Generator cooling

Provided is a method for controlling the cold gas temperature of a cooling gas of a closed generator cooling gas circuit of a generator having at least one cooler through which a cooling fluid flows. The method includes: a) defining cold gas temperature setpoint values in dependence on the stator and rotor current of the generator; b) detecting the current cold gas temperature; c) detecting the current stator and rotor current; d) determining the cold gas temperature setpoint value associated with the stator and rotor current detected in step c); e) regulating the cold gas temperature by changing the volumetric flow of the cooling fluid supplied to the at least one cooler as a function of the difference between the current cold gas temperature detected in step b) and the cold gas temperature setpoint value determined in step d); and f) repeating steps b) to e) at defined time intervals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/EP2017/065838, having a filing date of Jun. 27, 2017, based off of German Application No. 10 2016 213 469.8, having a filing date of Jul. 22, 2016, the entire contents of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a method for controlling the cold gas temperature of a cooling gas of a closed cooling gas circuit of a generator, which circuit has at least one cooler through which flows a cooling liquid.

BACKGROUND

The cold gas temperature of cooling gases of closed generator cooling circuits has up to now always been statically controlled. In other words, the controlling is designed in such a way that the cooling gas of the generator is adjusted just once when the generator cooling gas circuit is put into operation so that during the intended power operation a temperature distribution of the generator components is adjusted to a desired temperature level inside the generator. The cooling liquid volumetric flow through the cooler, then permanently selected, is therefore independent of the operating mode of the generator, which distinguishes startup operation, power operation, shutdown operation and turning operation. This basically leads to the generator being cooled over-dimensionally during power operation in the case of low load and partial load and also during startup and shutdown processes. In the case of the previous operating modes of conventional power plants during base load and medium load operation, in which the generator is operated with constant output over a very long period of time, this intense cooling during low load, partial load operation and during startup and shutdown processes played a very subordinate role, however.

The constantly increasing proportion of power generation from volatile sources, however, leads to the demands of the national electricity grid upon generators to be constantly changing. For the operating mode of a generator, this results in an execution of a large number of start-stop cycles and a frequent changing of the power point, that is to say a high number of steep load gradients and a frequent low-load and partial-load operation. Accordingly, generators in most cases are no longer in thermally steady-state stationary operation.

Resulting from this, the temperature levels inside the generator are subjected to very high and frequent fluctuations which, on account of an extremely wide variety of thermal expansion coefficients of the respective generator components, induce thermomechanical stresses, especially in the region of the winding bar connection. This leads to accelerated ageing of the generator components, which in particular affects the insulation system of the generator.

SUMMARY

An aspect relates to create an alternative method of the type referred to in the introduction which reacts to different operating modes of the generator in a flexible manner.

For achieving this aspect, embodiments of the present invention creates a method of the type referred to in the introduction which comprises the steps:

a) defining cold gas temperature setpoint values in dependence of the stator and rotor current of the generator, wherein the cold gas temperature setpoint values especially have a predetermined margin in relation to cold gas thermal protection values of a protection system of the generator;
b) detecting the prevailing cold gas temperature;
c) detecting the prevailing stator and rotor current;
d) determining the cold gas temperature setpoint value which is associated with the stator and rotor current detected in step c);
e) controlling the cold gas temperature by altering the volumetric flow of the cooling liquid which is fed to the at least one cooler in dependence of the difference between the prevailing cold gas temperature detected in step b) and the cold gas temperature setpoint value determined in step d); and
f) repeating steps b) to e) within defined time intervals.

Owing to the fact that the cold gas temperature setpoint values are defined in dependence of the stator and rotor current of the generator, to which they are indirectly proportional, and the cold gas temperature is then controlled in dependence of a cold gas temperature setpoint value which is associated with the prevailing stator and rotor current, it can be ensured that the cooling which is produced by the cold gas is adapted at least in most cases to the current operating state of the generator, as a result of which a more uniform temperature level is achieved inside the generator and the emergence or the frequency of thermomechanical stresses is reduced. The cold gas temperature setpoint values are in this case advantageously selected with a margin in relation to so-called cold gas thermal protection values of the protection system of the generator if a generator protection system is provided. Such a generator protection system serves for automatically shutting down or running down the generator when the cold gas thermal protection values are exceeded in order to prevent damage to the generator. The controlling of the cold gas temperature is carried out according to embodiments of the invention by altering the volumetric flow of the cooling liquid which is fed to the at least one cooler, as a result of which the current cooling capacity of the cooler is varied.

The cold gas temperature setpoint values defined in step a) in dependence of the stator and rotor current of the generator lie below the cold gas thermal protection values in the corresponding stator and rotor current. Therefore, it can be ensured that sufficient margin in relation to the critical temperatures, which lead to the shutting down of the generator, is maintained.

The prevailing cold gas temperature is advantageously detected in step b) at a number of positions at the same time and only the maximum prevailing cold gas temperature is taken into consideration during the controlling in step e). As a result of the redundant detection of the cold gas temperature, the reliability of the method is increased, especially when one of the temperature sensors malfunctions.

