THERMAL POWER PLANTS CONTROL SYSTEM

 

Thermal power plants have a great subject unresolved: The control system. There is a real necessity of carrying out all these projects as quick as possible to ensure the highest productivity. This situation makes control system detailed engineering is not developed enough as it should be required in a system of XXI century. Keep in mind that at a solar thermal power plant has almost all the equipment necessary to develop a complete automation. Thus, it is not an issue for both investment and equipment availability. It is a software matter, an appropriate control logic development.

By Santiago García Garrido

Technical Director RENOVETEC

www.renovetec.com

Full automation advantages

Currently power plants are completely manual. That means that starting, operation manner selection (there are more than 15 operation ways in solar thermal plants), temperature control, pressure control, cooling and control of all parameters that affect plant’s efficiency and power , are carried out manually, according to the plant’s operator decisions. What is the consequence? It depends on the control room panellist which can have a greater or less training, a good or bad mood, a greater or less ability, or even a personal opinion on how to operate the plant different from other panellists that works in other shifts. Thus, it is noticed how each panellist applies its own rules and customs, while others prefer to operate the plant in their way taking their own decisions.

Figure 1: Solar thermal power plant panellist giving instructions to an operator by the radio.

There are three clear advantages of having an automated control system. These advantages are:

-       Decrease the number of operators required. Rather, it could be said that, since the number of operators is fixed by budgetary issues, with a minimum number of operators, the plant can be operated safely and with the best efficiency.

-       The plant always operates at the optimum point, not at the operators preferred point. This is certainly the main advantage

-       The number of faults and incidents decreases. Only increase electronic problems related to control systems.

Current structure

Actually, a number of independent control systems coexist in the power plant and they only share a few signs. Firstly, solar fields control system. It consists of a central control system (FCS, Field Solar Controller). Secondly, individual controllers for each collector (usually a loop consists of four collectors, as shown in Figure 2)

Figure 2: Solar thermal power plant typical loop. It consists of 4 collectors and 48 modules. Cold HTY enters by one end and it goes out by the other with a net profit of about 100 oC temperature. Source: Solar Thermal Power Plant Engineering. RENOVETEC Course

This way, a plant consisting of 100 loops, would have a control system for the solar field FSC and 400 individual controllers, one for each collector. The FSC is responsible for guiding the modules. Like this, the proper angle position is reached to ensure the highest optical output. Also it is responsible for placing them in a safe position when atmospheric conditions are not appropriate, especially in the case of wind.

Control system is also responsible for making a temperature measuring. It makes five measures in total (one for each of the four collectors and one at the end of the loop), just before emptying the heat transfer fluid in the hot HTF control. In some plants the FSC can trigger an automatic valve at the entrance or exit of the loop. This way, FSC completely closes the fluid flow through it, and in a few plants (a minority) FSC is able to achieve automatically a pressure balance across the solar field. That ensures that both, the closest and the furthest loop of the pumping system, have exactly the same pressure and the same flow. 

Figure 3: Inlet valve located at the entrance of each loop in solar thermal power plant in construction. Source: RENOVOTEC File.

The control system complete automation keys are two:

-        To provide automatic valves at each loop entrance or exit, and a thermal probe at the loop exit.

-       To have sufficient meteorological data (radiation, humidity, wind speed, temperature) along the long the solar field, which may involve installing between 5 and 10 meteorological stations.

If these two

elements are installed in the plant, and almost all plants have them, it is possible to regulate the solar field control without operator intervention.

Figure 4: Control system screen of the solar field (FSC) of one of the first solar thermal power plants


Power control system is responsible for regulating fluid flow through different equipment so that, an exchange of heat between the thermal fluid (HTF) and the water-steam cycle is produced. Always keeping controlled (more or less) some of the variables such as temperatures, pressures, flows and levels. Thus, this system controls s HTF primary pumps speed variators, HTF main valves, steam generation system outlet valves, different by-pass, speed variators of water-steam cycle feed pumps, condensate pumps or the level of the tanks and boilers.

There is also the turbine control system, totally independent and provided by the manufacturer. This process is responsible for turbines starts and stops, load regulation and certain parameters vigilance that would trigger the assurances to protect the turbine.

