WO2017064820A1

WO2017064820A1 - Electric power generation system and its control system

Info

Publication number: WO2017064820A1
Application number: PCT/JP2015/079905
Authority: WO
Inventors: Ko Thet Aung; Noriaki Hino; Tomofumi Shiraishi; Naohiro Kusumi; Masatoshi Yoshimura
Original assignee: Hitachi, Ltd.
Priority date: 2015-10-13
Filing date: 2015-10-13
Publication date: 2017-04-20

Abstract

An electric power generation system (1) according to the present invention has a power generator (4) rotating by a turbine (304), a power assisted system (2) adjusts an electrical power on a side of a power system (5), wherein the power assisted system has a frequency converter (201) electrically connected between an electrical output of the generator and an energy conversion system (202) taking an electrical energy in or out, and a control system (203) controlling the frequency converter to adjust a power on a side of an output from the frequency converter. The control system includes a feedback-control portion which generates a first command based on a feedback of the power and a feedforward-control portion which outputs a second command based on the power, and the power is controlled by the first and second commands.

Description

Title of the invention: Electric Power Generation System And its Control System

Technical field

0001

The present invention relates to an electric power generation system and a control system used in the electric power generation system.

Background Art

0002

Due to an operating area of an electric power system that has been expanding and comprises many generators, transmission lines and variety of load patterns, a potential of occurring disturbance in the power system increases as the power system moves toward higher capacity and longer distances in power transmission and uncertainty in generation schedule and load variation. Maintaining the stability is one of objectives of the power system operation and an important issue.

0003

Nowadays, integration of an electricity generation from renewable energy resources into the power system is increasing. As the renewable energy resources have variable nature, the electricity generation from the renewable energy resources has characteristics of output power fluctuation and therefore, a gas turbine power generation system is interested to follow fast-load variations due to fast-power ramp rating capability.

0004

A prior art related to the gas turbine power generation system is disclosed by WO2014/020772A1 (Patent Literature 1). The Patent Literature 1 describes that a dual-shaft gas turbine power generation system includes an electric motor connected to a shaft of a compressor and a frequency converter, which drives the motor, connected between the motor and a synchronous power generator connected to a shaft of a low pressure turbine. An air-flow to a burner is controlled by a rotation of the compressor assisted by the motor for variation in an outside air temperature. Consequently, the gas turbine is continuously operated in high efficiency without reducing fuel burned in the burner. Citation List

Patent Literature

0005

Patent Literature 1: WO2014/020772A 1 Summary of Invention

Technical Problem

0006

However the dynamics of the response of change in active power and reactive power by the frequency converter is not enough to compensate a rapid load variation in the power system. Therefore, the rapid load variation causes an oscillation of a voltage and a rotational speed of the synchronous power generator even if the assisted system is operated.

0007

Accordingly, it is an object of the invention to provide an electric power generation system having a power assisted system of which an operation speed is improved and a control system used in the electric power generation system.

Solution to problem

0008 In order to solve the above mentioned problem, an electric power generation system according to the present invention has an electric power adjusting by means of frequency converter system, of which an operation speed is improved, is disclosed. The electric power generation system includes a power generator rotating by a turbine, a power assisted system adjusts an electrical power on a side of a power system, wherein the power assisted system has a frequency converter electrically connected between an electrical output of the generator and an energy conversion system taking an electrical energy in or out, and a control system controlling the frequency converter to adjust a power on a side of an output from the frequency converter. The control system includes a feedback-control portion which generates a first command based on a feedback of the power and a feedforward-control portion which outputs a second command based on the power, and the power is controlled by the first and second commands.

0009 Additionally, A control system for an electric power generation system including a power generator rotating by a turbine; a power assisted system adjusts an electrical power on a side of a power system, wherein the power assisted system has a frequency converter electrically connected between an electrical output of the generator and an energy conversion system taking an electrical energy in or out, and controlling the frequency converter to adjust a power on a side of an output from the frequency converter, includes a feedback-control portion which generates a first command based on a feedback of the power and a feedforward- control portion which outputs a second command based on the power The power is controlled by the first and second commands.

Advantageous Effects of Invention

0010

An electric power generation system and a control system according to the present invention result that an operation speed of a power-assisted system in the electric power generation system is improved. Consequently, oscillations of a rotational speed and a voltage due to a rapid fluctuation of a load are suppressed.

