Preventing Problems through Past Experience

With the potential for disaster at hydropower projects around the world, the lack of organized training programs becomes a prominent consideration in how companies manage their assets. Using case studies of preventable accidents, a model to help organizations better train and prepare their employees has been created.

By Enes Zulovic

The keys to professional risk management and asset management are the identification of potential operational risks and the development of a plan to eliminate or minimize them in a cost-effective manner. For hydroelectric facilities, the scope of asset management plans is primarily focused on the management of physical assets and asset systems.

However, human assets are critical to the successful delivery of optimized and sustainable asset management and require due consideration. Risks should be identified and managed as part of all asset management activities, considering the risks throughout the life cycle of hydroelectric power assets.

A large number of incidents that have occurred at hydropower plants are a result of a lack of operators and owners understanding the risks, which often results in low confidence from stakeholders and the general public. Gaining data from incidents to better understand and prevent them can be difficult, however, due to the sensitivity of the information, insurance, litigation and company image.

Presented here are a series of case studies detailing situations when operational and maintenance problems occurred and preemptive steps that could have been taken to avoid them.

Figure 1 — Pelton turbine at Poatina  A diagram showing the “push out” (left) and “cut in” positions with a vertical Pelton shaft turbine.

Case Study 1: Poatina plant Pelton turbine in frequency control mode
When a vertical-shaft Pelton turbine at Hydro Tasmania’s 300 MW Poatina plant operated in frequency control mode with the “push out” deflector arrangement (see Figure 1, below left), it caused water to discharge through the turbine’s air admission pipes into the underground machine hall. The facility is located in the Great Lake and South Esk catchment area in Tasmania.

The incident caused 250 liters of oil to be sucked from the turbine guide bearing oil pot into the river, where it developed into an environmental problem. In addition, the unit’s frequency control function could have been cancelled due to a shut down of the machine by its protection system, potentially causing a system blackout that would have cost the company millions of dollars.

The owner eventually determined that the development of negative air pressure around the Pelton runner and oil loss from the turbine guide bearing were causing the problem. The problem had been observed since the plant was commissioned in 1968 in situations when machines were being used for frequency control. It had been assumed over the years that this was due to an inadequate air admission system instead of the design of the deflector push out arrangement.

The design review during the modernization project failed to address the deflector design arrangement problem for times when the machine was operated in frequency control mode, making the situation a knowledge deficiency transfer problem.

The problem was solved by changing the deflector arrangement to use a “cut in” system, which solved the water discharge problem.

Water floods into the Poatina plant after a turbine operating in frequency control mode caused water to discharge through the turbine air admission pipes into the machine hall.

Case Study 2: Kaplan turbine load rejection and runner lifting and low pressure in draft tubes
A 30 MW vertical shaft Kaplan turbine at Hydro Tasmania’s 30 MW Paloona plant, located in Tasmania’s Mersey-Forth catchment, generates low pressure in the space between the guide vanes and the top of the runner under certain operating positions. This, coupled with relatively high tailwater levels and low rotating mass elements, results in an upward force on the runner large enough to lift the rotating element clear of the thrust pads.

During commissioning of the unit in 1972, the phosphor bronze reverse thrust ring above the turbine guide bearing failed when the machine was first synchronized, due to runner skating. The upthrust bearing was replaced with a new leaded-bronze design, which failed when the machine skated for an extended period as it was motoring on the system.

The conditions that led to the phenomenon at Paloona and the resulting consequences include:

— Runner skating at low guide vane openings: When operating below 7% of the guide vane opening, or around 2 MW generator output, a negative pressure develops above the runner and the shaft system lifts off the thrust bearing; and
— High upthrust during load rejections causes lifting of the shaft system at low and high loads. The water pressure above the runner briefly drops, causing the shaft system to lift up and drop back down onto the thrust pads when the pressure above the runner increases.

Pelton nozzles and the deflector “push out” arrangement at the Poatina project in their original arrangement.

The problem was solved by increasing the upthrust bearing’s clearance and material. White metal was used as a contact bearing surface to prevent the development of high metal temperatures due to friction caused by the steel mating surfaces.

A deficiency in knowledge transfer is again to blame for the problem, as air admission to the runner and draft tube was not considered during the unit’s design stage. The weight of the machine’s rotating parts was higher than axial hydraulic forces, and the closing law of the wicket gates did not develop negative pressure under the turbine runner blades.

Case Study 3: Catastrophic failure at Sayano-Shushenskaya
At the time of an August 2009 accident at RusHydro’s 6,400-MW Sayano-Shushenskaya, on the Yenisei River in Russia, nine of the plant’s 10 total units were available for operation and one was undergoing the final stage of a refurbishment program. An explosion within Unit No. 2 killed more than 70 people and caused the turbine room to flood, prompting a rehabilitation and reconstruction effort.

