WO2019053166A1

WO2019053166A1 - Adding inertia in electrical power systems, apparatus for adding inertia and method of increasing the inertia of an electric machine

Info

Publication number: WO2019053166A1
Application number: PCT/EP2018/074816
Authority: WO
Inventors: Guy Dawson NICHOLSON; Jason Edward Hill
Original assignee: Element Power Ireland Ltd
Current assignee: Element Power Ireland Ltd
Priority date: 2017-09-13
Filing date: 2018-09-13
Publication date: 2019-03-21
Anticipated expiration: 2020-03-13
Also published as: EP3682137A1; GB2566461A; GB201714708D0; AU2018334018A1

Abstract

The disclosure provides an electric machine comprising a stator and a rotor rotatable relative to the stator, and an additional mass attached to the machine so as to rotate with the rotor. The additional mass is pre-stressed before rotation.

Description

ADDING INERTIA IN ELECTRICAL POWER SYSTEMS, APPARATUS FOR

ADDING INERTIA AND METHOD OF INCREASING THE INERTIA OF AN

ELECTRIC MACHINE

Field of the Invention

This invention relates to a device for managing frequency fluctuations on electrical power systems.

Background.

Electrical power systems have to distribute electricity from generators to loads. Electrical power systems need to be managed so that generation and load are in balance to maintain the system frequency. Larger, geographically dispersed, electrical power systems operate with small variations in frequency.

Smaller, more isolated, electrical power systems e.g. supplied by inverters and batteries, can operate with fixed frequency. When there is more generation than load, the system voltage tends to increase and when there is more load than generation, the voltage tends to falls.

When there is more generation than load, the system frequency increases with the excess energy stored as increased kinetic energy in rotating plant (both rotating generators and rotating loads); when there is more load than generation the frequency falls.

As the load on the electrical power system is constantly varying, the grid frequency fluctuates; inertia limits the rate of change of frequency, known as RoCoF. Directly connected rotating machines (generators, loads and machines specifically added for this purpose) provide this inertia. in electrical power systems using renewable power sources, such as wind turbines, solar panels etc. more rapid frequency fluctuations affect the grid because, in contrast to conventional generators which are directly connected to the electrical power system, wind turbines, solar panels etc. are not directly connected to the electrical power system and thus do not provide inertia to the electrical power system. One solution would be to have a larger number of conventional generators operating at reduced output, rather than a smaller number of conventional generators operating at high output, so these extra generators provide the inertia, but this is inefficient, resulting in increased carbon dioxide emissions, and expensive, High inertia, slow speed electrical machines in which the rotating machine is deliberately manufactured to be heavier than is necessary for its electrical performance are also known. The additional mass is added to the rotating machine through manufacturing the rotor to be longer than required. This, however, restricts the options of the manufacturer in that the material used must be an electrical steel as required for the electrical properties of the machine. High speed flywheels to store energy are known. Generally, high speed flywheels are manufactured from carbon fibre as a solid cylinder, rotating at a high but non- synchronous (with the system frequency) speed. Such flywheels are not directly connected to the electrical power system and so do not provide inertia to the electrical power system.

There is therefore an increasing need for a device that can provide inertia to the electrical power system in an efficient, reliable and cost effective manner, and particularly, but not exclusively, in networks using renewable power sources. Summary of the Invention

According to one aspect, the invention provides a mass arranged to be attached to an electric machine so as to rotate with the rotor of the machine. In another aspect, the invention provides an electric machine comprising a stator and a rotor rotatable relative to the stator, and an additional mass attached to the machine so as to rotate with the rotor.

In another aspect, there is provided a method of increasing the inertia of an electric machine by attaching an additional mass to the machine to rotate with a rotor of the machine.

The mass may be connected to the rotor e.g. by means of a mechanical drive train or other connection means, and may be connected directly or via a gear mechanism, in the latter case, the additional mass can rotate at a different speed and/or in a different plane to the rotation of the rotor.

To avoid adverse effects of tension forces on the rotating mass, particularly when rotating at high speeds, preferred embodiments provide a pre-stressing (pre- compression) of the mass to compensate for (or to reduce) the tension forces during rotation.

One way of pre-stressing the mass is by tightening a band of material around the body of the mass in the direction in which the tension forces act.

An alternative way of compensation for the forces of rotation is to provide the mass as a filled container such that the filling (e.g. a fluid) is better able to withstand the forces rather than a solid mass. To avoid the fluid etc. rotating with the container, thus increasing the losses in the rotor, baffles can be provided inside the container to break the flow of the filling (fluid etc.).

A valve may also be provided to empty the container. Preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a perspective view of an electric motor with which the concepts of this invention can be used.

Figure 2 is a perspective view of an alternative type of motor with which the concepts of this invention can be used.

Figure 3 is a perspective view of an electric motor such as shown in Figure 1, modified according to one embodiment of the disclosure.

