US20150137786A1

US20150137786A1 - Overvoltage limiter in an aircraft electrical power generation system

Info

Publication number: US20150137786A1
Application number: US14/606,293
Authority: US
Inventors: Vijay K. Maddali; John F. Defenbaugh
Original assignee: Hamilton Sundstrand Corp
Current assignee: Hamilton Sundstrand Corp
Priority date: 2011-01-14
Filing date: 2015-01-27
Publication date: 2015-05-21
Also published as: EP2477294B1; US20120182648A1; EP2477294A1; US8970183B2

Abstract

A method of limiting a generator voltage in an overvoltage condition includes the steps of determining an amount of overvoltage of a generator output voltage exceeding a specified voltage and calculating a reference threshold voltage based upon the duration of overvoltage. A switch is modulated according to a voltage error between the output voltage and the reference threshold voltage. The current flow within the generator is interrupted based upon the voltage error to limit the output voltage to a desired voltage.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure is a divisional of U.S. patent application Ser. No. 13/006,576 filed Jan. 14, 2011.

BACKGROUND

This disclosure relates to a generator for a power generating system. In particular, the disclosure relates to an overvoltage limiting configuration and a method of limiting the output voltage of a generator to a desired voltage under overvoltage conditions.
One type of aircraft electrical power generating system includes a variable frequency generator. The variable frequency generator includes a permanent magnet generator (PMG), an exciter, and a main generator mounted for rotation on a common shaft. The shaft is driven by a prime mover.
A generator control unit (GCU) converts alternating current from the PMG to provide DC current to the exciter. Current from the exciter is fed to the main generator, which produces a voltage output.
Under some fault conditions, an overvoltage condition may result, which produces a higher than desired output voltage from the main generator. There are many strategies for limiting or preventing overvoltage conditions, but desired overvoltage protection remains lacking. For example, one typical overvoltage management strategy simply trips a switch to an open condition once a overvoltage threshold has been reached. Another strategy delays tripping the switch depending upon the duration of the overvoltage to avoid needlessly tripping the switch for a brief overvoltage spike. In both of the above strategies, once the undesired overvoltage has occurred, the generator is de-energized and effectively disabled, which may require the switch to be mechanically reset. Thus, the generator is not capable of supplying power during a persistent overvoltage condition.

SUMMARY

In one exemplary embodiment, a method of limiting a generator voltage in an overvoltage condition includes the steps of determining an amount of overvoltage of a generator output voltage exceeding a specified voltage and calculating a reference threshold voltage based upon the duration of overvoltage. A switch is modulated according to a voltage error between the output voltage and the reference threshold voltage. The current flow within the generator is interrupted based upon the voltage error to limit the output voltage to a desired voltage.
In a further embodiment of the above, the method includes using point of regulation voltage.
In a further embodiment of the above, the method includes decreasing the reference threshold voltage as the duration of the overvoltage increases.
In a further embodiment of the above, the switch is arranged between a permanent magnet generator and an exciter. The method includes interrupting the current between the permanent magnet generator to the exciter.
In a further embodiment of the above, the method includes interrupting the current along a return path from the exciter to the permanent magnet generator.
In a further embodiment of the above, the method includes tripping an overvoltage protection switch if at least one of a maximum overvoltage is exceeded or a duration of overvoltage condition exceeds allowable limits.
In another exemplary embodiment, a method of limiting a generator voltage in an overvoltage condition includes the steps of determining an amount of overvoltage of a generator output voltage exceeding a specified voltage. A switch is modulated to the specified voltage. The current flow within the generator is interrupted based upon the voltage air to limit the output voltage to a desired voltage.
In a further embodiment of the above, the switch is arranged between a permanent magnet generator and an exciter. The current between the permanent magnet generator to the exciter is interrupted.
In a further embodiment of the above, the current along a return path from the exciter to the permanent magnet generator is interrupted.
In a further embodiment of the above, an overvoltage protection switch is tripped if at least one of a maximum overvoltage is exceeded or a duration of overvoltage condition exceeds allowable limits.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a general schematic sectional view of a generator for a gas turbine engine.

FIG. 2 is a schematic view of an overvoltage protection arrangement for a generator.

FIG. 2A is a schematic view of an exciter field driver shown in FIG. 2 with an H-bridge including a pair of MOSFETs and a pair of flyback diodes.

