will be some slight variation in voltage/speed hence some amount of circulating current.

This is my opinion, well come to all for any comments.

Regards
Sunil Kumar

0 out of 1 members thought this post was helpful...
 Posted by Phil Corso on 13 March, 2012 - 6:35 pm
Sunil... I don't want to turn this thread into brouhaha, but synchronizing-power between alternators is essential. Even for multi-machine mega-systems. And, some believe lack of it could be responsible for inter-areas oscillations that have recently appeared especially when large wind-farms using induction-generators are involved.

In closing, this thread was intended to enlighten Control.Com forum readers. It is about synchronizing two alternators. Therefore, I encourage you to start a new thread related to Synchronizing-Power or Synchronizing-Torque!

Regards, Phil

 Posted by JC on 28 July, 2012 - 11:55 pm
From what I've learned during college.

To synchronize 3-phase alternators, the ff must true:
1. Same output voltage
2. Output voltage must be in phase
3. Same output frequency, regardless of the the rotor speed, poles, etc. Unless they share the same shaft.

The much larger scale example is connecting a Power Plant to the power grid.

Regards, JC

 Posted by Mark Allen on 31 July, 2012 - 5:11 pm
Hello Sunil

Sunil "As per my opinion we cannot parallel the machines while their polarity is opposite".

If I understand Phil's point the "polarity will always be opposite" can be seen this way:
`Electron flowGenerator/Alternator A Generator/Alternator B       -->>>>-l contactor for A phase l->>>>+ opposite but equal       -----l contactor for B phase l------ neutral and equal     +<<<<-l contactor for C phase l-<<<<-- opposite but equal`

From the point of view of Generator A phase A the electron flow is away from Generator A or negative polarity
From the point of view of Generator B phase A the electron flow is toward Generator B or positive polarity

The analogy of the two engines breaks down, as it relates to Phil's original post, in this respect: By introducing a ridged shaft connecting the engines you have introduced a stability that is non-existent when you have two Generator/Alternators operating as an island with no other load to serve.

The plant I normally work at is a Black-Start Hydro-Electric station. The station has three generators. When the units are operated in Black-Start mode only one of the units is used as a generator, the other two have the wicket gate opening reduced to something less than needed for synchronous speed and there for operate as synchronous motors and serve as a load, other wise the system is very unstable and is subject to split-phasing and tripping.

By your analogy the engines would have to able race or idle and do so with enough strength to break the ridged drive shaft you introduced in order to more closely fit Phil's original post.

To tell the truth about it, to me Phil's original post didn't address anything concerning introduced load or system stability. Phil's original post seemed to me to be showing how the average persons perception of polarities during the synching process might be wrong.

Dummy me, if someone had asked me if during the synching process if the polarities needed to be opposite or the same, before I read Phil's post, I would have said the same.

Now having read Phil's post I see where I would have been wrong.

Thanks
Mark Allen

 Posted by sunil on 2 August, 2012 - 1:40 pm
Hi Allen,

Thank you for your opinion. If you draw a vector diagram for the voltage for in phase voltage (0 degree) and out of phase (opposite, 180 degree) you may get clear picture about my previous explanation.

Thanks, Regards
Sunil

 Posted by Mustang on 1 August, 2012 - 4:09 pm
>> 2. Their voltage waveforms must be exactly opposite in phase.

> As per my opinion we cannot parallel the machines while their polarity is opposite.

>> single statement that "the polarities of the two sources of voltage must be
>> equal, but opposite at all times."If the terms 'equal', 'identical', 'same', and 'zero' were
>> used then Gurpreet's "floating generator" phenomenon would exist, but
>> no current will flow and therefore synchronism cannot be accomplished.

> Phil which current you are talking about? Of course there will not be any current within generators, we have to
> think about external circuit not within the closed loop!!!

> if the generator voltages are in phase (not opposite) definitely there will not be any current flow within generators
> (circulating current). but if you connect some external load, certainly there will be.

> If the voltages are exactly opposite in phase then there will be 100% (short circuit )circulating current.
> This is my opinion, I expect your thoughts.
Hello Sunil, it reads to me like you are used to paralleling Gensets.

When I parallel Genset to Genset (powered by a Diesel or Gas prime mover), I would normally check the Phase rotations are the same, Voltages are matched and Hz are matched.

The Genset closing onto the bus would normally go in at the same frequency or ever so slightly higher (50Hz Bus, 50.08Hz Gen).

I would only go in slightly faster if I wanted to take Kw load onto the incoming Genset. If there was no load, I would go in at same Hz.

If there is no load, then synchronizing with a frequency difference will normally end up with one Genset in Reverse Power.

I would normally always try and match RMS voltage exactly.

Once in parallel, at no load, I would want no Amps circulating between the machines, so I would be adjusting fuel (Hz) to ensure no Reverse Kw and Voltage to ensure no Reverse Kvar.

 Posted by sunil on 2 August, 2012 - 2:04 pm
Hi Mustang,

Thanks for sharing your experience, I totally agree with you and exactly mean the same as you mentioned in your thread.

Regards
Sunil

 Posted by nic on 31 July, 2012 - 3:08 pm
Phil,
I think some of these statements are a bit misleading, at least for me:

> 2. Their voltage waveforms must be exactly opposite in phase.

To me this implies that at any point in time the voltage phasor of one machine must be 180deg out of phase with the corresponding voltage phasor on the other machine. Can you clarify where I am mistaken?

> 3. The product of their poles and speeds must be the same.

> 6. Their rotational speeds (frequency), at the time they are switched together, should be closely matched.

3 and 6 above seem to contradict each other. I agree with three. I think 6 is confusing because rotational speed implies a mechanical measurement of rotational velocity, yet what it is trying to say is related to the electrical frequency measurement (electrical frequency must be closely matched, not mechanical speed, those can be way off, but must obey 3). 3 is a good way of stating the concept that mechanical speed and electrical frequency are related through the number of poles the machine consists of.

In regards to your comments about "closely matched" measurements - IEEE C50.12/13 defines the synchronization parameters of a machine as must being able to tolerate:

voltage difference of 0-5%
slip of +/-0.067Hz
phase angle difference of +/-10degrees

Hopefully that will give readers an idea of what "closely matched" means.

> D) Effects of Synchronizing-Power The synchronous-current, hence synchronizing-power, is circulating in
> the armatures of both machines. Assume that the excitation of B is such that Eb > Ea. Then, for B,

> Psb = Eb x Is x Cos (Theta), while for A,

> Psa = Ea x Is x Cos (180 - Theta) = - Psba.

You may want to elaborate on how and why the unit with the higher terminal voltage becomes the power producer and the other becomes the power consumer. Most people associate voltage with VAR production only.

Cheers,
Nic

 Posted by Phil Corso on 31 July, 2012 - 8:29 pm
Marc & Nic...

I personally used the procedure outlined while working as Chief Electrician on the Cruiseship "TSS Veracruz", in the mid '70s. I had to manually synchronize two (2) Turbo-Generators and five (5) Combustion-Engine Gensets.

Realizing that the discussion would present some problems... like describing a 'Ratchet' without using your hands... I offered "... pertinent Section(s) of the text (I used)... ", on request!

The offer still stands!

Regards, Phil Corso (Cepsicon@aol.com)

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