0011 Other objects, features and advantages of the invention will appear from the following description with the accompanying drawings.

Brief description of Drawings

0012 - - Fig. 1 illustrates an outline of an electric power generation system as an embodiment according to the present invention.

Fig. 2 illustrates an example of a circuit configuration of the frequency converter.

Fig. 3 illustrates a block diagram of the electric power generation system.

Fig. 4 illustrates a block diagram of the preceding power control system in the embodiment.

Fig. 5 illustrates a preceding active power control unit and a preceding reactive power control unit in the first embodiment.

Fig.6 illustrates a flowchart showing processes of the preceding power control system operating in a rotor vibration preference mode.

Fig.7 illustrates a flowchart showing processes of the preceding power control system operating in a voltage stability preference mode.

Fig. 8 illustrates examples of waveforms of a load variation, an active power variation which the power-assisted system outputs, a voltage of a power generator and a rotational speed variation.

Fig. 9 illustrates a block diagram of the conventional PI power control unit.

Description of Embodiments

0013

Preferred embodiments of the present invention directing to an electric power generation system will be described.

0014 Fig. 1 illustrates an outline of an electric power generation* system as an embodiment according to the present invention.

0015

The electric power generation system [1] has a power-assisted system (PAS) [2] to adjust .an electric power for a power system [5] by an electrical energy conversion system [202] consuming or generating an electric power, a turbine [3] and a generator [4], which outputs an electric ^■ power to the power system [5], rotating by the turbine [3].

0016

This power generation system [1] is connected electrically to the power system [5] to supply an electrical power for the power system [5]. The electrical power is supplied mainly by the generator [4]. Additionally, the electrical power is adjusted partly by the power-assisted system [2]. The power-assisted system [2] includes the energy conversion system [202] such as battery or an electric machine, a frequency converter [201], which is connected electrically between the energy conversion system [202] and an electrical output of the generator [4], and a preceding power control system [203] controlling the frequency converter [202] to suppress an oscillation of a voltage and a rotational speed U)_GEN of the generator [4]. Additionally, the power conversion system [202] takes an electrical energy in or out for charging, discharging, generating or utilizing the electrical energy.

0017

The mechanical energy or chemical energy stored in energy conversion system [202] can be transformed into the electrical energy and this electrical energy can be transported to the power system [5], when the frequency converter [201] operates varying the electrical frequency or voltage or both. These operations of the power-assisted system [2] including the energy conversion system [202] adjust an electric power including an active power and a reactive power for the power system [5] so as to increase a power capability of electric power generation system and to improve an active power response of it for voltage and frequency stability of the power system.

0018

Fig. 2 illustrates an example of a circuit configuration of the frequency converter [201] in Fig. 1. The frequency converter [201] is so-called "BTB (back to back)" . One of power conversion circuits [2010, 2011] such as three-phase bridge circuits is connected in series to the other of them through a DC link circuit [2012] having a smoothing capacitor [2013].

0019

When the energy conversion system [202] has an electric machine operating as a variable-speed motor, the power conversion circuit [2010] on the rotary electric machine side operates as an inverter converting a DC power of the DC link circuit [2012] to an AC power supplied to the rotary electric machine in the energy conversion system [202], and the power conversion circuit [2011] on the power system side operates as a converter converting an AC power of the power system side to the DC power of the DC link circuit [2012]. When the energy conversion system [202] has an electric machine operating as a generator, the power conversion circuit [2010] on the electric machine side operates as an converter converting an AC power generated by the rotary electric machine [202] to a DC power of the DC link circuit, and the power conversion circuit [2011] on the power system side operates as an inverter converting the DC power of the DC link circuit [2013] to an AC power supplied to the power system side.

0020

Additionally, power semiconductor switching devices such as IGBTs (Insulated Gate Bipolar Transistors) in the power conversion circuits [2010, 2011] are controlled with PWM (Pulse Width Modulation). The PWM signals to the power semiconductor switching devices are generated under a control of the preceding power control system [203] for suppression of the voltage and rotational speed oscillations of the generator [4]. Consequently, the rotary electric machine in the energy conversion system [202] can operate as a variable speed motor and it can operate as a generator supplying an electric power on a frequency of the power system [5] to the power system side. In this manner, the power- assisted system [2] can adjust the electric power for the power system [5] by the rotary electric machine in the energy conversion system [202] consuming or generating an electric power.

