WO2002038963A1

WO2002038963A1 - Active compressor stability control

Info

Publication number: WO2002038963A1
Application number: PCT/NO2001/000443
Authority: WO
Inventors: Jan Tommy Gravdahl; Svein Ove Vatland; Håvard DEVOLD
Original assignee: Abb Research Ltd.
Priority date: 2000-11-08
Filing date: 2001-11-08
Publication date: 2002-05-16
Also published as: NO20005676D0; AU2002214418A1; NO20005676L; NO313926B1

Abstract

A method and a device for actively controlling surge in a rotational compressor (1), the compressor being driven by an electric drive (2) and controlled by an active surge control system (7). The active surge control system (7). The active surge control system comprises an active surge controller (4) which measures and/or observes at least one system state (M1... Mn, Vs) of the compressor (1), and calculates a commanded output of the electric drive (2) for maintaining a stable operation of the compress or. Further, the compressor sends a signal (Us) comprising the commanded output to the electric drive and adjusts the output of the drive in accordance with the commanded output.

Description

Active compressor stability control

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for actively controlling surge in a rotational compressor, the compressor being driven by an electric drive and controlled by an active surge control system. The invention also relates to an active surge control system for actively controlling surge in a rotational compressor.

BACKGROUND OF THE INVENTION Operation of a rotating compressor may become unstable due to changes in various operating conditions such as flow rate or pressure. This causes a rapid pulsation in the flow, which is called surge. Surge is a problem that arises in all types of rotating compressors, such as a centrifugal or axial compressor. If the mass flow through the compressor, for some reason, falls below a certain lower limit known as the surge line SL, the compressor will go into surge. This means that the flow and pressure will begin to oscillate. The compressor is now unstable. The amplitude of the oscillations depends on the compressor configuration and may be of such high amplitude that flow reversal will occur in the compressor. This phenomenon is known as deep surge. Surge is a highly unwanted phenomenon, and may cause severe damage to the compressor. Figure 1 shows an example of a compressor characteristic with the surge line. When the compressor is working on the right-hand side of the surge line in figure 1, the compressor is working in a stable area.

Various surge avoidance schemes are used to protect compressors, axial as well as centrifugal, against surge. A surge control line SCL is drawn in the stable area of the characteristic, figure 1. This line is drawn at a certain distance from the surge line SL. The distance is called the surge margin SM, and may be defined in a number of ways. One typical definition is to measure it in percent of the mass flow at which surge occur, and a typical choice is to set the surge margin at 10%. This means that if the compressor were operating on the surge control line, a 10 % reduction in mass flow would result in surge. In such a system, if the compressor operates to the left of the surge control line, measures will be taken to bring the operating point back to the right-hand side of the control line. Such measures include, for example, recycling of the flow, bleeding of the flow or downstream throttling of the flow.

In U.S. Patent No. 5,306,116, a blow-off valve is continuously modulated in order to eliminate surge. An incipient surge condition is detected when discharge pressure drops faster than a predetermined value. In case of changing operating conditions, compressor speed is manipulated in order to achieve a minimum desired surge margin. In U.S. Patent No. 4,464,720 a recycle valve is used to keep the operating point of the compressor to the right of the surge line. An algorithm that compares the actual differential pressure with a computed desired pressure controls the recycle valve. In U.S. Patent no. 5,553,997 the compressor speed and guide vane position are modulated in order to place the compressor operating point on a dynamic or adaptive surge control line.

The use of surge avoidance and surge margins prevents the compressor from being operated in the neighborhood of the surge line. Although this ensures stable operation of the machine the maximum efficiency of the compressor may not be taken advantage of while using a surge margin as the efficiency usually is at its maximum close to the surge line. Also, the maximum pressure rise of the compressor is reached close to the surge line, and will therefore not be reached using surge avoidance. The use of surge avoidance and surge margins according to the above examples also requires extra equipment to be installed, for example a recycle valve.

