Water Power and Dam Construction. Issue November 2010

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OPERATION AND MAINTENANCE

BACKGROUND

Seventy percent of the load on the East Siberian grid consists of four

very large aluminium smelters. [12] The Bratskaya hydro plant

(4500MW) was normally the lead plant on the grid, with the respon-

sibility of following load and contributing to grid frequency control.

Sayano Shushenskaya was normally base loaded, and not following

load swings.

Beginning in 2002, the owner of the grid and most of the gener-

ating plants, RusHydro, ordered an improvement in the quality of

grid frequency control, which had been poor, due to the seriously

uctuating smelter loads, which are difcult to follow due to their

lack of rotating inertia. The objective was to serve growing non-

smelter loads, and to permit interconnection with other adjacent

power grids, in particular the Russian Far East power intercon-

nection, which is predominantly based on thermal generation. In

support of improving frequency stability, RusHydro mandated that

automated joint load control systems be installed in the Siberian

plants under its control. Previously, the unit load control had been

manual in all the plants. Automated joint load control was suc-

cessfully placed into commercial operation at Sayano Shushenskaya

during the rst half of 2009. [12, 13]

Due to a re at Bratskaya on the day before the accident, all that

plant’s load, and its load following requirements were transferred to

Sayano Shushenskaya by the grid load dispatcher. [9, 10]

During the night of 16-17 August 2009, Sayano Shushenskaya

experienced large and rapid load swings, with the total varying from

2800 to 4400MW. Unit 2 at the plant, with both a new governor

and a new automated joint load control system, had been set as the

lead unit, and experienced the largest load swings. [10] The accident

itself followed in the morning.

Very soon after the event, the Russian industrial safety agency,

Rostekhnadzor, studied the accident and attempted to determine its

causes. It issued an extensive report on 3 October 2009 in which it

attributed the cause to heavy vibration of Unit 2 in the plant, which,

combined with lax maintenance and inspection practices, resulted

in fatigue failure of the studs securing the turbine head cover to the

unit stay ring. Although two other units in the plant were similarly

destroyed, the Rostekhnadzor report did not address these failures,

nor did it identify any other issues that may have contributed sig-

nicantly to the accident. [10] It is understood that Rostekhnadzor

released its report only about six weeks after the accident took

place. They noted that it was an interim report, and that additional

work would follow. As of this writing, Rostekhnadzor has not

modied its conclusions that the accident was caused by fatigue

failure of the head cover studs.

The Sayano Shushenskaya project (1, 10, 14, except as noted)

The 6400MW Sayano Shushenskaya installation is one of four

very large hydroelectric generating stations that, at a total of

20,700MW, comprise over two thirds of the generating capac-

ity in the East Siberian electrical grid. The general arrangement

and size of the facilities at Sayano Shushenskaya are displayed in

the photographs in Figures 1 and 2. Figure 3 is a reproduction

of a drawing showing a vertical cross section through dam, pen-

stock and powerhouse. It shows the relative locations and sizes of

intakes, penstocks, turbines, generators, transformers and power

take-off facilities. Figure 4 is an interior view of the machine hall

looking across Unit 2 towards Units 3 through 10. Figure 5 is a

photograph of a cutaway model of one of the turbine generator

units showing its main components. Figure 6 is a cross section

drawing through a turbine. It contains some key dimensions, and

it claries the arrangement of the head cover, its structural support

for the thrust bearing, and the location of the studs connecting the

head cover to the upper stay ring.

OCTOBER REPORT OF ROSTEKHNADZOR

The Rostekhnadzor report was an extensive evaluation of the event and

the conditions which led up to it. Considering the short time available

to prepare it, the report was thorough, although its scope was clearly

limited, concentrating on Unit 2. It attributed the destruction of Unit 2

to fatigue failure of many of the 80 8cm diameter studs which held the

head cover in place. The report did not identify the exact location of

the studs, nor the manner in which they were loaded. [10]

Other information (see Figure 6) has indicated that these studs

were located at the extreme outer edge of the head cover, where

it rests on a narrow ange which is part of the upper turbine stay

ring. Both mating anges at the location of the studs are narrow

and relatively thin, as evidenced in the photographs (Figures 8 and

9). At full strength, the studs could be expected to carry a substan-

tial load. Assuming a mild steel material, each 8cm diameter stud

may be expected to carry a design load of 60 tonnes or more. It

is likely, however, that these studs were primarily for the purpose

of obtaining a seal where the head cover ange met the stay ring

ange. The joint probably contained a gasket, which was held by

the studs. From the available graphics (Figures 5 and 6, and some

in Reference 10), it appears that the main structural support of

the head cover in resisting uplift came from the conical structure

connecting the head cover to the generator bearing housing above

it. This structure comprised the unit thrust bearing support, so

Sayano Shushenskaya accident

Above: Figure 4 – Interior of

the powerhouse; Left: Figure 5

– Cutaway model of the generat-

ing unit (2, 15); Right: Figure

6 – Section through the turbine

(15). 1. Head Cover; 2. Guide

Vane (Wicket Gate); 3. Guide Vane

Servomotor; 4. Bearing hous-

ing; 5. Bearing seal; 6.Turbine

shaft; 7. Thrust bearing support;

8. Stay ring; 9. Spiral case; 10.

Lower ring; 11. Support ring; 12.

