Claims:We Claim:
1. A system (126) for detecting failure of a rotating diode in a brushless excitation system (100) under open-circuit/short-circuit condition, the system (126) comprising:
- a diode failure detection relay (101) having a signal conditioning unit (103) and a computational unit (104), wherein the diode failure detection relay (101) detects diode failure in a rotating bridge diode rectifier (111) connected to output of a main exciter (113) in the brushless synchronous generator (100) for initiating a protection action; and
- a communication device bi-directionally connected to the diode failure detection relay (101), wherein the communication device is configured to operate a human machine interface (105), for providing online monitoring of field current in the main exciter (113).

2. The system (126) as claimed in claim 1, wherein the signal conditioning unit (103) attenuates high frequency input signals, in which the high frequency input signals being provided as an input to an analog to digital (A/D) converter (301) present in the computational unit (104).

3. The system (126) as claimed in claim 1 or 2, wherein the computational unit (104) includes a Field Programmable Gate Array (FPGA) configured to extract AC harmonics from the digital signals fetched by the A/D converter in the computational unit (104).

4. The system (126) as claimed in claim 1-3, wherein the signal conditioning unit (103) is interfaced with voltage detected across a shunt resistor (102) connected in series with field winding (120) of the main exciter (113) and a thyristor bridge rectifier (110).

5. The system (126) as claimed in claim 1-4, wherein the diode failure detection relay (101) communicates with the human machine interface (105) to implement analytical functions for diode failure detection.

6. The system (126) as claimed in claim 5, wherein the communication between the processing unit and the human machine interface (105) is performed through TCP/IP communication protocol.

7. The system (126) on as claimed in claim 1-6, wherein the system (126) takes protective action based on type of diode failure detected through a voltage regulator connected to the diode failure detection relay (101).

8. A method implemented for detection of rotating diode failure condition using a system (126), the method comprising:

- extracting sum of peak of AC harmonics by a processing unit upto 6th harmonic and DC current in field current of an exciter;
- determining a AC-DC factor by the processing unit, comparing with the pre-determined range of AC-DC factor values within a pre-defined length of time;
- determining diode short circuit condition by the processing unit on ascertaining as determined AC-DC factor lying within the pre-determined range of operating AC-DC factor values for a predefined length of time;
- determining diode open circuit condition by the processing unit on ascertaining as determined AC-DC factor lying within the pre-determined range of AC-DC factor values for a predefined length of time;
- determining non-occurrence of diode failure by the processing unit on ascertaining as determined AC-DC factor lying beyond the pre-determined range of AC-DC factor values for the predefined length of time;
- receiving inputs from the processing unit by a computational unit (104);
- providing protection signal by the computational unit (104) to a voltage regulator (108), wherein the voltage regulator (108) increases/decreases the excitation of exciter by changing firing angle of a thyristor based on the type of diode failure condition; and
- communicating with a human machine interface (HMI) (105) by the computational unit (104).

9. The method as claimed in claim 8, wherein the human interface unit (105) communicates with the computational unit (104) for facilitating event visualisation, feeding reference settings, storing data and event recording for detection of diode failure condition.
Dated this 31st day of January, 2022

SOMA RANI MISHRA
PATENT AGENT
IN/PA – 1159
OF L. S. DAVAR & CO.,
APPLICANT’S AGENT

, Description:
 
A SYSTEM FOR DETECTION OF FAILURE CONDITION IN A ROTATING DIODE

FIELD OF INVENTION
[0001] The present disclosure, in general, relates to a system for detection of failure in a rotating diode. In particular, the present disclosure relates to the system including an advanced hybrid hardware platform accompanied with a human machine interface for providing diagnostic information regarding failure of the rotating diode. Further, the present disclosure also relates to a method of detection of the diode failure condition followed by an initiation of protection action in case of diode failure in a brushless excitation system.

BACKGROUND OF THE INVENTION
[0002] The background of the invention relates to a brushless excitation system which is used to provide field current to a synchronous generator without using slip rings and carbon brushes. In a typical synchronous generator, the direct current can be provided to the field winding by means of slip rings and carbon brushes. However, in a brushless synchronous generator, the direct current is provided by a supply circuit mounted on a rotor itself. Consequently, in these types of techniques, losses due to contact resistance of carbon brushes can be eliminated.

