Pilot automation system for gas turbines testing.

Dediu, Gabriel ; Popescu, Jeni ; Cuciumita, Cleopatra Florentina 等

1. INTRODUCTION

The dynamic characteristic of a gas turbine represents the multitude of regimes for which the gas turbine has a stable working, its utilization being extremely helpful in finding solutions for gas turbine connected problems.

The dynamic characteristic describes the dynamic properties of the gas turbine as a free system, unconditioned by any system law. In practice, only certain working regimes are necessary, depending on requests. In order to answer to these requests, the gas turbines are scheduled with automatic control systems accomplishing the command and control functions.

In industrial applications, the operator can only intervene on the gas turbine when it reaches the stable idle regime, all this time the gas turbine being totally controller by the automatic control system monitoring all gas turbine working regimes. (Calin, 1976)

2. OBJECTIVE

The technical evolution of gas turbines in terms of both industrial and aeronautical applications, as a result of the need to align with the nowadays demands of high efficiency and reliability, also impose their control with an automatic command and control system using a PLC (Programmable Logic Controller), high capacity and processing speed of the information regarding the functioning regimes.

Classical solutions for the gas turbines control, which can no longer accomplish the performances required must be replaced with last generation control systems, with a high level of intelligence, capable of communicating extremely fast with the gas turbine's PLC and provide the precise and unitary control of gas turbine's parameters at all regimes.

Current automatic control systems must solve the main performance and safety criteria:

--To provide an optimal response time at all functioning regimes;

--To have such a characteristic that, no matter the conditions, the following phenomena never occurs: the cycling of the compressor, the flame-out in the burning chamber, or any other phenomena that could lead to an abnormal functioning of the gas turbine;

--To maintain the stipulated values of the parameters without oscillation by automatically adjusting the control loop.

The objective of this paper is to provide theoretical and experimental data regarding the design, construction and testing of new types of automatic control systems for gas turbines. As applicability domain it can be mentioned: transient regimes control, high operating speed, automatic surveillance of the main functioning parameters of the gas turbines.

The results of the paper will contribute to the development of methods and systems for the automatic control of the gas turbines and to the attainment of the automatic control system for a practical example.

3. PILOT AUTOMATION SYSTEM STRUCTURE

For executing a new automation installation for gas turbines experimentation, the classical solutions of control systems have been replaced with last generation gas combustion and starting systems, able to communicate with the Programmable Logic Controller of the control system and to insure an unitary and clean control on gas turbine thermodynamic parameters in all working regimes.

The control algorithm of the automation system is able to command and control the contained equipments, such as the starter, the gas start dosimeter, the gas start electro-valves, the gas combustion dosimeter, the "stop" and purging gas combustion electro-valves, the ignition system and the gas combustion mass flow rate measuring system.

The design of the automation control system was focused on the areas which can usually generate significant problems. Therefore, there were investigated the speed control, the pressure and differential pressure monitoring, the setting of the general architecture of the Programmable Logic Controller in order to insure parameters' monitoring and the control of the working sequences based on specific gas turbines algorithms, the control of fuel dosage through a gas turbine specific dosimeter, the monitoring of gas leakage and emergency stop on 30% LIE level, the utilization of quick "stop" valves, below 100 msec.

The architecture of the pilot installation for gas turbines' testing involves the equipments to be described further down.

The Programmable Logic Controller is equipped with analogue and digital inputs and analogue and digital outputs. The analogue input modules are built on 16 bits with a precision of 0.05 in continuous current. The choice of the central unit module was based on facts referring to its processors, the possibility of floating point calculation due to its all-in mathematical co-processor, the comprisal of logarithmic, trigonometric and exponential functions and the "flash" high capacity memory. (www.gefanuc.com, 2007)

The transducers for monitored parameters consist in pressure transducers with EExd and Exi contacts, programmable to be used for several types of gas turbines. (Ionescu, 1985) The accuracy of the pressure transducers and the differential pressure is of 0.075%. The ST3000 pressure and differential pressure transducers, for measuring applications involving one of the basic types of pressures, such as differential, relative or absolute pressure, are able to measure the process pressure and send an unified output signal, proportional with the measured variable, in the range of a bifilar loop, of 4 - 20 mA.