FIG. 3 is a flow chart illustrating a method of limiting a generator voltage in an overvoltage condition.

FIG. 4 is a flow chart illustrating another method of limiting a generator in an overvoltage condition.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates selected portions of an example generator 10 driven by a prime mover 12 such as a gas turbine engine and for generating electrical current when being driven by the prime mover 12. The generator 10 may generally include a dynamoelectric portion 14, hydraulic pump 16 and a gear train 18, all contained within a common housing assembly 20.
The dynamoelectric portion 14 in the disclosed exemplary embodiment is a 3-phase machine that includes three machines 22, 24 and 26 mounted on a rotor shaft 28 along an axis of rotation A. Stator assemblies 22B, 24B, 26B of the three machines are installed in the housing assembly 20, and the three rotor assemblies 22A, 24A, 26A are installed on the rotor shaft 28. The housing assembly 20 may be closed with a drive end cover assembly or housing portion 20A, through which the rotor shaft 28 extends, and an end plate 20B.
The first machine 22 includes a permanent magnet generator (PMG) with a rotor assembly 22A and a stator assembly 22B. The stator assembly 22B supplies power for generator excitation, as well as power for other components of the electrical system. The second machine 24 includes an exciter with a rotor assembly 24A and a stator assembly 24B. The exciter receives field excitation from the PMG through a GCU 32 (Generator Control Unit). The output of the rotor assembly 24A is supplied to a shaft mounted diode pack 30. The diode pack 30 may be divided into six diode groups to provide a 3-phase full wave bridge rectification. The DC output of the diode pack 30 supplies the third machine 26, or main generator, which provides a desired output voltage from a POR 34 (Point of Regulation).
Portions of the GCU 32 are illustrated in more detail in FIG. 2. The GCU 32 includes a bridge rectifier 36 that converts 3-phase alternating current from the PMG 22 to DC power. The bridge rectifier 36 is arranged in a circuit 38 with an exciter field driver 44, which provides power to the exciter 24. In one example, the exciter field driver 44 includes an H-bridge having a pair of MOSFETs and a pair of fly-back diodes.
A capacitor 42 is provided in the circuit 38 to reduce the DC voltage ripple from the bridge rectifier 36. An exciter controller 45 is electrically connected between the POR 34 and the exciter field driver 44. The exciter controller 45 receives point of regulation (POR) voltage from the POR 34 and provides a desired voltage/current command signal 47 in response thereto to the exciter field driver 44 to achieve the desired output voltage from the main generator 26. A faulty desired voltage/current command signal may result in improper control of the exciter field driver 44 thereby resulting in an overvoltage condition. The exciter field driver may also include a conventional overvoltage protection switch that trips if a maximum overvoltage is exceeded or the duration of overvoltage condition exceeds allowable limits.
A switch 46, such as a MOSFET, is provided in the circuit in a return path 40 from the exciter field driver 44 to the bridge rectifier 36. The switch 46 includes open and closed conditions. Current flows through the circuit 38 in the closed condition, and current flow is interrupted in the open condition. An overvoltage limit controller 48 is electrically connected to the switch 46 and receives signals from the POR 34. The overvoltage limit controller 48 determines an amount of overvoltage exceeding a specified voltage in an overvoltage condition by detecting all three phase voltages. In one example, the specified voltage for over voltage condition for the main generator 26 may be 240 volts. The point of regulation (POR) voltage from the main generator 26 may be 300 volts for example, corresponding to an overvoltage condition. In one implementation of the voltage limiter, the overvoltage threshold for the limiter may be set at a fixed value of 280 V. The overvoltage limit controller 48 then modulates the switch 46 to limit the POR voltage 34 to 280 V as long as the overvoltage condition exists. Thus, the generator 10 can continue to supply power to a component, such as an aircraft system.
Referring to FIG. 3, a method of limiting overvoltage of the generator is generally indicated at 50. The actual POR voltage is measured, as indicated at block 51. The amount of overvoltage is compared to a specified voltage (for example, 280V), which can be the lowest threshold voltage considered as overvoltage, as indicated at block 52. The switch is modulated to maintain the output voltage at the specified voltage throughout the overvoltage condition, as indicated at block 54. As a result, the current flow between the PMG and exciter is interrupted to limit the voltage (block 56). If the overvoltage limit controller is unable to maintain the output voltage at or below the specified voltage and/or the output voltage exceeds a maximum overvoltage or the duration of overvoltage condition exceeds allowable limits, then the conventional voltage protection switch may be tripped, as indicated at block 58.
In another implementation, the overvoltage limit controller 48 uses a reference voltage threshold based upon the duration of the overvoltage condition, and modulates the switch 46 to limit the POR voltage to the reference voltage threshold; the longer the duration, the lower the reference voltage threshold will be. This varying threshold profile starts at an upper overvoltage threshold, which is less than the maximum overvoltage, and continues to decrease as a function of time. This process occurs iteratively such that the actual POR voltage converges on the desired voltage below the specified voltage.
A rate of change of voltage may also be used in combination with the POR voltage to allow limiting the voltage sooner in the case of rapidly changing POR voltages.
The overvoltage limit controller commands the switch open and closed to interrupt current flow within the circuit 38 based upon the error between the actual POR voltage, or a combination of actual POR voltage and weighted rate of change in voltage, and the reference threshold voltage to limit the output voltage (actual POR voltage) to the desired voltage. The rate of open and close of the switch is determined by the degree of hysteresis provided at the reference voltage threshold. That is, operation of the switch 46 based upon the reference voltage threshold will achieve the desired voltage at the main generator. For example, an output voltage of 300V may necessitate the switch to be modulated OFF and ON for 40 ms into the overvoltage condition to limit the output voltage to 300V and then continue to limit the output voltage to lower voltages as time progresses.
This method of limiting overvoltage of the generators is generally indicated at 60 in FIG. 4. The actual POR voltage is measured, as indicated at block 62. The amount of overvoltage is compared to a specified voltage, which is the lowest threshold voltage considered as overvoltage, as indicated at block 64. A reference threshold voltage is calculated based upon the duration of overvoltage (block 66). The switch is modulated according to a voltage error between the actual POR voltage and reference threshold voltage, as indicated at block 68. As a result, the current flow between the PMG and exciter is interrupted to limit the voltage (block 70). The steps of blocks 66, 68, 70 may be repeated to iteratively converge upon the desired voltage and achieve an output voltage at or below the specified voltage. If the overvoltage limit controller is unable to maintain the output voltage at the specified voltage and/or the output voltage exceeds a maximum overvoltage or the duration of overvoltage condition exceeds allowable limits, then the conventional voltage protection switch may be tripped, as indicated at block 72.
The switch 46 can be used and tripped when implementing the conventional voltage protection feature. That is, the same switch can be used for both overvoltage limiting and overvoltage protection.
Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.