Labyrinth seal; 13. Runner crown;

14. Runner cone; Studs indicated

at red arrows. Head cover lower

surface highlighted in blue

Generator bearing housing

Generator rotor

Generator stator

Head cover studs

Spiral case

Runner

Shaft

Gate levers

Head cover

Wicket gates

Draft tube

Flow

32 NOVEMBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

OPERATION AND MAINTENANCE

the entire weight of rotating parts and the hydraulic thrust on

the runner were supported by the turbine head cover, and would,

therefore, resist most, if not all of the hydraulic uplift. This is

conrmed by the photographs. [15, 5 through 8]

The stud fatigue failure was attributed to the large vibration which

had plagued Unit 2 for a very long time, both before and after the

refurbishment in the rst quarter of 2009. It appears that the runner

repairs that year were made in place, without removal of the head

cover, so the studs would not have been replaced. [10, 14]

The Unit 2 operating zones were dened in the report as acceptable

between 0 and 265MW (Zone 1), unacceptable between 265 and

570MW (Zone 2), acceptable between 570 and 640MW (Zone 3),

and prohibited above 640MW (Zone 4). The loads carried during

the nine hours leading up to the failure indicate that the rough zone

(Zone 2) was transited by the machine several times, and it was in

that zone at the time of the load rejection and failure. [10]

The report contains several interesting facts taken from logs and

automatic recording equipment. It notes that the Unit 2 load changed

12 times between midnight and 0230 on 17 August, and the range of

total plant load was 2800 to 4415 MW. Unit 2 was known to have

passed through its rough operating zones some six times in the few

hours preceding the failure. (10)

Some sample times and Unit 2 loads before and after midnight on

16-17 August (10) are shown in the table below:

At the time of the load rejection and failure, the load on Unit 2

was apparently being reduced. At 0800, Unit 2 was reported to be

carrying a 605 MW load, with a turbine owrate of 312m

/sec and a

gate setting of 72.5%. At 0813, Unit 2 was carrying 475MW, with a

ow of 256m

/sec at an opening of 69%. Twenty-ve seconds later,

the load was recorded as zero, indicating a sudden load rejection from

475MW. It should be noted that the load at 0813 was in the middle

of Zone 2, and considerable vibration should have been occurring,

based on previous experience with the machine. [10]

WHAT COULD HAVE CAUSED THIS ACCIDENT?

Early hypotheses

An early analysis [1] that was distributed via the internet only a week

after the accident postulated causes for the Unit 2 failure including

ingestion of debris and a broken governor oil pipe. The failures of

Units 7 and 9 were attributed to runaway of the units under ooded

conditions. As photographic evidence subsequently showed (Figures

10, 11, and 12), these explanations are not consistent with the events

that have been shown to have occurred.

Another early hypothesis was that one of the draft tube piers col-

lapsed causing a blockage of the draft tube. This seemed not to be

credible at the outset, and the Rostekhnadzor report makes no men-

tion of such a scenario, which would have left a clear set of evidence,

had it occurred.

An early hypothesis by a Russian engineer and hydraulic turbine

specialist, B. Kolesnikov [21], was that the accident was caused by the

seizure of either the turbine bearing or the upper runner seals causing

a very large twisting force to be transmitted to the head cover. The

fact that the studs were not sheared off eliminates this possibility, as

was pointed out by B. Kolesnikov in a later note written after addi-

tional photographs became available.

Hypotheses by Rostekhnadzor and a later writer

Rostekhnadzor attributed the accident to fatigue failure of the head

cover studs caused by the heavy vibrations that had plagued Unit 2

over considerable time.

An October 2009 presentation by E. Kolesnikov [19] expanded on

the Rostekhnadzor conclusions somewhat. He similarly attributed

the failure of Unit 2 to the fatigue failure of the head cover studs due

to the severe vibration the machine was experiencing, and he pointed

out that the vibration monitoring equipment on the unit was out

of service at the time of the failure. Neither he nor Rosteknadzor

alluded to a source of the large upward force necessary to lift the

machine, however. He did not attribute the failure to a load rejection

situation, implying that the failure occurred during an operating load

change. This does not seem likely, since, during even part load opera-

tion, there is a very signicant hydraulic down thrust exerted on the

runner (estimated to be of the order of 7500 tonnes) that is resisted

by the thrust bearing, which, in turn, is supported on the head cover.

This, combined with the weight of the parts supported by the thrust

bearing, would have left no load to be carried by the studs.