The Tachtrol tachometer is an instrument with one or two channels measuring the input frequencies and displaying the resulted quantities in terms of revolutions per minute (RPM), frequency report masking (FRM), % or Draw. The measurements use the periodic module, time per phenomenon, allowing a good combination of high accuracy and fast response. (www.aitek-usa.com, 2009)

The gas combustion dosimeter is a modern system for fuel dosage, utilized in a precise control of the gas turbine. It is controlled by the PLC according to the thermodynamic parameters measured during the working process and is characterized by high speed response.

For the cases of the ground applications involving gas turbines fuelled by gaseous fuels, and particularly for the present case, of a modified aviation gas turbine, an essential component of the automation and control system is the detection and alarm system. The OLCT IR gas detector is destined to the measurement of explosive gases in the atmosphere. It is based on an optical principle of gas detection through infrared absorption, registering very high detection reliability. It is programmed by the produced depending of the gas type of interest. Being fed in continuous tension, the detector releases a standardized 4-20 mA current, proportional to the measured gas concentration. The detector includes a local calibration device authorizing the ethalonation in the ATEX area without opening the case. It can be used in gaseous and powder explosive environments while responding to the European Directive ATEX 94/9/CE.

The MX48 group is conceived for medium units and does not necessitate installing a particular electric system. It has 4 8 independent channels, each of these being connected to one or several gas detectors installed in the surveillance areas. The measurement made by the detector is displayed by the MX48 group and continuously compared to the alarm threshold. In the cases of outran thresholds, the group activates the relays piloting the external equipments. The MX48 group includes one or two programmable electronic cards, each with four channels. (www.indsci.com, 2008)

[FIGURE 1 OMITTED]

4. PILOT AUTOMATION SYSTEM CHARACTERISTICS

For gas turbine control based on its working characteristics, a control algorithm was elaborated, based on which the command and control software was developed and implemented in the PLC, assuring all command and control sequences through commands and all the control signals provided by the equipments described in the previous section of the paper.

The software allows the gas turbine control in the following situations:

--Pneumatic or electric start;

--Gas turbine ignition;

--Gas turbine acceleration in safety conditions from the start to the idle working regime;

--Control of the gas turbine working regime between the idle regime and the nominal working regime;

--Gas turbine protection in the conditions of parameters outrunning the safety thresholds;

--Gas turbine stop in safety conditions.

[FIGURE 2 OMITTED]

5. CONCLUSIONS

The monitoring, command and control systems for aviation and industrial gas turbines fuelled either by liquid or gaseous fuels, require a high level of intelligence, capability of extremely fast communication with the gas turbine and precise and unitary control of gas turbine's parameters for all working regimes.

The modern systems, such as the pilot automation system described in the present paper, involves high number of measurement lines--both analogue and digital inputs and outputs, high speed for parameters' acquisition and execution elements command, high accuracy for all temperatures, pressures and mass flow rates measuring lines.

The system described in the paper is suitable for aviation gas turbines experimentations, turbo-compressor groups with industrial applicability, turbo-compressors and cogenerations groups driven by gas turbines, assuring an increase in gas turbines' working safety and a decrease in exploitation and maintenance costs.

Similar systems are already included in the activity of the Gas Applications Department and Combustion Chambers Department of COMOTI Romanian Research and Development Institute for Gas Turbines and several cogeneration applications, using gas turbines, in Romania.

The future work will focus on implementing the presented automation system for a test cell destined to experiment a microturbine, now in development, in two configurations: turbojet and turboshaft.

6. REFERENCES

Calin, S. (1976). Regulatoare automate (Automatic Control Systems) Advanced Energy Systems, Ed. Didactica si Pedagogica, Bucuresti, Romania

Ionescu, G., Dobrescu, R. & Popescu, D. (1985). Traductoare pentru automatizari industrial (Transducers for Industrial Automation), Ed. Tehnica, Bucuresti, Romania

*** (2007) http://www.gefanuc.com--PLC Datasheet, GEFanuc--Intelligent Platforms, Accesed on: 2007-06-20

*** (2008) http://www.indsci.com--MX 48 Litterature Sheet, Industrial Scientific Corporation, Accesed on: 2008-04-07

*** (2009) http://www.aitek-usa.com--TACHTROL 3 Instruction Manual, A.I.Tek Instruments, LLC, Accesed on: 2009-02-07