A CONTROL SYSTEM OF AN INTERNAL COMBUSTION ENGINE

Description

Field of technology

[0001] The invention relates to a control arrangement that controls generator sets when a grid trip occurs.

Prior art

[0002] The grip trip means that a part of a power network is disconnected from the utility grid/elecrical grid, i.e. from the rest main power network in practice. The rest of the network (the main network) is often called grid. The reason for the disconnection can, for example, be a short circuit or malfunction of a big transformer in the grid. Local power networks are often provided with a grid breaker arrangement for connecting and disconnecting the local network with the grid. The local network is, for example, a power network of a factory.

[0003] Figure 1 shows an example of a grid breaker arrangement having a grid breaker relay 3 that controls breakers 6 to switch on and/or off. If the local network 2 has to be disconnected from the grid due to a serious malfunction, the grid breaker relay reacts to the malfunction and drives the breakers. Figure 1 illustrates a normal situation wherein the local network is connected to the grid 1. As can be seen loads 5 and generator sets 4 in the local network are in connection with the grid. After the disconnection the local power network 2 with the loads 5 and the generator sets 4 forms a smaller power network being not in connection with the grid 1.

[0004] The generator set 4 is a combination of a generator and a prime mover. The prime mover rotates the generator that provides electric energy. The prime mover can, for example, be a reciprocating piston engine. The engines/generator sets are normally run in a control mode that is called a kW mode when they are in connection with the grid. In the kW mode a control device does not need to control the engine's speed (i.e. grid frequency as well), because the grid frequency is supervised elsewhere. In the kW mode the control device controls the fuel demand of the engine by comparing a load setting value (load reference value) to the measured engine load.

[0005] As said above if a problem occurs in the grid the local network may need to be disconnected from the grid. In this case the control of the engines must be changed from the kW mode to a speed droop mode. In the speed droop mode the engines in the local network control also the frequency of the grid. Therefore, for example, the fuel demand of the engine is controlled by comparing a speed reference and a measured engine speed.

[0006] Figure 2 illustrates the principle of the droop function in the speed droop mode. A line 21 is a droop curve that defines what is a speed setting for different loads. For example, in 100 % load the speed setting (speed reference) for the engine is the value of the y-axis at point 23. As can be seen in the figure the speed setting decreases when the load increases or vice versa. The point 22 at the droop curve in zero load (the point at the y-axis) is called speed set point for the droop curve. In other words, the droop function changes the speed reference for the speed control of the engine. The droop function is used because otherwise the engines do not share the load.

[0007] In addition to the speed droop control adjusts the speed reference for different loads it can be used for sharing loads between different engines. These features are utilized when the grid trip occurs. To provide the load sharing between the engines the slopes of the engine specific droop curves should be the same. The amount of load that the engine carries depends on the speed set point of the engine's droop curve. If the load of the local power network changes, the speed reference (the frequency reference as well) for the network is going to change as well. For offsetting the change of the speed reference i.e. returning the original speed (frequency) reference, the speed set points are adjusted.

[0008] The grid trip is usually associated with large load changes that affect on the engines. The engine load changes rapidly and therefore also the engine's speed changes rapidly. A usual situations is that the load decreases.

[0009] When changing from the kW mode to the speed droop mode the initial position of the speed droop curve is calculated in such a way that the curve goes through the point of a measured load value of the engine and a measured speed value. The measured speed value is used as an initial speed reference value in the droop curve. The load sensor/measurement is slow creating delay. The reaction of the grid breaker relay is also delayed. Therefore when the grid breaker relay reacts the speed of the engine has increased significantly that affects to the position of the speed droop curve. Yet further, the droop curve changes the speed reference higher in case where the load continues to decrease. Thus the engine overspeeds easily in this kind of situation. The engine goes into over-speed that may cause a shutdown of the engine and a black out of the local grid.

[0010] An example for changing a control mode of an engine of a generator set from a load mode to droop mode is disclosed in prior art document US2008314042A1.

Short description

[0011] The object of the invention is to alleviate or even eliminate the problems said above. The object is achieved in a way described in the independent claims. Dependent claims illustrate different embodiments of the invention.

[0012] In an event of disconnection of the local power network from the grid, the change of the control mode of the engine of the generator set in the local power network from the kW mode to the speed droop mode is arranged in a new way. The phase of changing comprises a delaying phase that in turn comprises parallel sub-phases to disable the droop function of the speed droop mode and keeping a speed reference value at a rated speed of the engine of the generator set. After the delaying phase an activation phase enables the droop function.

List of figures

[0013] In the following, the invention is described in more detail by reference to the enclosed drawings, where

Figure 1

illustrates an example of a connection arrangement between a main grid and a local network,

Figure 2

illustrates the principle of the speed droop mode,

Figure 3

illustrates an example of a control system to change the control of an engine from the kW mode to the speed droop mode according to the invention,

Figure 4

illustrates an example how the invention works at the grid trip situation, and

Figure 5

illustrates a flow chart example of the method according to the invention.

