Patent Number: 055639224
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawings, there is shown therein a nuclear reactor pressure vessel indicated generally as 1. Pressure vessel 1 contains a plurality of control rods 2 structurally coupled to respective drive rods 3. Control rods 2 and drive rods 3 are surrounded with water 4 which completely fills the pressure vessel 1. Under normal operating conditions, the water is at a high temperature and is pressurized so that no boiling takes place. The position of each control rod 2 is determined from measurements obtained from a sensing mechanism, indicated generally at 5, placed outside the pressure vessel 1. No mechanical penetrations of the pressure vessel 1 for purposes of control rod position sensing are permitted. Thus the area where sensing mechanism 5 can be placed is along a nonmagnetic rod travel housing 6. Drive rods 3, which are made of a metal having magnetic properties, move within their respective rod travel housings 6, which are sealed pipes formed integrally with and extending longitudinally upward from a head 7 of vessel 1. Sensing mechanism 5 is placed along rod travel housing 6 and can only sense the position of a drive rod 3. However, it is normally assumed that the fastening of control rod 2 to its respective drive rod 3 is reliable and therefore the displacement of drive rod 3 and control rod 2 are the same. Sensing mechanism 5 is shown in further detail in FIG. 2 and is of the linear voltage transformer type with which the method and system of the present invention are particularly useful. It should be understood that the present method is not restricted in application to the linear voltage transformer indicator but rather may be used with other types of sensing mechanisms. Sensing mechanism 5 includes a plurality of annular layered-wound primary coils P which are electrically connected in series to form a primary winding, and a plurality of annular, layered-wound secondary coils S which are electrically connected in series to form a secondary winding. The coils are stacked in tandem and are mounted on a coil stack indicated generally at 12. Coil stack 12 includes end plates 14 and 15 and a thin nonmagnetic stainless steel tubular substructure 13 that is slid over travel housing 6 which encloses drive rod 3. Secondary coils S are alternatively interleaved and inductively coupled with primary coils P. Coil stack 12 is preferably mounted on rod travel housing 6 so that at least the top portion 20 of drive rod 3 penetrates the bottom portion of the core stack. A sinusoidal current source 18 (FIG. 2) is connected for exciting a current in primary winding P which induces a voltage across terminals 21 and 22 of secondary winding S. In operation, the coupling between the primary and secondary windings increases as drive rod 3 moves through travel housing 6 causing a proportional increase in the magnitude of the voltage induced in secondary winding S. The secondary voltage thus also corresponds to the position of control rod 2 as it is withdrawn from the core of the reactor. While in theory the relationship between the secondary voltage and the rod position should be linear, in fact there are a number of variables which introduce error into the output of the secondary winding. These errors include changes in the primary and secondary circuit loop resistances due to temperature effects and connector contact resistance, variations in the primary excitation current caused by source variations, variations in the primary excitation current caused by changes in the magnetizing and leakage reactance of the coil stack which result from changes in rod position, primary current distortion and non-linear aspects of the coil stack magnetics which produce harmonic content in the secondary signal, or induced voltages in the secondary circuit from adjacent coil stacks. The system of the present invention includes a method and system for compensating the rod position indication system for such errors. FIG. 3 illustrates a general block diagram of the rod position indication system of the present invention indicated generally at 25. Sinusoidal current source 18 excites a primary coil stack 27 consisting of primary coils P, with a 60 Hz alternating current (AC). A secondary coil stack 28 consisting of secondary coils S, produces a secondary AC voltage roughly proportional to the excitation current multiplied by the percentage insertion of drive rod 3 into coil stack 12 plus a constant proportional to the excitation current only. A shunt 30 is connected between source 18 and primary coil 27 and provides current to an isolation transformer 31 whereby a primary AC voltage is induced across a secondary winding of isolation transformer 31 is shown in FIG. 4A. Secondary coil 28 and isolation transformer 31 are connected in series with a pair of low pass filters 32 and 33, respectively (FIG. 3). A pair of absolute value circuits or precision rectifiers 34 and 35 are connected in series and between low pass filters 32 and 33 and a second pair of low pass filters 36 and 37, respectively. Low pass filters 36 and 37 are both connected to an analog divider 38 which outputs a