Patent Description:
The electric energy sources, which contain at least one electric generator powered by the mechanical energy source, are the grounds of worldwide electricity production. Typically, these are a turbine-generator unit in thermal power plants. These rotating sources are unable to respond quickly to dynamic states in the grid. Frequency of these dynamic states increases hand in hand with development of difficult-to-predict renewable electric energy sources, or as a result of local overloading of a power system. The consequence is increased electrical and mechanical stresses applied on these power sources with an impact on their service life.

At present, the electric energy rotating sources are the ground of a wide range of power plants and heating plants - from conventional fossil fuel power plants and heating plants (combined cycle, coal, diesel power plants) through nuclear power plants and hydroelectric power plants. A common denominator of these rotating sources is the mechanical energy source (usually a turbine or combustion engine) mechanically coupled to an electric generator. These sources are connected to the power system and usually connected to a system of supporting services for keeping the power system stable (primary and secondary power control, minute back-ups, and more) and for securing a power balance. The control dynamics of these sources is limited by physical limits on the side of the mechanical, or thermal energy source in case of steam turbines (in particular due to permitted temperature gradients of materials used). Operating the sources at the very border of physical limits due to increasing demands for the control properties of these sources results in substantial reduction of their service life.

There may be many negative transients in the power system (typically due to reconfiguration of the power grid while stabilizing the power flows or switching high grid loads with low short-circuit impedance) acting directly on the connected electric energy rotating sources in the form of their increased electromechanical stress (increased vibration, voltage and current stress). These negative transients are much faster than the control capabilities of the mechanical energy sources used. Therefore, classic rotating sources cannot respond to the transients, or compensate them effectively.

Disclosed are technical solutions based on an accumulation element (usually an electrochemical accumulator) and an electronic power converter, e.g., in <CIT>. The elements are coupled to the power plant service consumption systems and designed to increase flexibility of the power control of the power plant, or for provision of back-up power. A similar technical solution, which increases flexibility of the power control of a power plant, is disclosed in utility model <CIT>, which uses an accumulation element with an electronic power converter connected directly to a common power node with a generator. Furthermore, there are documents dealing with the power balance and flexibility of a thermal power plant with the use of a superconductive magnetic energy accumulation element, e.g., <CIT>. Other solutions use the accumulation element directly connected to the power system, e.g., <CIT>. It improves stability of the power system based on power supplies or consumption at a frequency deviation. Document <CIT> discloses use of a centralized energy storage for stabilization of a whole power system. Likewise, <CIT> discloses a mass accumulator storage provided with a multi-port power converter, through which the storage is connected to alternating electric grid, and may optionally be connected to a renewable electric energy source (RES - e.g., photovoltaics). The accumulators or super-capacitors are used as e.g., disclosed in <CIT> for hybrid electric energy sources, consisting of the renewable electric energy sources (RES), in particular of combination of photovoltaics and wind power plant. Disclosed in <CIT> is an active power filter, i.e., AC/DC converter (voltage-source inverter) connected on the AC and DC part to the AC power grid and the accumulator, respectively, where the accumulator is used as an electric energy battery for operation of a filter. The combination of a generator and power semiconductor converter is known from the wind power plants. Disclosed in <CIT> is a wind power plant containing an electric generator and a throughput power semiconductor converter, which provides connection of the generator to the AC electric grid via an input converter (grid) transformer. <CIT> discloses a wind power plant containing an electric generator and throughput indirect frequency converter, which provides connection of the generator to the AC electric grid, and in which DC circuit, there may be an apparatus installed for accumulation of electric energy. However, these technical solutions concentrate on the issues related to the power control or power back-up. They do not mention use of an accumulation element and an electronic power converter in order to provide an active safeguard of a rotating source against negative electromechanical events from the power system.

<CIT> relates to an energy storage which is configured to emulate the behaviour of traditional synchronous generators to provide dynamic frequency control support by adding inertia to a microgrid during disturbances. Design parameters such as the maximum load change and an allowable frequency deviation are provided to a numerical model in order to determine an angular momentum of the energy storage which is used to control the output power of the energy storage.

<CIT> discloses a conditioning device for power supply networks wherein an energy storage is controlled to provide unbalance compensation, harmonic compensation and synchronous machine simulation.

Also known are passive generator safeguards (the safeguards that evaluate short-circuits and earth connection, and safeguards that evaluate abnormal operation states) that disconnect the generator from the power system when a hazardous state is detected. By definition, these safeguards are unable to actively intervene and mitigate or eliminate negative electromechanical transients acting on the electric energy rotating sources.

The technical solution described below actively safeguards the electric energy rotating source against negative transients and increases its service life and power flexibility.

