Static Excitation Pt. 3: Regulator Analysis 3

[n1mr0d]

Hi,

I am currently analysing the schematics of a static exciter of a 5MW generator (which i combined below), and i'm stuck with several questions.

Firstly,it appears to be a compound source exciter, which provides the voltage and current to the rotor field. It consists of a current transformer (see Pt. 2) which is shunted with a reactor and a potential transformer. A Basler whitepaper states: "Under normal steady-state conditions, the phasor summation between the PPT/linear reactors and the CTs provides the correct compensated voltage and current to the generator field for all loads
at any power factor.". Can anyone give a example how i can derive this phasor summation?

Secondly i am not very familiar with thyristors, and i am not exactly sure how the field regulator works. It seems to make a massive short circuit when the gates are activated, which is mitigated by the reactor (i think). I assume that two thyristors are used because of current latching/commutating. Notice the wire that runs from the middle phase of U2 to the anode of the lowest thyristor/ VRM module. Is this used to commutate the thyristor?

Thanks in advance.

 

[dpc]

Is this work related?

If so, you may want to find someone who has some experience dealing with these static exciters to work with you.

I would search websites of static exciter manuf for basic tutorial information. Instruction manuals are usually available for downloading and these generally have a section that briefly explains the theory of operation.

 

[ScottyUK]

there's just too much missing information to work out what the function of each component is. These things are pretty complex to understand even when you know how all the components behave individually. People make a living tuning AVRs to optimum performance. dpc has the right idea - find the explanatory notes which should accompany the drawing in the station's documentation.


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I don't suffer from insanity. I enjoy it...

 

[davidbeach]

Since you mention a Basler white paper, try

http://www.basler.com/html/sesae.htm

and find the excitation application engineer who covers your area.

 

[n1mr0d]

I've simplified things a bit; i left the core parts of the system. Unfortately the schematics are the only pieces of documentation available, and the original manufacturer has gone bankrupt. I won't bother Basler since they can't sell me anything.

Anyone have any suggestions about the thyristor part of the circuit

 

[ScottyUK]

I haven't got the time to try to simulate the circuit, but I wonder if the thyristors at the left hand side are there to initially establish forward current in the field, and the starting thyristors are commutated off by the connection from U2 centre phase? The other possibility would be that these thyristors are some form of regulator, but the principle of operation is not apparent. What is 'V_RM'?

T1 appears to be a phase shifting transformer designed to give a 12 pulse supply to the pair of rectifiers fed by U1 and U2.

My other thoughts are that the windings of T2 form part of a magnetic amplifier which is the means of controlling the field. This suspicion is raised by the pair of apparently uncontrolled rectifiers fed by U1 and U2. It would be interesting to know if anything else is wound on the core of T2 or the reactor but not shown on this drawing.

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I don't suffer from insanity. I enjoy it...

 

[n1mr0d]

I agree with you that the middle winding off u2 could be used to commutate the lowest thyristor, iff the amplitude of the current is high enough. VRM is actually the Voltage/Cos Phi Regulator Module (Holec UPF15). Even less is known about this thing than the rest of the circuit. I think it works like a switch or something. Could the left part of the circuit be some kind of crowbar?

T2, pictured below, show no secondary voltage/current so it must be an autotransformer, i think. Only 2 wires are connected to it, so it must be used as a coil. There are no dc control wires to saturate the coils. Unfortunately i do not have an inductance meter, but i'm currently building one from scratch.

PS: do you use Multsim for simulating circuits or some other program?

 

[ScottyUK]

I've two copies of spice, one being a command-line driven version for Unix, the other being pSpice which I acquired while at university. I don't do much circuit modelling these days - that's part of the reason why I haven't got time to trawl through my old notes and re-learn the bits I have forgotten! The former is unfriendly but powerful, the other is easier to use but the component libraries in my possession are limited.

I agree that U2 looks like an inductor rather than a transformer. Is it possible that the circuit is, to some extent, self-stabilising? As load current increases so does the boost to excitation provided by the compounding CT. There may be some very cute design with the magnetics which isn't apparent from the schematic. The left hand side of the drawing has me very puzzled.


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I don't suffer from insanity. I enjoy it...

 

[waross]

Hello n1mr0d

I agree that U2 looks like an inductor rather than a transformer. Is it possible that the circuit is, to some extent, self-stabilising? As load current increases so does the boost to excitation provided by the compounding CT. There may be some very cute design with the magnetics which isn't apparent from the schematic. The left hand side of the drawing has me very puzzled.

I agree with you completely, ScottyUK, including the puzzlement.
In regards to your suggestion that the circuit may be self regulating; I have seen this several times over the years.
Lister-Petter used an excitation scheme years ago that used a constant voltage transformer (saturated core) for basic, no load, excitation. Then the current boost was added to increase the excitation in relation to the load. These sets were small, about 15 KVA.
However, back in the 70's I installed a 500 KVA set with a similar excitation scheme. The literature claimed 5% voltage regulation with the basic exciter. The basic open loop voltage was adjusted with a wire wound resistor with a sliding tap. It could be used that way in open loop configuration. There was provision to add a conventional voltage regulator to implement closed loop control and improve the regulation to the limit of the added voltage regulator.
This may be similar. Do there appear to be any air gaps in T1?

It may help to analyze the circuit on the left to consider the regulator as a stand alone excitor and the circuit under question to be a "trimming" circuit.
Re the "crow bar" suggestion; Do the thyristers look as if the're physically large enough to survive crow-baring?
Could this be a surge supression circuit?
respectfully

 

[Skogsgurra]

Sorry to have been cut off from Eng-Tips for a while. Refreshing to be back.

This circuit is actually quite simple. U1 is the main excitation voltage. It is rectified and the result is fed to the field winding. The iron "knee" is utilized to obtain a stable operating point - it acts like a stabilizer circuit.

The L1 reactors make the U1 "soft" so that currents from the booster transformers have an influence on it. More current makes the U1 go up and maintain a nearly constant generator voltage.

The thyristors are for over-voltage protection. When an out-of-sync situation or a heavy transient occurs, the voltage to the left of the diodes/resistors gets high enough to trig the thyristors, which then short out the voltage and protect the rectifier. One thyristor will not work since it is not possible to turn it off (DC voltage). Therefore two thyristors with a connection to AC (for turning off) are used.

I am not sure about the functioning of U2. It looks like another compounding function, but I am not at all sure about that.

Gunnar Englund

http://www.gke.org

 

[waross]

It's good to see you back skogsgurra;
Re U2; Depending on the sizing of the components, it may be load compensation, or it may be an excitation boost for fault currents. Possibly both.
respectfully

 

[n1mr0d]

Skogsgurra thanks for your reply, but could you be more precise/mathematical. First of all, what do u mean with "iron knee", and "make the U1 "soft"". Secondly is the direction of the CT current is towards T1, in stead of to the rectifier bank? I wonder what Kirchoff would say about this.

The Voltage/Cos Phi Regulator module controls the lowest thyristor, so they can't be just for over-voltage protection.

Like mentioned before T1 is a phase shifting transformer, so you get 6 phases --> 12 pulse rectifier.