ratiometric signal 39 to a scaling and temperature compensation circuit 40. Compensation circuit 40 receives a temperature input 42 and a voltage reference 43 along with ratiometric signal 39 and is calibrated using zero adjustment 44, span adjustment 45 and a temperature compensation adjustment 46. Compensation circuit 40 outputs a signal 48 which is fed into a linear correction circuit indicated generally at 50. Linear correction circuit 50 includes a usual analog-to-digital (A/D) converter 49, an EPROM 59 and a digital-to-analog (D/A) converter 61. A/D converter 49 includes a voltage-to-frequency (V/F) converter 52 connected in series with a counter 54, and a clock 56 which drives both V/F converter 52 and a timing and control logic circuit 58. Counter 54 feeds EPROM 59 with a digital address 57 and EPROM 59 outputs a digital linear correction factor 60 to D/A converter 61 which converts the digital signal to an analog linear correction factor 62. Analog linear correction factor 62 received from D/A converter 61 is summed with signal 48 by a summing block 63. Summing Block 63 outputs a module output value 64 which is a linear representation of the control rod position. Output 64 is fed into a pair of output buffers 65 and 66 and a magnitude comparator 69 which compares the rod position with a rod bottom set-point 70. Comparator 69 energizes an output driver 71 which drives a plant annunciator 72 to alert plant personnel that a rod bottom condition exists. Output buffers 65 and 66 provide control rod position to a plant meter 67 and a plant computer 68, respectively. FIGS. 4A-4D show one of the many possible implementations and detailed circuitry of the method and system of the present invention. As shown in FIG. 4A, sinusoidal current source 18 provides an AC current to shunt 30 and primary coil stack 27. Shunt 30 includes a shunt resistor 80 and lines 82 and 84 which are connected to each side of shunt resistor 80. Shunt 30 functions as a current divider allowing current to flow from a first side of shunt resistor 80 through line 82 supplying current to isolation transformer 31 and back into the primary coil stack loop through line 84 on a second side of shunt resistor 80. Isolation transformer 31 is shown with a primary coil 85 and a secondary coil 86 which outputs an isolated AC signal along line 95. A test point A is connected in parallel with primary coils 85 of isolation transformer 31 to allow testing of the primary signal coming from sinusoidal source 18. Secondary coil 28 is connected to a pair of normally closed relays 87 and 87a. Relay 87 is connected between one end of secondary coil 28 and ground, and relay 87a is connected to a second end of secondary coil 28 and to a line 94. A test point B is connected between line 94 and ground to allow testing of the secondary signal induced on secondary coil 28. The details of the two low pass filters 32 and 33, precision rectifiers 34 and 35, and low pass filters 36 and 37 are substantially similar except for possible variations in component values. A resistor 88 is connected in line 94 after test point B and a second resistor 89 is connected along line 95 after secondary coil 86 of isolation transformer 31. A pair of capacitors 91 and 92 are connected between lines 94 and 95 and ground respectively, and along with resistors 88 and 89 provide initial filtering of the respective signals. A pair of usual voltage protectors 97 and 98 are connected to lines 94 and 95 after capacitors 91 and 92 and include forward-biased diodes 100 connected between lines 94 and 95 and positive 13 DC volts, and reversed-biased diodes 101 are connected between lines 94 and 95 and negative 13 DC volts. A pair of operational amplifier circuits 106 and 107 are connected to lines 94 and 95 respectively, and provide initial isolation to the respective signals with a high input impedance which factors out various resistance errors which may exist in the circuit cables and connectors. The outputs of op amp circuits 106 and 107 are fed into usual high order low pass filters 32 and 33 which are designed by cascading, three second order, low pass filter sections 32a, 32b, 32c and 33a, 33b, 33c, respectively. The output of low pass filters 32 and 33 are fed into usual precision rectifiers 34 and 35 (FIG. 4B) each of which consists of three op amp circuits 34a, 34b, 34c and 35a, 35b, 35c, respectively, that convert the secondary AC voltage and the primary AC voltage to a secondary DC voltage and a primary DC voltage, respectively, at points 41 and 41a. Precision rectifiers 34 and 35 are connected in series with usual, second order, low pass filters 36 and 37, respectively. Low pass filters 36 and 37 feed analog divider 38 which computes ratiometric signal 39. Zero adjustment 44 and span adjustment 45 are shown as potentiometers which use the reference voltage 43 to adjust a single op amp 47. Temperature input 42 is fed into temperature compensation adjustment 46 which outputs a temperature adjustment signal 51. The output of op amp 47 and temperature compensation signal 51 are both processed by scaling and temperature compensation circuit 40. Signal 48 is output from scaling and overall