B. Kolesnikov (21) hypothesized early that the event may have been

caused due to a seizure of the turbine bearing or the upper runner

seals causing a large torsional load on the head cover, but he later

rejected these ideas since, as shown in the photographs in Figure 13,

the studs failed in tension; not in shear.

DISCUSSION AND A NEW HYPOTHESIS

Unit 2 essentially exploded. The fatigue failure of the head cover studs

during operation of the machine, as suggested by Rostekhnadzor

and E. Kolesnikov, does not lead to a source for the enormous and

sudden upward force necessary to achieve this violent explosion.

That upward force caused runner, shaft, head cover, wicket gate

upper trunnions and operating arms, both bearings, the generator

rotor, and all associated structure of Unit 2 to be forced upward with

enough violence to crush the rotor spider and destroy the stator. It

had to be triggered by something that occurred suddenly and vio-

lently (Figures 7, 8, and 9).

Although the compromised condition of the head cover studs cer-

tainly contributed to the failure of Unit 2, stud fatigue failure alone

seems very unlikely to have occurred in all three failed units (Units 2,

7 and 9) at the same time. In the absence of any stud failure at Units 7

and 9, it is very difcult to envision what could have caused the damage

that is clearly visible in the photographs (Figures 10, 11, and 12). Thus,

it is likely that the studs of Units 7 and 9 did fail, but their failure was

caused by an upward force on the head cover of each unit.

Each rotor weighed 920 tonnes. Each runner weighed 156 tonnes.

The entire assembly as supported by the thrust bearing weighed about

1500 tonnes, not including the downward hydraulic thrust on the

runner or the resistance of the various mechanical connections associ-

ated with the gate trunnions and any generator guide bearing lateral

supports. [1, 9, 10] Each head cover was exposed to draft tube head,

plus a contribution from penstock head, which was exerted on the head

cover annulus outside of the upper runner seal, and particularly that

part outside the wicket gate trunnion circle (Figure 6). [15] This could

have provided enough upward force to lift the cover during a full shut

down if the studs were truly so weak as to contribute little or nothing

to the resistance to upward load, the down thrust on the runner had

become negligible at full ow stoppage, and no signicant resistance

was exerted by trunnion connections or any connections of the gen-

erator guide bearing to its lateral bracing. The tables and diagrams

in the Rostekhnadzor report suggest that the static tailwater load on

the head cover at the time would have been about 10m. [10] This is

equivalent to about 350 tonnes of upward force on the gross area of the

head cover inside the runner seal ring under static conditions; however,

during operation, and particularly during a load reduction, this upward

load would have been reduced substantially due to the effects of the

Sample times

Time Load (MW) Time Load (MW)

2315 50 0730 170

2317 110 0730 to 0745 170 to 260

2330 200 0746 610

2331 165 0747 to 0800 605

2344 to 0015 600 0812 575

0030 135 0813 475

0030 to 0703 10 to 255 0813 25secs 0

0703 to 0729 600

WWW.WATERPOWERMAGAZINE.COM NOVEMBER 2010 33

OPERATION AND MAINTENANCE

velocity head at the draft tube throat. Since the unit had experienced

considerable cavitation during operation, it is possible that the upward

load on the head cover from the draft tube was actually negative at

large turbine loads. [10] It is estimated that the static penstock pressure

load (neglecting penstock head losses) on the outer annulus of the head

cover amounted to something on the order of 2000 tonnes upward.

This compares with the machine weight of 1500 tonnes exerted down-

ward. During operation, there would also be the signicant hydraulic

thrust downward on the runner (7500 tonnes), which would have been

supported by the head cover. It is likely, therefore, that the head cover

would not have lifted during operation even if the studs carried no load

at all. Instead, the hydraulic thrust would have to have been reduced or

eliminated prior to the failure, implying a shutdown. A full shutdown

would generate a waterhammer pressure rise in the penstock associated

with the reduction of velocity. Assuming a rapid governor time of ve

seconds for full stroke, and a round trip pressure wave travel time in

the penstock of about 0.4 seconds (based on the penstock length of

about 200m), it is expected that the waterhammer pressure rise in the

penstock would amount to about 70m, equivalent to 33% of static

head – a relatively high, but not unreasonable waterhammer ratio for

a hydro plant. This would result in a peak pressure of about 290m on

the outer annulus of the head cover, which is equivalent to an upward

force of some 2550 tonnes on the 8.9m

annular area, or about 550

tonnes above static conditions.