Official reports and several technical discussions following the incident have drawn very general conclusions, attributing the failure to heavy vibration that might have been unchecked as vibration monitoring equipment on the unit was out of service at the time of the failure; and poor maintenance, associated with failed studs in the turbine head cover of Unit 2.

Another hypothesis is that the explosion was caused by water column separation in the draft tube. This condition can readily be caused by a rapid wicket gate closure during unit load rejection. Unit 2 experienced a load rejection the morning the turbine exploded, followed immediately by a loud bang heard in the administration and control building adjacent to the powerhouse.

The load rejection and development of the draft tube reverse water hammer precipitated a massive failure involving the lifting of the runner, shaft, head cover, turbine and generator bearings into the umbrella generator rotor spider, destroying it. The full penstock head was then released into the turbine pit, resulting in an enormous geyser and massive destruction.

This hydraulic transient phenomenon was probably caused by turbine governors that had been sped up — likely unknowingly — to an unsafe level in an attempt to improve frequency stability under changing electrical loads.

This reflects a problem in many electricity trading markets in that the supply of “ancillary services” — including frequency management, spinning reserve and synchronous condenser operation — is determined on an arbitrary basis or depending on the prices bid in by various generators at various times. This means it is no longer possible to ensure that the machines that are best-suited to provide frequency control management are selected to do so.

Hydraulic design of the runner and closing law of guide vanes (wicket gates) in relation to the Kaplan runner blades is to be set up to prevent occurrence of low pressure under the turbine runner, development of the reverse water hammer and uplift of the rotating parts. If large cavities can be detected at an early stage, full column separation can be avoided. The lower limit for the draft tube pressure is set to -3 m (water column) WC for the test to be further analyzed.

Poatina’s Pelton turbine deflector is pictured here in its new “cut in” arrangement.

Air admission system to the runner and draft tube (natural and or forced air admission) was not considered during the design stage, and design review did not recognise or raise this issue.

Identifying common problems in the knowledge transfer model
These case studies provide some interesting insights into the importance of hydro knowledge transfer deficiency when considering risk identification and risk severity and selecting risk management solutions.

The retirement of experienced hydro personnel leaves young engineers with fewer opportunities to gain experience, while the transfer of knowledge to this new generation also appears to be insufficient.

This has caused inexperienced manufacturers and plant operators attempting to design and run plants to take shortcuts in the process, giving the appearance that hydro operators are dismissing future problems such as deterioration risk and expenditure requirements to obtain short-term gains.

Many accidents at hydroelectric power plants can also be explained by a gradual drift toward unsafe conditions — many as a result of economic pressure, cost-cutting in maintenance, company reorganizations, or delayed retrofits and modernization efforts.

The turbine room at RusHydro’s Sayano-Shushenskaya plant, following an August 2009 accident that killed more than 70 people.

A lack of understanding of equipment by both equipment producers and operators can also be to blame, as designers and manufacturers have not yet succeeded in eliminating the possibility that the pulsation generated by a runner can create resonance within the complete plant hydraulic system.

Likewise, those responsible for the maintenance and operation of rotating machinery should be aware that the catastrophic failure of a critical machine can cause serious injury or death, the total loss of the machine, or extended shutdowns of an entire plant.

Utility trading plans committed to an unsustainable level of output are also to blame as they do not allow for necessary maintenance activities to be adequately carried out.

For these reasons, operators must take a proactive stance in not waiting until a machine fails – instead, taking a proactive stance toward knowledge transfer.

Knowledge-based mistakes occur when someone is confronted with a situation that has not occurred before and which has not been anticipated. It leads to errors and suggests that the most effective solution is to improve the knowledge of the people who have to make decisions.

Conducting risk analyses is often primarily associated with technological aspects. However, a risk analysis should integrate knowledge and organizational human factors to properly assess risks.

Solutions to solving the knowledge transfer model
There are new technologies, tools and benefits that have been proven in practice to be of measurable benefit to other hydro organizations. Active research is required to identify, investigate, try and evaluate such opportunities.

To transfer knowledge among workers, I have developed a proposed model that includes:

— Development of essential learning modules for knowledge about hydropower plants;
— Employing specialist organizations and personnel for design review and other critical activities;
— Improving graduate engineering development programs with mentor and technical coaches;
— Forums and seminars by professional bodies and trade associations to be tailored with a specific focus on knowledge transfer;
— Greater training involvement by original equipment manufacturers (OEMs) in the hydro industry;
— Research and development activities, with results of academic research to be published and readily available; and
— An institutional culture of life-long learning and cultivating on-the-job experience.

Hydro Tasmania has been developing an engineering development program and training courses that cover some of the specific topics previously described as indicative of the knowledge transfer deficiency.  

Enes Zulovic is a specialist mechanical engineer over major works, assets and infrastructure for Hydro Tasmania.

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