5. Simulation results

Figure 6. Single-line diagram of the test power system with an AC micro-grid including wind generation and the TCCC/

Figure 7. Capability curves of the wind farm: (a) voltage curve and (b) frequency curve.

VRFB system.

92 Redox - Principles and Advanced Applications

The frequency stability of the proposed MG is evaluated through simple events that impose high demands on dynamic response of the steam turbine and the TCCC/VRFB unit. In this regard, two case studies are considered. Case 1 evaluates the post-fault frequency stability of the MG when it is transmitting electric power to the 132 kV network. Case 2 examines the postfault frequency stability of the MG when it is importing electric power from the bulk power system. In addition to the frequency stability analysis, Case 3 evaluates the quality power supply during the isolated operation of the MG. The under-frequency load shedding system, which could help in maintaining the frequency drop after contingencies within acceptable values, is not considered in all the cases aiming at highlighting the benefits of the proposed solution.

#### 5.1. Case 1: unexpected disconnection of the MG during energy exportation

In this case, the frequency stability of the system is analyzed when an unexpected single-phase fault occurs at Line 1. In consequence, switch B1 opens 0.1 s after the occurrence of the event, causing the isolated operation mode of the MG. When the fault occurs at t = 150 s, the steam turbine and the wind farm are generating 9.5 and 3 MW, respectively. The load is 11 MW, so the power imbalance is 1.5 MW approximately.

Figure 8(a) shows the frequency deviation of the MG. Since the power generation is higher than the load, the frequency increases after the fault. When the control strategy developed in Ref. [21] is employed (previous control strategy), the frequency reaches the value of 52 Hz at t = 152.9 s; at this instant the wind farm switches off due to over-frequency. After that, the load is higher than the power generation, hence the system frequency decreases. The steam turbine switches off at t = 158.8 s due to under-frequency. Therefore, the MG collapses due to the lack of generation. A similar situation occurs when the new control strategy is applied and the rated power of the TCCC/VRFB is 1.0 MW. If the rated power of the TCCC/VRFB is 1.5 MW at least and the new control strategy is activated, the frequency reaches the value of 50.89 Hz at t = 153.4 s; after that the MG recovers the nominal value of the frequency (50 Hz).

Owing to the proportional characteristic of the PFC controller, the system frequency presents a steady-state error after the fault. In order to eliminate this error, the SFC controller participates in the frequency regulation. The steam turbine and the TCCC/VRFB compensator operate under this control scheme. These units restore the frequency after 70 s (Figure 8(a)).

The dynamic performance of the MG is represented in Figure 8(b)–(d). After the fault at t = 150 s, the short-term reserve generation is activated in order to compensate the power unbalance of the MG. Since the power generation is higher than the load, the active power outputs of the steam turbine and the TCCC/VRFB unit decrease according to Figure 8(b) and (d), respectively. According to Figure 8(d), when the 1.0 MW TCCC/VRFB is operating in the saturation region, there is no frequency control. Hence, the wind farm and the steam turbine switch off at t = 169 and 175 s, respectively. The incorporation of the 1.5 MW TCCC/VRFB compensator and the proposed control strategy avoid the collapse of the MG, in accordance with Figure 8(a).

#### 5.2. Case 2: unexpected disconnection of the MG during energy importation

In this simulation, the frequency stability of the power system is analyzed when a fault occurs at line 1, whereas the MG is importing energy from the bulk power system. An unexpected single-phase fault occurs at t = 150 s; in a consequence switch B1 opens 0.1 s after the occurrence of the fault, causing the MG to operate in isolated mode. When the fault occurs, the

Figure 8. Dynamic response of the MG in Case 1: (a) frequency deviation, (b) steam turbine power, (c) wind farm active power, and (d) TCCC/VRFB active power.

A New Control Strategy to Integrate Flow Batteries into AC Micro-Grids with High Wind Power Penetration http://dx.doi.org/10.5772/intechopen.69340 95

Figure 9. Dynamic response of the MG in Case 2: (a) frequency deviation, (b) steam turbine power, (c) wind farm active power, and (d) TCCC/VRFB active power.

steam turbine and the wind farm are generating 9.5 and 3 MW, respectively. The load is 14 MW, so the power unbalance is 1.5 MW approximately.

The frequency deviation of the MG is shown in Figure 9(a). In this case, the load is higher than the power generation, so the frequency decreases after the fault. When the previous control strategy is employed, the frequency reaches the value of 48 Hz at t = 155.6 s; at this instant the wind farm switches off due to under-frequency. In a consequence, the system collapses owing to the lack of generation at t = 156 s. If the new control strategy is activated and the rated power of the TCCC/VRFB is at least 1.5 MW, the frequency reaches the value of 49.15 Hz at t = 155.4 s; after that the frequency asymptotically recovers its nominal value.

