Synchronous Condensers, Anyone?

We are highly experimented 35 Year experience in Power plant with similar problem -

You understand that resolve a problem for a 150MW/e ,you may not help Remotely .

Joshua Gabriel SAADA

Engineer consultant.

Alternative Power Systems
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If the generators are designed for synchronous condenser operation, you should not have a problem on that end. If they are not designed for it, you need to look at the capacity and % regulation of the exciter and voltage regulator. You may not be able to provide enough excitation, or you may get an unstable output. You will also need to change your protective relay settings for the new operating design.

On the turbine end, the steam turbines will have issue with end-stage heating. If not already installed you may need to add an exhaust hood spray system. The aero will have issues with blade erosion and poor combustion at low loads, but both should be manageable with forethought and proper maintenance.

Economically, I can't see this project making much sense. The steam turbines are too large for the your VAR loading, and combustion turbines are deathly expensive in most parts of the world. You need to take a good hard look at the available revenues, especially whether or not you would have a guaranteed long-term income, before making any decisions.

How do you plan on getting this unit on line? If you simply run it to line speed and lock the control valves at the no load condition you may have problems with windage heating in the turbine stages.

The only way I have seen it accomplished was with two units specifically designed for the function, they had disconnect couplings and thrust bearings at the generator. The idea being to tie them together electrically on gear with one of them disconnected from the turbine, they would be tied to the system electrically as a pair. The driving unit could be used for load or taken off the line, the other could have the field varied to accomplish the synchronous condenser operation. These units were 1920' vintage.

My only comment on the economics would be that if you have the capability to improve power factor why wouldn't the power company pay for the benefit. They don't have too many qualms about charging for the effect.

If the plant is located in an industrial or dense urban area, it might make sense. Unless he's in a dense load center, the conductors on the poles would not be large enough to transmit that much reactive current to where it's needed.

We have 2 steam units (1965 & 1975) which are designed to be used as generators or sync condensers, and I've run them in both modes with no problem. We don't use them as condensers anymore because the nearby factories have all shut down, and it's cheaper to use line compensators where the new load on my system is located.

A little more information on my plant: there are 2 active units in an urban area with shipyards about 2 miles away. There is a 69/138 kV sub owned by the local utility. There are 2 active 69 lines and 3 active 138 lines.

Do your units use clutches? Can you use one as a condenser and the other as a generator? Are there other means of spinning up the generator besides a steam turbine? What kind of exciter is used? How about var control, AVR, stabilization, etc.?

Thanks for your comments.

Mine are both direct-drive, 3,600 RPM, GE synchronous generators, rated 50MW & 25MW. No clutches or gearing are involved, and the attached turbines (1 each) are the only source of rotation. The primary design is to provide real power (MW), with the condenser function simply an extra rating.

The static exciter/voltage regulators are GE SCT-PPT systems (Saturable Core Transformer with Power Potential Transformer), which was the standard for GE's marine turbine division up until the late '80's. I learned the same system in the Navy many years before. VAR control on mine is completely manual, based on the AVR setting. They are not designed to follow power factor, but rather for "base load" VAR compensation, so stability is not really an issue. The exciters are somewhat over-engineered, but I don't know if that's for the sync. condenser or just a consequence of designing a system before computer-assisted design was readily available.

If the utility has significant VAR load passing through the 69/138 sub, you might be able to convince them that you are a better deal than static compensation on the HV lines. If they have to transport it significant distance, the line losses will make your proposal less attractive. I get similar proposals about every other year (I am the utility here), and so far no one has presented an attractive enough package for me to sign on. However, your location and direct access to transmission may make your proposal very attractive. The utility will look at a) need (do they need that much VAR support?), b) relative cost to meet the need, and c) safety and reliability. Safety/reliability is a deal-breaker, but they won't bother to look there unless the economics work.

If you have no intention of using either unit as a MW generator again, I'd simply uncouple them from the turbines and connect an induction motor with belt & clutch to get up to speed. Your existing AVR's may or may not work, depending on the limitations of design. You will need to have PF-following or VAR setpoint capability, which may not be available on the existing units. Basler has a very nice line of relatively inexpensive AVR's which may suit your needs.

If you are in the situation that you intend to perform the double function of making power and operating in synchronous condenser mode, You should contact the OEM to ascertain if there is in fact a windage heating problem. If there is a problem it can probably be overcome with some simplistic resolution such as a properly placed line with a shut off valve and an orifice of the proper design. I don't think the amount of cooling steam involved would cause a concern with overspeed. Of course "trying this at home, unassisted" is strongly discouraged.