Electricity: single generator systems – Generator control – With excitation winding and/or circuit control
Reexamination Certificate
2001-08-17
2003-09-30
Ramirez, Nestor (Department: 2834)
Electricity: single generator systems
Generator control
With excitation winding and/or circuit control
C322S044000
Reexamination Certificate
active
06628104
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrical power generation.
2. Description of Related Art
Electrical power for military and commercial aircraft is typically generated by either an AC or DC generator that is controlled by regulating the voltage at a “point-of-regulation” (POR) to a specified level. To regulate the POR voltage, a generator control unit (GCU) modulates a generator excitation source voltage so that an ideal excitation current is maintained according to the load and speed conditions. When load on the generator is increased, the GCU must increase an exciter field current to compensate for a POR voltage drop caused by the extra load. When load on the generator decreases, the GCU must reduce the generator exciter field current so that the POR voltage will not be too high. In other words, the GCU must compensate for load transitions (e.g., high load to low load, or vice versa) by increasing or decreasing the exciter field current. If load transition compensation is not achieved quickly, the POR voltage could fall outside a specified limit, thereby causing utilization equipment malfunction and/or damage.
FIG. 1
illustrates a conventional generator control configuration for typical aircraft AC power generation systems. As seen in
FIG. 1
, a conventional generator control unit
10
includes the following main elements: (a) a field current modulation switch
12
; (b) a field current modulation switch driver
14
; and (c) and a free wheeling diode
16
. The generator control unit
10
is connected to an exciter stator winding
22
of a generator
20
via lines
17
and
18
to provide a field current I
f
. As is well known, the flow of field current I
f
through the exciter stator winding
22
of an AC generator induces a voltage in an exciter rotor winding
24
of the generator
20
, which is rectified by a rectifier
26
. The resulting rectified voltage is applied to a field winding
28
, which induces an AC voltage in a generator main winding
29
for distribution to electrical loads of the aircraft via feeders (not shown). The field current modulation switch
12
, which is typically either a MOSFET (metal-oxide-semiconductor field-effect transistor) or an IGBT (insulated-gate field-effect transistor), is connected to line
18
and between the exciter stator winding
22
of the generator
20
and ground. The field current modulation switch
12
is repeatedly switched between an ON state and an OFF state by the field current modulation switch driver
14
, such that the duty cycle (or ON/OFF pulse width) of the field current modulation switch
12
maintains the field current flowing trough the exciter stator winding
22
at a given level as a function of load.
During normal operation, the field current I
f
should be kept continuous to maintain a ripple-free POR voltage and to reduce the voltage noise across the field current modulation switch
12
during its switching. The free wheeling diode
16
is connected between lines
17
and
18
, the anode being connected to line
18
between an output end of the exciter stator winding
22
and the field current modulation switch
12
and the cathode being connected to line
17
between an input end of the exciter stator winding
22
and a DC power source (not shown), and bypasses excitation energy stored in the exciter stator winding
22
when the field current modulation switch
12
is OFF. When the field current modulation switch
12
is ON, the diode
16
is reverse-biased and is in a blocking state. Therefore, excitation current on line
18
from the exciter stator winding
22
goes to ground through the field current modulation switch
12
. When the field current modulation switch
12
is in the OFF state, the diode
16
is forced on by the induced voltage of the exciter stator winding
22
and the energy in the winding keeps flowing through the diode
16
via the line
18
so that the current flowing though the diode
16
is included in the field current I
f
that is fed to the input end of the exciter stator winding
22
via line
17
, thereby achieving a smooth continuous field current I
f
. Thus, the diode
16
creates a free-wheeling path for excitation energy from the exciter stator winding
22
.
For aircraft with traditional fixed frequency electrical systems that normally operate around 400 Hz, the conventional configuration in
FIG. 1
does not pose a serious performance problem. The inventors of this application have found, however, that problems may arise in variable frequency systems that have gained more attention in recent years. For a typical aircraft, electric power generated from a generator has to meet power characteristic requirements dictated by either the industry or military standards. One typical requirement is the maximum voltage overshoot and recovery time during a step load removal transient elsewhere in the electric power utilization system. In a variable frequency generator control system, in which the generator speed range can be wide from 10000 to 24000 revolutions-per-minute, if the energy stored in the exciter winding cannot be depleted quickly enough when the field current modulation switch is OFF, the voltage overshoot often exceeds acceptable levels when the load is taken off from the generator at high speed, especially when a large generator is used. Furthermore, during generator power-up at high generator speeds, the generator voltage can be excessive due to inability of the generator control unit to accurately ramp-up the excitation current because energy stored in the exciter stator winding does not decay fast enough. Because there is not much that can be done to solve these problems with existing conventional control compensation in standard generator control units, a product cannot be delivered to a customer if it fails to meet the customer's voltage overshoot tolerances.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above drawbacks of the conventional power generator control configuration are resolved through use of a generator control unit that selectively and temporarily introduces an energy absorbing circuit into an excitation current free-wheeling path to absorb residual energy from a generator winding, thereby accelerating the field current decay rate to reduce voltage overshoot in the generator. According to the present invention, the energy absorbing circuit is selectively and temporarily introduced into the excitation current free-wheeling path when the generator experiences a load transition (e.g., a transition from high load to low load) and/or during a power-up stage to reduce voltage overshoot.
According to an embodiment of the present invention, a power generator control unit includes a field current modulator that is repeatedly switched between an ON state and an OFF state to control the flow of field current to the an exciter winding of the power generator, and excitation current from the exciter winding is fed back to the generator via a free-wheeling diode when the field current modulator is OFF. An energy absorbing circuit is selectively added to the free-wheeling path. A by-pass switch is provided across the energy absorbing circuit, which my be an RC circuit, to provide, when in an ON state, a low-impedance path for excitation current to flow in the free-wheeling path. In this implementation, the generator control unit includes an impedance by-pass driver that changes the by-pass switch from an ON state to an OFF state as a function of one or more detection signals, e.g., indicating a load transition or power-up condition, to selectively and temporarily introduce the energy absorbing circuit into the free-wheeling path to accelerate decay of the excitation current from an exciter winding of the generator.
REFERENCES:
patent: 4035712 (1977-07-01), Yarrow et al.
patent: 4219769 (1980-08-01), Macfarlane et al.
patent: 4262242 (1981-04-01), Glennon
patent: 4442396 (1984-04-01), Hucker
patent: 4486801 (1984-12-01), Jackovich et al.
patent: 4973896 (1990-11-01), Shiga et al.
patent: 499
Malicki Wally
Yao Yuan
Cuevas Pedro J.
Honeywell International , Inc.
Palguta Larry J.
Ramirez Nestor
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