Patent Document

PRIORITY CLAIM 
     This application claims priority as a continuation of PCT application no. PCT/EP2008/056208 filed on May 20, 2008. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a control device and method of a gas turbine electric energy production plant. Specifically, the present invention relates to a control device and method of a gas turbine electric energy production plant connected to a network working in disturbed conditions. 
     BACKGROUND 
     As it is known, gas turbine electric energy production plants normally comprise a motor assembly (turbo assembly), to which belong a variable geometry stage compressor, a combustion chamber, a gas turbine and a generator, mechanically connected to the same turbine and compressor shaft and connected to an electric distribution network through a main switch. 
     Turbo-gas plants are further equipped with control devices, which implement the various operations needed for an appropriate plant operation and for meeting the standard requirements related to the performances of plants in terms of safety, stability and capacity of responding to variations in the demand for power by the distribution network. 
     Normally, when connected to the electric network, the plant outputs an electric power at a frequency which is stably maintained by control devices about a given frequency value, named nominal frequency (50-60 Hz). 
     Specifically, the known control devices perform the so-called primary setting, which stabilizes the plant frequency by varying the supply of fuel to the combustion chamber according to the difference between the nominal frequency and the plant frequency. The primary setting generally implements a proportional control logic. 
     However, the primary setting is not always sufficient to guarantee the stability of the frequency of the delivered electric power. 
     The plant is usually connected to a network comprising a plurality of electric energy production plants and loads, organized in a grid structure. In ordinary conditions, all the plants connected to the network participate to the frequency setting, which is stabile and subjected only to modest fluctuations. According to the diverse operation needs, portions of the network, including one or more plants, may be selectively isolated, e.g. to prevent the propagation of possible faults. 
     However, major frequency variations which the primary setting cannot compensate may occur in an isolated plant, especially because the isolation of the plant intrinsically implies evident imbalances between the power delivered by the plant and the power consumed by the loads. Specifically, the known control devices are not always capable of re-establishing a condition of balance (delivered power=consumed power) and thus of reaching the nominal frequency value again. 
     SUMMARY 
     It is an object of the present invention to make a control device which is free from the drawbacks of the known art herein described; specifically, it is an object of the invention to make a control device capable of maintaining the plant power delivery frequency close to the nominal value also in unusual operating conditions, and specifically in conditions of isolation from the network. 
     In accordance with such objects, the present invention relates to a control device and method of a gas turbine electric energy production plant as claimed in claims  1  and  9 , respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will be apparent in the following description of a non-limitative example of embodiment thereof, with reference to the figures in the accompanying drawings, in which: 
         FIG. 1  is a simplified block diagram of an electric energy production plant in which a control device according to the present invention is incorporated; 
         FIG. 2  is a block diagram of a detail of the control device incorporated in the plant in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a gas turbine plant  1  for the production of electric energy. Plant  1  is selectively connectable to a distribution network  2  through a main switch  3  and comprises a turbo assembly  5 , a generator  6 , a detection module  7 , a control device  8  and a reference value selection module  9 . 
     Turbo assembly  5  is of the conventional type and comprises a compressor  10 , a combustion chamber  11  and a gas turbine  12 . Combustion chamber  11  receives the fuel through a feeding valve  13 . 
     Generator  6  is mechanically connected to the same axis as turbine  12  and compressor  10  and is rotationally driven at the same angular rotation speed ω of turbine  12  and compressor  10 . Generator  6  transforms the mechanical power supplied by turbine  12  into active electric power, hereinafter simply named delivered power P and makes it available to distribution network  2  at a frequency f I . 
     Detection module  7  is in communication with a plurality of sensors (not shown) of plant  1  and supplies a series of parameters related to plant  1 , such as plant frequency f I , delivered power P, turbine exhaust gas temperature  12  etc., to control device  8 . 
     Reference value selection module  9  generates reference signals to be supplied to control device  8 . Specifically, reference value selection module  9  supplies a nominal frequency value f N  (50-60 Hz) and a set power value P SET  to control device  8 . Such reference values f N  and P SET  are generally established beforehand or manually entered by an operator. 
     Control device  8  uses the parameters from detection module  7  and from reference value selection module  9  to generate control signals adapted to adjust the supply of fuel to combustion chamber  11  and the flow rate of air fed to compressor  10 . Specifically, control device  8  generates a control signal U FV  which is sent to valve  13  to set the supply of fuel to combustion chamber  11 . 
     Control device  8  comprises a plurality of control modules (not shown in figure) by means of which the plant variables are controlled, such as for example angular rotation speed ω, delivered power P, turbine exhaust gas temperature  12 , etc. Specifically, control device  8  comprises a power control module  15 , a frequency control module  16  and a limiting module  17 . 
