Patent Description:
A power converter can be used as a power conversion system (PCS). A PCS can be used to convert electrical energy from one form to another, such as from ac to dc in which case it performs rectifier action or from dc to ac in which case it performs inverter action. If bidirectional flow is possible the PCS is usually referred to as a converter.

A PCS can be either current controlled or voltage controlled. When ac current controlled the current on the ac side of the converter is controlled to be at or close to a particular setpoint. An ac current controlled PCS can be referred to as a Current Source Inverter (CSI) since the ac current is similar to that coming from an ac current source. Similarly, when ac voltage controlled, the voltage on the ac side of the converter is controlled to be at or close to a particular setpoint. An ac voltage controlled PCS can be referred to as a Voltage Source Inverter (VSI) since the ac voltage is similar to that produced by an ac voltage source.

One use of a VSI is to create an ac electrical network or grid. The energy can come from an energy storage device, such as a battery or flywheel or super capacitor, or an energy producing device such as a generator. Since energy storage devices can be charged and discharged and VSIs are also bidirectional, the energy from an energy storage/VSI combination can be used by a load in which case they act as a generator or consumed from some other generation device in which case they act as a load. This ability to both produce and consume energy means the storage/VSI combination can create a grid and run as the only grid-forming device on the grid, i.e. stand-alone, or be used to stabilize the frequency or voltage of a grid formed by other devices.

If a VSI is given a fixed voltage and frequency setpoint and runs stand-alone there are no problems. However, if it runs in parallel with other voltage stiff devices, such as other VSIs or synchronous generators, then there are problems with these devices running in parallel.

A synchronous generator is the predominant power producing device in power systems. It comprises two parts: The prime mover and the synchronous machine. The prime mover is an energy conversion device which converts energy in some form such as steam or natural gas into rotational energy. The synchronous machine converts the rotational energy into ac electrical energy.

A synchronous generator is compliant, i.e. it initially reacts softly to load changes on it. This compliant behaviour occurs for both real load changes and reactive load changes. An example of a real power change is when the load increases on a synchronous generator, resulting in an initial dip in frequency. The speed or frequency controller, or governor, senses this speed decrease and responds by increasing the power output of the prime mover. This is usually done by increasing the fuel or steam flow into the prime mover. With the increased power output of the prime mover the speed and frequency is brought back to the setpoint. In the case of a reactive power load change, when the reactive load increases on a synchronous generator there is an initial dip in voltage. The automatic voltage regulator (AVR) senses this voltage decrease and responds by increasing the field current to the synchronous machine. This field current increase can be via a static or brushless excitation system. With the increased excitation the voltage output of the synchronous machine is brought back to the setpoint.

A standard VSI doesn't have the above compliance so it can be quite stiff.

There are problems when a standard VSI operates in parallel with another voltage source device, like a synchronous generator, another VSI, or a normal grid. These problems occur in both steady-state and during transients.

Voltage source devices, such as synchronous generators and VSIs that are operating in parallel and have the same frequency setpoint do not inherently share load; separate sharing algorithms are required. Similarly synchronous generators and VSIs that are operating in parallel and have the same voltage setpoint do not inherently share reactive load; separate reactive sharing algorithms are required.

During a load step the sharing between a VSI and generator is disrupted. If the generator has inertia and the VSI is a conventional type then the VSI will pick up the majority of the change in load. This puts a lot of stress on the VSI and may cause it to be overloaded, even if the total load is within the load capability of the combined VSI and generator.

When there is no fault and the system is at the steady-state, the generator and the VSI are operating in synchronism with each other with a static angle between the generator's internal emf and the VSI voltage. When there is a short circuit or fault on a power line, the line voltage on the network is suppressed to a lower value and the generators and VSIs connected to that line usually supply fault current. During the time that the voltage is suppressed the active power produced by the generators connected to the system changes from their pre-fault value; the generators speed up if the active power is lower, or slow down if the active power is higher. A normal VSI may or may not change its frequency, so the synchronism between the generator(s) and the VSI may be lost.

There are two problems that can occur due to this loss of synchronism. Firstly this loss of synchronism can cause the VSI and the generators to provide fault current at different phase angles, so the net current to the fault may be reduced from the sum, or even become zero, i.e. the VSI and generator fault currents are equal but out of phase. The second problem is that when the fault is removed there may be a large angle difference between the VSI and the generator which can cause large power flow between the generator and the VSI and/or large currents to flow. This can cause over-current and/or power protection devices to trip.

The standard conventional control methods used for VSIs don't have any mechanism to prevent the above issues.