Active surge control is an alternative approach to deal with the problem of compressor surge. In active surge control, feedback of compressor system states is used to stabilize the unstable regime of the compressor map. Different ways of dealing with the problem of compressor surge, including surge avoidance and active surge control are discussed in an article by de Jager, B., "Rotating stall and surge control: A survey", Proceedings of the 35^th IEEE Conference on Decision and Control. New Orleans, LA. pp. 1857-1862.

U.S. Pat. No. 5,005,353 discloses a method for continuous active control of unsteady motion phenomena such as blade flutter, surge and rotating stall in turbo compressors. This is achieved by feeding back one or several measurements of the compression systems states, for example mass flow or pressure, via a control law to one or several actuators. A variety of actuators are proposed including for example loudspeakers, bleed valves, flow and heat injectors, and drives for variable aerodynamic elements such as stator vanes.

In designing an active surge controller for a compression system, and proving its stability, there is a need for a model of the system to be controlled. Design of active control systems for compressors usually employs the model of Greitzer E.M., described in "Surge and rotating stall in axial flow compressors, Part I: Theoretical compression system model and Part II: Experimental results and comparison with theory",

Journal of Engineering for Power, vol. 98, pp. 190-217, or the model of Moore-Greitzer described in Moore F.K. and Greitzer E.M., "A theory of post-stall transients in an axial compression system: Part I—development of equations.", Journal of Engineering for Gas Turbines and Power., vol. 108, pp 68—76. In the above-mentioned patent, U.S. Pat. No. 5,005,353, the Moore- Greitzer model is used. Common to known active control systems for compressors is that the compressor rotational speed is considered constant when designing the control law. The assumption of constant rotational speed is made both in the Greitzer and Moore-Greitzer models.

A compressor under active surge control has effectively a shifted surge line, and the advantages of this is for example:

- Operation in the area of maximum efficiency is made possible. - Operation at maximum pressure rise is made possible.

- The range of mass flows over which the compressor may be operated without going into surge is also extended. In figure 1 the extended operating range EOR using active control is compared with the operating range OR when surge avoidance is used.

Although active surge control using the actuators listed above overcomes most serious drawbacks of the methods mentioned above regarding surge avoidance, the need for actuators, such as bleed valves or others, still remains.

In e.g. compression stations on pipelines for gas or fluid transport, recycle valves are commonly used for surge avoidance. These valves could also be used for active control, but it is desired to minimize the use of the recycle valves. Other actuators also suffer from the drawback that this is extra equipment having to be installed in the compression system. This adds cost and complexity to the compression system. For a long time there has been a need in the industry for achieving active surge control in a compression system without having to install extra actuator equipment.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for actively controlling surge in a compressor driven by an electric drive and controlled by an active surge control system, without the disadvantages mentioned above under background of the invention. This object is achieved by the active surge control system comprising an active surge controller performing said method comprising the following steps measuring and/or observing input data of at least one system state, Mi ... M_n, v_s, of the compressor,

- calculating a commanded output of the electric drive for maintaining a stable operation of the compressor,

sending a signal comprising the commanded output to the drive, and

adjusting the output of the drive in accordance with the commanded output.

A compressor characteristic shows the compressor pressure ratio versus the mass flow of fluid passing through the compressor. In the compressor characteristics a surge line, SL, is drawn. When the compressor is working on the right-hand side of the surge line, the compressor is working in a stable area. A drop in mass flow moves the operating point towards the surge line and to an unstable area on the left-hand side of the surge line. A surge control line SCL is drawn in the stable area of the characteristic at a certain distance from the surge line SL. This distance is called the surge margin SM. According to a preferred embodiment of the invention, the active surge controller comprises at least one processor to continuously carry out the steps of

- measuring and/or observing input data of at least one system state, M ... M_n, v_s, of the compressor,

calculating a commanded output of the electric drive for maintaining a stable operation of the compressor,

sending a signal comprising the commanded output to the electric drive, and

adjusting the output of the electric drive in accordance with the commanded output .

According to another preferred embodiment of the invention, the adjusted output of the drive is drive torque, drive speed or drive power.