     Power control module  15  controls power P delivered by plant  1  according to a reference power value P SETNEW . Specifically, power control module  15  receives as input a current delivered power value P ACT , from detection module  7 , and a reference power value P SETNEW , given by the sum of set power value P SET  from reference value selection module  9  and a power correction value PF SET  from frequency control module  16 , and generates control signal U FV  for controlling fuel feeding value  13  to combustion chamber  11 . Preferably, power control module  15  implements a PID (Proportional Integral Derivative) control logic based on a power error e p , i.e. on the difference between current power P ACT  and reference power value P SETNEW  (P ACT -P SETNEW ). 
     Frequency control module  16  receives as inputs nominal frequency value f N  and plant frequency value f I  coming from detection module  7 , and generates a (positive or negative) power correction value PF SET  which is added to set power value P SET  to form reference power value P SETNEW , according to a frequency error e F  (f I -f N ). In essence, frequency control module  16  provides a power correction value PF SET  such as to minimize frequency error e F , i.e. so as to maintain plant frequency f I  close to nominal frequency f N . 
     Limiting module  17  receives power correction value PF SET  as input and is made so as to limit both power correction value PF SET  and the excessively sudden increases of power correction value PF SET . Specifically, when power correction value PF SET  exceeds a certain threshold value, limiting module  17  deactivates frequency control module  16  so as to prevent correction value PF SET  from rising and compromising the operation of plant  1 , e.g. due to the reaching of the maximum flow rate limit of the gas turbine. For example, the deactivation of frequency control module  16  may be obtained by canceling the proportionality constants. 
     Furthermore, limiting module  17  avoids excessively sudden increases of power correction value PF SET  by limiting the derivative of correction value PF SET . The rising velocity of power correction value PF SET  must indeed be lower than a threshold value to prevent damage to plant  1 . In the described example, such limit is equal to 13 MW/min. If the derivative of correction value PF SET  reaches said threshold value, power limiting module  17  deactivates frequency control module  16  so as to prevent correction value PF SET  from rising and compromising the operation of plant  1 . Also in this case, the deactivation of frequency control module  16  may be obtained for example by canceling the proportionality constants. 
     With reference to  FIG. 2 , frequency control module  16  comprises a frequency error calculation module  18 , an activation module  19 , a proportional control module  20 , also named primary control module  20 , and a step control module  22 . 
     Frequency error calculation module  18  calculates frequency error e F  as the difference between plant frequency f I  and nominal frequency f N  (50-60 Hz). Frequency error value e F  is respectively fed to activation module  19 , proportional control module  20  and to integral control module  22 . 
     Activation module  19  evaluates the frequency error value e F  and selectively sends an activation signal to proportional control module  20  or to integral control module  22 . 
     Specifically, if frequency error e F  is comprised in a first safety range B1, generally equal to approximately ±6% of the value of frequency error e F , e.g. ±0.3 Hz, activation module sends an activation signal to proportional control module  20 , and an activation signal to integral control module  22  if the frequency error is beyond first safety range B1 (critical condition). Once the activation signal has been sent to integral control module  22 , activation module  19  cannot send any activation signal to proportional control module  20  until a deactivation of integral control module  22  occurs. The deactivation of integral control module  22  is preferably manual and performed by an operator. Alternatively, activation module  19  may perform an automatic deactivation of integral control module  22  when given conditions are satisfied, in terms of frequency error e F  and of other parameters of plant  1 . 
     The manual deactivation of integral control module  22 , and the consequent restoring of the initial selective activation of proportional module  20  or of integral module  22 , may only be performed if frequency error e F  is within a second safety range B2, generally equal to approximately ±2% of the value of frequency error e F , for example±0.1 Hz. 
     In practice, when frequency error e F  assumes critical values, integral control module  22  intervenes and proportional control module  20  is deactivated. This configuration remains unchanged until the operator reckons that the intervention of integral control module  22  is no longer fundamental. Generally, the operator deactivates integral control module  22  when plant  1  is connected to electrical network  2  again. 
     Proportional control module  20  provides a power correction value PF SETPR  by following a proportional control logic based on frequency error e F , while integral control module  22  provides a power correction value PF SETINT  following an integral control logic based on frequency error e F . 
     In practice, frequency control module  16  provides a power correction value PF SETPR  if frequency error e F  is comprised in first safety range B1, while it provides a power correction value PF SETINT  if frequency error e F  is not comprised in first safety range B1. 
     The present invention presents the following advantages. 
     Firstly, the control device according to the present invention allows to obtain a good and effective setting of the frequency in the plant also if the plant is isolated from the network. Specifically, control device is capable of setting the frequency of the plant taking the physical and dynamic limits of the plant itself into account. 
     The integral setting performed by the integral control module indeed contributes to restoring the nominal frequency with a slow, integrated action with respect to the frequency error, thus completing the proportioning control action of the primary setting. In this manner, the plant improves its capacity of tackling network emergencies. 
     Finally, the control device according to the present invention is easily installable in plants which are already running because no structural modifications are required. 
     It is finally apparent that changes and variations may be made to the device and method described herein without departing from the scope of protection of the accompanying claims.

Technology Category: h