Field of the Invention

The present invention relates to a method of controlling a turbogenerator, and more particularly to a method of controlling the start a turbogenerator when an electrical grid is unavailable. to the Invention

Turbogenerator systems are frequently used in energy recovery systems. In one example of such an energy recovery system, a turbine which forms part of the turbogenerator system is located within the exhaust flow of a prime mover. When the prime mover is in operation, the exhaust gases from the prime mover flow past and act upon the turbine blades of the turbine, rotating the turbine within the turbogenerator system. This rotation of the turbine is then used by the turbogenerator system to generate electricity, which is exported to an electrical grid. The turbogenerator may be connected to the electrical grid directly, or may be connected to the grid through power electronics.

Turbogenerator systems can become damaged if the turbine rotates too quickly, known as an overspeed. An overspeed situation is commonly prevented with the use of an electrical load on the turbogenerator, this electrical load is frequently provided by an electrical grid. However, it is sometimes necessary for a turbogenerator to initialise or start when an electrical grid is not readily available, or when an electrical grid is not stable. Such a situation may occur when the prime mover the turbogenerator itself is connected to is providing the grid, in the case of a fault on the system or in the case of a fault on the grid. Where the turbogenerator is connected to a grid via a power electronics system, such an overspeed situation can also damage the power electronics. A high rotational speed of the turbine within the turbogenerator results in a high output voltage, this high output voltage potentially damaging the power electronics themselves.

Where an electrical grid is not available to place a load on the turbogenerator, an alternative method of preventing an overspeed of the turbine is required. One such method is to reduce the output of the prime mover, such that the flow of exhaust gases past the turbine, and thus the speed of the turbine, is reduced. However, where a turbogenerator connects to a prime mover, the turbogenerator is usually the secondary system. Therefore, altering or ceasing the operation of the prime mover to accommodate the turbogenerator is undesirable, or even impossible. An alternative option is to ensure the flow of exhaust gases does not pass the turbine of the turbogenerator during the start-up phase. This bypassing of the main exhaust system may be undertaken via the use of valves, although such systems are costly.

Additionally, where there is no grid to place a load on the turbogenerator, the turbogenerator will also be unable to receive power from the grid. Here the turbogenerator must commence operation and restore the grid without relying on any external power source. A black start procedure is particularly problematic when considering both the issue of turbine overspeed.

A turbogenerator system comprises a turbine, power electronics used to control the rotational speed of the turbine and a generator. Typically, the turbogenerator is a highspeed turbogenerator, using high-speed power electronics.

One problem associated with preventing overspeed in a black start scenario is that initially, as there is no grid, there is no power supply to the to the power electronics. As such, by the time the grid has been established, and power to the power electronics has been restored, a damaging overspeed event may already have occurred. Additionally, the grid will take time to stabilise upon start-up of the prime mover, and any power export to the grid by the turbogenerator system before the grid has stabilised again has the potential to result in a damaging overspeed.

These issues are exacerbated in a‘fast start’ scenario. In a‘fast start’ scenario, the prime mover to which any turbogenerator system connected is able to rapidly move from idle to full power operation. A ‘fast start’ scenario can damage the power electronics of the turbogenerator as the turbine will rapidly achieve a high rotational speed within the generator. This high rotational speed results in a high output voltage capable of damaging the power electronics if the start-up procedure of the power electronics has not been completed. In a fast start procedure, the prime mover may reach maximum output level in as little as five seconds. Therefore, the power electronics must attain an operable condition in a very short amount of time, difficult especially where a fast start is combined with a black start in a‘fast black start’.

Embodiments of the present invention seek to address at least these problems.

Summary of the Invention

According to the present invention, there is provided a method of initialising a turbogenerator system for exporting electrical power to a grid, the method comprising providing a turbogenerator system comprising a generator, a turbine configured to rotate within said generator, and power electronics, providing electrical power to the power electronics, detecting a condition of the grid or the power electronics, limiting the rotational speed of the turbine within the generator to a first predetermined setpoint speed, monitoring the condition of the grid or the power electronics, determining the grid or the powerelectonics is in a condition suitable to receive electrical power, allowing the rotational speed of the turbine within the generator to increase above the first predetermined setpoint speed to reach a second predetermined setpoint speed, using the rotation of the turbine within the generator to generate electrical power and exporting said electrical power to the grid.