According to another preferred embodiment of the invention, the measured system states are at least one of an upstream system state i ... M_m, a downstream system state M_m+ι ... M_n and an internal system state. The measured system states comprise at least one of compressor rotational speed, flow and pressure.

According to another preferred embodiment of the invention, the active surge controller comprises a surge control algorithm which is based on the measured system states M ... M_n and on estimated states Oi ... O_k of the system states Mi ... M_n, where the estimated states are calculated by an observer. The surge control algorithm is designed by using a compression system dynamic model that takes account of varying compressor shaft rotational speed and the drive manipulating the rotational speed of the compressor. For example, the model presented in "Compressor surge and rotating stall, Modeling and Control", Springer-Verlag, London, 1999, by Gravdahl J.T. and Egeland 0. could be used. This model takes non-constant rotational speed into account and is an extension of the Greitzer model, mentioned under the background of the invention.

According to another preferred embodiment of the invention, the active surge control system comprises a compressor performance controller comprising a compressor performance algorithm. The compressor performance algorithm controls a set point level u_sp of a primary process variable, such as, for example, pressure rise. The compressor performance algorithm sends a signal u_p to the electric drive with a commanded set point. The active surge controller has access to the commands from the performance controller and vice versa.

The signal u_s from the active surge controller is based on feedback from the at least one system state Mi ... M_n in addition to the at least one estimated state Oχ ... O .

Another object of the present invention is to provide a compressor system comprising a surge control system for actively controlling surge in a rotational compressor, the compressor being driven by an electric drive. The active surge control system comprises an active surge controller which is arranged to measure at least one system state Mi ... M_n of the compressor, and to calculate the commanded output of the electric drive for maintaining a stable operation of the compressor. The active surge controller is arranged to send a signal u_s to the drive with the calculated commanded output and to adjust the output of the drive in accordance with the commanded output.

According to a further embodiment of the invention, a recycle valve (8) and the drive (2) are used for active control of the compressor. Another object of the present invention is to provide a computer program product, comprising computer code means or software code portions for enabling a computer or a processor to carry out a method for actively controlling surge in a compressor driven by an electric drive.

Another object of the present invention is to provide a computer program contained at least in part in a computer readable medium, comprising computer program code means to make a computer or processor carry out the following steps

- measuring and/or observing at least one system state, Mi ... M_n, v_s, of the compressor,

sending a signal comprising the commanded output to the drive,

- adjusting the output of the drive in accordance with the commanded output .

The method according to the present invention may be used in, for example, a compression station on pipelines for gas or fluid transport.

The present invention allows the compressor to operate at maximum efficiency and pressure by making the surge margin unnecessary. Also, the range of mass flows over which the compressor may be operated is extended as the present invention ensures that the compressor does not go into surge even if the compressor operates to the left of the surge line SL. In addition, less or smaller auxiliary process equipment, such as recycle valves for surge protection, is necessary, thereby saving cost and space. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained with reference to the appended drawings, wherein Fig. 1 is an example of a compressor characteristic showing a surge line, a surge control line and a surge margin.

Fig. 2 is a schematic block diagram of a compressor with a drive and a control system according to the present invention.

Fig. 3 is a schematic block diagram of a compressor with a drive, a recycle valve and a control system according to an embodiment of the invention.

Fig. 4 shows the influence of a preferred embodiment on the compressor characteristic.

Fig. 5 is an illustration of a compressor response due to 35% drop in mass flow caused by a disturbance.

Fig. 6 is a detailed plot of effects from the mass flow disturbance showed in figure 4.

Fig. 7 shows a simulation of surge plotted together with a compressor characteristic. Fig. 8 shows a simulation according to fig. 4, but with an active control system.

Fig. 9 shows a simulation of active surge control where the new operating point in the previous unstable region remains stable.

Fig. 10 shows different disturbances used in a simulation of an active surge control.

Fig. 11 shows a simulation of active surge control with disturbances .

