Excitation System Upgrades: Make the right choice!
1 February, 2021 | Blog
Excitation systems are a critical part of most power generation processes. Indeed, exciters are responsible for providing stable voltage levels and proper reactive power management, but also ensure adequate transient response to network perturbations (inrush, faults, etc.), in combination with speed regulation systems (governors).
With a significant proportion of power generating facilities in Canada being over 30 years old, the need to refurbish or replace aging equipment is now inevitable to ensure reliable generation. Indeed, spare parts are often scarce and qualified personnel are becoming difficult to find to maintain the legacy equipment.
Fortunately, rehabilitation projects can often be combined with capacity increases or plant upgrades to improve safety and flexibility, thereby helping justify the investment. In the case of excitation systems, modern technology offers advanced control algorithms that reduce the need for manual intervention on control setpoints and var sharing, and also enables enhanced monitoring capabilities. When used properly, this monitoring can help predict upcoming failures and implement efficient predictive maintenance tactics.
However, there is no one-size-fits all solution, as excitation systems are complex combinations of various electrical components that supply the alternator field circuit. Thus, when planning your excitation system upgrade, the design choices will not only affect direct costs, but also project downtime, system performance and reliability, along with future maintenance requirements. To properly consider the various alternatives, one must first understand the significant difference between the most commonly used exciter configurations.
Static exciters: optimal performance, high cost
Static exciters earn their name because the complete assembly is outside the generator and all components are therefore stationary. While this configuration offers excellent transient and steady-state performance, it is often the most expensive solution. Furthermore, it requires a large footprint in the generating station because of the space requirements for the power electronics and circuit breakers, in addition to the excitation transformer that needs to be connected to the alternator terminals to draw the power required by the excitation system.
Maintenance requirements are relatively low for this configuration, given the absence of any moving parts. However, field flashing is required to initiate voltage build-up in the event residual magnetism is not sufficient.
Rotating exciters: compact design, lower costs
The second option is rotating exciters, primarily because part of the excitation system is mounted on the generator shaft. The idea is to use a smaller generator to supply the main field current to the alternator. The voltage regulator (AVR) in this configuration acts on the field current of said smaller generator to effectively control the main field current, and subsequently the alternator voltage. This rotating exciter does add a time constant, however, which reduces the AVR response time compared to a static exciter, but the required footprint is significantly less. In some instances, replacing the AVR controller with a modern equivalent is enough to ensure system reliability, assuming the remaining system components are confirmed to be in good condition and can be properly maintained for years to come.
Several variations of the rotating exciter configuration are used, depending on the source of the excitation power, whether it be a DC source, such as a pilot exciter (other small DC generator mounted on the shaft) or a battery bank; or an AC source, such as an excitation transformer (EPPT), or even an external motor-generator (MG) set. Brushless exciters are also available and have significantly lower maintenance requirements.
These alternatives each have their pros and cons in terms of reliability, performance and cost. For instance, using an external supply source, which is independent of the generator output voltage, will provide sustained current and ensure proper voltage support and adequate protection coordination during a severe fault, which is especially important on off-grid or remote (weak) systems.
Make the right choice
This blog article gives an overview of exciter types and configurations to consider for a partial or complete upgrade, along with their main characteristics. Although some options may be more appealing at first glance, it is vital that owners properly define their needs against operational requirements and any applicable regulations and standards. Then, a thorough and comprehensive review of the available options is required to ensure an informed decision based on previously defined criteria. Also, keep in mind that a change from one configuration to another could have unforeseen impacts, such as a change on the waterwheel balance, if not properly planned.
Finally, owners must have the right tools to model various system components and perform simulations in the preliminary study phase to prevent potentially costly surprises at the end of the project. What used to be a nice-to-have, these simulations are increasingly becoming standard requirements to confirm compliance with utility performance specifications and relevant standards (e.g., NERC). Besides, it should be noted that the extent of the performance testing requirement following an upgrade on your excitation system may vary depending on the scope of the work performed.
BBA’s team is experienced in such projects and has developed flexible software interfaces to facilitate performance testing. Contact us should you wish to discuss your current and upcoming projects!
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