GB2490305A

GB2490305A - High frequency generator for isolated networks comprising a high frequency converter connected via an AC link

Info

Publication number: GB2490305A
Application number: GB1105707.2A
Authority: GB
Inventors: Maurizio Catucci
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2011-04-05
Filing date: 2011-04-05
Publication date: 2012-10-31
Also published as: GB201105707D0

Abstract

An isolated electrical network, comprising a high frequency generator 314 connected to a prime mover 312 and rotating shaft 316, a convertor 310 connected to the high frequency generator via a high frequency AC link (526 fig 5), and a load 320 electrically connected to the convertor to receive electrical power. The generator output may have a fundamental frequency of between 2KHz and 50KHz preferably above 5KHz and an output power between 50 and 500KW. The converter is preferably a matrix converter having cells of bidirectional switching arrangements. A phase locked loop is used to control the switching of the converter, which uses a pulse density modulation technique. The isolated network also comprises a controller configured to implement a predictive controller or hysteretic controller for the converter switching. A high frequency transformer 315 may be located between the converter and the electrical generator, which may be single phase. The converter may be controlled to switch at the zero crossing of the current or voltage. In one embodiment the electrical generation network (510, fig 5) incorporates an AC link (516, fig 5) which includes a ring circuit (522, fig 5) and which feeds multiple converters (520, fig 5) and their associated loads. The electrical generator is preferably powered by a mechanical drive such as a gas turbine, with a speed range within 40,000 to 100,000rpm, and is preferably a synchronous generator of either wound field machine type or of permanent magnet type. As the system is targeted towards aircraft, the use of an AC link eliminates the heavy and bulky items associated with the conventional DC link, such as smoothing capacitors, DC breakers etc.

Description

AN ELECTRICAL GENERATION SYSTEM

This invention relates to an electrical generation system.

Existing electrical systems for aircraft typically include one or more synchronous generators each mechanically driven by an engine. The electrical power is utilised by a variety of loads located within the engine and the airframe.

The electrical supply required by the loads varies but typically a frequency of approximately 400Hz is required. However, because the output of a synchronous generator is dependent on the rotational speed of the engine, which is in turn determined by the thrust requirement for the aircraft, the frequency outputted by a generator can be variable. Hence, there exists a general need to provide an electrical generation system which can provide a substantially fixed frequency supply from a variable speed mechanical drive.

Figure 1 shows a known system which uses back-to-back convertors to provide a desired frequency for the network. Specifically, Figure 1 shows an electrical generation system 10 including a prime mover 12 in the form of a rotating shaft of a gas turbine engine which mechanically drives the rotor of an electrical generator 14 via a shaft arrangement 16. The electrical generator 14 can be any suitable type but is typically a wound field synchronous generator arrangement which is fed from some form of shaft mounted exciter.

The three phase generator output 18 is fed into a power conditioning arrangement in the form of a back-to-back convertor 20. The first convertor 22 takes the three phase output from the generator 14 and converts it to direct current, DC. The DC is routed via a DC link 26 to the second convertor 24 which converts the DC into an alternating current supply at a required frequency for use by the electrical loads 28 on a network.

Where required, loads 30 can be coupled directly to the DC link to receive DC power.

Figure 2 shows a detailed view of a typical DC link 210 which is fed power from a first convertor (not shown) as described above. The DC link 210 includes a ring circuit 212 having a number of radial lines 214a-d, each of which connect to one or more loads via a convertor 216a-d which regulates the electrical supply for a specific load or loads (not shown).

Each radial line 214a-d incorporates an energy storage device 218a-d which acts to stiffen the electrical network and help reduce voltage droop which may occur during switching in and out of loads. A typical energy storage device 21 8a-d would be a capacitor. Each radial line also includes a circuit breaker 220a-d to isolate a specific convertor 21 6a-d and load in the event of a fault.

Although the above described generation system 10 can provide power from a variable speed mechanical drive to fixed electrical frequency loads, it is heavy due to the energy storage devices 218a-d in each radial line 214a-d. Further, the circuit breakers 220a-d need to be rated to switch high levels of DC which exist in fault conditions. This adds further weight to the system. Excessive weight is generally undesirable for aero applications.

The present invention seeks to provide an improved electrical generation system.

In a first aspect the present invention provides an isolated electrical network, comprising: a high frequency generator; a convertor connected to the high frequency generator via a high frequency AC link; and, a load electrically connected to the convertor so as to receive electrical power, when in use.

Having a high frequency generator to provide power to a convertor allows the removal of the DC link which is currently employed with back to back convertors.