Figure 5: A control system screen of a Siemens SST-700 turbine, the most commonly used in thermal power plants.

Besides these control systems that are practically independent, there are the controllers of each one of the auxiliary systems that make up the BOP (Balance of Plant). This way, systems such as treatment plant, auxiliary boiler, fire system, compressed air system or gas station, have their own control system, and they exchange certain signals with the control power block.

Disadvantages ofthis architecture

The first disadvantage to notice is that the plant operates normally outside its optimum design. It is often seen that the oil temperature rarely reaches the ideal temperature at the entrance of the exchangers (about 393 º C). At this temperature the plant thermal efficiency is maximized. The operator is who decides the number of open loops, the number of partially focused or unfocused loops, and in some cases, even the flow through the solar field. The fact of having too many possibilities and so many choices makes the operator choose not always the most appropriate. It is left to operator take decisions that do affect the plant efficiency, decisions that are not always optimal. It is easy to walk through a control room and check the temperature at which the plant is working at the entrance of the steam generation system. It is not common to see the plant working at its optimum point. But conclusions are worst when a complete energy balance is considered.

Secondly, and it is as bad as the previous point, there are many breakdowns that have been caused by poor operation. In the decision-making process, sometimes, to operate in an insecure way, is chosen. Like this, some punctures in the steam generation system, failure to pump seals, accelerated oil degradation, corrosion in some parts of the installation or the pressure imbalance from the solar field are often associated with unfortunate operations.

Total automation difficulties

It is not inconceivable that a solar thermal plant starts and stops completely automatically; also that it can approach ties or not in accordance with decisions implemented in the control system; or that auxiliary boiler will start up automatically when it is needed. Nowadays, honestly, seems inconceivable. But it also seemed unthinkable that a computer system is capable of winning any human at virtual chess. Also, for only 30 euro you can buy a machine that it is virtually unbeatable. And, of course, that machine should discuss more options and have less powerful processors that the ones installed on a solar thermal power plant.

First difficulty is the measure of atmospheric conditions. Current number of weather stations is insufficient, because radiation or temperature conditions present in an area of the plant may be different from those found in another area. But for a cost of less than 100,000 euros you can install all the necessary stations, a minimum of eight. This way, radiation measuring around the solar field would be almost perfect.

Figure 6: Meteorological station in a solar thermal power plant. Pyranometer and pyrheliometer, are common tools in these stations and used to take radiation measurements. Besides these, other measurements are necessary such as speed and wind direction, temperature and relative humidity. Source: Solar Country Course RENOVETEC

Secondly, the lack of automatic valves with a PID control to its regulation determines enormously, total automation possibility.

Third difficulty is the investment and time required for the development of a distributed control system plant, which embraces all the rest of the systems around the plant.

Finally, and probably the most hopeless of all, is the human resistance. Operators prefer to perform a manual control in spite of the serious disadvantages that it involves. Many plant managers have assumed that it must be in this way, and there is no alternative.

Control engineer

Control engineer as part of the Operation and Maintenance team, has become an essential figure in a solar thermal power plant. In many industrial plants, this person is linked to the operation and he/she just could modify a small number of parameters. In a solar thermal power plant, he/she has almost total access to the control program.

Control engineer has two main functions:

- To solve a lot of automatisms that are not well implemented in plants, such as level controllers, flow controllers, temperature controllers ... This lack comes from the implementation process, which normally is accelerated artificially leaving without ‘commission’ some systems, which are left forever in manual way.

-To improve the general control system, control engineer is responsible for devising control routines and adjusting PID controllers which ‘relieve’ the operator's job.

Conclusions

Control system can not only be improved, it must be done to achieve plant’s expected production plan. If the control system does not develop properly, in the following months, many plants will begin to fail its expected production, because they are working outside the design point. A poor operation causes great amount of damage and disability, also its production costs will be increased because more staff than initially considered will be necessary to be incorporated.

More information about solar thermal power plants

More information about solar thermal power plants and their control system in the following web sites: http:// www.renovete.com and http://www.centralestermosolares.com.

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