The dynamic response of the MG is shown in Figure 9(b)–(d). Because the load is higher than the power generation, the short-term generation reserve is activated in order to compensate the power unbalance. Therefore, the steam turbine and the TCCC/VRFB increase the power output according to Figure 9(b) and (d), respectively. Similarly in the previous case, when the 1.0 MW TCCC/VRFB is operating in the saturation region, there is no frequency control. Hence, the wind farm and the steam turbine switch off at t = 256.2 s. The incorporation of the 1.5 MW TCCC/VRFB unit and the proposed control strategy avoid the collapse of the MG as in the previous case (Figure 9(a)).

#### 5.3. Case 3: isolated operation of the MG

rated power of the TCCC/VRFB is 1.0 MW. If the rated power of the TCCC/VRFB is 1.5 MW at least and the new control strategy is activated, the frequency reaches the value of 50.89 Hz at

Owing to the proportional characteristic of the PFC controller, the system frequency presents a steady-state error after the fault. In order to eliminate this error, the SFC controller participates in the frequency regulation. The steam turbine and the TCCC/VRFB compensator operate

The dynamic performance of the MG is represented in Figure 8(b)–(d). After the fault at t = 150 s, the short-term reserve generation is activated in order to compensate the power unbalance of the MG. Since the power generation is higher than the load, the active power outputs of the steam turbine and the TCCC/VRFB unit decrease according to Figure 8(b) and (d), respectively. According to Figure 8(d), when the 1.0 MW TCCC/VRFB is operating in the saturation region, there is no frequency control. Hence, the wind farm and the steam turbine switch off at t = 169 and 175 s, respectively. The incorporation of the 1.5 MW TCCC/VRFB compensator and the proposed control strategy avoid the collapse of the MG, in accordance with Figure 8(a).

In this simulation, the frequency stability of the power system is analyzed when a fault occurs at line 1, whereas the MG is importing energy from the bulk power system. An unexpected single-phase fault occurs at t = 150 s; in a consequence switch B1 opens 0.1 s after the occurrence of the fault, causing the MG to operate in isolated mode. When the fault occurs, the

Figure 8. Dynamic response of the MG in Case 1: (a) frequency deviation, (b) steam turbine power, (c) wind farm active

power, and (d) TCCC/VRFB active power.

t = 153.4 s; after that the MG recovers the nominal value of the frequency (50 Hz).

94 Redox - Principles and Advanced Applications

under this control scheme. These units restore the frequency after 70 s (Figure 8(a)).

5.2. Case 2: unexpected disconnection of the MG during energy importation

Maintaining the voltage and frequency within certain limits in a MG is a basic operational requirement as many loads may be very sensitive to voltage and frequency deviations. Increasing wind penetration in the MG may lead to stability problems or produce unwanted voltage and frequency oscillation in isolated systems [2–4]. Bearing these aspects in mind, the dynamic performance of the MG is evaluated during the isolated operation. The operation conditions are as follows: the steam turbine is generating 4 MW (minimum operating power), the average wind farm active power is 3 MW, and the constant load is 7 MW. Therefore, the wind penetration is approximately 43%.

Figure 10(a) shows the frequency of the MG by considering two cases: with and without energy storage. When the TCCC/VRFB unit is not employed, the steam turbine presents problems in establishing the nominal value of the frequency as a result of random variations in wind power. When the TCCC/VRFB is used, the frequency deviations are less than 0.01 Hz approximately, enhancing the power quality supply of the MG.

Figure 10(b) shows the power output of the steam turbine and the wind farm. When the TCCC/VRFB unit is not employed, the steam turbine operates below the minimum operating power (4 MW) during several intervals. This situation is avoided when the TCCC/VRFB performs the load leveling of the wind power generation (Figure 10(c)), causing a reduction of the mechanical stress of the steam turbine. Figure 10(d) demonstrates that the steam turbine and the wind farm can control effectively the voltage of buses 2 and 3, respectively.

These simulations show that the TCCC/VRFB unit and the new control system enhance the dynamic response of MGs which incorporate wind generation, by performing the load leveling of wind turbines and carrying out the frequency regulation of the MG. It is important to notice that the DFIG wind farm and the TCCC/VRFB unit complement each other; the reactive power fluctuations generated by the TCCC are compensated by the wind farm, whereas the fluctuations of the wind power generation are smoothed by the TCCC/VRFB unit.

Figure 10. Dynamic response of the MG in Case 3: (a) frequency deviation, (b) steam turbine/wind farm power, (c) TCCC/ VRFB active power, and (d) bus voltage.