Description of the invention

[0014] Figure 3 illustrates an example of a control system according to the invention. In the kW mode 31 a control device 36 controls power output of the generator set 34, 35 by comparing a power reference value 38 with a measured power value 37 in the output of the generator 35 of the generator set. The measured power value is transmitted to the control device 36 via a feedback loop. The comparison can, for example, provide a difference between the power reference value and the measured value. The difference is used for forming a control signal that is transmitted to the engine 34 of the generator set. The control signal can, for example, adjust a fuel supply to the engine, which changes a power production of the generator set.

[0015] Normally, the kW mode cannot be used when the engine and the local network is disconnected from the grid. The control is changed to the speed droop mode in order that the engine can adjust the frequency of the local network with other possible engines that are connected to the local network. The main control unit 33 handles the change 318 of the control mode from the kW mode 31 to the speed droop mode 32. The changing is initiated when the main control unit receives info 314 about the disconnection. The disconnection info is transmitted, for example, from the grid breaker relay. In the speed droop mode 32 a speed control device 310 is used to keep the engine's speed as close as possible a speed reference value 312.

[0016] That is performed by comparing the speed reference value 312 with a measured speed value 39 in the output shaft of the engine 34 of the generator set. The measured speed value is transmitted to the control device 310 via a feedback loop. The comparison can provide a difference between the speed reference value and the measured value. The difference is used for forming a speed control signal that is transmitted to the engine 34 of the generator set. The control signal can, also in this case, adjust a fuel supply to the engine, which changes the speed of the output shaft of the engine.

[0017] In addition, there is a speed droop control device 311 that adjust the speed reference value 312 by providing a reference adjustment signal 317 that is transmitted to the speed control device 310. The adjustment signal can increase or decrease the speed reference value. The speed droop control device receives a power measurement 316 from the output of the generator 35. The functioning of the speed droop control has been described above.

[0018] Further, the grid trip control system according to the invention comprises a delaying unit 313 to disable a droop function of the speed droop control device 311 during a certain period, and a setting unit 315 to keep the speed reference value 312 in a rated speed of the engine of the generator set during said period. The disabling of the droop function can be achieved, for example, by keeping the droop % in zero percentage during the period. In other words, the droop curve is a horizontal line during the period. In the example of figure 3 the delay unit 313 is situated in the speed droop control device 311 and the setting unit 315 in the main control unit 33. However they can be situated in another way as well. For example, the delay unit can also be in the main control unit. After the delay period has expired, the system is arranged to enable the droop function. When the speed droop control device runs in the speed droop mode after the delay, it measures load of the generator set and utilizes the speed measurement of the engine for positioning the droop curve and offsetting the droop curve to the rated speed.

[0019] Figure 4 illustrates how the invention works. At moment 41 the main control device 33 detects the disconnection of the local network from the grid and the control mode is changed from the kW mode to the speed droop mode. The speed reference is kept in the rated speed of the engine during the delay period. The droop function is also disabled during the delay. As can be noted the grid and the local network has run at the reference speed or near. So, rated speeds of the engines are designed so that the speeds correspond the reference frequency of the power network through the generators of the generators sets.

[0020] The disabling of the droop function prevents the increase of the speed reference value and therefore alleviates the possibility that the engine races. Keeping the reference speed at the rated speed stabilizes the control during the delay period. The transients of the grid trip occur at the beginning of the grid trip. At least larger transients occur during the delay. After the delay has expired at moment 42, an activation phase begins when the droop function is enabled and the speed reference can be calculated according to the speed droop mode. The functioning during the delay period makes it possible that the control does not react too strongly to the transients and direct the engine to run at overspeed.

[0021] As can be seen in the example of figure 4 the engine's load 43 decreases due to the grid trip. The load starts to decrease before the control mode is changed to the speed droop mode at moment 41. The engine's speed 44 starts to increase before the moment 41 as well. The rated speed is a nominal speed of the engine. When running at the rated speed the frequency of the generators output is a desired frequency of the local network, for example 50 Hz or 60 Hz. During the delay the load decreases towards a state in the local network, and the speed control device drives the speed towards the rated speed.

[0022] The stabilized engine speed can, for example, be determined by utilizing a speed range around the rated speed value having upper and lower values 45. The engine speed may, for example, vary between +- 1% or +- 2% of the rated speed depending on the network and/or the engine itself. Speed can be defined to be stable is the engine speed is in the range. However, even if the engine speed is in the range it is practical, in addition, to have a stabilization time 46 during which period the engine speed is considered to be within the range. Figure 4 shows how the stabilization time 46 is in the delaying period. The stabilization time can, for example, be 1 - 10 seconds. If the engine speed remains within the range the delaying period can be ended. Otherwise, the delaying period is extended, for example, by another stabilization time, or the delaying period should be determined to be longer. The reference speed is kept or set to be at the rated speed during the whole delaying period.

