Load Rejection Transient

Fig. 5.15 shows the variations in turbo units‟ speeds and the bypass flow during a loss of load transient in the single and the three shaft configurations. In the three shaft system it can be seen that the controller managed successfully to prevent the power turbine over speed and to stabilise it at the normal operation conditions at 3000 rpm (50 Hz). This is done by means of opening the bypass valve. Additionally, the speeds of both high and low pressure turbines decrease substantially due to the reduced mass flow. Both units manage to reach a new steady state, stabilising the system at the new power level. It is also possible to see that during the event, the bypass flow reaches a maximum of 37 kg/s. In order to avoid a very large flow through the valve, 8 valves are used in parallel. The total flow which is bypassed is equivalent to about 150% of the power turbine nominal flow. Fig. 5.15 further indicates that the shaft speed of the singe shaft system reaches a value of approximately 3150 rpm before decreasing again. It is furthermore shown that the controller adjusted the helium mass flow which passes through the bypass valve, via a change in the valve diameter after sensing the power turbine speed. Each of the bypass valves employed obtains a maximum flow rate of 82 kg/s. The total maximal flow rate contributed to the three valves opening in parallel is about 190% of the nominal turbine flow. In both systems, the shaft speed increases before it reaches again the nominal speed value. This can be explained by the time it takes to reach a new pressure ratio in the system and the time needed for the redistribution of the mass between the high and the low pressure zones, especially in the three shaft case. The temperature transients are a non-avoidable consequence of the pressure ratio variations over the turbo-machines.

0 200 400 600 800 1000

0 5 10 15 20

Time (s)

Temperature (°C)

3-shaft - LP Inlet 3-shaft - HP Inlet

0 200 400 600 800

0 5 10 15 20

Time (s)

Temperature (°C)

1-shaft - LP Inlet 1-shaft - HP Inlet

Fig. 5.16: Recuperator‟s inlet temperatures during load rejection transient, single and three shaft configurations.

The impact of the temperature and pressure transients could lead to high stresses on materials of the main components. Therefore they must be taken into account at the design phase of the plant. Fig. 5.16 shows the temperature evolution in the recuperator heat exchanger at the inlet of the primary and of the secondary side of both single and three shaft configurations. It can

be seen that short-circuiting the system results in hot helium entering the primary side, i.e. the low pressure side of the recuperator. The secondary side experiences only a small decrease.

This is due to the large thermal mass of the recuperator, which delays a temperature increase of the high pressure gas. The recuperator low pressure inlet temperature increases in approxi-mately 250°C to 300°C during the event in both systems. Fig. 5.17 shows the change in sys-tem pressures during the event in the two different shaft configurations.

0 1500 3000 4500 6000 7500

0 5 10 15 20

Time (s)

Pressure (kPa)

3-shaft - LPC Inlet 3-shaft - Manifold

0 1500 3000 4500 6000 7500

0 5 10 15 20

Time (s)

Pressure (kPa)

1-shaft - LPC Inlet 1-shaft - Manifold

Fig. 5.17: System pressure at LP Compressor inlet and at the manifold during rejection tran-sient, single and three shaft configurations.

It can be seen that the circuit‟s pressure ratio decreases after opening the bypass valve. The single shaft configuration tends towards stabilisation at a new steady state with a significantly smaller pressure difference than the difference reached in the three shaft configuration, due to the difference in the valve opening between the two systems. Fig. 5.18 shows the variation in reactor temperatures during the transient.

0 200 400 600 800 1000

0 5 10 15 20

Time (s)

Temperature (°C)

3-shaft - PB Inlet 3-shaft - PB Outlet

0 200 400 600 800 1000

0 5 10 15 20

Time (s)

Temperature (°C)

1-shaft - PB Inlet 1-shaft - PB Outlet

Fig. 5.18: Pebble Bed Reactor temperatures during load rejection transient, single and three shaft configuration.

While the reactor inlet temperature slightly decreases and then significantly increases, the reactor outlet temperature remains almost constant in both shaft configurations. The steep change in the inlet temperature occurs due to the high level of interdependence between the

recuperator and the reactor. The large bypass flow, in combination with a decreased pressure ratio, causes a higher turbine outlet temperature, which feeds back through the recuperator and results in a higher reactor inlet temperature. On the other hand, the nearly constant value of the reactor outlet temperature reflects the rapid feedback from the fuel temperature to reac-tivity, by which the power output is adjusted to match the cooling capacity of the reduced helium flow. Fig. 5.19 shows the variations in turbo-machines‟ efficiencies during the event in the three and in the single shaft configurations respectively. The change in the helium flow velocities reduces the machines efficiencies. It is clear that during the event, the efficiency of the three shaft power turbine drops to zero. At this stage, the turbine will not have any electri-cal output, and the efficiency increases again as the resistor bank has been activated. On the other hand, in the single shaft configuration, since the shaft speed and consequently all other turbo-machines speeds are controlled to operate close to their design point at 50 Hz, the varia-tion in efficiencies is not as severe as in the three shaft configuravaria-tion. The temporary reduc-tion in the PCU efficiency must be retrieved by means of long term adjustments of storage or addition of helium in order to maintain a sufficient power level efficiency. Such changes will be discussed in the next transient analysis of load following.

0 20 40 60 80 100

0 5 10 15 20 25

Time (s)

Efficicnecy (%)

3-shaft - PT 3-shaft - LPT 3-shaft - HPT 3-shaft - LPC 3-shaft - HPC

78 80 82 84 86 88 90

0 5 10 15 20

Time (s)

Efficiency (%)

1-shaft - PT 1-shaft - LPC 1-shaft - HPC

Fig. 5.19: Turbo-machines‟ efficiencies, single and three shaft configurations.

In order to ensure stable operating conditions of the system it is also important to prevent the compressors from entering the surge region.

0 20 40 60 80

0 5 10 15 20

Time (s)

Surge Margin (%)

3-shaft - LPC 3-shaft - HPC

0 10 20 30 40 50

0 5 10 15 20

Time (s)

Surge Margin (%)

1-shaft - LPC 1-shaft - HPC

Fig. 5.20: Compressors‟ surge margins, single and three shaft configurations.

Fig. 5.20 shows that during the transient, the operating conditions of both high and low pres-sure compressors in both configurations and especially in the three shaft system change sub-stantially. This effect is contributed to the use of bypass valves, which causes a significant change in mass flow rate through the high pressure and the low pressure turbines in the three shaft system followed by a substantial drop in their speeds. It is seen that the compressors do not go into surge. A negative surge margin in the plot indicates that the compressor has crossed the surge line. The use of local compressor bypass valves in the three shaft configura-tion allows the compressors to maintain stable operaconfigura-tion. The analysis of a load rejecconfigura-tion tran-sient shows that both systems behave in a rather similar way. In both configurations, the oper-ating conditions of the high pressure and the low pressure compressors significantly change during the simulation. Yet, both compressors succeed in moving away from surge. The effi-ciencies of the turbo-machines in both configurations have degraded substantially, especially the efficiency of the power turbine of the three shaft configuration. Moreover, the power tur-bine in both systems speeds up, due to the larger excess in power after the event. In both sys-tems, the reactor outlet temperature hardly changes, yet the temperature of the recuperator low pressure side increases, and so does the core inlet temperature. A special care must be then given to avoid potential thermal stresses on material structures.