The paper "<NPL>, discloses an implementation of a virtual synchronous machine (VSM). The implementation is based on an internal representation of the synchronous machine inertia and damping behaviour, together with cascaded voltage and current controllers for operating a voltage source converter. The virtual synchronous machine is able to handle active load changes but provides poor voltage control, as there is no recovery mechanism in the event of a load step and it can therefore not act as a grid-forming device but only as a grid-following device.

<CIT> discloses a virtual synchronous generator-based photovoltaic power control strategy, which comprises a photovoltaic power generation system, an energy storage device, and an inversion device, wherein a power supply output end of the photovoltaic power generation system is respectively connected with an energy storage end of the energy storage device and an inversion end of the inversion device. The output end of the inversion device is connected with a power grid. The inversion device carries out a power control through a virtual synchronous generator method. The output power of the inversion device is adjusted through the virtual synchronous generator method to reach a stable state. The output power is adjusted according to load fluctuation.

<CIT> discloses a pre-synchronization control method based on a virtual synchronous generator.

<CIT> discloses a multi-virtual synchronous generator parallel network control method based on inverters.

<CIT> discloses a microgrid and microsource control method based on a virtual synchronous electric generator. The method comprises the steps of building a mechanical equation and an electromagnetic equation of the virtual synchronous electric generator, carrying out microsource active control through active adjustment of the virtual synchronous electric generator, carrying out microsource reactive control through reactive adjustment of the virtual synchronous electric generator and achieving seamless switching between different operating modes of a microgrid.

In view of the above, an object of the present disclosure is to provide a method of controlling a power converter which solves, or at least mitigates, the problems of the prior art.

There is hence according to a first aspect of the present disclosure provided a method of controlling a power converter, connected to an electrical grid, to mimic a synchronous generator, the method being performed by a control according to claim <NUM>.

An effect obtainable by being able to control active power and reactive power changes by means of the rotational frequency control and the output voltage control using closed-loop feedback is that multiple power converters, each separately controlled by means of the method, can operate in parallel and share well, during the steady state, and during transients and during faults. It also allows for paralleling with synchronous generators and/or with renewable energy generators such as solar, wind, tidal, etc..

Furthermore, the "virtual generator" which controls the power converter can be tuned to have the same characteristics as the conventional generation so they don't lose synchronism during a line fault. Moreover, the "virtual generator" can be tuned to be as compliant or as stiff as desired, which means it can mimic synchronous generators larger or smaller than its power rating. The power converter may furthermore operate as the sole voltage source in the grid and be the voltage and frequency reference which enables renewable energy sources to be used, both inverter connected, e.g. solar, and machine connected, e.g. wind turbine generators.

The power converter controlled by means of the method creates a positive sequence voltage source and is a good reference during line faults; the power converter can deliver fault current and maintain the rotating positive sequence voltage source.

There is according to a second aspect of the present disclosure provided a computer program comprising computer-executable components which when executed by processing circuitry of a control system causes the control system to perform the steps of the method according to the first aspect. There is according to a third aspect of the present disclosure provided a computer program product comprising a storage medium including a computer program according to the second aspect.

There is according to a fourth aspect of the present disclosure provided a control system for controlling a power converter configured to be connected to an electrical grid, to mimic a synchronous generator, wherein the control system comprises: processing circuitry, and a storage medium comprising computer-executable components which when run on the processing circuitry causes the control system to perform the method according to the first aspect.

There is according to a fifth aspect of the present disclosure provided a power converter system comprising: a power converter, and a control system as according to the fourth aspect, configured to control the power converter.

Preferred embodiments are described in the dependent claims.

All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.

It is herein presented a method and control system which mimics the operation and behaviour of a synchronous generator. A "virtual generator" functionality is thus provided, which enables a power converter configured to be connected to an electrical grid parallel to, and share power with, conventional synchronous generators, but also with other power converters. Since both the power converter and the conventional generators have similar properties, sharing occurs both in the steady-state and in transients, the latter case utilising the full capabilities of all generating assets. Moreover, the "virtual generator" enables the power converter to operate as the only grid-forming component within an electric grid, being the electric grid frequency and voltage master and provides frequency and voltage regulation.

<FIG> shows an example of a control system <NUM> for controlling a power converter to mimic a synchronous generator. The power converter may in particular be a voltage source inverter.

The control system <NUM> comprises processing circuitry <NUM> and a storage medium <NUM> comprising computer-executable components which when executed by the processing circuitry <NUM> causes the control system <NUM> to perform the method as will be disclosed in the following to mimic a synchronous generator.

