Addressing common causes of excitation system failure and how to prevent them
Whether it’s electronic component failure, loose connections, calibration drift or mechanical component failure, excitation systems require periodic maintenance to continue running reliably. With full system replacements often prohibitively expensive, it is crucial that maintenance programmes are effective and keep systems operating at peak reliability. Here Douglas Cope, director at excitation control system specialist Excitation Engineering Services, discusses common causes of excitation system failure and how to mitigate for them and keep systems up and running for the long term.
Age, contamination and mechanical and electrical stress all cause ongoing wear and tear in excitation systems, which, if ignored, can eventually lead to machine failure and costly downtime.
A significantly less expensive option than replacing whole systems is to adopt a preventative maintenance approach. This allows engineers to identify potential faults and fix them before they impact system performance and cause costly downtime.
An effective preventative maintenance programme should include routine inspections of all components, particularly mechanical ones such as contactors and motorised potentiometers. It should also include inspection of all connections, whether they are small control cables or high current busbar.
Testing should also be performed on any power supplies and transducers, along with injection testing to verify calibration and correct setting of limiters. Further testing should be performed with a running machine to assess the dynamic performance of the system. This will normally consist of open circuit step response tests, followed by tests on load, including step tests into the under- and over-excitation limiters.
Mechanical Component Failure
Even the smallest excitation systems will have some kind of excitation contactor. Contactors can fail to open or close due to misalignment, oxidisation or contamination. High ambient temperatures over a long period can cause contactor coils to fail, as can voltages that are outside the manufacturer’s recommendation.
As excitation systems go up in size, larger contactors require more maintenance. Static exciters typically have a field switch capable of controlling hundreds or thousands of amps, which require regular maintenance to prevent failure.
Older analogue excitation systems often have motorised potentiometers to provide setpoints for field voltage or field current control. These can wear down the track on the potentiometer around the nominal operating voltage of the machine, leading to transient spikes in voltage and reactive power.
Electronic component failure
Electronic components in excitation systems can fail due to a variety of causes. Overheating is one of the primary reasons for failure, as excessive temperatures can cause materials to degrade or circuitry to malfunction.
Voltage spikes or electrical surges can also lead to component failure, potentially causing immediate or cumulative damage. Mechanical stress, including vibration or physical shock, can lead to structural damage in components, impacting their performance or leading to outright failure. Environmental factors such as moisture or corrosive atmospheres can induce corrosion or create conductive pathways on circuit boards, resulting in failure or degradation of performance.
Additionally, manufacturing defects, such as poor soldering, inadequate quality of materials or flawed design, can predispose components or their soldered connections to failure under normal operating conditions. Over time, even natural wear and tear can degrade components, leading to failures as materials fatigue or as tolerances drift beyond operational limits.
Calibration Drift
Calibration drift in electronics refers to the gradual deviation of a device’s accuracy from its original output over time. This phenomenon is primarily caused by changes in the physical and electrical properties of the electronic components within the device.
Factors such as temperature fluctuations, humidity, mechanical stress and aging of components can alter the electrical characteristics such as resistance, capacitance and inductance, leading to changes in the instrument’s performance. This is particularly important in excitation control systems, as accurate measurements for stator voltage, field voltage, field current, real power and reactive power are essential for proper performance.
Maintaining a regular calibration schedule is crucial to avoid errors in measurements. Calibration drift is not only inevitable but also unpredictable, which underscores the importance of periodic checks and recalibrations to maintain the reliability and accuracy of an excitation control system.
Electrical connections
Degradation of electrical connections in terminals and busbar can significantly impact the performance and reliability of excitation control systems. This deterioration typically results from physical, chemical and environmental stresses.
Physical stress may include repeated mechanical movement or vibrations that loosen connections, leading to intermittent or permanent contact failure. Corrosion can also play a major role, as exposure to oxygen, moisture and pollutants can cause the metal components within terminals and connections to oxidise. This corrosion increases resistance, potentially leading to overheating.
Additionally, thermal cycling, where connections are repeatedly exposed to varying temperatures, can cause expansion and contraction of the terminal materials which can result in a loosening of the connection. To mitigate these issues, it is crucial to inspect and check all electrical connections at regular intervals.
A knowledgeable maintenance partner
It is important to partner with a maintenance specialist who understands all the faults covered here, and more, as well as the specific requirements of any given excitation system, regardless of the environments it is in.
Excitation Engineering Services’ engineers conduct maintenance work on all types of excitation equipment, from legacy analogue systems to the latest digital ones, ranging in type, size and age. Our maintenance services also cover systems that are no longer covered by OEMs and face obsolescence issues.
Typically, maintenance starts with static testing that includes visually inspecting the equipment, secondary injection and calibration checks, relay testing, checking the ancillary components and checking logic circuits.
Dynamic testing is then used to complete the maintenance and verify the performance of the excitation system. This is performed with the generator on open circuit, followed by online testing to assess excitation system performance, including limiters and protective features.
For more information on excitation system maintenance, visit https://excitationengineering.co.uk/maintenance/ or contact Excitation Engineering Services’ team on +44 (0) 1827 286100.
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