The maximum value of the detected prevailing stator current and of the detected prevailing rotor current is determined in step c), wherein the cold gas temperature setpoint value which is associated with the maximum value is determined in step d).

According to one embodiment of the method according to embodiments of the invention, the prevailing hot gas temperature is detected and taken into consideration during the controlling in step e). The detection of the prevailing hot gas temperature also serves for increasing the reliability of the method. Therefore, for example a maximum hot gas temperature, which must not be exceeded, can be defined.

The prevailing hot gas temperature is detected at a number of positions at the same time and only the maximum prevailing hot gas temperature is taken into consideration during the controlling in step e). As a result of these redundant measurements, the reliability of the method is increased, especially when one of the temperature sensors malfunctions.

The prevailing cooling liquid temperature is advantageously detected before entry into the cooler and taken into consideration during the controlling in step e). In this way, the accuracy of controlling can be increased.

According to a further embodiment of the method according to embodiments of the invention, the prevailing stator and/or rotor temperature of the generator is detected and taken into consideration during the controlling in step e), which also contributes to accuracy of controlling.

In the case of specific designs of the generator—with directly gas-cooled stator winding bars—the prevailing cooling gas temperature at the bar exit of the stator winding of the generator can also be detected and taken into consideration during the controlling in step e).

The differential of stator current or rotor current to the time is determined and taken into consideration as a vanishing impulse during the controlling in step e), as a result of which the accuracy and reaction speed of the controlling is improved.

The generator cooling circuit advantageously has a plurality of coolers, wherein the volumetric flows of cooling liquid which is fed to the respective coolers are altered in step e). A plurality of coolers have the advantage that cooling remains at least partially functional even in the event of a malfunction of one of the coolers.

A heating device is provided for optional heating of the cooling liquid which enters the cooler. Such a heating device is particular advantageous to the effect that cooling down of the generator during turning operation can be prevented, as a result of which the temperature difference to the mechanically stress-free state is reduced. The cooler can therefore be selectively also used as a heat exchanger which yields heat to the cooling gas.

According to a specific embodiment of the method according to embodiments of the invention, the generator cooling gas circuit is a component part of a power plant.

DETAILED DESCRIPTION

FIG. 1shows a closed generator cooling circuit1through which a cooling gas, for example in the form of air or molecular hydrogen (H2), is directed in a clockwise direction in the present case, and which has a generator2, provided with a cold gas thermal protection system, a cooler3, a cooling gas line4which interconnects the generator2and the cooler3, and a delivery device5. Flowing through the cooler3on one side is cooling gas coming from the generator2, which is also referred to below as hot gas, and on the other side a cooling liquid, for example in the form of cooling water, which is fed via a cooling liquid line6. The cooler is designed for the purpose cooling hot gas leaving the generator2by means of yielding heat to the cooling liquid for creating cold gas which is then fed again to the generator2. With constant volumetric flow of the cooling gas, the cooling capacity of the cooler3is in dependence of the volumetric flow of the cooling liquid directed through this and can be adjusted via the control valve8which in the present case is provided with an actuating drive7and is arranged downstream of the cooler3.

For controlling the cold gas temperature by altering the volumetric flow of the cooling liquid which flows through the cooler3, provision is made for a control device9, which is connected to the actuating drive7of the control valve8, and for a series of sensors or measuring devices which transmit the data detected by them to the control device9. Arranged in the generator2are current measuring devices10which detect the prevailing stator current and the prevailing rotor current of the generator2. Also, a plurality of temperature measuring devices11, which sense the prevailing temperatures of generator components, are positioned inside the generator2. Therefore, the temperature measuring devices11which are arranged inside the generator2can be provided for example in the form of slot resistance thermometers, to name but one example. Further temperature measuring devices11are arranged inside the cooling gas line4in the present case, specifically upstream between the cooler3and the generator2for detecting the cold gas temperature on one side and downstream between the generator2and the cooler3for detecting the hot gas temperature on the other side. A further temperature measuring device11is positioned in the cooling liquid line6upstream of the cooler3in order to detect the cooling liquid temperature before entry into the cooler3.

For controlling the cold gas temperature, according to embodiments of the invention cold gas temperature setpoint values were defined in advance in dependence of the stator and rotor current of the generator2and stored in the control device9, wherein the cold gas temperature setpoint values in the present case have a predetermined margin in relation to cold gas thermal protection values of the cold gas thermal protection system of the generator2, as is explained in more detail below with reference toFIG. 2.FIG. 2shows a graph, on the X-axis of which is plotted the stator and rotor current I in amperes, and on the Y-axis of which is plotted the cold gas temperature T in ° C. In the graph, the line12represents the cold gas thermal protection values of a cold gas thermal protection system which is common for example in power plants, that is to say those temperatures which when being reached a turbo generator set for example which is arranged upstream of the generator2, with correspondingly associated stator and rotor current, is compulsorily shut down, in order to prevent overheating of the generator2. The line13extends in the main parallel to the line12in the direction of lower temperatures and represents the warning values of the cold gas thermal protection system which when being exceeded an alarm is triggered for the operating personnel who are made aware of the fact that a shutdown of the turbogenerator set is shortly to occur.

