You are here

Hydro industry learns from mistakes

Poor maintenance, poor engineering and lack of emergency training were the main causes of catastrophic accident at one of the world’s largest hydropower plants at which 75 people were killed, generators destroyed and the powerhouse extensively damaged

Poor maintenance and engineering were the main causes of the explosion at the Sa
Poor maintenance and engineering were the main causes of the explosion at the Sayano Shushenkaya hydro plant

A few hours after unusual load demands were made on the Sayano Shushenkaya hydro plant in Siberia, one of its hydro-generators exploded and within seconds thousands of litres of water flooded the power house. Seventy five people were drowned or lost, the Siberian grid dropped ten per cent of its capacity, oil poured into the Yensei river, generators and transformers were destroyed and concrete structures severely damaged.

 

The problem began in a control room at the nearby Bratskaya 1400MW hydro plant on August 16, 2009 the night before the accident. A fire in the control room caused its grid regulating function to be transferred to Sayano Shushenkaya.

 

The turbo generators that took up the load, particularly unit 2 were old and badly maintained. The generally accepted view is that water-hammer in the penstock, caused this unit to fail and literally blow up, throwing the generator and turbine many metres into the air, and allowing water from the reservoir to flood the powerhouse.

 

When fully commissioned the Sayano Shushenkaya hydropower comprised ten 640MW units producing 24,000GWh a year at a 42 per cent capacity factor. The units were fed from a reservoir on average about 220m deep, created by a dam 245m high and 1km long across the Yenesi River. It was one of four hydroelectric stations with a joint capacity of 20,700MW, jointly providing more than two thirds of the East Siberian electrical demand. In terms of capacity it was the sixth biggest station in the world and the biggest in Russia.

 

The grid feeds four large aluminium smelters whose loads fluctuate wildly. Normally Bratskaya provided the regulation of the network and Sayano Shushenkaya the base load. The long-term aim of the plant’s owners, RusHydro was to supply electricity to more than the smelters, and interconnect with nearby grids, which made it especially important to regulate the grid. But although Bratskaya was able to carry out this function Sayano was not.

 

The load fluctuations were severe and rapid, a characteristic of aluminium smelters. Incipient faults, both electrical and mechanical, and there were many at Sayano, soon become active. Had the machines continued to supply the base load, the accident would not have happened at that particular time, but it was bound to happen sooner or later.

 

None of the machines at Sayano Shushenkaya were suitable for prolonged regulation service, but of the ten machines, unit 2 was selected as best available, and it was decided to use it for frequency control. One unit was on standby, another was under maintenance, and the remainder continued to supply base load. Unit 2 was near the end of its recommended useful life of 30 years. By August 2009 it had been operating on and off for 29 years and nine months.

 

However, it had been recently maintained and was thought to be the most reliable unit, although we will see later that this maintenance was grossly inadequate.

 

Operating demands were onerous. In the thirteen minutes before the accident the unit had swung from 170 MW to 600 MW six times against a rated output of 640MW. Every time the load changed, the machine had to pass through the so-called ‘unstable zone’ when vibrations were higher than normal.

 

In addition, the reservoir head was 212m as opposed to the design level of 197m. This provided the optimum conditions for mechanical failure. The turbine was frequently rotating in an unstable zone, with the added stress of decelerating and accelerating under a head 15m above the design level.

 

Although a new vibration trip for unit 2 had been installed it was not working. Weekly readings of vibration from April to August 11 showed that during in the week before the accident vertical displacements had reached 1500 micron, more than 700 per cent higher than the allowable maximum of 200 micron. At the point of failure it was 525 per cent higher. Three months earlier the level was 250 micron. Had the trip been working, the machine would have automatically dropped out at this point or soon after.

 

The bolts holding the head cover of the turbine in place played a critical part in the accident. The thrust bearing for the generator and the generator itself were immediately above the turbine head cover and the water hammer would have been enough to shear them, lifting the head cover and turbine above it and causing water to flood into the power house.

 

There were 80 80mm head cover bolts in place when the machine had been commissioned. At the time of the accident, at least six bolts were without nuts. When new, each bolt would have been able to carry a load of 60t. Subsequent laboratory tests on 49 bolts recovered showed that 41 had fatigue cracks, 49 had lost 65 per cent of their effective cross-section, and the remainder had lost anything from 80 to 96 per cent. Like the unit itself, they were also at the end of their working life with three months to go before reaching their 30 year end-of service date.

 

An alternative view is that the bolts were there principally to seal the flange of the head cover to the turbine body, rather than counter all of the thrust on the cover. There was a gasket between the turbine body and the cover, held in place by two flanges and clamped together by the bolts. The flange itself was relatively narrow and possibly its main purpose, with the bolts, was to provide a water-tight connection. Downward thrust on the head cover was provided by the generator and thrust bearing above it; possibly its weight was assumed to be adequate to resist water pressure irrespective of the bolts.

 

Photographs taken of the actual failure show that the head cover was lifted vertically by the in-rushing water, rather than to one side or the other. The explosion was so rapid and forceful that the bolts failed simultaneously rather than sequentially as might be expected as they were all at different states of disrepair. If their designed function was simply to ensure a good seal, then even if they had been in pristine condition they could not be expected to hold the head cover in place.

 

The head cover burst off at 08:13, some nine hours after the unit started its frequency control duties, and within seconds water, under a head 212 m burst, through the machine and into the machine hall throwing the turbine and its generator and its thrust bearing, weighing nearly 1,600t, many meters into the air and flooding the powerhouse. From that point on, the survivors had only torches to try and remedy the situation.

 

The powerhouse was flooded and 75 people were either drowned or killed by falling debris. Immediately after the collapse of the USSR clear lines of responsibility were not properly established so that at this time Sayano and many other plants were suffering from a lack of impartial inspections and an inadequate planned maintenance regime. Russia's emergencies minister Sergei Shoigu has described the accident as "the biggest man-made emergency situation in the past 25 years."

 

A similar accident occurred in 1992 when a head cover failed on unit 1 of the Grand Rapids Generating Station, operated by Manitoba Hydro. It caused flooding in the lower levels of the power house; the problem was subsequently traced to the bolts in the head cover, which apparently failed.

 

More than 2700 people carried out search and rescue work; 11 aircraft and 15 boats were also involved. About five thousand cubic metres of debris were cleared more than 277 thousand cubic metres of water pumped out and 324.2t of oily emulsion collected from the reservoir and river.