Figure 4 is a perspective view of an electric motor such as shown in Figure 1 , modified according to another embodiment of the disclosure.

Figure 5 shows how stresses act on the additional mass of e.g. Figures 3 and 4.

Figure 6 shows one embodiment of pre-stressing the additional mass.

Figure 7 shows another embodiment of pre-stressing the additional mass.

Figure 8 shows an embodiment of an additional mass with the valve to empty the filled container shown.

Description

A rotating electrical machine which is directly connected to the network provides inertia. The amount of energy stored in the electrical machine at a given rotational speed is proportional to the value of inertia. This inertia is a function of the mass of the electrical machine and where the mass is physically located e.g. whether the mass is at the centre of the electrical machine or at the rim of the electrical machine.

Figure 1 shows a generic (i.e. of any type) electrical machine [1] with the rotor [2] inside the stator [3] with an air-gap between the stator and the rotor [4]. The stator of the electrical machine is directly connected to the electrical power system [5]. This connection may include circuit breakers, disconnectors and / or isolators to allow the electrical machine to be disconnected from the electrical power system when required e.g. for maintenance or when the electrical power system does not require additional inertia.

Figure 2 shows an alternative arrangement with the rotor outside of the stator.

Electric machines are well known, including for use in electrical power systems. The relative rotation between the rotor and the stator generates forces for electricity generation. The rotor may be inside the stator as in the embodiment of Figure 1, or the stator may be inside the rotor as shown in Figure 2.

Any known type of electrical machine can be used with the concept of this disclosure, including induction machines, synchronous machines with a rotor winding connected to a dc source, synchronous machines with permanent magnets fitted to the rotors, doubly fed machines with rotor windings connected to ac sources, reluctance machines, dc brushed machines or the like. The electrical machine may be e.g. a generator with a prime mover attached to drive the generator, a load to convert electrical energy to another type of energy, or e.g. a lightly loaded electrical machine (importing power to supply windage, friction and electrical losses) connected to the network either for the specific purpose of adding inertia or for the purpose of controlling reactive power flows on the network, or both.

As mentioned above, to limit frequency fluctuations, a generating system requires inertia to provide a damping effect on frequency fluctuation due to imbalances between generation and demand on the network.

The arrangement of this disclosure effectively increases the inertia of the electrical machine by providing additional mass to the electrical machine e.g. by connecting or attaching an additional mass in some manner. This additional mass must also rotate. Figure 3 shows the addition of an additional mass [6], in accordance with the invention, connected to the rotor of the electrical machine [2] through a mechanical drive train [7], including an optional gearbox [8] which enables the additional mass [6] to rotate at a different speed from the rotor of the electrical machine [2]. In an alternative embodiment, as shown in Figure 4, the additional mass [6] is connected through a mechanical drive train [7] which includes a change in the direction of the axis of rotation between the additional mass [4] and the electrical machine rotor [2]. The connection between the additional mass and the machine can enable the mass to be attached to machines with different axes of rotation (e.g. vertical or horizontal)

Other ways are also envisaged for connecting or attaching additional mass to an electrical machine other than via a mechanical drive train and/or gear mechanism. For example, a direct connection could be made between the rotor and the additional mass or other fixation means of connectors or attachment means can be used.

It is desirable that the mass can be easily attached or connected to existing electrical machines without substantial modification.

In any case, the additional mass rotates with the rotor, providing inertia (or additional inertia) which will be dependent on the additional mass. As mentioned above, in rotating the additional mass, the material from which the additional mass is composed is subject to tensile stresses i.e. the stresses will tend to stretch the additional mass in the direction of the force.

If the tensile stress is above the material yield stress then permanent deformation to the additional mass will result. Ultimately, if the tensile stress is above the ultimate tensile strength then the additional mass will break apart, which is a safety issue. Figure 5 shows the directions of the stresses on the additional mass [6] as it rotates:

[9] is the stress in the radial direction which is tensile; [10] is the stress in the tangential direction, which is also tensile, and [11] is the stress in the axial direction, if any, which may be compressive or tensile. According to this disclosure, therefore, the ability of the additional mass to withstand the tensile stress can be increased by pre-stressing (pre-compressing) the material from which the additional mass is composed.

In one embodiment, the pre-stressing can be applied by winding a band of material around the additional mass (in the direction of the tensile stress) and tightening mis band, prior to spinning the additional mass.

This technique then allows the additional mass to be manufactured from a lower tensile strength material with the band composed from a higher tensile strength material, which is likely to be cheaper than making all of the additional mass from the higher tensile strength material.

Alternatively, instead of using a band of material, the pre-stressing can be applied using a solid container made from a high tensile material shrink (or interference) fitted around the solid, additional mass (the container itself contributing to the additional mass).