[0003] Now, the Brushless excitation system comprises of two major parts, namely, a pilot exciter and a main exciter. The pilot exciter can be a permanent magnet generator (PMG) where permanent magnets (field winding) of the PMG are mounted on a generator rotor shaft and armature winding of the PMG is mounted on a stator, which is a stationary part. The armature winding produces a three phase AC power by utilization of mechanical energy of the rotor. The pilot exciter output AC voltage is rectified by a thyristor bridge, after which the rectified DC voltage is fed to the field winding of the main exciter. The main exciter field winding, on the other hand, is wound on the stator, which is a stationary part and the armature winding is mounted on the generator rotor shaft. The main exciter generates high frequency AC output, which is rectified using a diode bridge rectifier and fed to the generator field winding. The diode bridge rectifier is also mounted on the rotor shaft and this does not allow the exciter output voltage, exciter output current, generator field voltage and generator field current to be measured as they are not accessible.
[0004] The rotating diode bridge rectifier is an important sub-system of the brushless excitation system. If a diode gets open-circuited in the bridge rectifier, the output capacity of the exciter may be reduced. In this case, though an alternator should still be able to deliver a rated output, but the transient capability is diminished. On the other hand, in case of a diode getting short circuited in the bridge rectifier, the output of the exciter is severely affected and the main alternator is unable to provide rated voltage, without overloading the exciter. If such a failure is prolonged, there is a risk of exciter insulation failure due to the stresses developed in the armature winding to maintain rated voltage of the generator.

[0005] In view of this, the detection of condition of a diode in the rotating rectifier is significant to ensure safe operation of the brushless excitation system. As the diode bridge rectifier circuit is not accessible directly, so it is challenging to the operator to detect the diode failure condition in the rotating rectifier. Hence, the present invention summarises a system that can be interfaced directly to the exciter field circuit which is a stationary part, to detect the diode failure condition. The system can monitor the diode condition on-line and provides diagnostic information in case of any diode failure in the bridge rectifier, so it will enhance the performance and reliability of Brushless Excitation System.

PRIOR ARTS OF THE INVENTION

[0006] With increase in demand for of brushless excitation system technology due to its technical benefits and the revolution in the electronics field, the development of rotating diode failure condition monitoring system for the rotating diode bridge rectifiers have been started a few years ago. The patents, which have already been filed in this field are as follows:

[0007] A US Patent No. 5453901 titled “Detection and protection of excitation system from diode failure” mainly discloses a diode short circuit detection and protection circuit for rotating rectifier exciter of the brushless excitation system. It describes a resistor - capacitor circuit which is used to detect the diode failure condition, and the protection of excitation system is taken care by operating a circuit breaker to disconnect the output supply of a permanent magnet generator, which is fed to the field winding of exciter, thus removing all the excitation to exciter field. In case of salient pole exciter field, the protection system is operated by adjusting a gate control voltage of a SCR, which in turn controls a field winding voltage. The gate control voltage of SCR is derived from a ripple voltage which appears across a resistor of RC circuit, connected in parallel to the field winding. This invention speaks about the excitation system and its analog circuit is used for detection of diode under short-circuit condition and subsequent protection of the excitation system. But the invention does not disclose about monitoring the diode in an open circuit condition. Additionally, the disclosed detection and protection scheme is bit complex with requirement of an additional circuit and may not be very much reliable as there are many elements in the scheme.

[0008] In a US Patent No. US 20150198655A1 titled “Rectifier diode fault detection in brushless exciters” a method and an apparatus for fault detection in the diode bridge rectifier of brushless exciter has been disclosed in which, the voltage across the field winding of generator, current through the field winding of generator is measured and harmonics analysis is performed to extract different harmonics, up to 6th harmonic value. The ratio of fundamental voltage component to 6th harmonic voltage component and also the ratio of harmonic voltage to harmonic currents are computed. The computed ratio is compared with the two threshold limit values and then analysed to determine the type of fault condition, i.e. whether the fault is due to diode open circuit condition or due to short circuit condition. All the computations are performed in a processor module. The transmission of control and trip signals from the processor module, mounted on a rotating system to a stationary system is transferred via radio telemetry wireless transmitter to a radio telemetry receiver module. This method uses sensors to measure the voltage and current of field winding of a generator, and the ratio of fundamental and 6th harmonic is computed, which is analysed to determine the type of fault. However, this method may not be performed accurately in real time as there may be some electromagnetic field induced in the rotor due to a leakage current, which may disturb the measurement accuracy of field voltage and current and also the harmonic quantity extraction may not be accurate. The output is transferred through a wireless communication, which may not be reliable in the rotating system. Moreover, the solution proposed may be costlier as the current and voltage is measured directly on the generator field.