The pictures show that the Unit 2 turbine lifted as a unit, and the

failed stud connection at the edge of the head cover is not marked by

any visible clues (Figure 9). The head cover ange through which the

studs passed appears to be completely undamaged by the failure of

the studs, suggesting that all studs failed simultaneously allowing the

head cover to rise vertically without distorting the ange. The conical

steel thrust bearing support connected the head cover structurally to

the generator bearing housing. This conical structure must have trans-

mitted an enormous upward force to the bearing housing, which then

lifted from its position, along with everything it supported, during

the failure. Had all this occurred due to stud failure, with only some

500 tonnes of unbalanced upward force (or less, if there was nega-

tive pressure under the center of the head cover), it seems likely that

the studs would have failed sequentially, and, when the anges at the

failed studs parted, the resulting ow into the turbine pit would have

substantially relieved any upward load associated with waterhammer

effects in the penstock. Moreover, lifting of the head cover on one

side would have caused the shaft, both bearings, and all parts con-

nected to them to tilt to the other side, causing the generator rotor to

collide with the stator windings before all of the head cover studs had

failed. Judging from the photographs of the damaged machine after

the pit was dewatered (Figure 9), the head cover ange was undis-

torted. This suggests that some additional transient loading occurred

at the time of the stud failures, since the photographic evidence sug-

gests a very sudden event and a very massive and completely vertical

upward acceleration, at least initially. [5 through 8]

Rostekhnadzor did not speculate as to the causes of the failures of

Units 7 and 9. They may take this up in their further analyses of the

accident. It is highly unlikely that they would have suffered simple

stud failures due to fatigue during the same incident that destroyed

Unit 2. The character of the damage as shown in the photographs

(Figures 10, 11, and 12) is not consistent with simple overspeed due

to governor failure. Both units appear to have moved vertically. Both

also appear to have tilted so that the generator rotors collided with

the stators, as shown by the stator damage clearly visible in Figure

13. Since the shaft alignment of each of these machines was gov-

erned by the turbine bearing and the generator guide bearing, both

of which were quite rigidly supported by the turbine head cover, the

only mechanism that could result in a tilt of that axis is the lifting of

the head cover itself. It is unlikely that the governors would have

failed soon enough to prevent wicket gate closure and allow a runa-

way, since some time would have passed between the electrical load

drop and the ooding of the powerhouse spaces near Units 7 and 9.

Moreover, wicket gates are usually designed to drift closed (but not to

slam) upon complete loss of oil pressure. The damage could not have

been caused by a short circuit, as E. Kolesnikov hypothesized, since a

short circuit would not have caused a movement of the rotor axis.

New hypothesis: water column separation and its effects

Each draft tube is long enough (about 35m) to suggest that a rapid

load rejection from a heavily loaded condition (e.g. 475MW, or 74%

of rated maximum) may have elicited water column separation in

the draft tube, followed by an extremely violent pressure rise as the

water column rejoined under the head cover. This could have caused

the explosion that occurred. [2, 10] The Rostekhnadzor report, E.

Kolesnikov’s presentation, and B. Kolesnikov’s commentaries do

not discuss the possibility of column separation and rejoining as the

mechanism responsible for the explosion. [10, 19, 22]

The Unit 2 turbine was known to have suffered from extensive

Below: Figure 7 – The machine hall on 17 August 2009 after closure of the head

gates (18); Right,top: Figure 8 – Unit 2 on 17 August (18); Right,bottom: Figure

9 – Unit 2 Damage After Dewatering on 3 September (18)

34 NOVEMBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

OPERATION AND MAINTENANCE

cavitation damage to its runner. This suggests that the local pressure

in the vicinity of the draft tube throat was fairly near vapor pressure

during steady state operation. This is to be expected in a region

where the velocity prole is extremely nonuniform, and there is a sub-

stantial whirl component of velocity. A sudden load rejection would

have caused a drop in pressure at the draft tube throat as the draft

tube water column was decelerated by the action of the closing wicket

gates. If the gate closure were fast enough, the draft tube pressure

would have been reduced to vapor pressure, leading to the formation

of a vapor cavity in the draft tube throat. This is a credible sequence

of events, as indicated by an approximate analysis of this condition

for various assumed governor gate closure times. [20] This analysis

indicates that column separation may be expected for governor times

faster than full stroke in about six seconds. The analysis was based

on mass surge assumptions and draft tube geometry reecting uni-

form area increase from the throat to the exit. It does not account for

the whirl component of velocity and related low pressure in the draft

tube associated with operation well off the machine design point, and

it is, therefore, probably not conservative in this instance.

As a result of the same gate closure, a simultaneous waterhammer

pressure rise in the penstock and spiral case would have been pro-

duced. This pressure rise, however, would have been simultaneous

with the pressure drop to vapor in the draft tube, and would have

preceded the collapse of the vapor cavity by an interval.

During the time that the vapor cavity persisted in the draft tube,

the transient ow condition between the throat and the draft tube

exit would have approximated a slow mass surge under the inu-

ence of the unbalanced head between vapor cavity and tailwa-

ter. As a result, it is likely that several seconds passed between

column separation and column rejoining at the collapse of the

vapor cavity. The waterhammer pressure rise in the spiral case

would have been relieved quite quickly due to the relatively short

penstocks. The penstock round-trip pressure wave travel time of

about 0.4 seconds was substantially shorter than the draft tube

mass surge time. A simplied mass surge analysis of the draft

tube ow during this postulated column separation indicates that

the time between opening and reclosing of the vapor cavity would

have been of the order of 2 ½ seconds. It further indicates that

the upward water velocity at vapor cavity closure would have been

approximately 2.4% greater than the initial downward velocity

when the separation began. This simplied analysis assumed

instantaneous draft tube inow interruption at the time of column

separation, so the results must be viewed as indicative only.

