SYSTEMS REQUIREMENTS
Station Operation Systems
The designers of the electrical systems should have a clear definition of the operating criteria required. Station technical particulars (STPs) should be developed during the early planning stages. They contain the requirements for the availability, operating flexibility, and control systems of the plant. They also include the technical requirements for the generator transformers, auxiliary systems, and protection arrangements.
The Appendix will detail the specifications that should be met. The minimum features of the electrical systems are determined based on the STPs.
Additional documentation is required to cover the safety aspects of nuclear power stations. It is normally presented in the form of a safety report. The designer will determine the electrical requirements based on the STPs. These requirements will generally include the following:
● The station-rated output is required over a frequency range of 59.5 to 60.5 Hz.
● A fault or a fire in any auxiliary system should not trip more than one generator.
● The electrical supplies to the auxiliary systems should remain stable for three-phase faults having duration up to 200 ms.
● The electrical supplies to the auxiliary systems should be designed to meet all the operating modes including part-loading and load rejection (sudden disengagement of the generator from the grid).
● The electrical supplies to the auxiliary systems should be designed to withstand any switching or transient overvoltages.
● The auxiliary systems should operate adequately within a voltage range of 95 to 105 percent of the rated value.
● The designer may supplement the STP requirements with additional features to improve the reliability of the electrical supply to the auxiliary system.
For example, the addition of alternate power supplies to selected equipment will reduce the outage time. This will consequently increase the revenues from power sales. The designer should provide economic justification for this design feature.
Grid Criteria
The grid system has criteria associated with frequency, voltage, and the partial or total loss of grid connections near the power plant. The frequency deadband of the turbine govern- ing system is around 36 mHz. An automatic trip should be initiated when the frequency reaches 57.5 Hz for 12 s. The auxiliary systems are also expected to withstand without trip- ping for periods not exceeding 15-min transient overfrequency excursions (up to 8 percent) that would occur following a load rejection. These overfrequency excursions will result in an increase in the current to the motors and a possible trip on overload. The grid also expects the power plant to continue delivering power while the grid voltage varies within ±5 percent of the nominal grid voltage.
The auxiliary equipment should be rated to withstand this variation in voltage. It is important to mention that when the grid voltage drops, the current drawn by the motors of the auxiliary equipment will increase.
This could lead to tripping of the motors on overload. These motors will not reset. Thus, the decrease in grid voltage could lead to impairments of many auxiliary systems. Therefore, the protection devices of the auxiliary equipment should be designed to tolerate the increase in current drawn by this equipment following a decrease in grid voltage.
The impact of partial or total loss of the grid on the power plant varies with the type of the plant. Nuclear power plants are most affected by these events. These plants rely on diesel generators or gas turbines to provide power to their safety systems following the loss of power from the grid. They also rely on large battery banks to provide power to the essen- tial safety systems.
Safety Requirements
The designer should abide by the standards, codes of practice, and rules of the safety acts to ensure the safety of the plant and personnel. All the electrical systems should incorporate features that allow the operation and maintenance of the equipment while complying with all safety requirements.
Operationally, the main design considerations are to ensure that the prospective system fault capabilities are not exceeded during normal and abnormal duties. The designer should rate the circuit for the required voltage and for normal and fault currents. The switchgear should be designed for making and breaking currents that occur during normal and fault conditions. The designer should also include interlocking and monitoring schemes to ensure that the equipment ratings are not exceeded due to an operational error.