[0009] In another US Patent No. 4559486 titled “Detector for shorted rotating diode of a brushless alternator” discloses a detection circuit used in the field winding to detect the diode short circuit condition in the rotating rectifier of the brushless excitation system. The detection circuit consist of a resistor and a transistor connected in series to the field winding, and an antiparallel diode connected across the transistor and field winding. The diode short circuit condition is detected by measuring a voltage polarity across the resistor. In normal operation, the current flow through the resistor is from the field winding to the diode bridge and in case of diode short circuited, the current flow direction reverses thereby also reversing the polarity of voltage across the resistor. The measured voltage across the resistor is interfaced to the sensing unit which senses the reverse in voltage polarity and generates a diode short circuit indication signal. But this invention does not describe about the detection of diode open circuit condition, and also the protection action to protect the exciter machine and regulator against the stresses will be developed during short circuit condition.

[0010] In US Patent No. 4486801 titled “Generator shorted diode protection system”, the detection of short circuit condition of a diode in the rotating rectifier of a brushless excitation system and also its protection system has been presented. The protection system includes a resistor - capacitor circuit arrangement for sensing the voltage difference across the field winding with respect to ground, and pairing the voltage difference with respect to a generator load current, which defines the set reference values for protection operation. In case of a diode short circuit, the voltage difference across the field winding exceeds the set reference value during normal generator load current, then the output signal is generated to interrupt the supply of exciter field. A microprocessor is used for storing the reference values, data processing and also it sends a protection activation signal to energise the relay. The energised relay gives a command to close a circuit breaker to short circuit the field winding through the resistor to damp out generator field excitation and the relay also gives a command to interrupt the input supply of exciter field by opening the circuit breaker contacts. However, this invention does not speak on the detection of open circuit condition of a diode in a rotating rectifier bridge. It also does not disclose about the electrical isolation of protection circuit from the field connections.

OBJECTS OF THE INVENTION

[0011] It is an object of the present subject matter to overcome the aforementioned and other drawbacks existing in the prior art systems and methods.
[0012] It is a principal object of the present subject matter to detect failure of a diode under an open circuit condition in a rotating bridge rectifier of a brushless excitation system.
[0013] It is another object of the present subject matter to detect failure of the diode in a short circuit condition in the rotating bridge rectifier of a brushless excitation system.
[0014] It is another object of the present subject matter to propose a system capable of pursuing an on-line monitoring of exciter field current, DC field current and sum of AC harmonics current up to 6th harmonic.
[0015] It is another object of the present subject matter to propose a system capable of generating alarm and trip event, storing data and trend recording by a human machine Interface.
[0016] It is yet another object of the present subject matter to propose a system that is capable of initiating a protective measure in case of a diode failure.
[0017] It is even another object of the present subject matter to propose a system that is reliable and advanced in nature.
[0018] These and other objects and advantages of the present subject matter will be apparent to a person skilled in the art after consideration of the following detailed description taking into consideration with accompanied drawings in which preferred embodiments of the present subject matter are illustrated.

SUMMARY OF THE INVENTION

[0019] This summary is provided to introduce concepts related to propose a system and unique architecture on an advanced hardware platform for detecting the diode failure under open circuit and short circuit condition in a rotating rectifier of a brushless excitation system. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0020] According to an embodiment of the present disclosure, there is provided a system for detecting failure of a rotating diode in a brushless excitation system under open-circuit/short-circuit condition. The system comprises of a diode failure detection relay having a signal conditioning unit and a computational unit, where the diode failure detection relay detects diode failure in a rotating bridge diode rectifier connected to output of a main exciter in the brushless synchronous generator for initiating a protection action and a communication device bi-directionally connected to the diode failure detection relay, where the communication device is configured to operate a human machine interface, for providing online monitoring of field current in the main exciter.

[0021] In an aspect, in the system, the signal conditioning unit attenuates high frequency input signals, in which the high frequency input signals being provided as an input to an analog to digital (A/D) converter present in the computational unit.

[0022] In an aspect, in the system, the computational unit includes a Field Programmable Gate Array (FPGA) configured to extract AC harmonics from the digital signals fetched by the A/D converter in the computational unit.

[0023] In an aspect, in the system, the signal conditioning unit is interfaced with voltage detected across a shunt resistor connected in series with field winding of the main exciter and a thyristor bridge rectifier.

[0024] In an aspect, in the system, the diode failure detection relay communicates with the human machine interface to implement analytical functions for diode failure detection.

[0025] In an aspect, in the system, the communication between the processing unit and the human machine interface is performed through TCP/IP communication protocol.

[0026] In an aspect, the system takes protective action based on type of diode failure detected through a voltage regulator connected to the diode failure detection relay.