0021

Gate Turn Off Thyristors (GTOs) and Power MOSFETs can be applied as the power semiconductor switching devices instead of the IGBTs.

0022

Fig. 3 illustrates a block diagram of the electric power generation system [1]. The turbine [3] converts a heat energy of high-temperature and high-pressure to a mechanical energy for driving the generator [4]. The generator [4] converts the mechanical energy from the turbine to an electric power.

0023

In Fig.3, a motor-drive system [204] in the power-assisted system [2] includes the rotary electric machine in energy conversion system [202] and the frequency converter [201] shown in Figs.1 and 2. The frequency converter [201] in the motor drive system [204] has a capability to control a reactive power QMAS and an active power P_MAS of the power-assisted system [2] on the power system [5] side with using PWM signals S_PWM output by the preceding power control system [203]. The preceding power control system [203] generates the PWM signals S_PWM for the frequency converter [201] based on a voltage V AS and a current I_MAS of the power- assisted system [2] on the power system side, a speed U)_GEN of the generator [4] detected by a position sensor [500] coupled to the rotating shaft [412] of the generator [4] such as a rotary encoder and an operation mode set to the preceding power control system [203].

0024

The voltage V_MAs and the current l_MAs are detected by a voltage sensor and a current sensor respectively, not shown in Fig. 3. These sensors are provided on the power system side of the frequency converter [201] in the motor-drive system [204]. Additionally, a voltage V and a current I are detected by a voltage sensor [206] and a current sensor [205] respectively, provided to an output of the electric power generation system [1]. 0025

According to the afore-mentioned manner, the motor-drive system [204] exchanges the power between the electric power system [5] and energy conversion system which could be a pump or compressor in the power plant. Therefore, auxiliary energy flow can be controlled by operation of the rotary electric machine [202] in the power-assisted system [2], between output of the generator [4] and energy conversion system. As the energy conversion system in the case of having capability of storing kinetic energy, or in the case of having stored electromechanical energy, the fast response of active power adjustment is done by means of active power control in the power-assisted system [2].

0026

Moreover, the frequency converter [201] of the power-assisted system [2] is controlled by the preceding power control system [203] that controls power of frequency converter [201] in the motor-drive system [204] for suppression of the rotational speed oscillation of the generator [4] in a rotor vibration preference mode (Fig. 6) and for suppression of the voltage oscillation of the generator [4] in a voltage stability mode (Fig. 7). These modes are set to the preceding power control system [203] as the operational mode in Fig.3.

0027

0028 A controller [2033] generates a reference of an active power AP_REF and a reference of a reactive power AQ_REF according to a fluctuation of a load APLOAD in the power system [5] and an operation mode. An information on APLOAD is given to the preceding power control system [203] from an outside. For example, the information is a load dispatch instruction given by a load dispatch center.

0029

A power calculator [2034] calculates an active power APMAS and a reactive power AQMAS of the power-assisted system [2] from the detected voltage V_MAS and current l_MAS output by the frequency converter [201] in the power-assisted system [2].

0030

Here, the notation "Δ" means a compensation or a variation. AP_REF, AQREF, APMAS and AQ _AS can be changed to P_REF, QREF, PMAS and Q_MAS respectively.

0031

ΔΡ R _E F and AP AS are provided to an active power control unit (APR) [2031], and AQ _REF and AQM_AS ^are provided to a reactive power control unit (AQR) [2032]. The active power control unit [2031] outputs a command Si on the basis of the provided Δ P _{R E F} and AP^AS_, and the reactive power control unit [2032] outputs a command S₂ on the basis of the provided Q_REF and QMAS- 0032

These commands S-i and S₂ are provided to a current regulator (ACR) [2035]. The current regulator [2035] generates a modulating signal for a PWM control on the basis of the commands ST and S₂. The modulating signal is provided to a PWM control unit [2036]. The PWM control unit [2036] generates the PWM signal S_PWM to drive the frequency converter [201] with comparing the modulating signal with a carrier signal, for example a delta carrier signal. The frequency converter [201] driven by SPW outputs the active power AP_MAS and the reactive power AQ_MAS- 0033

Fig. 5 illustrates the active power control unit [2031] and the reactive power control unit [2032] in Fig. 4.

