|Fig. 1: Nuclear power plant (NPP) interface with power transmission lines. (Source: Wikimeda Commons)|
Nuclear Power Plants (NPPs) use heat generated from nuclear fission to convert water into steam, and are used in many countries as an economical and environmentally friendly source of electrical generation. Interfacing NPPs with electric power grids, however, is fundamentally a very difficult engineering challenge. Not only does this require a country to build and invest in a massive electric grid infrastructure to connect the two systems, it also requires these large systems to be highly efficient while maintaining sufficiently safe, secure and reliable operation. The interconnected electric grids interfacing with NPPs can encompass several countries, consisting of hundreds of power suppliers, thousands of kilometers of transmission and distribution lines and millions of different electrical loads.  A typical NPP interface with power transmission lines is shown in Fig. 1 demonstrating how large these systems can be.
The complexity of interfacing NPPs and electric transmission grids is a result of several factors. First, the sheer size and highly controlled and dynamic interconnectivity of the electric grid is an incredible engineering achievement on its own. When coupled with NPPs, the system needs to balance electric supply and consumption throughout the grid at all times while meeting the safety requirements imposed on NPPs. The inherent symbiotic relationship NPPs have with electric grids makes the interfacing problem even more complex and difficult to maintain. NPPs are both electricity generators and customers of the electric grid: they supply large amounts of energy to the grid as well as rely on it to receive power for crucial safety operations, especially during emergency conditions.  Fear of grid disturbances, such as blackouts, also means installment of redundant transmission pathways and generating sources are absolutely necessary in establishing grid reliability. In addition to meeting certain electrical specifications during operation, NPPs also have stringent voltage and frequency requirements for long term startup and shutdown cooling to insure full operability during critical emergency situations. 
This reports addresses two of the major engineering challenges in designing the electrical grids to which NPPs are connected to, and how each design aspect must consider the safety issues associated with these large and highly complex interface systems. In particular, the following sections will address (1) concerns with magnitude and frequency of load rejections and loss of the load to the NPP, and (2) issues with grid transients with degraded voltage and frequency to the operation of the NPP.
Load rejections are sudden reductions in the electric power demanded by the grid to supply various consumers, often caused by a sudden opening of an interconnection with another part of the grid that has carried a large load.  In general, NPPs must be designed to cope with certain magnitudes of load rejections with tripping the system to maintain sufficient reliability. When designing NPP-electric grid interfaces to prevent serious grid disturbances, electrical protection systems are separated from the NPP, and backup power supplies such as DC batteries and onsite emergency power sources are used until the grid voltage and frequency are restored to acceptable values. This strategy to protect the NPP unfortunately places stringent requirements on the electric grid stability and requires design for two physically independent transmission circuits, one for the normal operation and one for the emergency onsite power system. The specifications needed for these two circuit systems requires them to be independent, testable, redundant and single failure tolerant. 
A loss of load is defined as a 100% load rejection, implying that all external loads connected to the output of the plant switchyard is suddenly lost or the breaker at the station's generator output fails.  In this case, the recommendation is to put the system in "house-load" operating mode, which essentially separates the NPP so that it powers only its own auxiliary systems and the reactor operates at a reduced power level that is still sufficient to power itself.  Despite all these precautions, it must still be recognized that there still exist scenarios in which the sudden shutdown of a large NPP can result in the collapse of the whole electric power grid if not properly considered. The major technical challenge in coping with sudden load rejections or loss of load is reducing the reactor power fast enough such that it does not trip the system, and then being able to quickly increase the power output back to the nominal value again once the fault has been cleared. The trade-off here is how much excess sizing should be provided in the main condenser unit to cope with these infrequent large load rejections without incurring a significant area and cost overhead.
Frequency and voltage level relationships on the NPP-electric grid operability are tightly coupled. As frequency decreases, higher and higher operating voltages are required in order to start up the system and continue running. Insufficient voltage leads to excessive current being drawn and cause overheating and opening of protective fuses or breakers in the NPP.  This implies there is a very small performance band window for these large systems to operate in. Studies on these system therefore suggest using methods such as automatic generation control (AGC) to very carefully regulate the frequency and voltage changes. 
In situations where the NPP is carrying a large portion of the electrical grid, a trip in the system can lead to a large discrepancy between available power generation and the corresponding load. When this happens, unless more power can be supplied fast enough through external grid connections or by generation, this will lead to degraded grid voltage or frequency. Degradation in voltage or frequency on the alternate off-site power connections leads to loss in off-site power supplied to the NPP. Electric grids running at around 50-60 Hz are typically maintained within a very small tolerance of 1%.  Even a small frequency droop due to an imbalance between generation and load, can lead to a power trip, which can shorten the lifetime of a NPP due to rapid changes in power, pressure and temperature. Measures to control this, include adding on more generation sources such as gas turbines or hydroelectric power to try to balance the generation versus demand.  If this is not possible, the grid voltage can also be reduced to meet the frequency requirements though these often lead to "brown-outs", which occur when the electrical supply is no longer sufficient to maintain normal lighting. Other suggestions to control droop in grid frequency include automatically or manually changing output by speed governors on the generating unit, and disconnecting selected loads (for example, customers) from the grid (this is also known as "load shedding"). 
As for voltage degradation, grid voltage decay transients occur during periods of low power system demands and is usually a result of insufficient power system reactance to cope with the system disturbances.  Voltage changes are especially difficult for NPP-electric grid interfacing because NPPs cannot rapidly change their power outputs to meet the high demand of electricity from the power grid. This concept, known as "load following", requires varying the output of the NPP in response to an instruction or control signal from the grid in a controlled, yet quick manner. For example, load following is especially difficult when the system requires reducing the output during situations in which the electrical load on the system is reduced (such as on nights, weekends, or holidays when major industrial loads are not present), and has to quickly increase the output to maximum capacity when the electrical load is high again. Load shifting, reducing customer demand during the peak period by shifting the use of appliances and equipment to partial peak and off-peak periods, is an example of one of the techniques of load management, and several studies have been done on developing systematic load shifting algorithms for this purpose.  The major technical concern with implementing techniques such as load shifting is that NPPs have normal operating systems (i.e. main reactor circulating pumps) and long-term decay heat removal that relies on highly stable electric power to function properly. 
Given the complexities and intricacies with NPP-electric grid design, it is imperative that operators of electric grids perform the proper electric power system simulation studies to study the effects of the system under both normal operating conditions as well as single faults (sudden losses of key transmission circuits or generating units). Key features to consider for these simulations include identifying the time dependent power system response in terms of voltage and frequency, physical limitations to prevent overloading transmission circuits, and the effects of automatic features such as automatic load shedding and emergency disconnects.  To address concerns with confirming that the physically separate transmission circuits are indeed independent and proper prevention against scenarios in which the NPP is unable to provide long term core decay heat removal, several barriers from nuclear safety regulations and standards have been developed. These barriers include provisioning of either an immediately accessible power source or independent connection from an off-site electric grid, and/or provisioning of redundant and reliable on-site power systems based on emergency diesel generators. 
© Lita Yang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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