[0027] In an aspect, there is provided a method implemented for detection of rotating diode failure condition using a system. The method comprises extracting sum of peak of AC harmonics by a processing unit upto 6th harmonic and DC current in field current of an exciter, determining a AC-DC factor by the processing unit, comparing with the pre-determined range of AC-DC factor values within a pre-defined length of time, determining diode short circuit condition by the processing unit on ascertaining as determined AC-DC factor lying within the pre-determined range of operating AC-DC factor values for a predefined length of time, determining diode open circuit condition by the processing unit on ascertaining as determined AC-DC factor lying within the pre-determined range of AC-DC factor values for a predefined length of time, determining non-occurrence of diode failure by the processing unit on ascertaining as determined AC-DC factor lying beyond the pre-determined range of AC-DC factor values for the predefined length of time, receiving inputs from the processing unit by a computational unit, providing protection signal by the computational unit to a voltage regulator, where the voltage regulator increases/decreases the excitation of exciter by changing firing angle of a thyristor based on the type of diode failure condition; and communicating with a human machine interface (HMI) by the computational unit.

[0028] In an aspect, the method also includes that the human interface unit communicates with the computational unit for facilitating event visualisation, feeding reference settings, storing data and event recording for detection of diode failure condition.

[0029] To further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit the scope of the present subject matter.

[0030] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING(S)

[0031] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the given figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which;
[0032] Figure 1 depicts an exemplary architectural layout of a brushless synchronous generator incorporating the proposed system in accordance with an exemplary embodiment of the present subject matter.
[0033] Error! Reference source not found. and Figure 3 shows an exemplary method demonstrating detection of diode failure condition and an initiation of protection signal in accordance with an exemplary embodiment of the present disclosure.
[0034] Figure 4 illustrates direct current (DC) and sum of peak of AC harmonic currents up to 6th harmonic of exciter field current, during operation of the brushless synchronous generator in accordance with an embodiment of the present disclosure; and
[0035] Figure 5 illustrates DC current and sum of peak of AC harmonics current up to 6th harmonic of exciter field current, in the Brushless Synchronous Generator under open circuit condition of the diode in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION OF INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS OF THE PREFERRED EMBODIMENTS

[0036] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[0037] While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure

[0038] The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such system, or assembly, or device. In other words, one or more elements in a system or device proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or device.

[0039] The present disclosure relates to a system (126) for detection of failure of a rotating diode in a brushless synchronous generator and an associated method of protecting the rotating diode during a failure. The diode bridge rectifier is mounted on the generator shaft and is therefore, not accessible directly. If anyone diode gets open circuited in the bridge rectifier, still the excitation system may operate by overloading/using full capacity of healthy diodes without effecting the output of generator. If this condition continues for longer time, there may be a chance of more diodes getting open circuited one by one due to avalanche effect and complete half-arm may get open circuited in the bridge rectifier, then immediately the diode failure relay detects open circuit condition and generates a protection signal. In case of diode short condition even one diode get short circuited then immediately diode failure relay detects and generates a protection signal. The generated protection signal, is fed to the voltage regulator to take necessary protection action.