0034

These control units correspond to power regulators generating commands which are given to the current regulator [2035] (Fig. 4) in the preceding power control system [203]. In Fig. 5, functions of the current regulator are included in transfer functions [2031 a, 2032a] representing responses (AP_MAS _, AQ_MAS) of the power-assisted system [2] to the commands generated by the active and reactive power control units. The responses Δ P MAS and AQMAS are an active power and a reactive power output by the power-assisted system [2], respectively.

0035

A reference of an active power AP_REF is input to the active power control unit [2031]. A deviation between AP_REF and AP AS is calculated by an adder. An integral control unit [2031c] having a gain K_M outputs a command signal s on the basis of the calculated deviation. On the other hand, a proportional control unit [2031 b] having a gain K_pi outputs a command signal s ₂ on the basis of the response ΔΡ_ΜΑ3 output by the power-assisted system [2] having a transfer function G^s) [2031a] for the active power. An adder calculates a command S-, for the active power output by the power-assisted system [2] with adding s₁₂ to S-M. The command ST is given to the power-assisted system [2] and then the power-assisted system [2] outputs the active power AP_MAs according to the transfer function G^s) [2031 a].

0036

As mentioned above, the active power control unit [2031] includes a feedback control portion which has the integral control unit [2031c] using APMAS as a feedback variable and a feedforward control portion which has the proportional control unit [2031 b] using AP_MAS as an input-signal. The feedback control portion controls AP AS to bring the steady-state deviation between AP_{RE F} and AP_MAS close to 0(zero) with the integral control unit [2031c], namely to bring AP_MAS close to AP_REF. Moreover, the feedforward control portion reduces a response-time of the power- assisted system [2] with the proportional control unit [2031b].

0037

A reference of a reactive power AQ_REF is input to the reactive power control unit [2032]. A deviation between AQ_REF and AQ_MAS is calculated by an adder. An integral control unit [2032c] having a gain K_!2 outputs a command signal s₂i on the basis of the calculated deviation. On the other hand, a proportional control unit [2032b] having a gain K_p2 outputs a command signal s₂₂ on the basis of the response AQ_MAS output by the power-assisted system [2] having a transfer function G₂(s) [2032a] for the reactive power. An adder calculates a command S₂ for the reactive power output by the power-assisted system [2] with adding s₂2 to s₂i . The command S₂ is given to the power-assisted system [2] and then the power-assisted system [2] outputs the reactive power AQ_MAS according to the transfer function G₂(s) [2032a].

0038

As mentioned above, the reactive power control unit [2032] includes a feedback control portion which has the integral control unit [2032c] using AQMAS as a feedback variable and a feedforward control portion which has the proportional control unit [2032b] using AQ MAS as an input-signal. The feedback control portion controls AQ AS to bring the steady-state deviation between AQ_REF and AQ_MAS close to 0(zero) with the integral control unit [2032c], namely to bring AQ_MAS close to AQ_REF. Moreover, the feedforward control portion reduces a response-time of the power- assisted system [2] with the proportional control unit [2032b].

0039

According to the active and reactive power control units in this embodiment, an operation speed of a power-assisted system in an electric power generation system is improved. Therefore, an oscillation of a voltage and a rotational speed of the power generator due to a rapid load variation are suppressed.

0040

Fig.6 illustrates a flowchart showing processes of the preceding power control system operating in a rotor vibration preference mode as the operation mode for suppression of the voltage and rotational speed oscillations of the generator. 0041

The preceding power control system [203] has an objective to keep the rotational speed (CO_GEN) of the generator [4] at constant. In this operation, the preceding power control system [203] monitors voltage (V) and current (I) of the electric power generation system [1], voltage (V_MAS) and current (IMAS) of the power-assisted system [2] and the rotational speed (COGEN) of the generator [4] [203_a]. These voltages and currents are monitored by the voltage sensors and current sensors respectively as above mentioned on Fig. 3.

0042

Then, active power (P) and reactive power (Q) of the electric power generation system [1] are calculated on the basis of V and I, and active power (PMAS) and reactive power (QMAS) ^{of tne} power assisted system [2] are calculated on the basis of MAS and IMAS [203_b].

0043

Then power factor (cos Θ), where Θ is an angle between voltage and current of the power-assisted system [2], are observed on the basis of (VMAS, IMAS) or (PMAS.QMAS) [203_C].

