1. Field of the Invention
The present invention relates to a nuclear power plant, and more specifically to a nuclear power plant capable of adjusting the position of a turbine by-pass valve based on the steam pressure generated in a nuclear reactor.
2. Description of the Related Art
FIG. 1 is a diagram showing a main steam system and a turbine by-pass system of a nuclear power plant.
A main steam supply system 01 is constituted such that steam generated in a nuclear reactor 1 is supplied to a steam turbine 8 via a main steam header 4, a main steam stop valve 5, and a main steam control valve (CV) 6, respectively.
Specifically, steam from the nuclear reactor 1 is supplied to the main steam header 4 disposed outside a primary containment vessel 3. The steam thus supplied to main steam header 4 then flows to the steam turbine 8 via the main steam stop valve 5 and the main steam control valve 6. The main steam stop valve 5 isolates steam in the steam turbine 8 in case of stopping operation thereof, and the main steam control valve 6 adjusts the flow rate of steam to the steam turbine 8. The steam from the nuclear reactor 1 rotates the steam turbine 8, and a generator 9 connected directly to the steam turbine 8 generates electric power.
Steam that passes through the steam turbine 8 is then guided to a condenser 10. Cooling water such as seawater enters the condenser 10, and a heat exchange is made between the cooling water and the steam. Steam thus cooled is condensed to water and is circulated back to the nuclear reactor 1.
A turbine by-pass steam supply system 02, independent from the main steam system 01, branches from the main steam header 4. The turbine by-pass system 02 guides steam from the main steam header 4 to the condenser 10 via the turbine by-pass valve.
In a regular operation mode of the nuclear power plant, steam pressure generated in the nuclear reactor 1, which is specifically pressure on the main steam header 4 detected by a main steam pressure detector 2 or pressure detected by a reactor dome pressure detector 11, is adjusted by the main steam control valve 6 in order to meet a predetermined pressure value. The turbine by-pass valve 7 is fully opened in this situation. Meanwhile, when the nuclear power plant is in a starting or a stopping mode, or when an accident happens to a power supply system, the position of the main steam control valve 6 restricted. In this situation, the turbine by-pass valve 7 adjusts the main steam pressure 2 in the main steam header 4.
Further, when a load is deprived, such as load isolation of the generator 9 and turbine trip, turbine-trip, or the like, both the main steam stop valve 5 and the main steam control valve 6 are closed rapidly, stopping the steam flow to the steam turbine 8. This causes an increase in the pressure in the nuclear reactor 1 and of the main steam. To relax this pressure, the turbine by-pass valve 7 rapidly opens and the main steam is bypassed to the condenser 10.
A conventional turbine controller for the nuclear power plant is explained referring to FIG. 2. A regulating controller in the steam turbine controller 12 controls the position of the main steam control valve 5 and the turbine by-pass valve 7.
Main steam pressure signals are output signals from the main steam pressure detector 2 connected to the main steam header 4 and enter the steam turbine controller 12. The signals thus entered are compared to the predetermined pressure value in a main steam pressure setter 23, and a pressure deviation signal 29 is carried out by a first pressure deviation calculating unit 24. Here, the pressure deviation signal 29 is entered into a pressure control calculating unit 25, and a pressure control signal 30, which is proportional to the pressure deviation signal 29, is input into a first low value selector 18 as a pressure control signal 30.
In the first low value selector 18, the pressure control signal 30 is compared to a velocity/load control signal from a speed/load control calculating unit 15, a load limit signal from a load limiter 16, and a maximum flow rate limit signal from a maximum discharge restriction unit 17, respectively. After the comparison, the first low value selector 18 chooses a minimum signal from among those signals and outputs the minimum signal as a valve position demand signal 26 of the main steam control valve 6.
Further, the pressure control signal 30 carried out by the pressure control calculating unit 25 and the valve position demand signal 26 of the main steam control valve 6 obtained by the first low value selector 18 are input into a first deviation calculating unit 20, and a deviation signal is calculated. The maximum discharge restriction signal carried out by the maximum discharge restriction unit 17 and the valve position demand signal 26 of the main steam control valve 6 obtained by the first low value selector 18 are input into a second deviation calculating unit 21, and a deviation signal is calculated.
The deviation signals from the first deviation calculating unit 20 and the second deviation calculating unit 21 are input into a second low value selector 22. These deviations are then compared therein, and the lower signal is chosen as a valve position demand signal 31 of the turbine by-pass valve 7.