[0040] Hence, the present embodiment summarises, a system (126) that can be interfaced directly to the exciter field circuit which is a stationary part, to detect the diode failure condition. The mentioned invention provides an accurate and reliable method to detect a diode failure condition either open circuit or short circuit in the rotating diode bridge rectifier of excitation system. Additionally, the present disclosure also presents a method of communication implemented for communicating the diode failure relay using a remote human machine interface. These are detailed in the subsequent sections.
[0041] Error! Reference source not found. illustrates a schematic of the brushless excitation system of a synchronous generator (100) incorporating the proposed system (126) for detection of failure of the rotating diode and thereafter protecting the diode once failure has been detected. The system (126) comprises of a diode failure detection relay (101) having a signal conditioning module (103) and a computational unit (104). The data as obtained can be fed to a Human Machine Interface (105), where the communication may be implemented bi-directionally as well. The working of the system (126) has been explained in subsequent section after illustrating association of the system (126) with the brushless excitation system of synchronous generator (100), as shown in Figure 1.
[0042] In an aspect, it is observed from the figure that, a brushless synchronous generator (100) is coupled with a rotating diode rectifier (111) on a shaft (116), having a shunt resistor (102) in series with the main exciter (113). The output of the shunt resistor (102) is fed into the system (126) which is connected to a voltage regulator (108) for generating protection signal.
[0043] In an aspect, the brushless synchronous generator (100) is having a shaft (rotor) (116) coupled to a turbine prime mover where the shaft (116) rotates with a speed of generator synchronous speed, Ns (117). Further, the brushless excitation system of synchronous generator (100) comprises of a Pilot Exciter (112) and a Main Exciter (113).
[0044] In a preferred embodiment, the pilot exciter (112) is a Permanent Magnet Generator, having permanent magnets (119) mounted on a rotor and a stationary three phase armature winding (118).
[0045] In an aspect, output of the three phase armature winding (118) is connected to a three phase thyristor controlled bridge rectifier (110). The thyristor controlled bridge rectifier (110) converts the three phase AC voltage into DC voltage and the rectified output DC voltage is controlled by varying firing angle (109); that is being fed into the thyristor. The firing angle of the thyristor is controlled by a voltage regulator (108).
[0046] In another aspect, the main exciter (113) includes a stationary field winding (120) and a rotating three phase armature winding (121). The stationary field (120) winding receives DC supply voltage from the thyristor controlled rectifier. The three phase armature (121) output voltage of exciter is further connected to a 3-phase full wave rotating diode bridge rectifier (111). The rotating diode bridge rectifier (111) converts AC voltage into DC Voltage and feeds same to the rotating field winding (123) of a generator (114). The armature winding (122) of the generator (114) is a stationary part, which generates the three phase output AC voltage. The output (124) of the generator is connected to the AC Grid through a step-up transformer
[0047] In a preferred embodiment, operation of the exciter (113) has a significant impact on the dynamic, transient performance, reactive power generation of the generator (114), thereby determining quality of output voltage.
[0048] In another aspect, all the rotating elements including the diode rectifier bridge (111) is mounted on a generator shaft (116) to make it as a rotating structure (115). The rotating structure (115) tries to maintain the synchronous speed (117) of the generator (114) during steady state operation. The frequency of output AC voltage of pilot exciter (112) is dependent on the plurality of poles in the rotor (119) and the stator (118). The frequency of output AC voltage of the main exciter (113) is also dependent on plurality of poles in the stator (120) and the rotor (121).
[0049] In a preferred embodiment, the main function of excitation system is to provide controlled DC current to the field winding of the generator (114) to regulate the output (124).
[0050] In another aspect, the DC current of generator field winding (123) is controlled by the voltage regulator (108) of the generator (114). The voltage regulator (108) senses the generator terminal voltage (124) and compares it with a set reference voltage and generates a firing angle/pulse according to voltage error. The generated firing angle/pulse is fed to the 3-phase thyristor bridge rectifier (110) to vary the field current of exciter (113), which in turn changes exciter output voltage. The change in output voltage of exciter (113) leads to the change in output voltage of the diode bridge rectifier (111), which is connected to the field winding (123) of a generator thereby maintaining terminal voltage of the generator. The voltage regulator (108) receives the diode failure condition signal from diode failure detection relay (101) and then initiates a protection action to decrease/increase the excitation voltage accordingly by changing firing angle/pulse of the thyristor bridge rectifier (110). If required, the voltage regulator (108) can also generate and send a block signal (109) to the thyristor bridge rectifier (110) for blocking thyristors to de-energise the excitation system, based on the type of diode failure condition. It can also generate a trip signal (127) and fed it to a trip relay to shut down the generator, thereby relieving stresses on the exciter and the voltage regulator (108) caused due to failure of a diode in the bridge rectifier. The de-energisation of field winding can also be carried out using a discharge resistor and a metal oxide varistor.