Fig. 12 shows a simulation of active surge control with disturbances plotted together with a compressor characteristic.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Figure 2 shows a schematic block diagram of a rotational compressor 1 with a drive 2 and an active control system 7 comprising an active surge controller 4 according to the present invention. The rotational compressor is an axial or centrifugal compressor driven by a shaft 3 connected to the drive 2. The drive is an electric motor.

The active surge controller 4 in figure 2 comprises an active surge control algorithm, implemented in software, which uses measurements Mι,...,M_n of at least one system state. To carry out the active surge control, the active surge controller 4 comprises at least one processor or computer. The active surge control algorithm calculates the torque, speed or power of the drive necessary to keep the compressor out of surge. The active surge-control algorithm is derived using non-linear control theory, and it is capable of stabilizing the previous unstable equilibrium points to the left of the surge line SL. The active surge control algorithm has the following form:

u_s=f (Mι,...,M_n,Oι,...,O_k, U_p)

where u_s is a control signal to the drive, commanding a change in the output of the drive when a disturbance is changing the operating characteristics of the compressor. The choice of commanded speed, torque or power depend on the nature of the drive governor. The function f may be non-linear in its arguments, and a mathematical model of the compression system is used to derive the function f. This mathematical model is a part of the active surge controller 4. An example on how to derive this mathematical model is shown in "Compressor surge and rotating stall, Modeling and Control", Springer-Verlag, London, 1999, by Gravdahl J.T. and Egeland 0. The control system uses measurement of n states for feedback. The measurements Mi to M_m are measured upstream of the compressor, while the measurements M_m+ι to M_n are measured downstream of the compressor. In addition, measurements may be performed internally in the compressor. Typical measurements are, for example, temperature, mass flow or volumetric flow, pressure, density, molecular weight, shaft speed and/or torque.

The surge control system 7 comprises a compressor performance controller 5, which comprises a compressor performance control algorithm ensuring that a primary process variable, e.g. pressure rise, is maintained at its set point level u_sp. The performance controller 5 achieves this by commanding a set point Up to the drive 2 or by manipulating some additional actuator, such as a valve or guide vane.

In the event that not all of the necessary states are available for measurement, they are estimated by the use of an observer 6. The observer comprises an observer algorithm, implemented in software, which estimates process variables that are unavailable for measurement. Inputs to the observer comprise the measurable system states Mι,...,M_n, the outputs u_s from the active surge controller 4 and the outputs u_p from the performance controller 5.

The k observed states are termed Oι,...,O_k in figure 2. Solid lines in figure 2 represent signal paths that must be present, while at least one of the signal paths represented by dashed lines must be present.

The drive torque or speed from the drive 2 is calculated in part by the active surge control algorithm and in part by the performance control algorithm. The resulting compression system consisting of the compressor 1, the drive 2, the performance controller 5 and the active surge controller 4 is capable of operating on the left-hand side of the surge line without going into surge.

According to a preferred embodiment of the invention, the drive is equipped with a control system that allows for input of commanded speed, torque or power from the active surge controller 4 and the performance controller 5. In order to avoid conflicting of commands, the surge controller 4 has access to the commands from the performance controller 5 and vice versa.

In a further embodiment of the invention, figure 3, the control system comprises a recycle valve 8, and the active surge controller 4 gives commands u_s, u_r both to the electrical drive 2 and the recycle valve 8. This makes it possible to use a smaller recycle valve, i.e. reduced dimensions and reduced recycle flow, than in a conventional surge avoidance system.

EXAMPLES

In the following a proposed control scheme for carrying out the invention is simulated. In the first example it is shown that the model is capable of demonstrating surge. This is accomplished by imposing a mass flow disturbance to a model of a pipeline compressor for natural gas, driving the operating point over the surge line. The resulting surge oscillations are clearly visible in the simulation. In the second and third examples the present invention is used to stabilize the compressor in the previous unstable area of the compressor characteristics, that is, to the left of the surge line. The relevant parameters for the system are: Natural gas: c = 2064X^J— kgK where;

K = Cp/ C_v

Cp = Specific heat of gas at constant pressure c_v = Specific heat of gas at constant volume At the design point, the conditions are:

m=100^ s p_n = 60 bar = 60 * 10⁵ Pa T_n = 293.15 K = 0°C π_c = 1.5 where; = Mass flow p = Plenum pressure

T = Temperature π_c = Pressure ratio

In the examples the compressor has the main dimensions in table

1.