The present invention is defined by independent claims <NUM> and <NUM>. The present disclosure discloses a technical solution of a unique connection and method of control for increasing the service life and flexibility of an electric energy rotating source. The apparatus comprises the electric energy rotating source with an electric generator connected to a mechanical energy source, a power system interface, a power semiconductor converter, an electric energy accumulation device, a micro-grid, a control system with a rotating source active safeguard and a power controller, and a measurement block.

The electric energy rotating source and the power semiconductor converter output are jointly connected to the power system interface. The power semiconductor converter input is connected to the electric energy accumulation device. The electric generator, the mechanical energy source, the power semiconductor converter, the electric energy accumulation device, the control system, the measurement block, and the power system interface jointly form the micro-grid. The micro-grid is adapted for control by a control system.

The control system includes an active safeguard of the rotating source against transient electromechanical stress. The active safeguard refers to an apparatus, which based on measuring of electric voltages and currents, generates voltage and current components in the power system interface that counteract a transient that negatively influences the service life of the rotating source. The active safeguard counteracts current and torque surges caused by voltage or current oscillations. To do so, it uses the measurement block in the power system interface and a numerical model of the electric energy rotating source. By solving a system of differential equations, the numerical model evaluates electric and mechanical stress of the electric energy rotating source. The active safeguard generates the voltage and current components using the power semiconductor converter and the electric energy accumulation device. To a maximum extent possible, the negative transients and their impact on the connected electric energy rotating source are mitigated or eliminated. Furthermore, the control system includes a power controller. The measuring inputs for the active safeguard and the power controller are connected to the measurement block. The measurement block is connected to the power system interface. The micro-grid is connected to the power system through the power system interface.

The electric energy accumulation device refers to a device being able to store and then supply electric energy - typically the electrochemical accumulators or a mechanical energy accumulator (e.g., a flywheel).

The power semiconductor converter refers to an electronic device being able to change the characteristics and parameters of electric energy. It is typically used for change the characteristics of electric energy from AC to DC (also called a rectifier) or from DC characteristics to AC (also called inverter). The parameters of electric energy refer to in particular frequency, voltage, and current.

The electric energy rotating source refers to a unit of the mechanical energy source (usually a steam turbine or an engine, in particular a combustion engine) and of the electric generator (typically a synchronous generator).

The power system refers to a device for electric energy transfer from the point of production (power plant) to the point of consumption. The power plant refers to a complex installation in heart of which is the electric energy rotating source, including all systems needed for operation of the same.

Increasing the service life of electric energy rotating source is achieved by a connection and a micro-grid control method. The function of the active safeguard of the electric energy rotating source against negative electromechanical transients is thereby provided. The transients are excited including, but not limited, by the side of the connected power system. The transients are referred to the electric and mechanical stresses, in particular undesired current and torque changes.

The flexibility of the electric energy rotating source is increased by the method of connection and control of the power semiconductor converter and the electric energy accumulation device. They then act as a secondary electronic controlled electric energy source connected to the common power system interface with the electric energy rotating source. The control system evaluates requirements for the micro-grid power or for the secondary electronic controlled source, and generates a request for the power controller. The evaluation occurs based on a request from superior control system of a power plant or totally independently based on measuring of electric voltages and currents using the measurement block in the power system interface. The secondary electronic controlled electric energy source can operate at much higher control dynamics compared to a conventional electric energy rotating source. Hence, the dynamics of the electric energy rotating source control or of the power plant as a whole is increased. The primary electric energy rotating source (usually in a configuration of a steam turbine and a synchronous generator unit) may operate in this way under optimum operation range without excessively burdening dynamic changes, and accordingly, its service life can be significantly improved.

The micro-grid and its control system are designed so that upon commissioning, a fully independent operation and provision of the active safeguard feature of the electric energy rotating source are possible. It means that the micro-grid and the control system are eligible for operation without the need of connecting to the superior control system of the power plant, or to the control system of the electric energy rotating source. However, unidirectional or bidirectional connection with the superior control system of the power plant is favourable for increased dynamics and precision control while increasing flexibility of the electric energy rotating source, or for diagnostic data communication. The measurement block may be used for operation of the micro-grid even when the electric energy rotating source is out of order at that moment. It means that the micro-grid is adapted to operation with the electric energy rotating source active or inactive.

Communication between the electric energy accumulation device and the power semiconductor converter is provided via the control system or directly between the individual blocks.

The exemplary embodiment of the proposed solution is described with reference to the drawings where.

In the present exemplary embodiment, the apparatus for the increased service life and flexibility of the electric energy rotating source is applied in a system of a thermal power plant <NUM> - refer to <FIG>.

