FOR A FAILED BRIDGE POWER RECTIFIER

AND AN ELECTRICAL SYSTEM EMPLOYING SAME

Field of the Invention The instant invention is related to protection circuits, and more particularly to a

detection and isolation circuit capable of sensing a failed diode in a bridge power rectifier circuit and a variable speed constant frequency (VSCF) electric power generating system employing same.

Background Art Electrical power generation and distribution systems typically employ various means for protecting both the distribution equipment (e.g., generators, controllers, distribution wiring

and feeders, etc.) and the utilization equipment connected thereto (e.g., computers, lighting

systems, fans, motors, control units for other systems, etc.) from faults which could otherwise

cause extensive damage. Protective functions such as over voltage, under voltage, over frequency, under frequency, etc., are designed so that the utilization equipment are not damaged by a failure within the controls or equipment which regulates the power produced by

the electrical system. At the same time, functions such as over current, shorted generator

diode, differential current, etc., protect the generation equipment from damage which may result as a result of failures within the generators, along the distribution feeders, or even within

the utilization equipment, e.g. short circuits, open phase wires, etc.

Of these protective functions, the differential current protection is particularly

important. A differential current fault includes short circuits, high impedance to ground faults,

and line-to-line faults which occur somewhere along the feeders between the generator and the utilization equipment. These faults are of particular importance because at least some of the current injected into the feeders by the generator is going to a fault and not to the loads. This differential current may result in fire or other damage. For many ground based electrical

systems, protection against this type of fault is provided by installing circuit breakers or like

devices at the input to the distribution system which go open circuit, or "blow", when one of these faults occur due to the substantially increased current flow along the feeders. The typical time for actuation of one of these devices is related to the amount of current flowing, and may be as long as 10 seconds depending on the application. For some high impedance faults, however, the threshold current level may never be reached, and the fault could exist indefinitely, thus reducing the efficiency and safety of the system.

For airborne electrical systems, however, the electrical feeders often traverse fuel tanks

and pass through volatile areas. Allowing a short circuit or other differential fault to last for

such time frames as are allowed in ground based systems could result in fire, explosion, and

crash of the aircraft. To protect against such disasters, airborne electric power generation and distribution systems utilize a much more sophisticated approach to detect and isolate these types of faults. The differential current protection system used on aircraft works under the

simple principle that what goes in, must come out. For electrical systems the principle states that the amount of current injected into a feeder must equal the amount of current supplied to

the loads (the utilization equipment). The amount of current generated at the source is monitored and compared to the amount of current supplied to the loads. Any difference

between these two values indicates that a fault exists somewhere in between. Upon detection

of such a fault, the system controller can isolate the fault and reconfigure the system to compensate within 20 to 120 milliseconds.

As shown in FIG. 1, the input monitoring devices 100 and the output monitoring devices 102, e.g. current transformers, define a differential protection (DP) zone. Protected within this DP zone for a typical 400 Hz system are the generator windings 104, main line

contactors 106, terminal connectors, etc. Inclusion of the entire system from the generator

108 to the loads 110 is possible because the current generated by the generator 108 is delivered to the loads 110 without conversion, i.e. the power generated is 115 Nac, 400 Hz and the power used is 115 Vac, 400 Hz.

As shown in FIG. 2, another topology system exists, however, wherein the power generated is converted prior to delivery to the loads. This type of system makes it difficult to compare the current generated by the generator to that which is ultimately delivered to the loads due to the varying efficiencies of the conversion equipment under the various loading conditions. For this reason, DP zones are established for each stage of power conversion. A

first DP zone exists from the generator 112 to the converter 114, a second from the converter

1 14 to the loads 110. Unfortunately, these DP zones do not overlap, and therefore cannot sense faults within the converter 114.

A popular converter topology used for airborne applications is known as a dc link

system. This topology transforms the input variable frequency electric power generated by the

wild frequency generator 112 into dc electric power via an input bridge rectifier 116. This dc electric power is then input via a dc link 118 to a switching network 120 which produces constant frequency electric power for use by the utilization equipment 110. Although the

sophisticated control of the switches can detect and isolate faults which occur at its stage of

conversion (dc to constant frequency ac conversion), no system of protection could detect and isolated a fault occurring at the other stage of conversion within the converter (wild frequency

ac to dc conversion). The result is that a short circuit across one of the rectifiers in the ac to dc conversion stage could go undetected, depending on generator speed and loading conditions, resulting in reduced safety and efficiency, and potential damage to the converter as well as to the aircraft. The likelyhood of detection is reduced at high generator speeds and low loads, which is the normal operating mode for back-up and emergency power systems. The present invention is directed to overcoming this problem.

Summary of the Invention It is the principle objective of the instant invention to provide a new and improved detection and isolation circuit. More specifically it is the principle objective of the instant invention to provide a new and improved detection and isolation circuit for a failed bridge power rectifier circuit whereby a short circuited diode may be detected and the fault isolated to protect the system from further damage as a result of the failed diode.