The processing circuitry <NUM> uses any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate arrays (FPGA) etc., capable of executing any herein disclosed operations concerning power converter control.

The storage medium <NUM> may for example be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory.

<FIG> shows a power control system <NUM> including the control system <NUM> and a power converter <NUM> configured to be controlled by the control system <NUM> and to be connected to an electrical grid. In <FIG>, there are shown a plurality of functional blocks typically implemented by the computer-executable components, or software, included in the storage medium <NUM> and configured to be executed by the processing circuitry <NUM>.

The functional blocks provide both frequency control and voltage control of a power converter, with closed-loop feedback, whereby the power converter is able to act as a grid-forming device. The power converter is thus able to follow the load on the grid and adapt the output voltage and output frequency based on the actual reactive load and active load conditions.

Among the functional blocks, there is an automatic voltage regulator block <NUM> and a synchronous generator model block <NUM>. The synchronous generator model block <NUM> includes a mathematical model of the excitation system of a synchronous generator. The output voltage of a synchronous generator is a function of the field current, i.e. the magnetisation current in the field windings of the rotor, and of the reactive power.

The control system <NUM> is configured to receive the actual voltage Uact output by the power converter <NUM>. The actual voltage Uact that is output by the converter can for example be obtained by measurement at the terminals of the power converter <NUM>.

The control system <NUM> is configured to determine a voltage control error with respect to the actual voltage Uact output by the power converter <NUM> and a setpoint voltage Uset, as shown in adder block <NUM>.

Optionally, according to one variation voltage droop Udroop may be used to determine the voltage control error in adder block <NUM>, as shown in <FIG>.

The voltage control error is input to the automatic voltage regulator block <NUM>. The automatic voltage regulator block <NUM> is a closed-loop controller. The automatic voltage regulator block <NUM> may for example comprise a PI-regulator.

The PI-regulator of the automatic voltage regulator block <NUM> may be provided with maximum and minimum reactive power limits and built-in integrator anti-windup. The dynamics of the PI-regulator may be set and an optional linear voltage droop can be used.

Based on the voltage control error, the automatic voltage regulator block <NUM> determines an exciter parameter, and regulates the output voltage U of the synchronous generator model block <NUM> by means of the exciter parameter. The exciter parameter can for example be the exciter current or the field current, i.e. the magnetisation current of the field windings of the rotor.

In this manner, the power converter may be controlled based on the output voltage U. In particular, the actual voltage Uact that is output by the power converter <NUM> may be controlled, thereby providing adaptability to reactive power changes in the electrical grid.

Among the functional blocks, there is also a speed governor, or frequency governor, block <NUM> and an inertia model block <NUM>. The inertia model block <NUM> includes an inertia model which is a mathematical model of the inertia of a synchronous generator, and can thus mimic the inertia of a synchronous generator. The inertia model has an inertia constant H, which indicates the amount of the spinning mass that it is representing and has unit of seconds. The inertia model may according to one variation also include a damping component, which is present in a synchronous generator. The damping component mimics the damping that exists in a synchronous generator. The rotational frequency of the inertia model is related to the power balance, i.e. the difference between the power going into the inertia, which is the input power, and the power coming out of it, which is the actual active power. When the input power going into the inertia model is less than the power delivered by the power converter, the frequency decreases. Conversely, if the input power is higher than the power coming out of it, the frequency increases. This is identical to a conventional synchronous generator with the prime mover providing the power, and the output power being that delivered by the synchronous generator.

The control system <NUM> is configured to receive an actual frequency fact of the power converter. The actual frequency fact may for example be measured at the output of the power converter. Alternatively, the rotational frequency f output by the inertia model block <NUM>, may be used as the actual frequency fact, as shown in <FIG>.

The control system <NUM> is configured to determine a frequency control error with respect to the actual frequency fact and the setpoint frequency fset, as shown in adder block <NUM>.

Optionally, according to one variation frequency droop fdroop may be used to determine the frequency control error in adder block <NUM>, as shown in <FIG>.

The frequency control error is input to the speed governor block <NUM>. The speed governor block <NUM> is a closed-loop controller. The speed governor block <NUM> may for example comprise a PI-regulator.

The PI-regulator of the speed governor block <NUM> may be provided with maximum and minimum power limits and built-in integrator anti-windup. Typically, the power limits are set to the energy storage limit of the power converter <NUM> and can be asymmetric.

Based on the frequency control error, the speed governor block <NUM> determines an input power, and regulates the rotational frequency f of the inertia model included in the inertia model block <NUM> by means of the input power.