At a load of 475MW, the wicket gate setting was reported to be

69%, and the machine ow was 256m

/sec. [10] The average ver-

tical velocity across the draft tube throat at that condition would

have been 8.3m/sec. [15] If water column separation occurred at

that point, and the vapor cavity persisted until the gates were com-

pletely closed, then the mass surge analysis indicates that the velocity

attained after the ow in the draft tube reversed and just at the point

of the vapor cavity volume reaching zero would have been 8.5m/sec

upward. The instantaneous head rise resulting from the collision of

the water column with the head cover, according to the fundamental

waterhammer equation [21], will be:

∆H = - (a/g)(∆V),

where a is the celerity of an elastic pressure wave, g is the acceleration

of gravity, and ∆V is instantaneous change of ow velocity, -8.5m/

sec. This will amount to -(900/9.81)(-8.5), or about 780m. This head

rise multiplied by the area of the head cover inside the wicket gates of

49m

(see Figures 6 and 13), is equivalent to about 38,000 tonnes of

upward force. This is based on a very simplied analysis, so the value

must be taken as indicative of an upper limit, however, even a small

part of this force would have been very destructive.

The time that this force would exist would, of course, be very brief.

Based on a 35m draft tube length, it is estimated that the pressure spike

would last about a tenth of a second. Assuming the parts of the machine

that were lifted weighed 1500 tonnes, this pressure spike could have

lifted the machine a meter and a quarter during the tenth of a second

duration. This is about as close to a true explosion as it is possible to get

with an incompressible uid. It would explain the nature of the damage

that is visible in the photographs of Unit 2 (Figures 7, 8, and 9).

If the 80 studs holding the head cover to the stay ring were all

intact and in good condition, assuming that the stud material had an

ultimate strength of 550 MPa, they would have all failed under an

uplift force of about 23,000 tonnes. This implies that Unit 2 might

have failed even if the studs were new.

Between late August and early December 2009, B. Kolesnikov

posted several internet commentaries on the Rosteknadzor report and

various publications in the mass media. [22] He accompanied his

posts with three gures which included photographic evidence that

strongly supports the column separation hypothesis presented here,

although he did not mention column separation as a possible cause.

His gures are reproduced in Figure 13. The photographs show that

the wicket gate stems were all broken off at the tops of the gates,

while the bottom trunnions were undamaged. This could have hap-

pened only if the gates had been lifted vertically until the trunnions

were fully out of the lower bushings before the 300mm diameter

stems were snapped off. Moreover, the pictures show that the gates

were forced outward toward the spiral case; not inward toward the

runner, because the runner blades show no evidence of the physical

damage that would be expected from collisions with the broken gates.

The pictures also show some of the studs remaining in the stay ring

ange, which demonstrate that they were not sheared off, but failed

Above, left to right: Figure 10 – Unit 7 on 18 August (18); Figure 11 – Unit 9

on 18 August (18)

WWW.WATERPOWERMAGAZINE.COM NOVEMBER 2010 35

OPERATION AND MAINTENANCE

in tension. This set of conditions can be explained only by a very

large pressure exerted upward from the draft tube. B. Kolesnikov

stated that there had to have been a large upward force on the head

cover, but he did not speculate as to its cause. His information and

observations have been very helpful.

In summary, the photographic evidence of the sequence of failure

events seems to be very powerful. That the wicket gates were blown

outward after their lower trunnions were pulled out of their bushings

seems very clear from the pictures. Thus, it seems unavoidable to

conclude that the very large pressure spike originated on the inside

of the gate ring, which leads to the hypothesis that it started in the

draft tube.

Units 7 and 9 apparently had sudden load rejection conditions

imposed on them as a result of the Unit 2 failure. It appears that

both Units 7 and 9 experienced draft tube water column separation

followed by powerful uplift, causing severe damage to the generators

and surrounding structure.

The evidence from the photographs and video recorded sounds

suggests that Units 7 and 9 were similarly forced upwards, but with

less violence than in the case of Unit 2. Since it is likely that the

head cover studs on these two machines were in better condition

than those of Unit 2, any upward force would have been resisted

to a greater extent than in the Unit 2 case. It is possible, therefore,

that the studs of Units 7 and 9 failed sequentially, causing the rotat-

ing parts to tilt as they were being thrust upwards. This would

result in a collision between rotor and stator before the rotating

parts had moved far enough upwards to release penstock pressure

into the turbine pits.