Table 1

The design was chosen to satisfy the boundary conditions of an electric drive with a specification according to table 2.

Table 2

EXAMPLE 1

A conventional compressor system is simulated when it is driven into surge by a drop in mass flow. The compressor response to this disturbance is shown in figure 4. A constant drive torque is used. The compressor undergoes deep surge with oscillations in mass flow, pressure rise and shaft speed. The compressor is initially operating in a stable mode at = lOOkg/s. When a drop in mass flow of 35% in about Is occurs at t=5s, it results in deep surge. A constant drive torque of τ_d = 7957 Νm, which is the maximum torque of the drive, is used at all times. The destabilizing mass flow drop is shown in figure 5, where also a more detailed plot of the surge oscillation in mass flow and pressure rise is shown. The drop in mass flow is shown in the upper plot of figure 5. The two lower plots of figure 5 show the surge cycles in mass flow and pressure rise in more detail. As can be seen, the surge frequency is about 1.6 Hz, which is typical for a compressor of this size. In figure 6, the surge cycle is clearly visible in the compressor characteristics. The operating line OL and the surge line SL are shown in figure 6. The compressor becomes unstable when its operating point crosses the surge line, in figure 6 the speed oscillates around 11250 rpm. The reason for the higher rotational speed of the compressor during surge is that the drive torque is kept constant during simulation, and the compressor load torque is lower for this lower- mass flow.

EXAMPLE 2

The compressor system is simulated when the compressor is controlled with the active surge control system according to the present invention. The compressor speed is controlled with feedback from mass flow and speed so that the compressor operates in a stable mode even to the left of the surge line, thereby avoiding the unstable operation shown in example 1 above.

This active surge controller 4 is implemented with a commanded drive-torque according to the following:

τ_A = u -k_NAω ^■ ^■ k„Λm - - k_j \Aωdt

where: k_N is a constant gain,

Δω is the deviation (from the operating point) in rotational speed, k_m is a constant gain,

Δ is the deviation (from the operating point) in mass flow rate, k_j is a constant gain This equation describes the mechanism behind the stabilization. The terms - k_NAω - kj \Aωdt may be regarded as part of the performance contro system, keeping the compressor at the desired rotational speed. The term -k_mAm contributes to the surge stabilization by increasing drive torque, and thereby the speed, when Am is negative, that is

™_ac,_uai < m_des,_red . When Am is positive, that is m_actual < m_desιred , the term - k_mAm contributes by decreasing the drive torque, and thereby the speed.

In the unstable area of the compressor map, the slopes of the constan speed lines are positive, and in the stable area, the slopes of the constant speed lines are negative. This is a well-established fact in the literature (see e.g. [deJager95] or [Greitzerl976] ) . For an operating point that is nominally unstable and located to the left of the original surge line, the term -k_mAm controls the speed in such a that the compressor experiences speed lines with negative slopes, and therefore does not enter surge.

The integral term k_} \Aωdt is included in order to keep the compressor at a desired speed. This may be regarded as part of the performance control system.

The controller is active at all times, and as the mass flow drop is introduced at t=5s, the compressor remains stable. This is shown in figure 7, where the mass flow, pressure rise, shaft speed and drive torque are plotted as a function of time. The simulation is also shown in figure 8, where a disturbance is driving the initial operating point IOP over the surge line to the left-hand side. The new operating point NOP on the left- hand side of the surge line is stabilized with the active surge controller, and no surge oscillations occur. EXAMPLE 3

In the following the compressor system is simulated when the compressor is controlled with the active surge control system 7 and process disturbances are introduced to the system. The disturbances used in this example, i.e. the amplitude frequency, are considered very rough to the system. Process disturbances in the form of mass flow fluctuations, measurement noise in the mass flow measurement and a time delay in the measurement will be considered:

- The mass flow drop of 35% that drives the compressor into surge takes place at a time scale of Is. In the simulations this is a step which is filtered through a time constant T=l. A typical disturbance in a gas pipeline might be a 10% drop in flow rate over a period of 5 minutes.