[0051] In a preferred embodiment, output current of the thyristor bridge rectifier (110) is not a pure DC, it includes some AC harmonics, which depends on frequency of output voltage of the permanent magnet generator (112), the plurality of thyristor bridges used in the rectifier (110), output voltage frequency of the exciter (113) and the plurality of diodes bridges used in the bridge rectifier (111).
[0052] In another aspect, a shunt resistor (102) is connected in between the thyristor bridge rectifier (110) and exciter field winding (113), which is placed in series in the circuit, to monitor the field current by measuring the voltage across it. The field current is not pure DC, it consists of DC component and some AC harmonics. The measured voltage signal (125) is interfaced to signal conditioning module (103) of the system (126).
[0053] In a preferred embodiment, the shunt resistor is a very low resistance, high power and high accuracy shunt employed in the field circuit to sense the exciter field current accurately.
[0054] In another aspect, the signal conditioning module (103) present in the system (126), provides a high voltage isolation of the field circuit that should attenuate some undesirable high frequency signals which superimposes on the field signals. Since, the system (126) is expected to operate in harsh environment under severe fault conditions, so it is very essential to protect the system (126) from high voltage and high frequency noise entering into the system either by conduction or by radiation. Consequently, the cut-off frequency for the filtering circuit is set so as to attenuate all the high frequency signals.
[0055] In a preferred embodiment, the signal conditioning module (103) also provides flexibility to select the desired gain amplification to suit the input range of an analog to digital (A/D) converter. Since the processor can operate only in the digital domain, the field signal need to be converted into digital data using the A/D converter, whose input ranges are approximately ±10V.
[0056] In another aspect, the computational Unit (104) connected to the signal conditioning module (103), provides data acquisition to input voltage signal, signal processing for extracting the different harmonic components, performing computations for detecting diode failure condition and generating a protection signal based on the type of failure of diode either in open circuit or in short circuit condition. The computational unit (104) is an advanced hybrid hardware, comprising of a field programmable gate arrays (FPGA) and a processing unit. The A/D conversion of input voltage received from the signal conditioning module (103) is implemented in the FPGA with very high cycle time processing to generate precise digital sample data for the received analog input voltage signal. Harmonic components have been extracted from the digitised sample data using principle of a discrete Fourier transform. . The discrete Fourier transform technique has been implemented using the FPGA with high cycle processing for extracting, preferably, up to 6th harmonic frequency of exciter machine and also for achieving high accuracy in the harmonic component values. The processed voltage signal data and extracted harmonic component data is sent to the processor for performing slow cycle time processing events, above 1ms cycle time, such as computations and communication software. The processor reads digital data of voltage signal from the FPGA followed by performing computations for detecting diode failure condition and initiating a protection action. The diode failure detection and protection techniques is implemented in the processor. In case of any diode failure in the bridge rectifier of excitation system, it immediately determines diode failure as well as the type of diode failure and sends a protection signal (107) to the voltage regulator (108).
[0057] In a preferred embodiment, the A/D conversion is implemented in the FPGA with cycle time that is < 10us.
[0058] Further in an embodiment, the principle of discrete Fourier transform is implemented in the FPGA with high cycle processing up to 50us for extracting up to 6th harmonic frequency.
[0059] In another aspect, the system (126) also provides the on-line monitoring of exciter field current and fault signature current in the user friendly Human Machine Interface (105) . The Human Machine interface, HMI (105), is implemented in a personal Computer and communicates with the processor over a communicating network that may include an ethernet cable (106) using TCP/IP Communication Protocol.
[0060] In a preferred embodiment, the system (126) may operate in an online mode where the operator can view various parameters through the display and can simultaneously edit the measurable parameters as per requirement. Moreover, alarms and event records can be viewed on a graphical user interface (GUI) installed in a personal computer, which can also be used in post-fault analysis. All such records are stored in a cyclic buffer for viewing later. This makes the system (126) very convenient for achieving the results accurately for diagnostic purposes.

