SAFETY P R A C TIC E S - gnssn - International Atomic Energy Agency

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This publication is no longer validPlease see http://www-ns.iaea.org/standards/3.3.1.4. Voltage considerationsSystem voltage studies are performed for the EPS to optimize and establishoperating voltage levels during normal operation, anticipated operational occurrencesand accident conditions. The analysis should consider the effects of normaloperation of the main unit generator and grid as well as transient conditionsassociated with startup, shutdown, unit trip, postulated accidents and abnormal gridconditions. The analysis will include the situations where the off-site grid is the onlyelectrical power source available and the grid voltage is at the minimum level calculatedwith the nuclear unit in any anticipated condition and the situation where thegrid voltage is at the maximum level calculated with the minimum loads suppliedfrom the EPS, e.g. cold shutdown or refuelling.The voltage studies cover the complete EPS, from the power supplies to theloads, at all voltage levels, to ensure that the loads are supplied with electrical powerat acceptable voltages so as to permit the required performance of the EPS and equipmentimportant to safety. The voltage studies should consider the effects of connectingloads to the EPS in proper sequence under the various conditions outlined aboveand the effects of inrush currents on starting motors.Overvoltage and undervoltage protection will be provided to prevent adverseoperation of the EPS and possible damage to safety system electrical equipment asthe result of sustained abnormal voltage conditions of the EPS.Basis for acceptanceSafety Guide 50-SG-D7, paras 408-412, 508, 520, 522, 526, 531, 566.Assessment questions(1) Is the voltage at the terminals of the loads within the manufacturer’s ratedcontinuous operating ranges for operating states and accident conditions whenthe EPS is supplied from:(a) the grid;(b) the unit main generator;(c) the stand-by generator(s);(d) a neighbouring unit;when the voltage and frequency of each is at its maximum or minimum anticipatedvalue?(2) Considering all the conditions described in (1) above and, in addition, thevoltage drops caused by the transient currents of load energization, can theloads be started safely and within the time limits required, so that the safetyfunctions of equipment can be accomplished? For example:— contactors or relays could drop out or fail to pick up;22
This publication is no longer validPlease see http://www-ns.iaea.org/standards/— motors could fail to accelerate in an acceptable time;— power supplies to the EPS could become overloaded.(3) Are the voltages on the buses of the EPS and off-site power supplies indicatedin the control room with abnormal high and low level alarms annunciated?(4) Is the voltage sensing logic, incorporated in each division of the EPS to detectdegraded conditions in the normal power supply, designed:(a) to initiate protective action in sufficient time to ensure safe and orderlyload shedding, startup and loading of the stand-by power supply?(b) to be properly co-ordinated with the protective and control systems of theEPS so as to avoid, for example, spurious separation of the normal powersupply when supplied from the grid, or spurious shedding of loads fromthe stand-by power supplies due to motor starting transients?(c) to comply with protection system requirements?3.3.1.5. Overcurrent and short circuit considerationsIn order to achieve the protection sought with minimum disturbance, orunnecessary system service impairment, the protective devices must be calibratedand designed to interrupt the circuit without damage to themselves and in propersequence, i.e. the protective device closest to the faulted component on the powersupply side should open first in sequence and so on.Overcurrent protective device co-ordination studies are conducted to ensurethat the various devices are selective in their operation, i.e. co-ordinated with eachother. In these studies transient inrush currents from energizing equipment such astransformers and motors as well as maximum and minimum short circuit currentsfor various given faults are considered. Proper co-ordination of protective devicesallows energization and operation of the EPS, startup and operation of requiredloads, and the selective isolation of faults so as to localize the disturbance andminimize the effects on the EPS.Short circuit studies are also performed for the EPS and are used to:(1) Select overcurrent protective devices such as circuit breakers and fuses whichmust isolate the faults at a given location safely with a minimum of damageto circuits and equipment.(2) Determine the maximum short circuit currents which pass through the variouscomponents of the EPS and for which the components must be designed tosafely withstand the effects.(3) Establish the short circuit currents used in co-ordinating the overcurrentprotective device and/or protective relay settings.Basis for acceptanceSafety Guide 50-SG-D7, paras 533-535.23
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