The photographs (Figures 10, 11, and 12) indicate that Unit 7 was

somewhat more severely damaged than Unit 9. This is interesting,

since the Rostekhnadzor report shows that at 0813, Unit 7 was car-

rying only 85MW, while Unit 9 was carrying 570MW. Although

Unit 7 was carrying the lightest load in the plant at the time of the

accident, the sudden load rejection by Unit 2 would have caused Unit

7, with its very light load, to start to accept the load dropped by

Unit 2 when its breaker opened. Thus, during the period between

the Unit 2 load rejection and the Unit 7 load rejection, Unit 7 would

have experienced a signicant load acceptance transient. When the

massive electrical disturbances that followed the destruction of Unit 2

caused Unit 7 to shut down, it could have been from a heavily loaded

condition, with substantial transients remaining in its hydraulic pas-

sages. [5 through 8, 10]

New hypothesis: potential causes of water column separation

Observations at hydroelectric installations over the past 40 years have

indicated that plant operators sometimes try to improve the responsive-

ness of their generating units by various adjustments to the equipment.

One such adjustment is to modify the orice control of the wicket gate

servomotor oil pressure system in order to speed up the wicket gate

movement. Occasionally, this has resulted in cases where draft tube

column separation has occurred causing loud banging sounds, pres-

sure spikes, and sometimes damage to the machines. Normally, gov-

ernors are designed with considerable margin allowance in the sizing

of oil piping, leaving the speed control up to the orice plates (or needle

valves in some cases) that are installed to limit the velocity of oil ow.

How much margin is allowed is determined by the governor designer,

but oil piping is normally not a large portion of the cost of a governor,

so designers can be conservative with piping sizes and remain com-

petitive. Thus, it is often possible for governor speeds to be changed

signicantly by replacing orice plates with larger ones.

The fact that Unit 2 at Sayano Shushenskaya had had a new gov-

ernor installed in early 2009 is suggestive, as is the fact that this par-

ticular unit, in spite of its vibration problems, was chosen as the lead

load-following unit and was under the control of the new joint load

control system. It is possible that the governor had been adjusted to

increase the speed of wicket gate movement, which would have been

expected to improve the machine’s load-following capability. This

would have been consistent with the very fast load changing capa-

bilities of the joint load control system installed in June. It has been

reported that the joint load control was set to change unit loads at the

rate of 30MW/sec. [13] It is likely that the governors were set to an

even faster rate. If the governor gate speed were too fast, the transient

pressure drop in the draft tube accompanying a load rejection would

have caused water column separation as described earlier.

Unfortunately, the lack of data on these machines and the nature

of the complex pressure and velocity elds in the draft tubes make

it technically infeasible to predict analytically the governor time that

would cause column separation in the draft tubes. Normal water-

hammer assumptions of uniform velocity distribution simply do not

come close to actual draft tube conditions. Turbine designers require

extensive physical model testing to determine draft tube behavior.

Moreover, wicket gate position versus turbine ow behavior is quite

non-linear. The last part of a closing stroke changes ow very dra-

matically compared with conditions near full load. Thus, it is pos-

Sayano Shushenskaya – key facts

Initial operation date: 1978

Dam: Concrete gravity-arch, 245m high, 1066m long at crest

Reservoir: Sufﬁcient storage for regulation over a full year

Powerhouse:

t Installed Capacity: 6400MW

t Annual Generation: 23.5TWh (42 % Plant capacity factor)

t Number of Units: 10

t Turbine Type: Francis

t Rated Capacity: 650MW (turbine), 640 MW (generator)

t Rated Discharge: 358.5m

/sec. per unit

t Rated Speed: 142.86 rpm (50 Hz)

t Rated Net Head: 194m

t Submergence: approximately 10m.

t Runner Diameter: 6.77m

t Runner Weight: 156 tonnes

t Rotor Weight: 920 tonnes

t Generator Type: synchronous, umbrella-type [16]

t Joint Load Control load-change rate: 30MW/second [13]

Particular turbine features:

t Generator thrust bearing structurally supported on turbine head cover [15]

t Each of 20 wicket gates supplied with an individual hydraulic servomotor [16]

t Draft tubes 27.3m long (horizontal projection) and 16.9 m high. [16] Total

hydraulic length about 35m.

t All units had narrow ranges of satisfactory operation, with large rough-

running zones

t Due to rough zones, the units were not suitable for load-following operation,

therefore plant was normally operated as a ﬁxed base loaded installation

t All units initially sized to accommodate overloads up to 750MW [13]

t Field tests showed draft tube pulsation resonance with penstocks at loads

over 640MW, therefore operation above 640MW forbidden [13]

Special issues related to turbine unit 2:

t Fitted with new speed governor during ﬁrst quarter 2009

t Had runner cavitation damage weld repairs ﬁrst quarter 2009

t Runner was not rebalanced after weld repairs

t Experienced severe vibration problems both before and after runner repairs

t Connected with automated joint load control in June 2009 [13]

t Selected as lead unit due to recent repair and new governor

36 NOVEMBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

OPERATION AND MAINTENANCE

sible to effect load changes in operation without producing the same

transient conditions that would accompany a complete load rejection.