temperature compensation circuit 40 and is input to V/F converter 52 (FIG. 4C) which includes zero and gain adjustment potentiometers 53a and 53, respectively. V/F converter 52 is driven by oscillator/clock 56 which outputs a 4 Hz signal 55 into the timing and control logic 58. Timing and control logic 58 includes nand gates 73, 74, 75 and 76 which are connected to drive counter 54. Oscillator/clock 56 drives V/F converter 52 through line 77 and V/F converter 52 feeds clock input 78 of counter 54. Counter 54 feeds a 12 bit digital address, indicated generally at 57, to EPROM 59 which outputs an 8 bit digital linear correction factor indicated generally at 60. Correction factor 60 is processed by D/A converter chip 79 before being input into op amp 81 of D/A converter 61 and is converted to analog linear correction factor 62 before being summed with ratiometric signal 48 by summing block 63 (FIG. 4D). Summing block 63 includes a linear scaling potentiometer 83 and op amp circuit 90. Module output value 64 is fed into output buffers 65 and 66 and rod bottom detector 69. Output buffer 65 includes a gain adjust 108 to allow for a variety of meter movements. Output buffers 65 and 66 include voltage protectors 93 and 93a respectively, which are the same as voltage protectors 97 and 98 described above (FIG. 4A). The buffered control rod position is then fed into plant meter 67 and plant computer 68 for further processing. The rod bottom detector includes a comparator 69 which compares module output value 64 with rod bottom threshold set-point 70 which is set by potentiometer 96. Plant annunciator 72 is represented by a relay coil and is energized if module output value 64 is below the value set by potentiometer 96. FIGS. 5 and 6 show the deviations in step position when a resistance is applied to a prior art rod position indication system and the rod position indication system of the present invention. Line 150 of FIG. 5 shows a prior art rod position indicating system of which the present invention is an improvement thereon, when unwanted resistances from 0 to 20 ohms are applied to the primary coil stack circuit as can occur during operation from a variety of uncontrolled sources. As this resistance is added to this prior art circuit or system the measured output of the rod position decreases linearly until a position of approximately 205 steps is indicated when the resistance is 20 ohms. A deviation of approximately 23 steps from an expected 228 steps calculates to an error of approximately 10%. Line 151 shows the effect on the position indicated output of the system of the present invention with the same resistances applied to the primary coil stack. A position of approximately 225 steps is measured by the system of the present invention when the same 20 ohms is applied to its primary circuit. A deviation of approximately 3 steps from an expected 228 calculates to an error of only slightly greater than 1%. The graph in FIG. 6 shows the output of the two systems when a resistance of up to 30 ohms is added to the secondary coil of the respective circuits discussed above for FIG. 5. Line 155 shows the effect of the added resistance to the secondary of the same prior art system shown by line 150 of FIG. 5, and shows a deviation from the expected 228 step position by slightly more than 8 steps, while line 157 shows the effect of this added resistance to the secondary of the system of the present invention, showing no deviation from the expected control rod position. In operation, sinusoidal current source 18 applies a 120 volt, 60 Hz sinusoidal or AC current to primary coil stack 27. As magnetic drive rod 3 enters travel housing 6 the magnetic coupling between primary coil stack 27 and secondary coil stack 28 increases while a voltage is induced on secondary coil stack 28. Shunt 30 provides current to isolation transformer 31 and as the current passes through primary coil 85 a voltage is induced across the secondary coil 86. The voltage induced across secondary coil 86 is proportional in value to the primary current used to induce a voltage across secondary coil stack 28 (FIG. 4A). The secondary and primary voltages are initially filtered by the RC-circuit formed by resistors 88 and 89 and capacitors 91 and 92, respectively. The signal along lines 94 and 95 is then fed into op amps 106 and 107 respectively, to provide initial isolation of the signals which makes the circuit immune to any substantial secondary circuit resistance changes. Low pass filtering is applied to both signals by filters 32 and 33 which are tuned for a cutoff frequency of approximately 90 Hz to adequately pass the 60 Hz signals and to reject any high harmonic noise. The secondary and primary AC voltages are then converted to a secondary and primary DC voltage by precision rectifiers 34 and 35. The secondary and primary DC voltages are then passed through second low pass filters 36 and 37 to smooth the output of the absolute value circuit and to discriminate against harmonic frequencies generated in the conversion process. The secondary and primary DC voltages are then fed into analog divider 38 whereby the secondary DC voltage is divided by the