Patent ID: 12259422

DETAILED DESCRIPTION

In order to address the above issues, the present invention provides a method, as well as a system that monitors the health of insulation in a power system. Whilst the examples described herein are described with reference to an aircraft's power districts and motor drive systems, as would be understood, the present invention may find use in other applications, and is not limited to its use within aircraft.

FIG.1shows and example motor drive system capable of monitoring the insulation at various points within the motor drive system.

The system comprises a power source1connected to a motor harness8, and a motor9via a motor drive100. The motor drive100comprises an EMC filter2, bidirectional switches3aand3b, an impedance network4, a power bridge/inverter5, current monitoring circuits6aand6b, and switches7aand7b.

In normal operation, DC power from the power source1may be filtered by an EMC filter, and passed through a closed bidirectional switch3aconnected to the positive terminal, and a closed bidirectional switch3bconnected to the negative terminal of the power source1. The closed bidirectional switches may be, for example, thyristors. The output of the switches3aand3bis connected the impedance network4(e.g. a lattice network with two inductors L1, L2, one on each rail, and two capacitors C1, C2in an X-shape across the inductors, or an equivalent circuit), which functions as a local energy storage for the power bridge/inverter5. High voltage DC (HVDC+) is provided on the HVDC+ rail to top switches5a,5b,5cof the inverter5, and low voltage DC (HVDC−) is provided on the HVDC− rail to bottom switches5d,5e,5fof the inverter5. The power/bridge/inverter then provides AC power to the motor9via harness8. The inverter module5can be based on any switching devices, for example IGBTs, SiC or relays.

By providing the impedance network4in the motor drive100, it is possible to boost the input voltage by shorting HVDC+ to HVDC− in a resonant interaction. Such a resonant interaction causes the input current to drop to zero, thereby allowing the voltage at its output to be greater than the voltage at its input. Additionally, with such a zero current at the input, it is possible to disconnect the bidirectional switches3aand3bfrom the DC grid (i.e. the power source1) during the resulting zero current condition. In this way, the impedance network can function as a decoupling circuit, thereby allowing for decoupling of the load (i.e. the inverter5and motor9) from the source (power supply1, and filter2by opening the switches3aand3band acts like a zero current disconnect for the DC grid.

In order to determine whether there has been any degradation of the insulation at various points within the system, the voltage in the load may be boosted, as outlined above, to a desired level to determine the characteristics of the circuits at said level. For example, it may be desirable to determine whether there is any discharge at the level of normal operation. In modern aircraft systems, this can be in the excess of 1000 v or higher. Therefore, in such systems, it is desirable to boost the voltage in the load to such a level, and then to decouple load.

Additionally, by boosting the voltage in the load and reducing the current demand to zero, it is possible to turn off switches3aand3b. For example, these switches may be thyristors, which may be turned off when there is zero current demand. Once the load has been decoupled, then it is possible to determine whether there is any excessive leakage current which may be attributed to a failure in the insulation. In an ideal system, with perfect insulation, there would be no leakage, or limited leakage current from the load and system insulation. The voltage would therefore remain at the boosted level.

Therefore, in order to test for faults in the insulation of system1, firstly, the DC link voltage is boosted to the desired level. In the example ofFIG.1, this could may be achieved by controlling the inverter module5to cause a momentary short circuit between HVDC+ rail and HVDC− rail. For example, this may be done by turning switches5a∥5d,5b∥5e,5c∥5fon, or all the switches on at the same time. This results in a momentary short, and during this operation, the equivalent capacitance and inductance of the impedance network causes a slightly increased current flow through its inductive part. When the switches are disconnected, the energy that is stored in the inductive components creates a voltage boost, which is then stored in the capacitors. As would be appreciated, the boost operation is controlled by the length of time the short is applied, and may be controlled in, for example, a PWM fashion. The result is an increase in the DC link voltage due to the presence of the impedance network4.

Once the DC link voltage is boosted, the motor dive circuit may be isolated. This is achieved by turning off switches3aand3b, which is possible during zero current demand from the power source1. Such a zero current demand results when the inverter switches are turned on momentarily to short HVDC+ and HVDC−, as described above.

At this point, when the DC link voltage has been boosted, and the motor drive circuit isolated, then it is possible to perform isolation tests.

In a standalone inverter (i.e. where the motor cables are fixed to the output of the motor drive100), it is possible to test insulation of the motor drive first and then insulation of the load. For example, in the example ofFIG.1, the following tests may be performed:

TABLE 1Test No.Test objectivesMethod1Test the insulation of inverterTurn OFF 5a, 5b and 5ctop switches 5a, 5b, 5c onlyand 7b,Turn ON 5d, 5e, 5f and 7a.2Test the insulation of inverterTurn ON 5a, 5b, 5c and 7b,bottom switches 5d, 5e, 5f,Turn OFF 5d. 5e, 5f and 7a.motor windings and harness 8.3Test the insulation of inverterTurn OFF 5d, 5e, 5f and 7a,bottom switches 5d, 5e, 5fTurn ON 5a, 5b, 5c and 7b,onlyand disconnect the load (e.g.the motor windings and theharness)

In this way, by providing only certain current paths in the floating system, it is possible to isolate certain parts of the system, and therefore identify the exact source of any leakages (and therefore, where there may exist a failure in the insulation).