The turbine by-pass valve position demand signal 31 output from the regulating controller 13 and the valve position demand signal 26 are entered into a valve position control unit 32 having an amplifier, and a deviation signal carried out by the valve position control unit 32 is entered into a servo valve 33. The servo valve 33 controls the valve position of the turbine by-pass valve 7 to a value required by the steam turbine controller 12, by adjusting the amount of oil in an oil cylinder 38 that operates turbine by-pass valve 7.
The oil cylinder 38 connects a fast acting solenoid valve 37; the fast acting solenoid valve 37 accepts a fast open acting demand to turbine by-pass valve 36 and makes turbine by-pass valve 7 realize a rapid valve-opening operation in an emergency as well as in a performance test. In the regular operation mode, the fast open acting demand to turbine by-pass valve 36 is not generated, and therefore, the oil cylinder 38 is controlled only by turbine by-pass valve 7. However, if the fast open acting demand to turbine by-pass valve 36 is generated due to detection of a power load unbalance such as a load isolation, the turbine by-pass valve 7 is fully opened regardless of the control signal from the servo valve 33. Usually, a plurality of turbine by-pass valves 7 are equipped in a plant, however, only the valve which accepted the fast open acting demand to turbine by-pass valve 36 can be fully opened.
For reliability reasons, the main steam pressure detector 2, the regulating controller 13 and the like are multiplexed. Therefore, FIG. 2 shows the case where the triplex main steam pressure detectors 2 and the triplex regulating controller 13 are arranged. The medium value among the output signal from the triplex main steam pressure detectors 2 are chosen by the first medium value selector 27, and each of the triplex regulating controllers 13 operates the pressure control signal 30 and the valve position control unit 32 for the plant control.
Further, the number of turbine by-pass valves 7 varies from each nuclear power plant. The valve position control unit 32, the servo valve 33, the fast acting solenoid valve 37, and the oil cylinder 38 are identical in each turbine by-pass valve 7, and therefore, only one turbine by-pass valve 7 and the peripherals are illustrated in FIG. 2.
In a nuclear power plant having multiplexed regulating controllers 13, if one regulating controller 13 has a problem or an unusual condition in its regular operating mode, the other regulating controllers can compensate the unusual condition and maintain the operation. Moreover, if the unusual condition is found, the system can recover from any problems. However, if there is an unusual condition in hardware or software that affects all the regulating controllers 13 commonly, such unusual condition may not be found and the operation may continue.
If an unusual condition over plural regulating controllers happens, the ability to adjust the position of the turbine by-pass valve 7 is lost, and a turbine trip occurs before the unusual condition is detected, the turbine by-pass valve 7, which is usually opened when the main steam stop valve 5 is fully closed, may not operate. Because the turbine by-pass valve 7 keeps closing in this situation, pressure inside the nuclear reactor 1 is rapidly increased and will be in critical thermal condition.
The present invention has been made in view of the above-mentioned circumstances and is intended to solve the above-mentioned problems. In particular, the purpose of the present invention is to provide a steam turbine controller for a nuclear power plant capable of avoiding a rapid increase in pressure in the nuclear reactor even if the function of the turbine by-pass valve is lost.
The present invention provides a nuclear power plant having a nuclear reactor, including: a first steam supply system connected between the nuclear reactor and a steam turbine, a second steam supply system branched from the first steam supply system and connected downstream of the steam turbine, a first valve in the first steam supply system for adjusting steam pressure to the steam turbine, a second valve in the second steam supply system for adjusting branched steam pressure, a first controller that generates a first opening/closing signal for the first valve and a second opening/closing signal for the second valve, and a second controller that generates a third opening/closing signal for the second valve, the third opening/closing signal having priority over the second opening/closing signal.
Here, the third signal may be generated if the second valve is closed and the pressure in the steam turbine decreases. The third signal may include an opening signal for the second valve.
Further, the third signal may be generated if the second valve is closed within a predetermined time period after receiving the second signal. The third signal may be released if the second valve is opened within a predetermined time period after receiving the third signal.
The second valve may be multiplexed, and each second valve may accept the second signal and the third signal.
Furthermore, the third signal may be released if the steam pressure from the nuclear reactor is in a predetermined value. The third signal may be generated only once.
The third signal may be generated at least when the plant is not in regular operating mode. The third signal may act to avoid closing both the first valve and the second valve.