In a preferred embodiment of the instant invention, a protection circuit for a bridge rectifier having a plurality of ac inputs coupled to a plurality of line voltages, and a dc output, comprises monitoring circuits coupled to the line voltages for monitoring the line voltages and

generating sequential line-to-line voltage signals, conduction circuits coupled to the

monitoring circuits for conducting that portion of each of the sequential line-to-line voltage signals having a given polarity, the conducting circuits generating sequential half wave voltage

signals having a given polarity magnitude, and discriminating circuits coupled to the

conducting circuits for discriminating a faulted rectifier by monitoring the given polarity

magnitude of each of the sequential half wave voltage signals. The discriminating circuits generate a protection signal in response to one of the magnitudes of the sequential half wave

voltages signals being less than a threshold.

In an alternate embodiment of the instant invention, an electric power generating system comprises a generator, having a poly-phase ac output, being drivably coupled to a

prime mover whereby a first poly-phase ac electrical waveform having a frequency component which varies with speed is generated on the poly-phase output. An electric power converter is

electrically coupled to the poly-phase output and converts the first poly-phase ac electrical waveform to a second poly-phase ac electrical waveform having a controlled frequency

component which may be varied independent of generator speed. In this embodiment, the converter comprises an input bridge rectifier which converts the first waveform into a dc

waveform on a dc link, and a switching network which then converts the dc waveform into the second waveform. The converter further comprises a circuit which detects and isolates a

failure in the input bridge rectifier. This detection and isolation circuit de-energizes the

generator in response to the fault.

Brief Description of the Drawings

While the specification concludes with claims particularly pointing out and distinctly

claiming that which is regarded as the present invention, the organization, the advantages, and

further objects of the invention may be readily ascertained by one skilled in the art from the

following detailed description when read in conjunction with the accompanying drawings in

which:

FIG. 1 is a single line diagram of a single channel of a typical aircraft electric power

generating system illustrating the differential protection (DP) zones;

FIG. 2 is a single line diagram of a single channel of a variable speed constant frequency (VSCF) electric power generating system illustrating the differential protection

(DP) zones; FIG. 3 is a single line diagram of a single channel of a variable speed constant frequency (VSCF) electric power generating system of the instant invention;

FIG. 4 is a circuit block diagram of an embodiment of the detection and isolation circuit of the instant invention;

FIG. 5 is a waveform diagram illustrating the line-to-line voltages during a shorted rectifier fault;

FIG. 6 is a schematic diagram of an embodiment of the detection and isolation circuit of the instant invention; and

FIG. 7 is a schematic diagram of a further embodiment of the detection and isolation circuit of the instant invention

Description of the Preferred Embodiments

A preferred embodiment of the instant invention, as illustrated in FIG. 3, comprises a variable speed generator 112 having a poly-phase ac output (shown schematically as phase

windings 122 _A , 122 _B , and 122 _c ). The generator is drivably coupled to a prime mover, such as the aircraft engine (not shown), and produces a first poly-phase ac electrical waveform having

a frequency component which varies as a function of the generator speed. The output of the generator 112 is coupled by feeders 124 _A , 124 _B , and 124 _c to electric power conversion means,

such as converter 114, for converting the first poly-phase ac electric waveform to a second

poly-phase ac electric waveform on feeders 126A, 126B, and 126C. This second waveform has a controlled frequency component which may be varied independent of generator speed by varying the switching frequency of the converter 1 1 This constant frequency output is

coupled through main line contactor 106 to a load bus 128, to which are coupled the various

system loads 110.

In the preferred embodiment of the instant invention, the converter 114 comprises an input rectifier means 116 for converting the first waveform into a dc waveform on the dc link

1 18. A switching network means 120 is coupled to the dc link 118 and converts the dc

waveform to the second poly-phase constant frequency waveform on feeders 126 _A , 126 _B , and 126 _c . Differential protection for this system is provided by several zones of protection. The start of DP zone #1 is defined by the generator current transformers 130 _A , 130 _B , 130 _c and its

end by current transformers 132 _A , 132 _B , 132 _c . This zone includes generator windings 122 _A ,

122 _B , 122 _c , and feeders 124 _A , 124 _B , 124 _c . DP zone #2 is defined by current transformers 134A, 134B, 134C and current transformer pairs 136 and 138 for each phase. These pairs are cross coupled, thereby canceling any current delivered by the tie bus 140. Protection from failures within the switching network means 120 is provided by its own sophisticated control

and protection. Protection against failures within the input rectifier means 1 16 is provided by

detecting and isolating means which will de-energize the generator in response to a fault.

As shown in FIG. 4, the detecting and isolating means comprises a monitoring means

or circuit 142 coupled to each phase 124A, 124B, 124C of the first poly-phase ac waveform

for monitoring the phase voltages. This monitoring circuit 142 generates sequential phase-to-

phase voltage signals on lines 144, 146, 148 therefrom. Polarity conducting means or circuit

150 is coupled to the monitoring circuit 142 and conducts that portion of each of the

sequential phase-to-phase voltage signals having a given polarity, e.g. only the positive portions. The conducting circuit 150 generates sequential half wave voltage signals having the given polarity magnitude on lines 152, 154, 156. Discriminating logic means or circuit 158 is

coupled to the conducting circuit 150 and discriminates a faulted input bridge rectifier 116 (see FIG. 3) by monitoring the given polarity magnitude of each of the sequential half wave

voltage signals on lines 152, 154, 156. The discriminating circuit 158 generates a protection signal on line 160 in response to one of the magnitudes being less than a given threshold. It

should be noted that the use of the term circuit is generic only and will be recognized by one skilled in the art as allowing for implementation in hardware or software depending on the particular application.