primary DC voltage to produce a ratiometric signal 39. In the ideal case, the primary current remains constant while the secondary voltage changes as a function of deliberate changes in rod position. In reality, there is a small interaction introduced as the coupling coefficient of the coil stack core changes with rod position. The ratiometric signal operates on the principle that if the secondary circuit of a transformer is open circuited, the output voltage will be proportional to the input excitation current and the effective turns ratio of the primary and secondary circuit. Moving the drive rod inside the rod travel housing changes the effective turns ratio of the coil stack. A value proportional to the excitation current is ratioed with the secondary voltage to produce a signal which is proportional only to effective turns ratio of the coil stack. Filtering and rectifying the primary coil current signal and secondary coil voltage signal produces a signal proportional to the original fundamental AC signals without distortion present in the raw AC signals. Forming the ratio of these filtered and rectified signals produces a value for turns ratio which is tolerant to self-generated and applied harmonics. The design of the present invention is virtually immune to routine fluctuations in AC excitation current regardless of source. Harmonics are removed in the filtering applied to both primary current and secondary voltage by filters 36 and 37. The plant metering circuits see only the magnitudes of fundamental signals applied to and obtained for the coil stack. The process of producing DC values from the AC primary and secondary signals reduces these signals to scalar values. Induced 60 Hz coupling from adjacent coil stacks add vectorially, with the noise signal substantially quadrature to the signal induced by primary current on the coil stack. For noise signals -3 dB or less, such vectorial addition will produce negligible effect on the scalar equivalent of the resultant vector voltage. The ratiometric approach makes the design immune for all primary circuit resistance changes except those resulting from gross circuit failure. The high impedance input, made possible by the inherent noise immunity of the circuit design allows for substantially secondary circuit resistance changes without adversely impacting the accuracy of the measurement. The ratiometric signal is fed into scaling and overall temperature compensation circuit 40 (FIG. 4B). External adjustments on the front of the system control panel provides zero, span, and temperature correction gain, as well as mode selection, rod bottom set point, and meter output calibration or gain for the module. The compensated signal 48 is fed into linear correction circuit 50 (FIG. 4C). EPROM 59 is programmed on-site or off-site with up to eight sets of correction curves with 4096 linearity correction data points that are interpolated from a few sample data values for a given coil stack. Linear correction circuit 50 (FIG. 3) provides for piece-wise linear correction that can easily be customized for each rod position indicator to compensate for inherent non-linearities of individual coil stack magnetics. In accordance with one of the many features of the system of the present invention zero adjustment 44 and span adjustment 45 (FIG. 4B) have been decoupled allowing for a shorter calibration time during start-up critical path. The control rod is fully inserted into the reactor core and the zero adjustment is calibrated, the control rod is then completely removed from the reactor core and allowed to thermally stabilize for an hour. The span adjustment, being decoupled from the zero adjustment is calibrated without affecting the calibration of the zero adjustment. Accordingly, the method and system for indicating the position of control rods of a nuclear reactor provides a method and system for desensitizing the coil stack output against excitation source fluctuation by computing the quotient of secondary voltage and primary current. The method and system makes for a more stable rod position indicator and performs a function that is typically handled with a microprocessor-based system. The method and system's elegantly simple design yields high-tech performance without attendant concerns for the application of high-tech upgrades to older operating plants. The method and system achieves all the enumerated objectives, provides for eliminating difficulties and inaccuracies encountered with prior devices, and solves problems and obtains new results in the art. In the foregoing description, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirement of the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the description and illustration of the invention is by way of example, and the scope of the invention is not limited to the exact details shown or described. Having now described the features, discoveries and principles of the invention, the manner in use, the characteristics of the construction, and the advantageous, new and useful results obtained; the new and useful structures, devices, elements, arrangements, parts and combinations, are set forth in the appended claims.