Hello, we have a 132kV system with GT's supplying mains power at one end and 100kms away a production site stepped down to 6.6kV. For black starts and high load starts we have (x6) 1.5MW 6.6kV DGs. When we start from black the procedure is start a DG, synchronize with the GT's, set the DG's to droop slowly build load by starting small production agitators, conveyors etc.
The problem we have is with all the small loads started and x4 DGs running in droop, when we start the first big unit 5000kW the 6.6kV voltage drops to 5.8kV which is close to tripping.
Question -
1. Will putting the DG's in (kVar) control vs pf control assist with the voltage drop. ?
2. Do I need to switch them back and forth between pf and kVar control during startups from blackout?


Previous to my role the DG's were run in kVar but placed into pf control during care and maintenance and the production site was running in island.

Thankyou

 

@Toby_Eadie,

I'm very confused. When I think of droop I think of the prime mover governor running in droop speed control--which means the droop machine isn't going to change load if the frequency of the system it is synchronized with deviates from normal.

Yes; excitations systems can have a droop mode also, but in my experience the term droop is not commonly used with exciters.

What's missing from this post is whether or not there is some kind of external frequency and/or voltage control for the system that is configured to issue commands to running machines to change load to maintain speed/frequency as the loads connected to the system are started or stopped, and which might also be configured to adjust excitation to keep it near nominal as loads connected to the system are started or stopped. Otherwise, the human operators are going to have to be adjusting speed/frequency and excitation as system load changes.

Sometimes these external control systems are called or referred to as PMSs--Power Management Systems.

What we also don't know is what the loads are on each of the running DGs. If the four running DGs are nearly fully loaded and are the only machines running when a large 5000kW load is added to the system then it's very likely that the system frequency and voltage are going to deviate from normal.

We also don't know if the GTs are running under this scenario that's been presented or if the starting means for one or more of the GTs is the 5000kW load.

For me at least, it's not really clear about the details. A black start condition is mentioned but we don't know if this scenario is a black start condition or not.

For me, unless there is a very capable and proven PMS (external speed/frequency and voltage control system issuing commands to maintain frequency and voltage) then it's incumbent on the human operators in the control room to monitor conditions and make changes as necessary to keep everything humming along and not getting close to a trip condition. Also, to my way of thinking/operating a power island using VAr- or power factor control is mostly intended for steady-state operation, and starting 5000kW loads doesn't meet my definition of steady state operation. It would take strong excitation controls to be able to put them in VAr- or power factor control during such a high load starting condition. And most typical DGs I'm familiar with don't have such excitation systems--they are either rotating exciters or small AC powered excitation systems (120 VAC or 220 VAC).

Balancing exciters on small systems of multiple generators synchronized together with large load swings is not an easy task. And I want to qualify that statement by emphasizing if proper components are chosen for the system being assembled then it's not that difficult. But, often economic concerns can cause equipment to be selected instead of engineering concerns and that can lead to some difficult times during commissioning and during atypical operational situations. Unfortunately we just don't have enough information. In recent decades when a plant is being built it's common to prepare a very good written description of expected operations (including to the extent possible atypical situations such as black starts or high load changes (added or deleted!)). That document becomes the definition for choosing equipment and configuring it to meet the operational description. AND, it's very common that the operational description is included with the Operation Manuals provided with the plant/equipment that is turned over to the Customer. So, if you can, try to find the plant Operation Manuals and see if there's something applicable in there, such as the plant operational description. It can be a GREAT help, and is very often overlooked--because, who reads manuals (paper manuals or digital manuals). I've even seen training courses put together for new plants/operators that did not include the available operational description which would have been an incredible addition to the training and familiarization of the operators, and in some cases, their supervisors. And when new operators are hired over time, everyone thinks that OJT (On-the-Job Training) from experienced operators is all these new operators need to quickly become effective and conscientious operators. Often, the new hires aren't even shown the original training manuals....

So, to be honest, I think getting the missing information is going to be near impossible to completely understand the situation. So, I'm going to drop out of this thread, but will continue to monitor it because I quite often learn great things just by following threads with lots of input.

Best of luck.

 

@WTF? - Thankyou for the reply, I know the GT powerhouse remotely shifts the 6.6kV voltage up and down in preparation of starting large loads. Therefore it sounds like the voltage regulation is manual.
I will look for the manuals to get a better idea and do some more research. Thanks

 

@Toby_Eadie,

Automation (Automatic controls) are not the answer to every problem. And every problem needs to be analyzed and properly understood before spending lots of time and effort on problem-solving when the root cause of the problem wasn't properly identified. Trying to use simple (to my mind, VAr- and power factor control are simple controls--again intended for steady-state operational situations) automatic controls for atypical operational situations is one of the things that leads to the "control system" being improperly blamed for not working properly.