The measurement noise is implemented in the simulations as band limited white noise with a power of 0.20 and a sampling interval of 0.010s. This gives a measurement error in the range of ± lOkg/s. The time delay for the mass flow measurement is set at 0.050s. Figure 9 shows the different disturbances that the compressor is experiencing in the simulations. The upper plot in figure 9 is the 35% drop in mass flow driving the compressor into surge. The middle plot is the process disturbances and the lower plot is the measurement noise.

As may be seen in figures 10 and 11, the active surge controller keeps the compressor stable. The controller is active at all times, and as the mass flow drop is introduced at t=5s, the compressor remains stable. This is shown in figure 10, where mass flow, pressure rise, shaft speed and drive torque are plotted as a function of time. The simulation is also shown in figure 11, where a disturbance is driving the initial operating point IOP over the surge line to the left side. The new operating point NOP on the left-hand side of the surge line is stabilized with the active surge controller, and no surge oscillations occur.

Claims

C l a i m s

1. A method for actively controlling surge in a rotational compressor (1) , wherein the compressor is driven by an electric drive (2) and controlled by a surge control system (7), characterized in that the surge control system (7) comprises an active surge controller (4) which performs the following steps in order to stabilize the compressor that otherwise would be unstable :

- measuring parameters representing the system state (Mi ... M_n, v_s) of the compressor (1) ,

calculating from the measured parameters a commanded output, τ_d, of the electric drive (2),

sending a signal (u_s) comprising the commanded output to the electric drive, and

adjusting the output of the electric drive in accordance with the commanded output for maintaining a stable operation of the compressor at an operating point.

2. A method according to claim 1, characterized in that the active surge controller (4) calculates the commanded output τ_d of the electric drive (2) by means of the equation: τ_d = u = -k_NAω - k_mAm - k, \Aωdt, where: k_N is a constant gain,

Aω is the deviation (from the operating point) in rotational speed, k_m is a constant gain, Δm is the deviation (from the operating point) in mass flow rate, k; is a constant gain.

3. A method according to claim 1 or 2, characterized in that the active surge controller (4) comprises at least one processor to continuously perform the steps of measuring the parameters of the compressor (1), calculating the commanded output of the electric drive (2) , sending a signal (u_s) comprising the commanded output to the electric drive and adjusting the output of the electric drive in accordance with the commanded output .

4. A method according to one of the preceding claims, characterized in that the output of the electric drive is one of drive torque, drive speed and drive power.

5. A method according to one of the preceding claims, characterized by measuring at least one system state (Mi ... M_m) upstream of the compressor.

6. A method according to one of the preceding claims, characterized by measuring at least one system state (M_m+ι ... M_n) downstream of the compressor.

7. A method according to one of the preceding claims, characterized by measuring at least one internal system state

(v_s) .

8. A method according to claim 5, 6 or 7, characterized in that the measured system states are at least one of compressor rotational speed, mass flow and downstream pressure.

9. A method according to one of the preceding claims, characterized in that the active surge controller (4) comprises a surge control algorithm which is based on the measured system states (Mi ... M_n) and on estimated states (Oi ... O_k) of the system states (M ... M_n) , where the estimated states are calculated by an observer (6), and where the surge control algorithm calculates the commanded output (u_s) to the electric drive.

10. A method according to one of the preceding claims, characterized in that the surge control algorithm is designed by using a compression system dynamic model that takes account of varying compressor shaft rotational speed.

11. A method according to one of the preceding claims, characterized in that the surge control system (7) comprises a compressor performance controller (5) comprising a compressor performance algorithm which controls a set point level (u_sp) of a primary process variable, and that the compressor performance algorithm sends a signal (u_p) with a commanded set point to the electric drive.