In this way, there is provided a method which allows a turbogenerator to initialise and commence the export of power to an electrical grid with a reduced risk of damage to the turbogenerator or system as a whole. This advantage is provided as the detection of the state of the electrical grid, and the prevention of the turbine from reaching a speed higher than the first setpoint speed until the grid is suitable to receive exported power prevents any overspeed of the turbine due to the premature supply of power to the grid from the turbogenerator.

It is understood that the step of limiting the rotational speed of the turbine within the generator to a first predetermined setpoint speed encompasses the method whereby the rotational speed of the turbine is allowed to vary between the first setpoint speed and any speed below the first setpoint speed. Here, the first setpoint speed acts as a limit the rotational speed of the turbine is not allowed to exceed. The rotational speed of the turbine within the generator may vary, as long as the first setpoint speed is not exceeded.

Preferably, the step of providing power to the power electronics comprises the step of providing electrical power to the power electronics via a battery. Such a feature may be advantageous in allowing operation of the power electronics to be initiated when not connected to a grid. Preferably, the step of providing power to the power electronics comprises the step of providing electrical power to the power electronics via a battery external to the turbogenerator system. Such a feature may be preferred as an external battery may have a capacity higher than that of a battery within the turbogenerator system, and therefore provide a more reliable means of providing power to the power electronics in the power-up phase.

Preferably, the step of detecting a condition of the grid comprises detecting the grid is unsuitable to receive electrical power from the turbogenerator. More preferably, the step of detecting the grid is unsuitable for power to receive electrical power from the turbogenerator comprises detecting the grid is unstable or fluctuating. Still more preferably, the step of detecting the grid is unsuitable to receive electrical power from the turbogenerator comprises detecting the grid voltage or grid frequency is unstable.

Preferably, the step of detecting the grid is unsuitable to receive electrical power from the turbogenerator comprises detecting the grid is absent.

Preferably the step of limiting the turbine speed to a first predetermined setpoint speed using the power electronics comprises applying a brake to the turbine. More preferably, the brake comprises a physical brake. Preferably, the brake comprises an electrical brake. More preferably, the brake comprises a brake circuit.

Preferably, the step of allowing the turbine speed to increase above the first predetermined setpoint speed to reach a second predetermined setpoint speed comprises releasing the brake.

Preferably, the step of determining the grid is in a condition suitable to receive electrical power from the turbogenerator comprises receiving a signal or message from a source external to the turbogenerator. More preferably, the signal is received from the grid. More preferably, the signal is received from an external controller.

Preferably, the step of determining the grid is suitable to receive electrical power from the turbogenerator comprises determining or establishing the grid is present. Preferably, the step of determining the grid is in a condition suitable to receive electrical power from the turbogenerator comprises determining or establishing the grid is stable. Preferably, the step of detecting the grid is suitable to receive electrical power from the turbogenerator comprises detecting the grid voltage or grid frequency is stable compared to a predetermined threshold or tolerance.

Preferably, the second predetermined setpoint speed is the speed at which the turbogenerator system provides an optimum output of electrical power. Such a feature may be advantageous as it ensures the operation of the turbogenerator is optimum once the grid is in a condition where it is ready to receive power from the turbogenerator. Preferably, the step of detecting a condition of the power electronics comprises determining the power electronics is in a condition unsuitable to receive power from the generator. Such a feature may be advantageous as it allows the system to ensure the power electronics are in a condition suitable for the export of power to the grid before any power export commences, preventing potential damage to the power electronics.

Preferably, the step of detecting the power electronics is in a condition suitable to receive electrical power comprises detecting the power electronics is configured to receive electrical power from the generator.

Preferably, the method further comprises the step of determining the rotational speed of the turbine within the generator. More, preferably, the method comprises determining the rotational speed of the turbine within the generator is above a predetermined value. Preferably, this predetermined value is l OOOrpm.

Preferably, the step of determining the rotational speed of the turbine occurs simultaneously with the step of determining a condition of the power electronics or determining a condition of the grid.

Preferably, the method comprises both determining a condition of the grid and determining a condition of the power electronics.

Preferably, the method comprises both determining the grid is in a condition suitable to receive electrical power and determining the power electronics are in a condition suitable to receive electrical power.

Preferably, the step of limiting the rotational speed of the turbine within the generator to a first predetermined setpoint speed comprises maintaining the rotational speed of the turbine at the first predetermined setpoint speed.