[0061] Error! Reference source not found. illustrates an example method for determining diode failure condition and initiation of protection action using the proposed system (126).
[0062] At block (301), the method includes reading input voltage signal by the FPGA and implementing data acquisition, thereafter. Herein, the input voltage is read from the signal conditioning module (103) and then data acquisition is performed in order to convert analog signal to digital data samples.
[0063] At block (302), the method includes extracting the AC harmonic data, upto 6th harmonic component of exciter frequency by application of discrete fourier transform technique input data samples. Further, there is provided a timer to prompt the FPGA for processing various events of occurrences.
[0064] At block (303), the method further includes operating the timer circuit in order to select the cycle time, preferably less than 100us.

[0065] Figure 3 depicts an example method illustrating whether the failure has been detected in the rotating diode under open circuit condition or under short circuit condition.
[0066] At block (401), the method includes reading data of voltage values and extracting the harmonic component data from the FPGA present in the computational unit (104) of the system (126).
[0067] At block (402), the method includes extraction of DC component from voltage samples. Herein, the DC Component data is extracted from the digital data samples of voltage received through the FPGA module and implements the operation in which, the peak values of harmonics is added to calculate the sum of peak values of AC harmonics, up to 6th harmonic, in the exciter field current.
[0068] At block (403), the method includes determination of AC-DC factor by calculating ratio of sum of peak values of AC harmonics to DC current.
[0069] At block (404) the method includes, comparing by the processor range of AC-DC factor, in which, the determined AC-DC factor value is compared with the values range from 0.3 to 0.85, pre-defined for diode short circuit failure condition. This is followed by generating a signal for checking time delay in the timer circuit on ascertaining that the AC-DC factor lies within a predefined range of values. Otherwise the program loop continues to run for identifying the diode failure condition.
[0070] For instance, the processor implements the operation in which , the diode short circuit alarm signal ‘DO’ status is set as logic ‘1’, the same signal is connected to annunciator in the control panel and generates an Alarm. Alarm activated status indication will also be appearing in the HMI. Further, the processor implements the operation in which, the diode short circuit trip signal ‘DO’ status is set as logic ‘1’, the same signal is sent to the voltage regulator for taking necessary control and protection based on the diode short circuit condition detection. Trip status indication is simultaneously checked in the HMI.
[0071] In a preferred embodiment, time delay lies within a range of 0.5 sec to 2 sec. If it is meeting the specified time delay, then it sends a signal to activate diode short circuit alarm signal and trip signal.
[0072] At block (405), the method includes, comparing by the processor range of AC-DC factor, in which, the determined AC-DC factor value is compared with the values range from 0.1 to 0.3, pre-defined for diode open circuit failure condition. This is followed by generating a signal for checking time delay in the timer circuit on ascertaining by the processor that the signal to be tested lies within a predefined range of values. Otherwise the program loop continues to run for identifying the diode failure open circuit condition.
[0073] For instance , the processor implements the operation, in which, the diode open circuit alarm signal, ‘DO’ status is set as logic ‘1’, the same signal is connected to annunciator in the control panel and generates an alarm. Alarm status indication will also be appearing in the HMI. Further, the processor implements the operation, in which, the diode open circuit trip signal ‘DO’ status is set as logic ‘1’, the same signal is sent to the voltage regulator for taking necessary control & protection based on the diode open circuit condition detection.. Herein, also, the HMI notified regarding status of the trip signal.
[0074] In a preferred embodiment, time delay lies within a range of 2 sec to 5 sec. If it is meeting the specified time delay (214), then it sends a signal to activate diode open circuit alarm signal and trip signal.
[0075] At block (406), the method includes indicating cycle time by the processor for processing various applications. In a preferred embodiment, the chosen cycle time is more than 1 ms.
[0076] For an instance, say in normal operation, all the diode in the bridge rectifier (111) are intact and there is a small AC harmonic ripple present in the field current of the exciter. The AC harmonic ripple depends on the pilot exciter output voltage frequency, thyristor bridge configuration, exciter output voltage frequency and diode bridge configuration. Normally, the pilot exciter (112) and the main exciter (113) output voltage frequencies are chosen higher to minimise AC harmonics in the DC field current. If the diode rectifier is of 6 pulse rectifier, then in the normal operation minimum AC harmonic in the field current is six times the exciter (113) output voltage frequency. The exciter machine (113) frequency reflects on the field current, so the fundamental frequency is considered as exciter output voltage frequency for implementing diode failure condition detection technique. The AC harmonics (H1, H2…H6) up to 6th harmonic of exciter frequency is extracted in the field current for determining the diode failure condition. The extracted AC harmonics peak values (H1, H2…H6) are added to calculate the total peak value of AC harmonic ripple in the field current. The DC of field current is also extracted from the field current, then the AC to DC (AC-DC) factor is determined by calculating the ratio of sum of peak of AC harmonics values to the DC current. In normal operation, all the diodes are intact, this factor is well below 0.1. In the case of diode open circuit condition the factor is in between 0.1 to 0.3 and for diode short circuit condition the factor is in between 0.3 to 0.85. The range may not be same for all brushless excitation systems, it may vary bit based on the type of the bridge rectifier (111), the exciter machine (113) characteristics. The time delay selected for detecting the diode short circuit condition should be very low i.e. 0.5 sec. to 2 sec., because in this case the output of the exciter is severely affected and the generator is unable to provide rated voltage, without overloading the exciter. If such a failure is prolonged, there is a risk of exciter insulation failure due to the stresses developed in the armature winding to maintain rated voltage of the generator. The AC-DC factor varies little bit from no load to full load for a generator but still it will be within the defined AC-DC factor values range. The time delay selected for detecting the diode open circuit condition is bit higher i.e. 2 sec. to 5 sec., because in this case, the output capacity of the exciter may be reduced. In this case, the generator should still be able to deliver rated output, but the transient capability will be diminished. The methods as discussed above constitute the working principle of the invention as well.

[0077] It is to be noted that the processor/processing unit may comprise one or more microprocessors, microcomputers, microcontrollers, digital signal processors, central processing units, logic circuitries, sensors, interfaces and/or any devices that manipulate data based on operational instructions. Among other capabilities, the processor is configured to fetch and execute computer-readable instructions stored in its memory. The memory may store one or more computer-readable instructions or routines, which may be fetched and executed. Additionally, the processor may be implemented as a combination of hardware and programming (for example, programmable instructions) to implement one or more functionalities of the processor. In examples described herein, such combinations of hardware and programming may be implemented in several different ways. In one example, the programming may be processor executable instructions stored on a non-transitory machine-readable storage medium and the hardware may include a processing resource (for example, one or more processors), to execute such instructions. In other examples, the processor may be implemented by electronic circuitry.