Line14represents the cold gas temperature setpoint values according to embodiments of the invention in dependence of the stator and rotor current of the generator2. In the present case it extends in the main parallel to the line12in the direction of lower temperatures and advantageously has a measured safety margin in relation to the line13in order to prevent the cold gas temperatures being able to reach the level of the warning values of the cold gas thermal protection system during an uninterrupted operation of the generator. The safety margin in relation to line13should, however, be small enough to permit temperatures which are as high as possible. The safety factor advantageously lies within the range of between 2.5 and 7.5%.

During the operation of the generator2, the prevailing cold gas temperature is detected by means of the temperature measuring devices11which are arranged in the cooling gas line4between the cooler3and the generator2, the maximum value of the detected temperatures is determined and transmitted to the control device9. The redundant detection of the prevailing cold gas temperature and the determination of the maximum value serves in this case for increasing the reliability and the accuracy of controlling. Also, the prevailing stator current and the prevailing rotor current are detected by means of the current measuring devices10. Then, based on the previously defined cold gas temperature setpoint values, the cold gas temperature setpoint value which is associated with the maximum detected current value is assigned to the maximum detected current value which is taken into account in the controlling. In addition, in the present case the prevailing cooling liquid temperature before entry into the cooler3is detected using the temperature measuring device11which is arranged at a corresponding position in the cooling liquid line6, and is transmitted to the control device9. This is especially of importance if the temperature of the cooling liquid fluctuates to a considerable degree, for example if river water, which is exposed to extremely variable ambient temperatures due to the seasons of the year, is used as cooling liquid. The prevailing cooling liquid temperature can then be taken into account in the controlling by the cold gas temperature setpoint value being multiplied by a suitable factor, to name but one example. Also, in the present case the prevailing hot gas temperature is repetitively detected using the temperature measuring devices11which are arranged in the cooling gas line4between the generator2and the cooler3, the maximum value of said temperature then being transmitted to the control device9. The prevailing hot gas temperature can also be taken into consideration by a corresponding factor or the like during the controlling. The redundant temperature measuring also serves here for increasing accuracy and reliability. Finally, in the present case prevailing temperatures of individual generator components are also detected using the temperature measuring devices11which are arranged inside the generator2and the average value of the detected temperatures is formed, which average value is transmitted to the control device9and taken into account in the controlling. Via the control device9, the actuating drive7of the control valve8is finally operated in order to alter the volumetric flow of the cooling liquid and therefore the cooling capacity of the cooler3in such a way that the prevailing cold gas temperature is adapted to the cold gas temperature setpoint value.

During on-load operation of the generator2, the basic task of the previously described cold gas temperature controlling entails providing an adequate cooling capacity in order to compensate load change processes. It is the primary aim of the controlling in this case to achieve a generator2which is as hot as possible in order to reduce thermomechanical loads of the generator components. At low loads, the cold gas temperature setpoint value has to be correspondingly increased, at high loads correspondingly reduced, in order to keep the temperature level inside the generator as high and constant as possible regardless of the type of load operation (full load or partial load).

When the generator2is being run down with subsequent turning operation without intended shutting down, that is to say for achieving a so-called hot standby mode, it is desirable to maintain a temperature level which is as high as possible inside the generator2in order to bridge the turning operation. To this end, the volumetric flow of the cooling liquid is set to minimum in the case of low stator and rotor current. In the case of turning operations of frequently long duration, it can be advantageous to temporarily use the cooler3as a heating device in order to supply heat to the generator2. This can be achieved for example via the circuit15, shown by dashed lines inFIG. 1, which has a cooling liquid branch line16, which branches from the cooling liquid line6upstream of the cooler3and joins with this again between the cooler3and the control valve8, and also a heating device16and a separate delivery device5. In the circuit which is formed when the control valve8is shut off, the circulating cooling liquid can be heated by means of the heating device16and the absorbed heat in the cooler3can be yielded to the cooling gas.

When the generator2is being run down with subsequent turning operation and shutdown of short duration, it is the aim to ensure a quick and adequate cooling of the generator2, especially of its rotor, in order to compensate thermally different expansion coefficients of the materials of the generator components and to counteract thermal expansions. To this end, the volumetric flow of the cooling liquid which is fed to the cooler3is to be selected as high as possible, which can be carried out by means of a separate control program which can be activated by the operating personnel and renders the previously described controlling inoperative.

FIG. 3shows an alternative generator cooling circuit1which in the main differs from the generator cooling circuit1shown inFIG. 1to the effect that provision is made for two coolers3which are arranged in parallel, as a result of which reliability and also cooling capacity are increased. Although it is not shown inFIG. 3, the depicted arrangement can also be provided with a correspondingly adapted circuit15.

FIG. 4shows by way of example a possible arrangement of temperature measuring devices11, designed as slot resistance thermometers, in a stator plate packet17of the generator2, in which arrangement the temperature measuring devices11are positioned in each case in the region of cooling grooves19which are provided between the plates18and through which flows cooling gas.

LIST OF DESIGNATIONS

1Generator cooling circuit

4Cooling gas line

6Cooling liquid line

10Current measuring device

11Temperature measuring device

17Stator plate packet