Monitoring systems for emergency diesel generators

A safe and reliable emergency power supply is required to operate any nuclear facility. The primary function of the emergency diesel generators (EDGs) is to supply the emergency power required during a loss of offsite power (LOOP) and to guarantee the required voltage and frequency during the emergency power mode. 

Accordingly, nuclear power plants must have a reliable emergency power system on standby. This reliability can only be achieved if the physical condition of the diesel generators is checked and monitored frequently. Such monitoring helps to anticipate maintenance needs and avoid time-intensive repair work. Permanent monitoring systems help to collect equipment data that can be analysed to make informed decisions about further action. In several cases, a single measurement does not give a clear indication of an issue. Consistent monitoring is needed to analyse trends and characterise the complete behaviour. 

What is an EDG monitoring system?

In general, there are two categories of monitoring systems for emergency diesel generators:

• Those required under nuclear safety standards (KTA 3702 (2014-11); Emergency Power Generating Facilities with Diesel-Generator Units in Nuclear Power Plants) for operation of the generators. 
• Those that are not mandatory for operation but improve the complete handling of the component itself.

According to nuclear standards, a safety-relevant qualified generator set (genset) is required on site. It must have had several running hours in the supplier’s facility and during commissioning. It must have regular test runs (typically monthly) that follow the same principles and test procedures that, with the corresponding monitoring, enable a strong basis for a comparison and respective analysis of the behaviour of the genset or of the single auxiliary components in the nuclear power plant.

The monitoring can be based on measurement parameters such as temperature, vibration, pressure, level, speed, start behaviour or electrical values (active power, voltage, current, cos phi, etc).

To evaluate the correct conclusions of each parameter, continuous monitoring of the measured values is required (known as condition monitoring).

Additional functionalities increase the quality and efficiency of the monitoring tasks. These functionalities include further parameter processing (such as experts’ logic, fast Fourier transform (FFT) analysis, maximum/minimum comparisons, calculations, surveillance of limit thresholds, etc.), visualisation, and automatic data mining and reporting. 

The measured values of the parameters listed above can be recorded in a customised manner and enable pure data recording and visualisation, which provides a basis for further analysis and actions. This information also offers insight into the instrumentation and control (I&C) of the unit and the plant. It is also possible to compare measured values from various test runs of the same and multiple types of diesel generators to detect degradation.

A monitoring system that is independent of the original equipment manufacturer also offers the advantages of a uniform system platform, which can be used for a variety of emergency power systems. Several types of generator sets may be used in one facility. 

Why is it beneficial to have a monitoring system for emergency diesels?

Although the monitoring system is not mandatory for operation of the emergency diesel generators, it offers several advantages. In some circumstances a monitoring system will be recommended or even required for finding a solution, or example if damage has occurred that could have been avoided by observing specific trends or identifying defects in which the root cause is not known. 

A monitoring system can be implemented at any time. The advantages include:

• Understanding equipment condition. Monitoring data provides useful information about the behaviour of equipment. For example, if there is concern regarding the behaviour of bearings, it can be checked easily by comparing the temperature measurements from the last tests with the different divisions. Performance and condition trends can be checked and appropriate measures, such as more detailed examinations or maintenance work, can be scheduled. This knowledge also supports discussions between the nuclear authority, plant owner, supplier and equipment manufacturer.
• Facilitating operational processes for monthly tests. A permanent monitoring system reduces setup times for logging monthly tests. Data are recorded continuously and independent of events, so different data sets can be accessed in a time-definable manner to evaluate events. Uniform and generically formulated protocols ease preparation, execution, and completion of the test runs on-site. The maintenance team can focus on the goal of conducting the operational tests.
• Efficient troubleshooting. A detailed alarm and event list with dates and timestamps enables faster, easier and more precise troubleshooting. This can reduce the downtime of the gensets. The monitoring system can help the plant owner identify the root cause of problems with the equipment. For example, with fatigue issues, it is mandatory to find the correct source of the problem to define the proper corrective measures. 
• Schedule of maintenance. Usually diesel generators are maintained in a very conservative way, given their high safety relevance. A component may still fail earlier and lead to complete unavailability of the unit. A monitoring system can give an early indication of issues arising, which helps to schedule the required action in due time, including preparation works. Varying values or a permanent increase in the monthly test runs would give the operator a clear indication they should perform an immediate check or analysis of the component. During normal plant operation, the time for maintenance is strictly limited. Maintenance works must be completed in a certain time, or the plant has to shut down. It is important to have a clear and reliable maintenance programme for continued efficient and reliable operation. Optimisation potential for critical components can be defined in advance and short-term spare parts availability can also be planned.
• Prevention of wear. The most significant advantage of a monitoring system is its ability to help prevent wear during operation. For an emergency power system, this is even more important. High vibrations that are not detected can lead to critical fatigue failures of key components. Performance and condition trends can be identified and localised easily and at an earlier stage. 
• Implementation and extension. Adding an additional monitoring system is always feasible, during each project phase and stage of the plant’s life.

In contrast to safety-relevant measurements and detections like overspeed, which are always part of the basic configuration, the monitoring system is non-safety relevant. This means that documentation and qualification efforts are comparably small. Nevertheless, the mounting stability of the parts is analysed to avoid subsequent damage on safety-relevant parts. The monitoring can be handled as an “add-on” to the already available and functional system. Therefore, the existing qualification is not affected. Once a monitoring system is implemented, and if the required space inside the cabinet is still available or can be provided, it can be extended with additional sensors that use the same monitoring for their graphs and trends.

When and why start monitoring emergency diesel generators? 

A monitoring system has several advantages for every emergency diesel generator. Technical and financial effects include minimising the risks, improving maintenance scheduling, and preventing wear during operations.

The system can be customised to fit each plant or facility’s needs. It generates graphical data, trends, and developments over several runs of the gensets to help operators analyse data and make informed decisions. Although ‘the earlier the better’, a monitoring system can be implemented at any time of the project and plant lifetime.

The monitoring can provide immediate support to evaluate the condition of components with many running hours. As a result, a monitoring system helps to ensure a reliable, safe and economic lifetime of emergency diesel generators. 

About the authors

Manuel Güldner is EDG product portfolio manager, Framatome

Günter Rott is EDG design lead engineer and technical expert, Framatome