[0023] After the delaying period has expired, the speed droop mode is enabled and the reference speed is not just kept at the rated speed, but it is calculated according to the speed rate mode making it possible to share the load between the other engines in the local network. The time of the delay is selected to take into account the engine's size and type, the transients and load change at the grid trip, and the properties of the speed droop control mode.

[0024] Figure 5 illustrates a flow chart example of the inventive method for controlling a grid trip in an event of disconnection of the local power network from the grid. The method concerns the phase to change the control mode of the generator set in the local power network from the kW mode to the speed droop mode. The phase of changing comprises a delaying phase 51 that comprises parallel sub-phases to disable 53 the droop function of the speed droop mode and keeping 54 the speed reference value at the rated speed of the engine of the generator set. After the delaying phase the method further comprises an activation phase 52 to enable 55 the droop function. A method can also comprise a speed range around the rated speed having upper and lower values 45. The speed of the engine is defined to be stable if the engine speed is in the range within a stabilization time 46. As Figure 4 illustrates the stabilization time belongs to the delaying phase 51.

[0025] The activation phase comprises functions to measure load of the generator set and speed of the engine, to position a droop curve using said measurements, and offsetting the droop curve to the rated speed.

[0026] It is important that the speed/frequency control of the engines works rapidly in order to keep the local power network on running. However as the load transients associated with grid trip event affect the engine's speed reference via the droop compensation. The invention disables the droop function until the load transients (at least the greatest) have passed. As the delay has expired the droop function runs.

[0027] It is evident from the above that the invention is not limited to the embodiments described in this text but can be implemented in many other different ways within the scope of the independent claims.

Claims

1. A method of controlling an internal combustion engine of a generator set in an event of disconnection of a local power network, which comprises the generator set, from a grid, the method comprising a phase to change a control mode of the generator set in the local power network from a kW mode to a speed droop mode, characterised in that the phase of changing from the kW mode to the speed droop mode comprises a delaying phase (51) that comprises parallel sub phases to prohibit (53) a droop function of the speed droop mode and keeping (54) a speed reference value at a rated speed of the engine of the generator set, and after the delaying phase the method further comprising an activation phase (52) to enable the droop function.

2. A method according to Claim 1, characterised in that method comprises a speed range around the rated speed having an upper and lower values (45), speed of the engine being defined to be stable if the engine speed is in the range within a stabilization time (46), the stabilization time belonging to the delaying phase (51).

3. A method according to Claim 1 or 2, characterised in that the activation phase (52) comprises sub phases to measure load of the generator set and speed of the engine, to position a droop curve using said measurements, and offsetting the droop curve to the rated speed.

4. A control system of an internal combustion engine of a generator set comprising a main control unit (33) that in an event of disconnection of a local power network, which comprises the generator set, from a grid is that can be arranged to change a control mode of the generator set in the local power network from a kW mode (31) to a speed droop mode (32), and further comprising a speed control device (310) and a speed droop control device (311), characterised in that the system comprises a delaying unit (313) to prohibit a droop function of the speed droop control device (311) during a certain period, and a setting unit (315) to keep a speed reference value at a rated speed of the engine of the generator set during said period, which system is arranged to enable the droop function after the period.

5. A system according to Claim 4, characterised in that the speed droop control device (311) is arranged to utilize a measurement of load of the generator set and a speed measurement of the engine for positioning a droop curve using, and to offset the droop curve to the rated speed.

Ansprüche

1. Verfahren zum Steuern eines Verbrennungsmotors eines Generatorsatzes bei einer Trennung eines lokalen Stromnetzes, welches der Generatorsatz umfasst, von einem Netz, wobei das Verfahren eine Phase umfasst, um einen Steuermodus des Generatorsatzes in dem lokalen Stromnetz von einem kW-Modus in einen Drehzahl-Droop-Modus zu ändern, dadurch gekennzeichnet, dass die Phase des Wechsels vom kW-Modus in den Drehzahl-Droop-Modus eine Verzögerungsphase (51) umfasst, die parallele Unterphasen umfasst, um eine Droop-Funktion des Drehzahl-Droop-Modus zu verhindern (53) und einen Drehzahlreferenzwert bei einer Nenndrehzahl des Motors des Generatorsatzes zu halten (54), und das Verfahren nach der Verzögerungsphase ferner eine Aktivierungsphase (52) umfasst, um die Droop-Funktion zu aktivieren.