[0078] It is to be further noted that the proposed system (126) and the method is not limited for implementation in the brushless synchronous generator excitation system (100) and can be implemented in like equipment as well.

[0079] Test Results
The described invention operation and performance of the system (126) has been evaluated by carrying out various tests on the Test Bed of the Brushless Excitation System (100). The simplified test setup comprises of a brushless exciter, which is mechanically coupled to a calibrated DC drive motor, with its gearbox (1:4 speed ratio) and slip ring shaft to run at its rated speed on the test bed. The rated speed of 3000 rpm of the exciter shaft is achieved by running the DC drive motor at the speed of 750 rpm. The controlled single phase AC supply voltage is rectified using full wave bridge rectifier and fed to the field winding of Exciter machine. In this, the field voltage of the exciter machine is controlled by varying single AC supply voltage. The exciter machine output is connected to a variable resistive load (water load) and the load resistance was adjusted to match with the load characteristics of the exciter machine. The protection system operation is not interfaced with the voltage regulator and trip relay of the generator. In the field circuit of exciter, a shunt resistor of 0.1 Ohms is connected in series with the field circuit to monitor the field current by measuring voltage across the shunt resistor. The voltage measured across the shunt resistor is interfaced to diode failure detection relay to detect the diode failure condition, to monitor the DC field current and sum of the peak values of harmonics, up to 6th harmonic in the field current on-line. The field current has been recorded during normal operation, diode open circuit and trends data was stored in the Human Machine Interface (HMI). The settings were also fed to the device through the HMI.
Normal operation and diode open circuit condition test cases were carried out to evaluate the relay operational performance and are illustrated in Figure 4 and Figure 5 respectively.
Normal Operation: In normal operation, at round 11:12:500 hrs on X-axis, the exciter output is connected to the water load and the single phase AC supply voltage is varied to increase the exciter field current, subsequently, the DC field current and sum of peak values of AC harmonics current are also increased as illustrated in Figure 4. In this case, the AC-DC factor is < 0.1 and the diode failure relay did not detect any diode failure condition.
Diode Open Circuit Condition Operation: The diode open circuit condition is created by removing the healthy diodes of one arm of 3-phase rotating bridge rectifier and replaced with open/faulty diodes. As shown in Figure 5, at round 13:40:00 hrs on X-axis, the single phase supply voltage is varied to increase the field current of exciter machine and the exciter machine output is connected to the water load. When the machine is loaded, immediately the diode failure relay is operated and hence, the diode open circuit condition is detected. At this instant, the DC field current is approx. 1.45 Amp & sum of peak values of AC harmonics current is approx. 0.55 Amp as illustrated in Error! Reference source not found.. In this case, the AC-DC factor is in between 0.1 & 0.4, and the diode open circuit detection alarm is generated as soon as the exciter output is connected to the load because, when the exciter is in open circuit condition then there is no current flow through the diode hence there is no harmonic/unbalance current in the field circuit. In this case, pre-defined range for diode open circuit condition is set as 0.1 and 0.4 and pre-defined range for diode short circuit condition is set as 0.4 and 0.85.
From the test results, it is observed that the relay performance is found to be satisfactory. In the testing, during the normal operation case, the diode failure relay is not operated/detected. In the diode open circuit condition case, the relay does not detect/operate the diode open circuit condition under exciter no-load condition and the relay is detected/operated when it is connected to water load. In the actual brushless excitation system, the exciter machine no-load condition scenario will not come into picture as the exciter output is always connected to the field winding of the generator.
[0080] ADVANTAGES OF THE INVENTION
All in all, the system (126) described in the present disclosure is having the following advantages:
a) A simple and efficient architecture to achieve the system operation.
b) Presence of a Human machine interface (105), for the purpose of online monitoring.
c) An accurate and reliable method to detect a diode failure condition either in open circuit or in short circuit condition in the rotating diode bridge rectifier (111) of the excitation system.

[0081] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various systems that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.

[0082] Although embodiments for the present subject matter have been described in language specific to package features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/device of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

[0083] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0084] It will be further appreciated that functions or structures of a plurality of components or steps may be combined into a single component or step, or the functions or structures of one-step or component may be split among plural steps or components. The present invention contemplates all of these combinations. Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other dimensions or geometries are possible. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention. The present invention also encompasses intermediate and end products resulting from the practice of the methods herein. The use of “comprising” or “including” also contemplates embodiments that “consist essentially of” or “consist of” the recited feature.