Detailed Description

Embodiments of the present invention are now described by way of example only and with reference to the accompanying drawing, in which:

Figure 1 is a schematic flow diagram of the outlined method. Referring to Figure 1 , the first stage of the method 101 includes the provision of a turbogenerator system including a turbine and power electronics. Such systems are well known and are frequently attached to the exhausts or fluid outflows of prime movers to harvest energy from these fluid flows and export this energy as electrical power to a grid. Such an arrangement is desirable, as the energy harvested by the turbogenerator system would otherwise be wasted. Therefore, the inclusion of a turbogenerator apparatus alongside a prime mover increases the efficiency of the system as a whole, maximising the output of electrical power for any given input of materials.

In the present embodiment, the described method commences with both the turbogenerator and the power electronics in an unpowered state. Such a situation may occur when both the prime mover and the turbogenerator are idle, or where the turbogenerator is not operational for any reason. In the present embodiment, the prime mover is also idle.

Initially, electrical power is supplied to the power electronics 102, and operation of the prime mover is commenced. As power is supplied to the power electronics, this enables the power electronics to commence operation and determine both the state of the turbine and the grid itself.

The power supply to the power electronics may be from several sources. Usually, power is supplied to the power electronics via the electrical grid. However, in the present embodiment of the invention, the prime mover has only just commenced operation and, therefore, the electrical grid is unavailable. As such, this initial supply of electrical power to the power electronics and turbogenerator system as a whole is provided by a battery located within the prime mover. This battery contains sufficient power to commence operation of the power electronics.

After power is supplied to the power electronics, the power electronics continue the start-up sequence by detecting a condition of the grid. In this detection step, the power electronics may monitor the voltage and /or frequency of the grid to determine the stability of the grid. Additionally, the detection step may involve detecting the presence of the grid. If the grid is both present and stable, then the turbogenerator may begin operation, as usual, exporting power to the grid.

However, as the prime mover has commenced operation at the same time as the turbogenerator, it is likely that the grid will not yet have been established, or that the grid remains unstable. In this case, the grid is in a condition where it is unsuitable to receive power from the turbogenerator. This unsuitability will be determined by the power electronics of the turbogenerator when detecting the condition of the grid.

If the power electronics determine that the grid is unsuitable to receive electrical power from the turbogenerator, it is necessary to limit the speed of the turbine within the turbogenerator. If the speed of the turbogenerator is not limited, it may begin to export power to a grid that is not in a suitable state. In this case, if the turbogenerator begins to export power to a grid that is unstable or absent, there is a significant risk of an overspeed damaging both the turbine and the electrical components of the turbogenerator. The overspeed may occur as the grid is unable to provide a stable load, or any load at all, on the turbogenerator.

Additionally, the power electronics themselves take some time to become fully operational after they receive power from the battery. Therefore, where the grid is not ready for power export, or the power electronics are not fully operational, the power electronics will detect these conditions and limit the turbine speed to a first predetermined setpoint speed 104. This first predetermined setpoint speed is below the speed at which the turbogenerator begins the export of power to the grid. Therefore, limiting the turbogenerator to this first predetermined setpoint speed ensures the turbogenerator will not begin to export power, eliminating the risk of damage to the system due to an overspeed or premature power export.

The turbine’s speed may be limited to the first predetermined setpoint speed using a brake or brake system. This brake may be in the form of a physical brake which engages with the turbine to slow its rotation. Alternatively, the brake may be an electrical brake in the way of a brake circuit.

After limiting the speed of the turbogenerator to the first predetermined setpoint speed, the power electronics will continue to monitor the condition of the grid 105. This monitoring of the grid may include monitoring the voltage and frequency of the grid to determine its stability.

During this monitoring process, the power electronics will determine the grid is in a condition suitable to receive electrical power from the turbogenerator 106. This determination may be made when the stability of the grid voltage achieves a predetermined level set within the power electronics. Alternatively, this determination may be made when the stability of the grid frequency achieves a predetermined level set within the power electronics. Commonly, this determination will be made when the power electronics receives a‘grid healthy’ signal from an external source, such as the prime mover or an overall control circuit.

Once the grid is determined to be in a condition suitable to receive electrical power from the turbogenerator, the power electronics will allow the turbine speed to increase above the first predetermined setpoint speed to reach a second predetermined setpoint speed 107. This increase in speed is allowed as the power electronics release the brake, allowing the turbine to accelerate. The second predetermined setpoint speed may be the speed at which the turbogenerator begins to export power to the grid 108. Alternatively, the second predetermined setpoint speed may be the speed at which the export of electrical power from the generator is optimised. As such, the turbogenerator begins to export electrical power to the grid, and the system as a whole is fully operational.