Figure 6 shows the application of a band of material [12] to pre- compress the additional mass [6]. Figure 6 also shows one method of tightening the band in which the band [12] is fixed to the additional mass [6] at [ 13] . The band then passes through a hoop [14], to prevent the band moving outward radially as the band is tightened, which is attached [IS] to a pulley [16]. In this method of tightening the band, the band [12] is manufactured with saw-teeth [17] so that, as the band is tightened by pulling the band in the direction [18], it cannot slip back in the opposite direction. When the required compression is applied, the band is to be fixed to the additional mass and the hoop and pulley arrangement [14, IS and 16] removed before rotating the additional mass.

The band could be around the entire circumference of the mass, or only around a part of the circumference or even around the entire body of the additional mass (like a sleeve). Other methods of compressing the additional mass can, of course, also be used such as the container enclosing the solid, additional mass being fitted:

through an interference fit;

by pre-heating the container to expand it if it has a positive coefficient of thermal expansion and allowing the container to shrink onto the solid core of the container;

by pre-cooling the container to expand it if it has a negative coefficient of thermal expansion and allowing the container to shrink onto the solid core of the container,

by pre-cooling the solid core to shrink it if it has a positive coefficient of thermal expansion and allowing the solid core to expand inside the container;

by pre-heating the solid core to shrink it if it has a negative coefficient of thermal expansion and allowing the solid core to expand inside the container.

A further method to safely deal with the tensile stresses and avoid the risk of the additional mass breaking apart is not to manufacture the additional mass as a solid object but as a filled container. In the steady-state with the additional mass rotating, the tensile stresses on the material of a solid object are highest in the centre and decrease with increasing radius. Using a filled container means that in the regions of highest stress the deformation of the solid object is avoided. The container may take various shapes and configurations and may be made of different material compared to the fluid inside. The container may be filled (or partly filled) with a fluid. Suitable fillings include liquid, powder, granular, solutions, paste, gels, colloids and suspensions. A filled container can also be provided with a band as described above.

To avoid increased losses from the filled container over a solid additional mass from friction of the material inside the container, baffles may be included to break the path of the content as the container rotates so as to prevent the material rotating with the container itself.

Figure 7 shows the addition of baffles [19] inside the filled container. Other shapes and distributions of baffles could, of course, be used.

An additional advantage of using a filled container is that, in the event of excessive vibration then, for safety reasons, a valve could be opened spilling the contents of the filled container in a controlled way rather than a catastrophic failure.

Figure 8 shows the addition of a valve [20] to open in the event of excess vibration, with a local control unit [21] which can be signalled remotely. Different types of valve could be used including single and multiple-use valves. If signalling is used to control the valve, this could be done by e.g. a direct mechanical connection, electronically hardwired or remotely by e.g. radio, laser, infra-red, Bluetooth, wireless or similar. Thus, the present disclosure provides a solution to the problem of no, or insufficient inertia in an electricity network, which can lead to undesirable frequency fluctuations. The solution involves providing an additional mass to be connected or attached or affixed to the machine rotor, or to rotate with the rotor. The additional mass can be attached to any existing machine and the size of the mass can be selected accordingly without the need for a completely new machine.

Because the mass is an additional component, there is more flexibility as to how it can be attached to the machine, e.g. in view of space or speed constraints, some examples being shown in Figures 3 and 4.

The ideas described herein would find application in conventional, terrestrial electrical power systems from small single-generator networks to multi-national grids, or e.g. for networks on board ships, aircraft, submarines, cars, trains, rockets, satellites, space vehicles, and other vehicles or locations. The disclosure can find application in power systems operating at different frequencies, including fixed and variable frequencies e.g. 50Hz, 60Hz or 400 Hz or 360 - 800 Hz.

Claims

A device comprising a mass body configured to be attached to an electric machine so as to rotate with the rotor of the machine.

The device of claim 1 comprising means for pre-stressing the mass body.

The device of claim 2, wherein the means for pre-stressing the mass body comprises a band tightened around a body the mass body.

The device of claim 2 or 3, wherein the mass body comprises a container and the means for pre-stress the mass body comprises a content of the container.

5. The device of claim 4, wherein the content comprises one of a fluid, liquid, powder, gel, solution, paste, granular, colloid and suspension.

The device of claim 4 or 5, further comprising a valve in the container for dispensing the content from the container.

The device of claim 6, further comprising means for controlling operation of the valve.

The device of any of claims 4 to 7, further comprising baffles inside the container.

9. The device of claim 3 or any claim dependent thereon, wherein the band is provided around at least part of the circumference of the mass body.

10. An apparatus for adding inertia to an electric machine, comprising: an electric machine having a rotor and a stator, and a device as claimed in any preceding claim attached to the machine to rotate with the rotor.

11. A method of increasing the inertia of an electric machine by attaching an additional mass to the machine to rotate with a rotor of the machine.

12. The method of claim 11, wherein the additional mass is a device as claimed in any of claims 1 to 9.