Isochronous Load Sharing on Dissimilar Generators

A

Thread Starter

Ali

Hello,

I am an E&I engineer at FFC, a fertilizers plant.

Our power generation system is based upon two 8 MW steam turbine driven generators and one 15 MW gas turbine driven generator. The gas turbine has digital synchronizer and load control (proprietary of Turbomach) and is operating in isochronous mode. It supplies a load of 10 MW under normal conditions. The STG's operate on droop mode and share a load of 7-8 MW equally.

Now according to my understanding every time there is a increase or decrease in system load the iso machine picks the load and drops it on to the droop machine. However due to lag in droop response we suffer frequency and voltage drops in our system. This is obviously unacceptable (though I would like that you point out why).

Now we are currently installing an electronic governor (woodward) to replace the hydraulic one on one of the steam turbines. We will use a DSLC2 module for digital synchronizing and load control. Now I have a number of questions regarding the possibilities of operating modes in which we can run our machines.

1. Is it possible to operate two machines (GT and STG with new system) in ISO mode while the other STG runs in droop mode?

2. Should we install the same system on the second STG too? can we implement Isochronous load sharing on the three machines? If so , then how?

There is an MSLC2 module that is a master module for communicating b/w the DSLC2 for load sharing purpose. From what I understand the three machines will communicate the loads to the master controller via DSLC2's. We would have set the percentage of load we want each generator to pick in order maintain the frequency. The master controller will control all the governors' output to control the speed until the bias signals to each governor is equalized.

3. If we are able to successfully install isochronous load sharing on the three gens then how will this scheme be better then our current scheme of iso-droop-droop?

These were some of my queries. I want to thank you in advance. I know it is long but I wanted to explain as much as possible so there was no confusion.
 
Hello,

It all really depends on the capabilities of the governors. Some don't operate so well in Isoch-Droop configuration when in parallel on an isolated load/grid. Some operate perfectly fine in such a configuration. Others don't operate well at all. A lot of it has to do with governor tuning and control valve response and other subjective elements (every engineer has their own opinions--most are correct, in their own way, but not always explained or described fully or correctly). Just about anything is possible, but there are always limitations. One of the biggest problems I see with a lot of small steam turbines is that they use pneumatically-operated control valves, and those are notoriously problematic.

Whenever the load increases on a small isolated plant with a single machine operating in Isoch and one or more other units synchronized and operating in Droop mode the Isoch unit takes the load. That's it's job--to keep the frequency constant. The Droop units are just "along for the ride" and while they will change their output (power) when the grid frequency changes they won't do very much to return the grid frequency to normal--that's not their job. The Isoch machine's job is to change load to maintain frequency, and when it's working correctly the load on the Droop machines won't change--because the frequency is not changing because the Isoch machine is changing it's load to maintain frequency.

All of this happens as long as the load on the Isoch machine doesn't exceed the ability of the Isoch machine to produce power ("nameplate"), or the load on the Isoch machine doesn't drop below 0 MW. It's the operator's job to adjust the load on the Droop machine(s) to keep the load on the Isoch machine above 0 MW (usually 10-20% of rated) and below rated power output (usually 10-20% of rated) so that sudden changes in load can be absorbed by the Isoch machine without affecting frequency.

That's where most sites/operators/supervisors get into trouble--load on the Isoch machine changes automatically as the total load on the system changes. But, it's the operator's job to make sure that anticipated changes in load don't go above the Isoch machine's ability or below 0 MW--and they do that by changing the load on the Droop machine(s).

If the total load on a system is 10 MW, and the Isoch machine is rated for 5 MW and is carrying 4 MW, and two other machines are each rated for 4 MW and are each carrying 3 MW (for 4 + 3 + 3 = 10), if the operators want to decrease the load on the Isoch machine by 1 MW they need to increase the load on one or both of the Droop machines by 1 MW. So, let's say they add 0.5 MW to each of the Droop machines while the load is stable at 10 MW, the load will be (3 + 3.5 +3.5 = 10 MW).

It's as simple as that.

Isoch load sharing will usually automatically split the load between the units according to some configuration determined when the system is installed so that the load will be "shared" by all machines in accordance with their capability. So, in the above example, two machines are rated for 4 MW each, for a total of 8 MW, and the third machine is rated for 5 MW, for combined total of 13 MW. The two 4 MW machines would be set to carry approx. 30.77% of the load, and the 5 MW machine would be set to carry about 38.46% of the load. So the 5 MW machine would be carrying approx. 3.846 MW of a 10 MW load, and the two 4 MW machines would each be carrying about 3.077 MW, for a combined total of 10 MW. Any change in load would be split according to the above ratios.

As for why frequency excursions are undesirable, well, most electric motors run at speeds that are directly proportional to the frequency of the AC that is being applied to them. That means they won't run at rated speed if the frequency isn't at rated. Also, their power output will vary when frequency changes, so the plant will experience problems with throughput and output if frequency isn't at or very near rated. Some pumps and fans and such do not change their output linearly with speed, but exponentially, so some are more "affected" by changes in frequency (motor speed).

Sorry, but that's about the extent of what we can offer based on the information provided.

Hope this helps.
 
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