It will be understood that an isolated network is one with a low electrical inertia. That is, the isolated electrical network may have a floating voltage and current. The isolated network may include between one and ten high frequency generators. The high frequency generators may be low power. For example, the generators may be rated below 1 MW of power to the network. Each generator can be rated to provide between approximately 25 kW to 500 kW. The generator can be rated to provide between approximately 200 kW and 300 kW.

The isolated electrical network may be part of a vehicle or vessel. The vehicle may be an aero vehicle. The aero vehicle may be a plane or a helicopter. The vessel may be a surface going water vessel or a submersible water vessel.

The generator may be powered via a mechanical drive. The mechanical drive may be a gas turbine engine. The gas turbine engine may be an aero engine. Alternatively, the gas turbine may be part of a marine drive system. The rotational speed of the mechanical drive can be variable. The rotational speed of the mechanical drive may vary within the range bounded by the values 4000 rpm and 8000 rpm. The rotational speed of the electrical generator's rotor may be between 40,000 rpm and 100,000 rpm.

The electrical generator, convertor and load may be co-located or located remotely from each other. For example, the electrical generator may be located within the gas turbine engine and the convertor and electrical load may be located on an airframe.

Multiple loads may be connected to a single convertor. The network may include multiple convertors.

The electrical generator can be a synchronous machine. The synchronous machine may be a wound field machine. Alternatively, the synchronous machine can be a permanent magnet machine.

It is to be understood that high frequency can include an electrical fundamental frequency which is above approximately 2 kHz. The fundamental frequency may be in the range bounded by approximately 2 kHz and 50kHz. The fundamental frequency may be in the range bounded by approximately 20 kHz and 40 kHz. The fundamental frequency can be above approximately 5 kHz.

The switching frequency of the convertor may be between 4kHz and 100kHz. It will be understood that the switching frequency of the convertor would ideally be twice that of the fundamental frequency of the electrical generator.

The convertor may be a matrix convertor. The matrix convertor may include a plurality of cells having bidirectional switching arrangements.

The network may further comprise a phase locked loop arranged to receive an input from the high frequency AC link or directly from the output of the electrical generator.

An output of the phase locked loop may be used to control the switching of the convertor.

The convertor is switched using a pulse density modulation technique.

The network may further comprise a controller configured to implement a predictive controller scheme for controlling the convertor switching. Alternatively, the controller may be configured to implement a hysteresis controller.

The network may further comprise a high frequency transformer located between the convertor and the output of the electrical generator.

The output of the electrical network can be single phase. Alternatively, the output of the electrical network can be three phase.

The convertor may be controlled to switch at the zero crossing of the current or voltage.

The output power of the electrical generator may be in the range between approximately 50 kW to 500 kW. The output power may be in the range between 200 kW and 300 kW.

Figure 1 shows a known arrangement of an electrical generation system used in aero applications.

Figure 2 shows a known DC link arrangement of the electrical generation system shown in Figure 1.

Figure 3 shows an electrical generation system of the present invention.

Figure 4 shows a bidirectional switching arrangement.

is Figure 5 shows an AC link arrangement of the electrical generation system of the present invention.

In Figure 3 there is shown an electrical generation system 310 including a prime mover 312 in the form of a rotating shaft of a gas turbine engine which mechanically drives the rotor of an electrical generator 314 via a shaft arrangement 316. The output of the generator is fed into a power convertor arrangement 318 for conversion into the frequency required by the electrical loads 320a, 320b.

The electrical generator 314 is a high frequency permanent magnet synchronous generator which outputs a single phase high frequency alternating current. The high fundamental frequency output of the generator 314 of the embodiment is approximately kHz. However, it is to be appreciated that a higher fundamental frequency output can be used. The electrical generator of the embodiment is capable of supplying power in the up to approximately 500 kW, but lower or higher power generators may be used where required. The electrical network includes a high frequency transformer 315 as are known in the art for converting the output of the generator to a required voltage.

In the described embodiment, the power conversion arrangement is in the form of a conventional single phase to three phase matrix convertor. Matrix convertors (otherwise known as cyclo-convertors and AC Link convertors) are known in the art.

Typically, a matrix convertor includes a plurality of cells arranged in a matrix, each cell includes a switch. In the present embodiment, the arrangement of switches allows for bidirectional power flow which can be achieved with a number of arrangements. Figure 4 shows a typical single switching cell 410 which includes a pair of IGBTs, insulated gate bipolar transistor, 412a, 412b, each having collector, C, emitter, E, and gate, G, connections. The emitters of the IGBT's are connected together such that bidirectional current flow is possible.

The invention uses a high frequency generator as an input to the convertor which converts the input frequency to a desired output frequency, be that AC or DC, as required by the various loads. The switching of the convertor is restricted to zero crossing of the current or voltage so as to reduce the switching losses in the convertor.

