Normal AC power supply

This section shall describe the normal AC power supply, emphasizing the design and performance characteristics. 

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If the normal feedwater is lost at a WWER-1000/320 plant, the auxiliary feedwater pumps would start from a low SG water level and the emergency feedwater pumps would start from a very low level. The auxiliary feedwater pumps do not have diesel backed power supply, and thus the loss of normal AC power supply leaves the emergency feedwater system as the only means for feedwater supply. The emergency feedwater (temperature around 20°C) is not heated, and therefore every startup of the emergency feedwater system involves a thermal shock to the SGs. 

For the Zaporozhe NPP Units 5 and 6, the auxiliary feedwater pumps which suck hot water from deaerator are backed up by the additional diesel generator to avoid possible damage of steam generators by cold water from the emergency feedwater tanks in case of loss of normal AC power supply. 

Each start-up feed water pump and its associated motor operated isolation valve must be able to receive power from one of the two onsite standby diesels in the event of loss of the normal ac power supply, with the other pump supplied from the other diesel. 

The non-safety-related Standby ac Power Supply consists of two diesel generators. Each diesel generator (DG) provides 6.9 kVac power to one of the two load groups whenever the main turbine generator and the normal preferred off-site power source are not operating. When operating, the standby ac power supply provides power to safety- related loads and to non-safety-related investment protection loads. Other non-safety-related loads are not powered from the standby power source. 

The Class IE vital ac power supply provides redundant, reliable power to the safety logic and control functions during normal, upset and accident conditions. 

The onsite standby diesel generator backed 400 Vac distribution system provides the normal ac power to the battery chargers. Industry standard stationary batteries are provided to supply the dc power source in case the battery chargers fail to supply the dc distribution bus system loads. The batteries are sized to supply the system loads for a period of at least two hours after loss of all ac... 

Two onsite standby diesel generators power the onsite standby power system. This supplies power to selected loads in the event of the loss of the normal or the preferred ac power supplies. These loads provide defence-in-depth functionality. The diesel generators are automatically started and connected to their respective buses. In the event of a fast bust transfer, the diesel connection to the bus is delayed such that the fast bus and residual transfer is allowed to initiate. 

If normal electrical power is unavailable, the diesel generators (non-class IE, class IE) act as a back-up source. In the event of a station blackout (loss of off-site and on-site AC power supply), the alternate AC diesel generator (class IE) supplies power to the class IE loads to maintain the reactor in a safe shutdown condition. The batteries have adequate capacity to supply DC power to perform required functions in an accident assuming a single failure. To ensure the safety of the reactor, the electric power supply for the safety related systems is designed as a highly reliable power source (class IE). Two physically separate power sources are provided to the safety related system. 

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

When a short circuit occurs in a power supply, larger than normal currents are caused to flow into the short circuit. The magnitude of the short-circuit current is determined by the impedance of AC systems, or the resistance of DC systems, that exists between the short circuit and the sources of voltage. That impedance or the resistance will be called the source impedance in the discussions that follow. In DC systems the source impedance is often the series addition of the supply cable resistance, the rectifier or thyristor internal resistance and any other resistance that may be connected in the circuit. The calculation of the short-circuit current in a DC circuit is therefore a reasonably simple process once the resistance data are known. 

ELECTRIC POWER SYSTEM Clas I,II and III Electnc System CSAB290 5 The Class III system pow ers designated safety -related and economic equipm i protection loads pump motors, valv es etc) Normally Class IV power supplies the Class III power W. cn the Class IV system fails two redundant standby diesel generators provide Class ni power The ac Class II and dc Class I systems supply un-mtermptible power to the control and safety svstems... 

FIGURE 10.162 How the normal mode noise attenuation of an isolation transformer combines with the filtering characteristics of the AC to DC power supply to prevent noise propagation to the load. 

A modular power center, commonly found in computer-room installations, provides a comprehensive solution to AC power-distribution and ground-noise considerations. Such equipment is available from several manufacturers, with various options and features. A computer power-distribution center generally includes an isolation transformer designed for noise suppression, distribution circuit breakers, power-supply cables, and a status-monitoring unit. The system concept is shown in Fig. 10.268. Input power is fed to an isolation transformer with primary taps to match the AC voltage required at the facility. A bank of circuit breakers is included in the chassis, and individual preassembled and terminated cables supply AC power to the various loads. A status-monitoring circuit signals the operator if any condition is detected outside normal parameters. 

The seals of the reactor coolant pumps (RCPs) are cooled by the CVCS. Part of the make-up flow of the CVCS pumps is directed to the RCP seals. The pumps of the CVCS are supplied by the Diesel Generators in case of LOOP. Only in case of station black out (loss of all AC power) the seals are without cooling and will heat up. To avoid a small break LOCA under such conditions, a special seal at the shaft of the RCPs - which is normally not in operation - has to be closed remotely. The behaviour of this final seal has been tested in a test house and it was demonstrated that this seal is capable to withstand the primary coolant temperature. 

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