2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass das Verfahren einen Geschwindigkeitsbereich um die Nenngeschwindigkeit mit einem oberen und unteren Wert (45) umfasst, wobei die Drehzahl des Motors als stabil definiert wird, wenn die Motordrehzahl im Bereich innerhalb einer Stabilisierungszeit (46) liegt, wobei die Stabilisierungszeit zur Verzögerungsphase (51) gehört.

3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass die Aktivierungsphase (52) Unterphasen umfasst, um die Last des Generatorsatzes und die Drehzahl des Motors zu messen, eine Droop-Kurve unter Verwendung der Messungen zu positionieren und die Droop-Kurve auf die Nenndrehzahl zu versetzen.

4. Steuerungssystem eines Verbrennungsmotors eines Generatorsatzes, umfassend eine Hauptsteuereinheit (33), die im Falle einer Trennung eines lokalen Stromnetzes, das den Generatorsatz umfasst, von einem Netz angeordnet werden kann, um einen Steuermodus des Generatorsatzes im lokalen Stromnetz von einem kW-Modus (31) zu einem Drehzahl-Droop-Modus (32) zu ändern, und ferner umfassend eine Drehzahl-Steuervorrichtung (310) und eine Drehzahl-Droop-Steuervorrichtung (311), dadurch gekennzeichnet, dass das System eine Verzögerungseinheit (313) umfasst, um eine Droop-Funktion der Drehzahl-Droop-Steuervorrichtung (311) während eines bestimmten Zeitraums zu verhindern, und eine Einstelleinheit (315), um einen Drehzahlreferenzwert bei einer Nenndrehzahl des Motors des Generatorsatzes während dem Zeitraum zu halten, wobei das System eingerichtet ist, um die Droop-Funktion nach dem Zeitraum zu aktivieren.

5. System nach Anspruch 4, dadurch gekennzeichnet, dass die Drehzahl-Droop-Steuervorrichtung (311) angeordnet ist, um eine Messung der Last des Generatorsatzes und eine Drehzahlmessung des Motors zum Positionieren einer Droop-Kurve mit und zum Versetzen der Droop-Kurve auf die Nenndrehzahl zu verwenden.

Revendications

1. Procédé de commande d'un moteur à combustion interne d'un groupe électrogène en cas de déconnexion d'un réseau électrique local, qui comprend le groupe électrogène, d'un réseau, le procédé comprenant une phase pour changer un mode de commande du groupe électrogène dans le réseau électrique local d'un mode kW à un mode de statisme de vitesse, caractérisé en ce que la phase de passage du mode kW au mode de statisme de vitesse comprend une phase de temporisation (51) qui comprend des sous-phases parallèles pour interdire (53) une fonction de statisme du mode de statisme de vitesse et maintenant (54) une valeur de référence de vitesse à une vitesse nominale du moteur du groupe électrogène, et après la phase de temporisation, le procédé comprenant en outre une phase d'activation (52) pour activer la fonction de statisme.

2. Procédé selon la revendication 1, caractérisé en ce que le procédé comprend une plage de vitesse autour de la vitesse nominale ayant des valeurs supérieure et inférieure (45), la vitesse du moteur étant définie pour être stable si la vitesse de moteur se situe dans la plage pendant un temps de stabilisation (46), le temps de stabilisation appartenant à la phase de temporisation (51).

3. Procédé selon la revendication 1 ou 2, caractérisé en ce que la phase d'activation (52) comprend des sous-phases pour mesurer la charge du groupe électrogène et la vitesse du moteur, pour positionner une courbe de statisme à l'aide desdites mesures, et décaler la courbe de statisme par rapport à la vitesse nominale.

4. Système de commande d'un moteur à combustion interne d'un groupe électrogène comprenant une unité de commande principale (33) qui, en cas de déconnexion d'un réseau électrique local, qui comprend le groupe électrogène, d'un réseau est qui peut être agencé pour changer un mode de commande du groupe électrogène dans le réseau électrique local d'un mode kW (31) à un mode de statisme de vitesse (32), et comprenant en outre un dispositif de commande de vitesse (310) et un dispositif de commande de statisme de vitesse (311), caractérisé en ce que le système comprend une unité de temporisation (313) pour interdire une fonction de statisme du dispositif de commande de statisme de vitesse (311) pendant une certaine période, et une unité de réglage (315) pour maintenir une valeur de référence de vitesse à une vitesse nominale du moteur du groupe électrogène pendant ladite période, lequel système est agencé pour activer la fonction de statisme après la période.

5. Système selon la revendication 4, caractérisé en ce que le dispositif de commande de statisme de vitesse (311) est agencé pour utiliser une mesure de charge du groupe électrogène et une mesure de vitesse du moteur pour positionner une courbe de statisme à l'aide, et pour décaler la courbe de statisme à la vitesse nominale.

Drawing