Simulating the realities of cogeneration

21 January 2000

Most CHP plants are operated from the steam host control room and feed the local electricity grid. Training operators to operate such plants is becoming increasingly important. Using simulators to enable a trainee to learn how to operate a plant is one possible solution.

In late 1994, Tractebel Energy Engineering (TEE) commissioned the world's first GE aeroderivative LM6000 dry low emission gas turbine at SPE Ghent combined cycle plant. Since then, TEE has started up eleven other LM6000 DLE generator sets in various CHP schemes. Nine additional LM6000-based cogeneration plants will be commissioned before the end of 2000. TEE and its main customer Electrabel have already accumulated over 100 000 hours of operational experience in CHP plants using the DLE LM6000 engine.

To support this programme, TEE has developed a computer simulation tool which models a generic cogeneration unit comprised of GE’s LM6000PD dry low emission gas turbine, a single-pressure boiler with post combustion and a back-up boiler.

Other simulator tools have been developed for CHP projects using other gas turbine models.

LM6000 PD generator set model The characteristics of the LM6000PD are very different from those of other industrial gas turbines. This results from the LM6000 aeroderivative twin shaft design as well as from its dry low emission combustion system including three premixer rings with staging valves.

To simulate the dynamic behaviour of a LM6000PD generator set and its influence on the power system, TEE has used EUROSTAG software for simulation of power system dynamics. The model has been tested by comparing its output with data from existing units.

It is now possible to simulate transient operating conditions such as islanding on the steam host plant loads after a short-circuit cleared by the protective system in back-up.

The results of the simulations extended to the steam host plant loads, network and protective system allow the operator to define and tune the best strategy to:

• Protect the plant;

• Increase the security of operation;

• Enhance its global dynamic performance.

For an investor, this new simulation facility is able to efficiently address two unavoidable questions:

• How to demonstrate to the transmission system operator that the new facility complies with the grid code;

• How to assess the impact of power system disturbances on his investment.

Why a multifunctional simulator?

Before being allowed to operate a power plant, the trainee has to understand a dynamic and complex system, and develop the skills that will be needed to deal with emergencies and accidents. This cannot be done with a real system.

Staff training has many benefits:

• It provides better and more efficient system operation.

• Safety improvement.

• To anticipate incidents.

• Reduce risks of faulty trips.

• Efficiently manage emergencies.

Simulators are the most efficient means of training operators to be able to cope with extreme behaviour in the system.

The simulator can be made to be more cost-effective if it is not dedicated only to training. The simulator can be used to test any planned modifications of the system or of the control system. Such a simulator has to be very flexible and easy to modify using modular and open tools with libraries or reusable and generic models. The use of the real control system, either by emulation or by simulation, can be very useful.

The simulator allows visualisation and monitoring of all the parameters calculated in real time by the model. The trainee sees the information displayed as it would be in real-life, and has to respond accordingly. For training purposes, some additional displays with more information and/or dynamic visualisation of the underlying control loops are provided.

What is the scope of the simulator? There are increasing numbers of cogeneration units installed around the world. As a result, there is not a great deal of point building a simulator specific to one particular unit.

Instead, a generic simulator is more sensible, as it will provide basic training covering normal and emergency operation of plants for all operators and managers of such units.

The cogeneration simulator has two major components:

• Gas turbine and fuel, post-combustion and gas supply system.

• Recovery boiler and feedwater tank. The steam is produced in a vertical-type forced-circulation single-pressure-level recovery boiler comprising an economiser, a vapouriser, and a two-part superheater.

A steam turbine is simulated, and a back-up boiler is planned on the HP and LP steam line to ensure continuity of process steam to the client. This will offer a broad enough spectrum of operating conditions that may be encountered.

The following operations can be performed by operators on the simulator:

• Cold or hot start, automatic or manual.

• Complete shutdown of the installations, automatic or manual.

• Tripping of a gas turbine, steam turbine or boiler.

• Shutdown of the entire power plant.

• Restarting after an incident.

• Emergency shutdown.

• Coupling generator to grid.

Man-machine interface

The images which make up the simulator interface are as follows:

• Gas turbine main display image.

• Generator image.

• Lube oil generator image.

• Variable geometry image.

• Gas control image.

• Trim display image.

• Casing ventillation image.

• General overview image.

• Steam header image.

• Feedwater and boiler image.

• Post-combustion image.

The instructor station

The instructor station provides all standard functions available on training simulators, with the following additional functions being the most commonly available:

• Freeze/run function.

• Initial states administration.

• Snapshot management.

• Local control management.

• Malfunction and incidents management.

• Replay function (up to 60 minutes backtrack).

• Model measurements monitoring.

• Display of operators images.

• Alarms and events management.

• Exercise function.

Safe and profitable operation

The training is given to four trainees using four screens (see photograph on previous page). The plant is mouse-controlled, as in modern installations.

For experienced operators, the training takes five days and for inexperienced operators, 2-3 weeks. This training is a vital and crucial element in ensuring both safe and profitable plant operation, allowing experience to be gained without any operational risk.

The simulated plant


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