Synchronization & Start Up

[joan271273]

I would like to know any experiences that people might have had when starting up gas turbines in an industrial facility that had steam generators already supplying power.

I had some experiences in the past were during initial start up the bringing up some steam turbine generators to synchronize for first time caused the tripping of some gas turbines. Some of the reasons I rememeber hearing during the course of several meetings were items such as:

1.- Gas Turbines will pick up load faster than the steam turbines , therefore overloading themselves.
2.- Load sharing was not properly set up ( droop resistance,incorrect value).
3.- Operators were not properly monitoring situation.

What is the group experince in this ares, and can someone explain item #2

 

[dpc]

When paralleling synchronous generators of any type, consideration must be given to controlling how the machines will share load, both real power and reactive power. Either one can cause problems. You don't indicate why the units are tripping, so it's difficult to speculate.

Sharing of real power (kW) is a function of the governors and their settings. The simplest approach is to set all governors for simple droop control when operating in parallel. The droop settings must be similar to permit equal load sharing, within the capabilities of the generators. There are other, more complicated options, depending on the overall electrical system and your requirements. But the governor instruction manuals should have a section dealing with load sharing.

Sharing of reactive power (kVAR) is a function of the voltage regulator settings and configuration. These can be set up to operate in a droop mode, similar to the governors with all units sharing roughly equally in the production of vars. Again, your AVR manuals should provide some guidelines for your particular regulators.

If your units are tripping on reverse power, or over/underspeed, the problem may be with the load sharing amongst the governors. If they are tripping due to over/undervoltage or similar, the problem may be with the reactive power sharing settings in the regulators.

 

[joan271273]

The steam units were 48MW units and the Gas Turbines were LM2500 and LM6000. One of the things that came up but is still an open issue is what should have the engineering firm done to prevent this:

a) System Studies
b) Start Up verifications.

I saw that some of the traditional studies were issued by the Engineering firm ( fault study, power flow), but I really did not see anything else. Equipment suppliers have stated that the droop values should set up by customer or whoever is handling the overall "master plan".

For example what should I look for during design and previous to start up in the form of documentation. Should we have an integration document with all type of droop settings for the regulators before and after the generator expansion.

 

[dpc]

There should always be some condideration given to the interaction between units when adding additional generation. I suspect there must have been, but you may be experience some transient problems that would not be apparent in a typical power flow analysis.

During commissioning, there should be been some tuning of the governor and VR response characteristics.

But the design documents should cover how the units were intended to operate in parallel mode, and someone should calculated at least a starting point for the droop settings.

What is causing the units to trip?

 

[joan271273]

The whole in under review for now. I am just trying to verify typical steps to see if we the group did the job or we just miss some issues.

 

[busbar]

The tasks of 'startup verifications' are mentioned. If thorough commissioning tests have not been documented, it's rather difficult to speculate on the actual cause, and may turn into a very expensive omission.

 

[joan271273]

Manufacturer of the STG has provided calculations showing incorrect droop circuit resistance settings are the culprit of the tripping. This resistance value usually comes from an overall study that includes all the machine parameters and type of machines (GTG or STG) and with proper calculations will provide how the droop values need to be adjusted,so all unist can properly interact. I was able to see just a condensed report, has anyone done a complete calculation of this type.