A thermal power plant <NUM> comprises a micro-grid <NUM> and a service consumption grid <NUM> being connected in a common power system interface <NUM>. The micro-grid <NUM> includes an electric energy rotating source <NUM>. In this case, the rotating source <NUM> is a turbine-generator unit consisting of a mechanical energy source <NUM> in the form of a steam turbine and synchronous generator <NUM>. Further, the micro-grid <NUM> includes a control system <NUM> with an active safeguard <NUM> and a power controller <NUM>, an electric energy accumulation device <NUM>, and a power semiconductor converter <NUM>. The service consumption grid <NUM> of the thermal power plant <NUM> consists of a service consumption <NUM> and a back-up power <NUM>. The power system interface <NUM> includes a measurement block <NUM> and connects to a block transformer <NUM> to output the thermal power plant <NUM> power to a power system <NUM>.

The control system <NUM> of the micro-grid <NUM> and the control systems of the electric energy rotating source <NUM> and the service consumption grid <NUM> are connected to the power plant superior control system <NUM> via communication lines. Further, the control system <NUM> of the micro-grid <NUM> connects by means of communication line to the control system of the electric energy rotating device <NUM>. The service consumption <NUM> connects to the power semiconductor converter <NUM> via the communication and power line.

For exemplary function of the active safeguard <NUM> of the electric energy rotating source <NUM>, refer to <FIG>. A fast transient (caused e.g., by switching high grid loads with low short-circuit impedance, due to reconfiguration of the power grid while stabilizing the power flows) occurs in the power system <NUM> at time t1. When the active safeguard <NUM> of the electric energy rotating source <NUM> is not active, a transient torque response occurs on the synchronous generator <NUM>. Thereby, the electric energy rotating source <NUM> is undesirably stressed and its service life reduces. The effects of the transient fade out at time t2. When the active safeguard <NUM> of the electric energy rotating source <NUM> is active, the undesired transient is evaluated at time t1 by means of the measurement block <NUM> and a numerical model of the rotating source <NUM>. By means of the measurement block <NUM>, the active safeguard <NUM> measures electric voltages and currents in the power system interface <NUM>. These quantities enter in the numerical model of the electric energy rotating source <NUM>. By means of the numerical model solving a system of differential equations, the active safeguard <NUM> evaluates electric and mechanical stress of the rotating source <NUM>. By means of the power semiconductor converter <NUM> and the electric energy accumulation device <NUM>, the active safeguard <NUM> then generates the components of voltages and currents in the power system interface <NUM>. These components counteract the transient that has negative impact on the service life of the electric energy rotating source <NUM>. Ideally, the active safeguard <NUM> suppresses the transient in the synchronous generator <NUM>, which is obvious from the torque curves in <FIG>. The exemplary function for increasing the flexibility of the electric energy rotating source (e.g., support service start-up speed) is shown in <FIG>. At time t1, there is a request for change to the electric energy rotating source <NUM> power from P2 value to P3 value. The electric energy accumulation device <NUM> and the power semiconductor converter <NUM> accommodate the power request by change to their power to P1 value at time t2. Thereafter, its power decreases correspondingly to increasing power of the electric energy rotating source <NUM>. At time t3, the electric energy rotating source <NUM> assumes accommodating of the full power request, and ramps up to the power P3 value. Subsequently, at time t3, the electric energy accumulation device <NUM> recharges as well. This is done by further increasing of the electric energy rotating source <NUM> power to P4 value and equivalent reduction of the electric energy accumulation device <NUM> power and of the power semiconductor converter <NUM> to a negative P5 value. A rate of change to the electric energy accumulation device <NUM> and of the power semiconductor converter <NUM> power to adjust the recharge value must not be higher than maximum rate of change to the electric energy rotating source <NUM> power.

Claim 1:
A method of controlling an apparatus for increasing service life and flexibility of an electric energy rotating source (<NUM>) where a power semiconductor converter (<NUM>) and an electric energy accumulation device (<NUM>) are used, by means of a measurement block (<NUM>) an active safeguard (<NUM>) measures electric voltages and currents in a power system interface (<NUM>) while the rotating source (<NUM>) is in operation, the measured quantities of electric voltages and currents are input to a numerical model of the electric energy rotating source (<NUM>) being calculated in real time in the active safeguard (<NUM>), wherein by solving a system of differential equations in the numerical model, electric and mechanical stresses of the rotating source (<NUM>) are evaluated in the form of changes to voltages, currents and an electric generator (<NUM>) torque, and by means of the power semiconductor converter (<NUM>) and the electric energy accumulation device (<NUM>), the components of voltages and currents are generated in the power system interface (<NUM>) as calculated by the active safeguard (<NUM>) that counteract the undesirable changes to voltage, current and the electric generator (<NUM>) torque.