Also, many operating load changes may be gradual enough that the

governors are not saturated, or operating at maximum speed, while a

load rejection will always saturate a governor to minimize overspeed

of the generator.

The character of the damage to Units 7 and 9 suggests that they,

too, may have had their governors speeded up, which could have

caused column separation upon load rejection.

The reports of severe vibration in Unit 2 suggest possible causes

for the initial load rejection that apparently precipitated this mas-

sive failure. There may have been a protective relay that respond-

ed to excessive vibration, as is often the case in large rotating

electrical equipment. It has been reported, however, that the Unit

2 vibration monitoring equipment may have been disabled. [19]

Excessive vibration could also have caused heavy loads on the

guide bearings, which might have caused overheating and pre-

cipitated a trip. Also, if the powerhouse relay panels dated from

the 1970’s, and the relays were not of the more modern solid-

state types, vibrations carried through powerhouse structure to

the relays in the panels could have caused the contacts mounted on

the delicate carriages of an electro-mechanical relay to have closed

inadvertently. There are, of course, many other possible causes for

a load rejection to have occurred.

CONCLUSIONS

The conclusions of this article are based on an analysis that is specula-

tive, since complete technical information on the machines and their

governors and other ancillary and control equipment has not been

available to the writer. Nevertheless, the importance of this incident

to the safety of hydroelectric installations everywhere demands that

this evaluation, however imperfect and incomplete, be made available

to everyone in and around this industry.

1. The fundamental conclusion from this examination is that the explo-

sion of Unit 2 and the destruction of Units 7 and 9 were very prob-

ably caused by water column separation in the turbine draft tubes

during unit load rejection. This hydraulic transient phenomenon

was probably caused by turbine governors that had been speeded up

(probably unknowingly) to an unsafe level in an attempt to improve

frequency stability under changing electrical loads.

2. This project serves to re-emphasize the need to stress both model

and eld testing of hydraulic turbomachinery. Although the rough

operating zones of the Sayano Shushenskaya turbines were able

to be identied in the laboratory, the problem of resonance in the

penstocks as excited by draft tube pulsations at overload condi-

tions could only be identied in the eld under full scale operat-

ing conditions. Such testing was done early on and established

limits to safe operating zones which prevented resonance prob-

lems in operation. Had these limits not been established or been

violated in practice, consequences as dire as those experienced on

17 August could have occurred. Fortunately, they did not, due

to adequate eld testing and implementation of results. Much

progress has been made in recent times in the eld of computa-

tional uid dynamics (CFD), however, the uid mechanics of tur-

bomachinery, especially in unsteady ow regimes, still remains

beyond the abilities of present day CFD modeling.

3. If the turbine governors in this plant were adjusted in recent

times to increase wicket gate operating speed above safe levels,

this may have been done in good faith by operations person-

nel who were not familiar with hydraulic transient phenomena,

and the attendant limitations that the design of this installation

imposed on operation.

4. This sort of adjustment has been observed on other plants, both

in the USA and in other countries. Operators, left to their own

devices, will attempt to maximize the output of their plant, while

ensuring that it reacts to load changes in the fastest way possi-

ble. In this case, the plant operators were clearly under pressure

from the owners and grid operators to improve system frequency

stability, and, therefore, load following capability. Starting with

the rst implementation of automated and fast joint load control,

the operators of this plant had a strong incentive to speed up

Left, top: Figure 12 – Unit 7 generator after dewatering on 30 August (6)

Left, bottom: Figure 13 – Unit 2 damage details (22)

WWW.WATERPOWERMAGAZINE.COM NOVEMBER 2010 37

OPERATION AND MAINTENANCE

the governors, which could have been accomplished easily by

replacing orice plates or adjusting needle valves. [13] Full load

rejections in hydro plants are not very common. There may

not have been any severe ones at Sayano Shushenskaya prior to

August 2009.

5. Turbine mechanical designs should be carefully evaluated rela-

tive to the safety and redundancy of connections. This mechani-

cal design, which exposed a signicant head cover area to static

penstock pressures at the outer periphery of each, placed a great

burden on a set of relatively small and rather inaccessible studs.

Inspection of the studs was not easy due to their location in

recesses that were small and not particularly visible. This could

be considered a deciency on the part of the machine designers

6. Plant designers typically prepare Operating and Maintenance

Manuals as part of the design documentation. These manuals

are for the use of operations personnel, and they include both

equipment manufacturer recommendations and limitations and

those of the overall plant designers. A vital responsibility of the

designers of these plants is to state clearly in the O&M manuals

the design limitations inherent in the plant and its equipment.

Clear warnings should be stated about such matters as speeding

up governor times without allowing for the hydraulic transient

effects thereof. Operators may not be trained in the mechanics

of hydraulic transients, and their understanding of these phe-

nomena must not be taken for granted. They must be warned

what not to attempt and why, as well as what good practices to

follow. It must be kept in mind that operators may be widely

separated from designers by both distance and time, and addi-

tionally in technical knowledge. The O&M manual may be the

only link between the two.