With this arrangement, the high frequency input provides the basic unit of synthesis for constructing the lower frequency output, using a pulse width modulation control philosophy. The zero crossing results in a relatively small sized unit when compared with a conventional back to back DC link convertor arrangement.

The convertor can be controlled using a predictive controller 319 as is known in the art for high frequency AC Links. An example of a suitable predictive controller is described in "ZCS Predictive Controllers for High Frequency AC-Link Resonant Convertors", Catucci, M; Clare, J; and Wheeler, P; EPE-PEMC conference, Portoroz, Sept 2006.

However, the skilled person will appreciate that other control methods may be used.

For example, the controller may be an analogue controller in the form of a hysteresis controller as are known in the art.

In the case of a synchronous generator, the variable speed of rotor will directly affect the generator output frequency and thus the input frequency of the convertor and the associated switching required to produce the desired output frequency for the load. For this reason, a phase locked loop 317 may be incorporated to track the output frequency of the generator. The output of the phased locked loop can inputted into the controller to provide a reference signal for controlling the switching frequency of the convertor.

Another advantage offered by the high frequency switching device is that the energy storage device which is required to maintain the energy requirements of the network are not needed. This is because the convertor can be controlled to match the load requirements with the required power being provided by the high frequency generator.

Nevertheless, the skilled person will appreciate that the convertor may include a filter arrangement which may incorporate some energy storage.

Figure 5 shows a schematic of an electrical generation network 510 having a high frequency generator 512 which feeds multiple convertors 520a-c and loads (not shown) via a high frequency single phase AC link 516. As with the embodiment shown in Figure 3, the high frequency generator 512 is driven via rotating shaft coupled to a mechanical drive, for example, a gas turbine engine (not shown). Although the AC link and electrical generator of the embodiment are single phase, it will be appreciated that the invention may incorporate a three phase electrical generator and corresponding AC link.

The AC link 516 includes a ring circuit 522 having a number of radial lines 524a-d, each of which connect to one or more loads (not shown) via the matrix convertors 520a-d which regulate the electrical supply for the specific load or loads. Each radial line includes a circuit breaker 526a-d to isolate a specific convertor 520a-c and load in the event of a fault. It will be appreciated that the size of the breakers can be reduced when compared to the DC link equivalent arrangement due to the zero crossing current switching which can be employed.

It will be appreciated that a gas turbine rotor has a variable speed. The speed of rotation of a given shaft is dependant on which shaft it is, i.e. low pressure, intermediate pressure of high pressure. Typically, the high frequency generator will be driven by the intermediate pressure shaft which rotates between 5000 rpm and 7000 rpm. However, the skilled person will appreciate that gearing can be used which results in a rotational speed of up to 100,000 rpm. Other rotational speeds may be used depending on the engine configuration.

Claims

CLAIMS1 An isolated electrical network, comprising: a high frequency generator; a convertor connected to the high frequency generator via a high frequency AC link; and, a load electrically connected to the convertor so as to receive electrical power, when in use.
2 An isolated electrical network as claimed in claim I wherein the electrical generator outputs a fundamental frequency above approximately 5 kHz.
3 An isolated electrical network as claimed in claims I or 2 wherein the convertor is a matrix convertor.
4 An isolated electrical network as claimed in claim 3 wherein the matrix convertor includes cells of bidirectional switching arrangements.
5 An isolated electrical network as claimed in claims 1 to 4 further comprising a phase locked loop arranged to receive an input from the high frequency link, wherein an output of the phase locked loop is used to control the switching of the convertor.
6 An isolated electrical network as claimed in any preceding claim wherein the convertor is switched using a pulse density modulation technique.
7 An isolated electrical network as claimed in any preceding claim further comprising a controller configured to implement a predictive controller for controlling the convertor switching.
8 An isolated electrical network as claimed in any of claims 1 to 6 further comprising a controller configured to implement a hysteresis controller for controlling the convertor switching.
9 An isolated electrical network as claimed in any preceding claim further comprising a high frequency transformer located between the convertor and the output of the electrical generator.
An isolated electrical network as claimed in any preceding claim wherein the output of the electrical generator is single phase.
11 An electrical generation system as claimed in any preceding claim wherein the convertor is controlled to switch at the zero crossing of the current or voltage 12 An electrical generation system as claimed in any preceding claim wherein the output of the electrical generator has a fundamental frequency in the range between approximately 2 kHz to 50 kHz.13 An electrical generation system as claimed in any preceding claim wherein the output power of the electrical generator is in the range between approximately kWto 500 kW.