By providing the above tests, it is possible to determine not only the overall leakage within the system, but also the leakages separately of the inverter top switches, the inverter bottom switches, as well as the load (motor/motor harness). In this way, these tests allow for a determination of leakage of the inverter top switches only, and a determination of the leakage of the bottom switches only. Then, by connecting the load, it is possible to determine the combined leakage of the inverter bottom switches, and the motor/motor harness. This will be a sum of the leakage within the inverter bottom switches, as well as the leakage in the motor/motor harness. By subtracting the known leakage in the inverter bottom switches from the combined leakage of the inverter bottom switches and the motor/motor harness, it is possible to work out the leakage (and therefore the insulation condition) of the motor/motor harness itself, thereby more accurately identifying the source of any issues. Of course, a similar method may be used by connecting the load in test1, thereby measuring the combined leakage of the top switches and the load.

The load may be disconnected in any known manner. For example, a multiplexer may be used to disconnect the load. Additionally/alternatively, the load may be disconnected by a physical removal or by isolating the load using switches.

During each test, the current is monitored by current monitoring circuits6a, and6b(whichever is connected), which are able to detect any partial discharge or excessive leakage occurs. It is possible to store those measurements and form predictions about remaining life or present state of insulation. Specifically, switches7aand7bmay switch ON or OFF, thereby creating a reference point for the current to go through, and enabling the relevant monitoring circuit6aand/or6bto measure the current passing through.

FIG.2shows a power district architecture example where multiple motor drives100may be combined in parallel to drive one or more motors11, depending on the system requirements. This is managed through a reconfiguration matrix10(for example, a multiplexer) to connect the required motor(s)11to the required motor drive(s)100, thereby allowing each motor drive100to drive any one of the motors11, M1, M2etc. Such a power district may be found in, for example, an aircraft.

The insulation testing method set out above may still be used in this setup, and a single motor drive can test all the connected motors. For example, motor drive1can have the circuit configuration100shown inFIG.1. Following the above steps above, test2(shown in Table 1 above) can be used to test all of the remaining motor drives, along with all the motors11and their associated harnesses with only a single detection circuit6a,6b. In this way, whilst the reconfiguration matrix10may be is used to multiplex loads (i.e. motors) during normal operation, it may be used to test other insulation within the system. For example, the insulation testing motor drive1(having a configuration100as shown inFIG.1) can be used as an “insulation tester” and connect to the outputs of motor drive2via the reconfiguration matrix10. The connection of another motor drive to the outputs of motor drive1allows for the insulation of motor drive2(or any motor load within the system) to be checked. It is also possible to test insulation of other power blades in the power rack. As the leakage of the inverter of the motor drive can be tested in isolation, then it is possible to attribute any further leakage to the load that is connected by the multiplexer.

Such a method allows the system to monitor the health of the insulation in the power district as a part of start-up or diagnostic routine. For example, the above method may be performed upon a regular start-up of an aircraft, in order to accurately track any potential leakage, and how it might be developing over time. This data may be used during a scheduled repair, or used to prompt the scheduling of a repair to fix the identified issues. Additionally, or alternatively, such a routine may be performed during maintenance in order to determine whether certain components in a motor drive, or within a motor should be serviced.

By monitoring partial discharge, it is possible to assess whether there has been any degradation of the insulation on a certain channel. If partial discharge is detected, then a scheduled repair may planned be to address the source of the partial discharge, before it results in accelerated ageing of the insulation. Additionally or alternatively, in the case where partial discharge is detected at a particularly high voltage, then a decision may be made to reduce the operating voltages to a safe level where discharge no longer occurs, until a repair may be made.

By effectively monitoring the health of the insulation of a power district, whether that be as part of aircraft start-up or diagnostic routine, it is possible to mitigate the issues outlined above by detecting degradation of insulation before it results in total failure. Failure of insulation will increase leakage current to a very high level, which may eventually be detected by the aircraft power distribution system (GCI), where the faulty power branch will be disconnected and backup functionality will be deployed. This reduces functionality which can lead to a worsened customer experience, and result in unscheduled maintenance. At this point, it would otherwise be necessary to investigate during maintenance to identify the cause of the issues. However, the circuits and methods outlined herein would allow for a single weight neutral test routine to be used to monitor the state of the insulation of a standalone motor drive or whole power district.

Further, the method considered herein may be used during the design of aircraft power districts, for example to optimize isolation thickness in cables. During design, by knowing that the state of the isolation is going to be monitored, less isolation can be used, which can in turn help to manage the heating effects of bundled wires.

Such a method provides the ability to carry out diagnostic and prognostics for insulation in increasingly challenging environments for a selected load, and the ability to detect failures and identify its origins before they occur naturally helps in improving safety. This may be implemented with a cheap add on circuit that is capable of detecting leakage current and partial discharge, and which is also compatible for testing all elements of power district scalable systems. By monitoring the state of the insulation, and thereby by detecting when there is an appreciable amount of degradation in the insulation, potential faults may be identified at their exact location and addressed before they become critical. This reduces down time, and unexplained aircraft protection trips, which can take substantial amount of time to identify the source of the failure. Further, in an emergency, the resonance network can be used as a HVDC zero current safety disconnect.