Simulation tests for Primary Frequency Response requirements

requirements

Using the requirements of Grid Code for Primary Frequency Response regarding the increase in power output by the power plant, several simulations for different operating loading levels were conducted, with steam pressure considered to be in fixed pressure control mode. Having the power plant operating at steady state, a step change was applied to the load demand signal at time t = 60 s. The simulation data was processed in Fig. 4.8 – Fig. 4.12 presented below.

FIGURE 4.8 Power output response and steam pressure evolution for 60 MW load demand step increase from the power plant loading level of 350 MW

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FIGURE 4.9 Power output response and steam pressure evolution for 60 MW load demand step increase from the power plant loading level of 400 MW

FIGURE 4.10 Power output response and steam pressure evolution for 60 MW load demand step increase from the power plant loading level of 450 MW

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FIGURE 4.11 Power output response and steam pressure evolution for 50 MW load demand step increase from the power plant loading level of 500 MW

FIGURE 4.12 Power output response and steam pressure evolution for 25 MW load demand step increase from the power plant loading level of 550 MW

The response time, power output overshoot and pressure drop values obtained for each simulation test are presented in Table 4.2.

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TABLE 4.2 Results obtained from the simulation tests for Primary Frequency Response requirements with the plant operating in fixed pressure control mode

Loading level

Fixed Pressure Control Mode Load demand step increase Response time Power output overshoot Steam pressure drop %RC MW %RC MW s % MW % MPa 58.33 350 10 60 128 1.56 6.41 2.43 0.61 66.66 400 10 60 136 1.50 6.90 2.54 0.64 75 450 10 60 136 1.68 8.56 2.68 0.68 83.33 500 8.33 50 147 1.08 5.91 2.19 0.55 91.66 550 4.17 25 160 0.66 3.78 1.15 0.29

The response time is defined as the time elapsed between the moment when the load demand step change signal was applied (t = 60 s) and the first time instance when the increase in the power output matches the load demand.

Looking at the simulation response time values from Table 4.2, it shows that the time required by the power plant to increase its power output according to specified load demand step changes is increasing with the increase of the operating loading level. All the results show that the power plant cannot fulfil the Primary Frequency Response regulations specified by the Grid Code, being far more than the required response time of 10 s. By analysing Fig. 4.8 – Fig. 4.12, it results that after a new load demand signal is given to the power plant, there follows an immediate linear increase of the power output, with a high rate of change, which is due to the fast opening of the control valve, allowing more steam to flow into turbine. As the stored energy of the boiler is limited, this high rate of change of the power output comes to

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an end after a certain time. The rest of the necessary power increase is delivered at a smaller rate, as there is a time delay until the new fuel demand signal is reflected by the coal mills’ output and then a further delay is added by the combustion and steam generation process in the boiler.

The fast opening of the control valve generates an increase in the steam flow going through turbine, which means as well a steam pressure drop from its set point. These values and the ones of the power output overshoot obtained from the simulation tests are presented in Table 4.2. The power output overshoot expressed in MWs represents the maximum power generated minus the load demand step change value. This result can be expressed in percentages if it is further divided by the load demand step change value and multiplied with one hundred. The steam pressure drop is calculated as the difference between the minimum value of the steam pressure and its set point value. By dividing this result with the set point value and then multiplying it with one hundred, it can also be expressed in percentages.

It can be observed from Table 4.2 that when the same load demand step signal (60 MW) is sent to the power plant, operating at three different loading levels, the overshoot of the generated power (expressed in MW) is increasing with the increase of the loading level. This can be justified by the initial fuel demand signal given to the coal mills, which increases with the operating loading level. As power output reaches the set point setting, the fuel demand signal is decreased, but there will continue to be an increase in generated power due to the thermal inertia constant of the boiler.

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The pressure drop values (expressed in MPa) for all the simulation tests are of small value, but it can be observed a slight increase with the increase of the operating load level. This is due to the wider opening of the control valve.

Although operating with sliding pressure control mode is not suitable for achieving a fast power plant response, simulation tests were run considering the same Primary Frequency Response requirements specified by the Grid Code. The results were processed in the graphs presented in Fig. 4.13 to Fig. 4.17.

FIGURE 4.13 Power output response and steam pressure evolution for 60 MW load demand step increase from the power plant loading level of 350 MW

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FIGURE 4.14 Power output response and steam pressure evolution for 60 MW load demand step increase from the power plant loading level of 400 MW

FIGURE 4.15 Power output response and steam pressure evolution for 60 MW load demand step increase from the power plant loading level of 450 MW

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FIGURE 4.16 Power output response and steam pressure evolution for 50 MW load demand step increase from the power plant loading level of 500 MW

FIGURE 4.17 Power output response and steam pressure evolution for 25 MW load demand step increase from the power plant loading level of 550 MW

It can visually be observed from the figures above, that the response times of the power plant are longer than the ones obtained when operating in fixed pressure control mode. This is expected, considering that the control valve is kept more than

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90% opened, which means that the boiler has no stored energy to use for fast changes in the load demand. The response time, power output overshoot and pressure drop values obtained for each simulation test are presented in Table 4.3.

TABLE 4.3 Results obtained from the simulation tests for Primary Frequency Response requirements with the plant operating in sliding pressure control mode

Loading level

Sliding Pressure Control Mode Load demand step increase Response time Power output overshoot Steam pressure drop %RC MW %RC MW s % MW % MPa 58.33 350 10 60 153 1.44 5.89 4.43 1.01 66.66 400 10 60 155 1.50 6.89 4.49 1.05 75 450 10 60 175 1.93 9.82 4.57 1.09 83.33 500 8.33 50 189 1.52 8.37 3.08 0.74 91.66 550 4.17 25 202 1.01 5.80 1.21 0.29

Similar with the case of power plant operating with fixed pressure control mode, it can be noticed from Table 4.3, that the response time is increasing with the increase of the operating loading level. Analysing the figures in Table 4.3 for power output overshoot, it can be observed that for the same load demand step increase (60 MW), the overshoot of the power output (expressed in MW) is increasing for operating at a higher operating loading level. The values are higher than when the power plant was operating in fixed pressure control mode. The same conclusion can be drawn for the values of the pressure drop. The response time values obtained during simulation tests, for the power plant operating under fixed pressure and sliding pressure control mode are plotted together in Fig. 4.18.

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FIGURE 4.18 Power plant response times for the boiler operating in fixed pressure and sliding pressure control mode

It can be observed from Fig. 4.18 that the difference between response times for different pressure control mode is even more consistent for operating loading levels situated above 400 MW (~ 65% RC). The numerical comparison is summarised in Table 4.4.

TABLE 4.4 Power plant response times for boiler operating in fixed/sliding pressure control mode Loading level Response time Fixed Pressure Control Mode Sliding Pressure

Control Mode Time difference

%RC MW s s s % 58.33 350 128 153 25 19.53 66.66 400 136 155 19 13.97 75 450 136 175 39 28.67 83.33 500 147 189 42 28.57 91.66 550 160 202 42 26.25 100 150 200 250 350 400 450 500 550 Chart Title Fixed Pressure Sliding Pressure Loading Level (MW) Time (s)

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Table 4.4 shows that the difference in response times is quite consistent, being between 19 s and up to 42 s. The simulation results prove that the fastest response of the power plant is obtained when steam pressure is controlled in fixed pressure control mode.