Claims

What is claimed is:

1. A method of limiting a generator voltage in an overvoltage condition comprising the steps of:

a) determining an amount of overvoltage of a generator output voltage exceeding a specified voltage;

b) calculating a reference threshold voltage based upon the duration of overvoltage;

c) modulating a switch according to a voltage error between the output voltage and the reference threshold voltage; and

d) interrupting current flow within the generator based upon the voltage error to limit the output voltage to a desired voltage.

2. The method according to claim 1, wherein step a) includes using point of regulation voltage.

3. The method according to claim 1, wherein step b) includes decreasing the reference threshold voltage as the duration of the overvoltage increases.

4. The method according to claim 1, wherein the switch is arranged between a permanent magnet generator and an exciter, and step d) includes interrupting the current between the permanent magnet generator to the exciter.

5. The method according to claim 4, wherein step d) includes interrupting the current along a return path from the exciter to the permanent magnet generator.

6. The method according to claim 1, comprising step e) tripping an overvoltage protection switch if at least one of a maximum overvoltage is exceeded or a duration of overvoltage condition exceeds allowable limits.

7. A method of limiting a generator voltage in an overvoltage condition comprising the steps of:

b) modulating a switch to the specified voltage; and

c) interrupting current flow within the generator based upon the voltage air to limit the output voltage to a desired voltage.

8. The method according to claim 7, wherein the switch is arranged between a permanent magnet generator and an exciter, and step c) includes interrupting the current between the permanent magnet generator to the exciter.

9. The method according to claim 8, wherein step c) includes interrupting the current along a return path from the exciter to the permanent magnet generator.

10. The method according to claim 7, comprising step d) tripping an overvoltage protection switch if at least one of a maximum overvoltage is exceeded or a duration of overvoltage condition exceeds allowable limits.