12. A method according to one of the preceding claims, characterized in that the active surge controller (4) has access to the commands from the performance controller and vice versa.

13. A method according to one of the preceding claims, characterized in that the signal (u_s) from the system surge controller (4) is based on feedback from the at least one system state (Mi ... M_n) in addition to the at least one estimated state (Oi ... Ok) .

14. A method according to one of the preceding claims, characterized by adjusting at least one recycle valve (8) and the drive (2) for actively controlling the compressor.

15. A compressor system comprising a surge control system (7) for actively controlling surge in a rotational compressor (1) , the compressor is driven by an electric drive (2) , characterized in that the surge control system (7) comprises an active surge controller (4), where the active surge controller (4) is arranged to measure input data of at least one system state (Mi ... M_n, v_s) of the compressor (1) , and to calculate the commanded output of the electric drive (2) for maintaining a stable operation of the compressor, the active surge controller (4) being arranged to send a signal, u_s, to the electric drive with the calculated commanded output and to adjust the output of the electric drive in accordance with the commanded output.

16. A compressor system according to claim 14, characterized in that the active surge controller (4) comprises at least one processor arranged to continuously perform the steps of measuring and/or observing the input data of the compressor (1) , to calculate the commanded output of the electric drive (2), to send a signal (u_s) comprising the commanded output to the electric drive and to adjust the output of the electric drive in accordance with the commanded output.

17. A compressor system according to claim 14 or 15, characterized in that the output of the electric drive is one of drive torque, drive speed and drive power.

18. A compressor system according to any of claims 14-16, characterized in that the at least one of the measured system states (Mi ... M_n) is the system states (Mi ... M_m) upstream of the compressor.

19. A compressor system according to any of claims 14-17, characterized in that the at least one of the measured system states (Mi ... M_n) is the system states (M_m+ι ... M_n) downstream of the compressor.

20. A compressor system according to any of claims 14-18, characterized in that the at least one of the measured and/or observed system states is an internal system state (v_s) .

21. A compressor system according to claim 17,18 or 19, characterized in that the system states are at least one of compressor rotational speed, mass flow and downstream pressure.

22. A compressor system according to any of claims 14-20, characterized in that the active surge controller (4) comprises a surge control algorithm which is based on the measured system states (Mi ... M_n) and on estimated states (Oχ ... O_k) of the system states (Mi ... M_n) , where the estimated states are calculated by an observer (6), and where the surge control algorithm is arranged to calculate the commanded output (u_s) to the electric drive.

23. A compressor system according to any of claims 14-21, characterized in that the active surge control algorithm is designed by using a compression system dynamic model that takes account of varying compressor shaft rotational speed.

24. A compressor system according to any of claims 14-22, characterized in that the active surge control system comprises a compressor performance controller (5) comprising a compressor performance control algorithm arranged to control a set point level (u_sp) of a primary process variable, and that the compressor performance control algorithm sends a signal (u_p) with a commanded set point to the electric drive.

25. A compressor system according to any of claims 14-23, characterized in that the active surge controller (4) has access to the commands from the performance controller and vice versa.

26. A compressor system according to any of claims 14-24, characterized in that the signal (u_s) from the active surge controller (4) is based on feedback from the at least one system state (Mi ... M_n) in addition to the at least one estimated state (Oi ... O_k) .

27. A compressor system according to at least one of the claims 14-25, characterized in that at least one recycle valve (8) and the drive (2) are adjusted for active control of the compressor.

28. A computer program product, comprising computer code means or software code portions for enabling a computer or processor unit means to carry out a method according to any of claims 1- 13.

29. A computer program contained at least in part in a computer readable medium, comprising computer program code means to make a computer or processor carry out the steps according to any of claims 1-13.

30. Use of a method according to any of claims 1-13 in a pipeline for gas for stabilizing the operation of a compressor in an unstable area of the compressor characteristics.