FIG. 5 illustrates the line-to-line voltage waveforms present on lines 144, 146, 148

(see FIG. 4) during a failed input bridge rectifier condition. The output of the polarity conducting circuit 150, if the given polarity is set to positive, during this condition will be a positive half wave on line 152, a positive half wave on line 154, and zero voltage on line 156.

Under these conditions the discriminating logic circuit 158 will generate the protection signal on line 160 to de-energize the generator to isolate the failed input bridge rectifier 1 16 of FIG. 3.

As illustrated in FIG. 6, the phase voltages produced by the generator on lines 124 _A ,

124 _B , 124 _c are coupled both to the input bridge rectifier means, which comprises a diode bridge in this exemplary embodiment to rectify the three phase voltages to a dc voltage on the

dc link 118, and to the monitoring circuit 142. This monitoring circuit 142 comprises differential summing amplifiers 162, 164, 166, each of which monitor two phase voltages and

produce a single phase-to-phase voltage output on lines 144, 146, 148 which are coupled to

the polarity conducting circuit 150. This polarity conducting circuit 150 passes only a single polarity, and comprises diodes 168, 170, 172. In this exemplary embodiment the diodes 168, 170, 172 are coupled with their anodes coupled to the monitoring circuit 142 and their

cathodes coupled to the discriminating logic circuit 158 to pass only the positive portions of the phase-to-phase voltage waveforms on lines 144, 146, 148. If the diodes 168, 170, 172

were coupled in the opposite direction (with their cathodes coupled to lines 144, 146, 148 and their anodes coupled to lines 152, 154, 156) this polarity conducting circuit would pass only

the negative portion of the phase-to-phase waveforms. As shown, however, the positive

portion of the phase-to-phase waveforms are passed via lines 152, 154, 156 to the

discriminating logic circuit 158 which determines from these waveforms the presence of a fault in the input bridge rectifier circuit 116. Specifically, the discriminating logic circuit 158 monitors the positive half- wave signals and generates a protection signal on line 160 if any one

of the magnitudes of these positive half-wave signals is less than a given threshold.

In a highly preferred embodiment of the instant invention, as shown in FIG. 7, an ideal

diode circuit comprising a diode 174 and an operational amplifier 176 replaces the diodes 168,

170, 172 (see FIG. 6) in the polarity conducting circuit 150 to eliminate the diode voltage

drop from this stage of the circuit. Additionally, the addition of a multiplexer 178 allows only one ideal diode to be used in the polarity discriminating circuit 150. In this way, the discriminating logic circuit 158 sequentially monitors the waveform magnitudes to discriminate

the presence of a fault. This embodiment of the instant invention monitors the line-to-line

voltages in the order of A to B, B to C, and C to A. During unfaulted operation, the

measured voltages are equal, and the polarity conducting circuit 150 outputs consecutive

positive waveform values which will be read by the discriminating logic circuit. Each of these

non-faulted voltages will be of approximate equal amplitude.

During faulted operation, e.g. diode 180 of the input bridge rectifier circuit fails short circuited, a half wave short circuit will exist from phase A (line 124 to phase B (line 124 _B ), and from phase A (line 124 to phase C (line 124 _c ). Since the voltage sensing is accomplished using line-to-line voltages of V _A . _B , V _B-C , and V _C . _A , the V _A . _B voltage will be only

positive going (see FIG. 5) because the negative going voltage, using phase A as the reference, is essentially shorted when the phase B diode 182 is positive biased. The V _B _c

voltage waveform will maintain both positive and negative going (see FIG. 5) because the fault is not within the path between phase B and phase C. The voltage variation seen in FIG. 5 is caused by the unbalanced loading on the generator. This variation on the voltage amplitude can be neglected. The V _C . _A voltage waveform will be only negative going (see FIG. 5) because the positive going voltage, using phase C as the reference, is essentially shorted when the phase C diode 184 is positive biased As described above, these voltages are multiplexed by multiplexer 178 with the output going through the polarity conducting circuit 150 prior to

being sensed by the discrimination logic circuit 158. The polarity conducting circuit 150 only

allows the positive going voltage to pass. Therefore, when a fault occurs, the discrimination logic circuit 158 will sense two voltages with approximately normal amplitudes and the other

voltage with essentially zero amplitude. In this situation, the protection signal on line 160 will be generated and the generator will be de-energized.

Numerous modifications and alternative embodiments of the invention will be apparent

to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art

the best mode of carrying out the invention. The details of the structure may be varied

substantially without departing from the spirit of the invention, and the exclusive use of all

modifications which come within the scope of the appended claims is reserved.