7. The importance of hydraulic transient phenomena cannot be

overemphasized, as witnessed by the tragic outcome of this spec-

tacular failure. Designers must be cautious, and must impart

that caution to operations people. Hydro plants are inherently

safe structures, and hydro machinery is robust, conservative, and

safe. All that can be negated by misoperation and by lack of

maintenance and inspection, so these must be avoided through

every avenue available. The continued safety of our hydro

resources depends on it.

F. A. Hamill, P. E., Life Member ASCE, Member

USSD, Project Engineering Manager, Hydro and

Water Resources, Bechtel Corporation, San Francisco,

California, USA

The author would like to thank Dr. Alexander Gokhman

and Dr. Fred Locher for their encouragement and

technical assistance in the preparation of this article

References

1. Cruz, Euler, and Rafael Cesario, ‘Accident at Russia’s Biggest

Hydroelectric Sayano Shushenskaya – 2009 August 17”, PowerPoint

presentation posted on: http://www.slideshare.net/guestdff6c6ec/

accident-at-russias-biggest-hydroelectric-rev-00-2436565, and several

other web sites accessible through Google.

2. Pre-accident photographs posted on: http://4044415.livejournal.

com/51285.html

3. Pre-accident photographs posted on: http://4044415.livejournal.

com/51685.html

4. Pre-accident photographs posted on: http://4044415.livejournal.

com/51884.html

5. Post-accident photographs posted on: http://drugoi.livejournal.

com/3031739.html#cutid1

6. Post-accident photographs posted on: http://drugoi.livejournal.

com/3032285.html#cutid1

7. Post-accident photographs posted on: http://drugoi.livejournal.

com/3033773.html#cutid1

8. Post-accident photographs posted on: http://englishrussia.

com/?p=4853.

9. Fleming, Diarmaid, “Catalogue of failure led to Russian hydro plant

disaster”, New Civil Engineer, 8 October 2009, http://www.nce.co.uk.

10. Rostekhnadzor, “Technical Investigation Report: Causes of the August

17, 2009 Accident at the Sayano-Shushenskaya Hydroelectric Power

Station (the P.S. Neporozhnij Sayano-Shushenskaya Hydro-Power Plant,

an Afﬁliate of RusHydro, Inc.),“ in Russian, 3 October 2009, posted at:

http://www.gosnadzor.ru/news/aktSSG___bak.doc.

11. Video from mobile phone downstream from dam, 17 August 2009,

http://www.youtube.com/watch?v=3cHMS_7oqvI&NR=1

12. Podkovalnikov, Sergei, “Power Grid Interconnection in Northeast Asia:

View from East Russia,” First Workshop on Power Grid Interconnection

in Northeast Asia, Beijing, China, May 2001. http://nautilus.org/

archives/energy/grid/index.html

13. “Expert: Technical director for Power Machinery about Act of

disastrous failure at Sayano-Shushenskaya hydro power plant,” (in

Russian), http://energyfuture.ru/ekspert-texnichekij-direktor-silovyx-

mashin-ob-akte-avarii-na-sshg/.

14. “2009 Sayano-Shushenskaya hydro accident,” in Wikipedia, the

free encyclopedia, http://en.wikipedia.org/wiki/2009_Sayano-

Shushenskaya_hydro_accident

15. “Hydro-power and Auxiliary Equipment of Hydro Power Plants”,

Hand Book, Vol.1, “Energoatom Publishing House”, 1988 (in

Russian, Gidroenergeticheskoe I Vspomogatelynoe Oborudovanie

Gidroelektrostantsii).

16. “Reference Book on Water Turbines,” Manufacturing, 1984 (in

Russian, Spravochnik po gidroturbinam, Mashinostroenie, 1984)

17. Video assembled from CCTV security cameras, 17 August 2009,

http://www.youtube.com/watch?v=V9QiP26Ea_w

18. Boston.com, “The Big Picture,” Boston Globe web site, 9 September

2009, http://www.boston.com/bigpicture/2009/09/the_

sayanoshushenskaya_dam_acc.html#photo11

19. Kolesnikov, Eugenio, “Reﬂections on Russian Accident on August

17th 2009 – 6400-MW Sayano-Shushenskaya Hydroelectric Power

Plant,” Presentation, Miami, 12 October 2009, revised 20 October

(unpublished as of 20 October, distributed via e-mail).

20. Montreal Engineering Company, Limited, Manual of Hydraulic

Engineering Practice, Section 260, Hydraulic Turbines, 2nd Edition

Revised and Augmented, February 1981.

21. Wylie, E. B., and V. L. Streeter, Fluid Transients, Corrected Edition,

1983, FEB Press, Ann Arbor, Michigan, p. 3, eq’n. 1-1.

22. Kolesnikov, Boris, “Failure of Sayano-Shushenskaya Hydroelectric

ru/2009/12/10/189.

IWP& DC

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