0044

After observing the power factor, a rotor vibration is checked with using a generator speed sensor, a filter or an other such device, or system oscillation information [203_d].

0045

If the rotor vibration is not occurring [203_d, No], then the preceding power control system [203] keeps monitoring in the step [203_a]. 0046

If the rotor vibration is occurring [203_d, Yes], an active power compensation Δ PMAS and a reactive power compensation AQ_MAS are calculated under a setting of the rotor vibration preference mode to the preceding power control system for following a load-fluctuation AP_LOAD [203_e].

0047

The following [MATH 1] is used to calculate AP_MAs and AQMAS in the step [203_e].

0048

0049

The [MATH 1] shows a relation between variations (ΔΕ, Δω) of an induced voltage and a rotational speed of the generator [4] and variations (AQMAS, APMAS) of reactive and active powers output by the power- assisted system. The matrix S in MATH 1 is a sensitive matrix according to a structure or a characteristic of the power system [5].

0050

A type of equation such as the [MATH 1] is usually used to compensate the acceleration power difference between mechanical and electrical power of a generator for minimizing the speed fluctuation (Δω). The inventors of the present invention derive the [MATH 1] from the type of equation and apply the [MATH 1] for the calculation of the compensations APMAS and AQMAS- 0051

The ΔΡ MAS and AQMAS f°^r following the load-fluctuation APLOAD are calculated with the [MATH 1] on the basis of the availability of observed reactive power in the step [203_c] to minimize the fluctuation (Δω) of the rotational speed U)_GEN of the generator [4] due to the vibration. Here, it is desirable that a power factor of the power-assisted system [2] is set within the acceptable range in order to get the best effort in maintaining the power factor of the power-assisted system [2].

0052

After the step [203_e], the references AP_REF and AQ_REF are set to the active power control unit [2031] and the reactive power control unit respectively on the basis of the calculated compensations AP_MAS and AQ_MAS [203_f].

0053

After the step [203_f], the active power ΔΡ MAS and reactive power

AQ _AS of the power-assisted system [2] are adjusted to meet the references AP_REF and AQ_REF respectively by the active and reactive control units [2031, 2032] in Fig. 5 [203_g]. Therefore, the power-assisted system increases or decreases quickly its output powers by AP_REF and Δ Q _R . Consequently, the vibration of rotational speed of the generator [4] due to a rapid load variation are suppressed.

0054

After the step [203_g], the operation mode is checked [203_h]. If the operation mode is not changed [203_h, No], the preceding power control system [203] executes the step [203_a] again under the rotor vibration preference mode. If the mode is changed [203_h, Yes], then the operation mode of the preceding power control system [203] is changed [203_i].

0055

Fig.7 illustrates a flowchart showing processes of the preceding power control system operating in a voltage stability preference mode as the operation mode for suppression of the voltage and rotational speed oscillations of the generator. The preceding power control system [203] has an objective to keep the voltage of the generator [4] at constant. Differences between the flowcharts shown in Figs.6 and 7 are mainly explained, as follows.

0056

After observing the power factor, a voltage oscillation is checked on the basis of the monitored V [203_d2]. In the step [203_d2], a voltage oscillation may be checked with checking a power oscillation on the basis of the monitored P, Q.

0057

If the voltage oscillation is not occurring [203_d2, No], then the preceding power control system [203] keeps monitoring in the step [203_a].

0058

If the voltage oscillation is occurring [203_d, Yes], then response rates of the active and reactive powers of the power-assisted system are culculated [203_e2]. In the step [203_e2], the gains (K_p1, K_p2) related the response rates in the proportional control units [2031b, 2032b] are adjusted for stable responses of the transfer functions (G-i(s), G₂(s)) [203_f].

0059

The gains (K_p1, K_p2) for stable responses of the transfer functions (G-i(s), G₂(s)) may be set to the preceding power control system [203] in advance.

0060

After the step [203_e2], the references AP_REF and AQ_REF for following the load-fluctuation AP_LOAD are set to the active power control unit [2031] and the reactive power control unit [2032] respectively under setting the calculated response rates [203_f]. Here, it is desirable that a power factor of the power-assisted system [2] is set within the acceptable range in order to get the best effort in maintaining the power factor of the power- assisted system [2].