The phase angle θ is also obtained. The phase angle θ is closely related to the rotational frequency f; it is the integral of the rotational frequency f with some modification in case the damping component in the inertia model is present. Hence, the input power also regulates the phase angle θ. The rotational frequency f and the phase angle θ are provided as outputs from the inertia model block <NUM>.

In this manner, the power converter may be controlled based on the rotational frequency f. In the present example, it is the phase angle θ derived from the rotational frequency f that is used for controlling the power converter. The power converter can thus be controlled to provide adaptability to active power changes in the electrical grid.

The control system <NUM> is configured to control the power converter <NUM> based on the rotational frequency f, in this example the phase angle θ derived from the rotational frequency f, obtained from the inertia model block <NUM> and based on the output voltage U obtained from the synchronous generator model block <NUM>. This control may for example be by means of PWM, i.e. by using PWM for switching the switches, for example insulated gate bipolar transistors (IBGTs) of the power converter <NUM>.

The output of the power converter may be filtered by a sine wave filter, creating a low-harmonic sinusoidal output entirely comparable to that of a conventional synchronous generator.

The output provided by the power converter <NUM> is a true voltage source. The real and reactive power delivered by the power converter <NUM> controlled by the control system <NUM> is related to the loads that are connected to it, if operated islanded, or its frequency setpoint and voltage setpoint relative to a macrogrid if grid-connected.

A power meter <NUM> may be provided to measure and capture the actual voltage Uact output from the power converter <NUM>, the actual active power Pact and actual reactive power Qact and in certain examples also the actual frequency fact, for use in the control loops described above.

In addition to the previously described functional blocks, there may also be provided a virtual impedance block <NUM>. The virtual impedance block <NUM> includes a virtual impedance of a stator of the synchronous generator model. In particular, the virtual impedance may include stator leakage inductances and a stator resistance.

In a variation which includes the virtual impedance block <NUM>, the output from the synchronous generator model block <NUM>, i.e. the output voltage U, and an output from the inertia model block <NUM>, in the present example the phase angle θ is provided into the virtual impedance block <NUM>. Alternatively, the rotational frequency f could be provided as an input to the virtual impedance block <NUM>. In this manner, the output voltage U is adjusted to obtain an adjusted output voltage U' based on the virtual impedance, and the phase angle θ is also adjusted to obtain an adjusted phase angle θ' based on the virtual impedance. The controlling of the power converter <NUM> is thus in this case based on the adjusted phase angle θ', and on the adjusted output voltage U'. If instead the rotational frequency f is input into the virtual impedance block, the rotational frequency f is adjusted based on the virtual impedance and the adjusted rotational frequency and the adjusted output voltage U' are used for controlling the power converter <NUM>.

It is possible to change the characteristics of the functional blocks adaptively in real time, i.e. adjust the parameters such as the inertia constant H, the gains Kp and Ki of the speed governor block and maximum and minimum limits of the speed governor block, to make the control system <NUM> controlled power converter mimic the characteristics of the conventional generators to enhance the ability to maintain synchronism during line faults.

Claim 1:
A method of controlling a power converter (<NUM>), connected to an electrical grid, to mimic a synchronous generator, the method being performed by a control system (<NUM>), wherein the method comprises:
determining a frequency control error with respect to a setpoint frequency (fset) and an actual frequency (fact) of the power converter (<NUM>),
determining an input power to an inertia model of a synchronous generator based on the frequency control error, which inertia model mimics the inertia of a synchronous generator,
regulating by means of the input power a rotational frequency (f) of the inertia model,
determining a voltage control error with respect to a setpoint voltage (Uset) and an actual voltage (Uact) output by the power converter (<NUM>),
determining an exciter parameter of a synchronous generator model based on the voltage control error,
regulating by means of the exciter parameter an output voltage (U) of the synchronous generator model, and
controlling the power converter based on the rotational frequency (f) and on the output voltage (U),
wherein the control system (<NUM>) includes processing circuitry (<NUM>) and a storage medium (<NUM>), wherein a plurality of functional blocks implemented by computer-executable components is included in the storage medium (<NUM>), wherein the functional blocks include a speed governor block (<NUM>), and an inertia model block (<NUM>) including the inertia model, which is a mathematical model of the inertia of a synchronous generator, and wherein the method is characterised by comprising:
changing the characteristics of the functional blocks adaptively in real time to make the control system-controlled power converter mimic the characteristics of conventional generators to enhance the ability to maintain synchronism during line faults.