0061

According to the processes of the preceding power control system operating in a voltage stability preference mode shown in Fig. 7, the power-assisted system increases or decreases quickly its output powers by APREF and AQ_REF. Consequently, the oscillation of the voltage of the generator [4] due to a rapid load fluctuation are suppressed.

0062

Fig. 8 illustrates examples of waveforms of a load fluctuation AP_LOAD_> an active power AP_MAS which the power-assisted system outputs, a voltage V, of the generator and a rotational speed variation Δω of the generator in both of the embodiment and a conventional PI (Proportional and Integral) control. A block diagram of the conventional active power control unit, which the conventional PI control is applied to, is shown in Fig. 9. The conventional active power control unit includes a feedback control portion which has a integral control unit and a proportional control unit without a feedforward control portion, while the embodiment includes a feedback control portion which has a integral control unit with a feedforward control portion which has a proportional control unit as shown in Fig.5. Additionally, a conventional reactive control unit not shown in Fig. 9 is similar to the conventional active control unit shown in Fig. 9.

0063

As shown in Fig. 8, AP AS according to the embodiment reaches to

ΔΡ R E F more quickly than AP_MAS according to the conventional PI control unit. Consequently, oscillations of V_t and Δω due to the embodiment are reduced in comparison with the conventional PI control unit.

0064

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Reference Signs List

0065

1 Power generation system 2 Power-assisted system (MAS)

3 Turbine

4 Generator

5 Power system

201 Frequency converter

202 Energy conversion system

203 Preceding power control system

204 Motor-drive system

205 Current sensor

206 Voltage sensor

312 Rotating shaft

412 Rotating shaft

500 Position sensor

2010 Power conversion circuit 2011 Power conversion circuit

2012 DC link circuit

2013 Smoothing capacitor

2031 Active power control unit

2031 a Transfer function

2031 b Proportional control unit

2031c Integral control unit

2032 Reactive power control unit 2032a Transfer function

2032b Proportional control unit 2032c Integral control unit 2033 Power controller

2034 Power calculator

2035 Current regulator

2036 PWM control unit

Claims

Claims Claim 1

An electric power generation system comprising: a power generator rotating by a turbine;

a power assisted system adjusts an electrical power on a side of a power system, wherein the power assisted system has a frequency converter electrically connected between an electrical output of the generator and an energy conversion system taking an electrical energy in or out, and

a control system controlling the frequency converter to adjust a power on a side of an output from the frequency converter,

wherein the control system includes a feedback-control portion which generates a first command based on a feedback of the power and a feedforward-control portion which outputs a second command based on the power, and the power is controlled by the first and second commands.

Claim 2

An electric power generation system according to claim 1, wherein the feedforward control portion has a proportional control unit transforming the power to the second command.

Claim 3

An electric power generation system according to claim 1 or claim 2, wherein the feedback control portion generates the first command to bring the feedback of the power near a reference power. Claim 4

An electric power generation system according to claim 3, wherein the feedback control portion has an integral control unit transforming deference between the reference power and the power to the first command.

Claim 5

An electric power generation system according to claim 3 or claim 4, wherein the reference power is set according to a fluctuation of a load.

Claim 6

An electric power generation system according to claim 5, wherein the power reference is set based on a calculated power compensation on the side of the output from the frequency converter for suppressing a vibration of a rotational speed of the electric generator.

Claim 7

An electric power generation system according to claim 2, wherein a proportional gain in the proportional control unit is adjusted for a stable power response on the side of the output from the frequency converter for suppressing an oscillation of a voltage of the electric generator

Claim 8 An electric power generation system according to claim 1, wherein the power includes an active power and a reactive power that are controlled by the frequency converter.

Claim 9

An electric power generation system according to claim 8, wherein the frequency converter is operated by semiconductor switching devices.

Claim 10 A control system for an electric power generation system including a power generator rotating by a turbine, a power assisted system adjusts an electrical power on a side of a power system, wherein the power assisted system has a frequency converter electrically connected between an electrical output of the generator and an energy conversion system taking an electrical energy in or out, and controlling the frequency converter to adjust a power on a side of an output from the frequency converter, comprising:

a feedback-control portion which generates a first command based on a feedback of the power;

a feedforward-control portion which outputs a second command based on the power, and

wherein and the power is controlled by the first and second commands.