Performance Validation

The performance of the proposed hybrid microgrid and control strategy are validated using PSIM simulations only. As mentioned earlier, the hardware validation for the hybrid microgrid is not done due to limited resources. The considered hybrid microgrid is rated at 3 kVA , 120V , and the control and system parameters are _rms

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Figure 4.4: Hybrid microgrid Scenario A (a) Simulation setup (b) Active power droop curve (c)Reactive power droop curve

DG#12 60V,0.75kVA Slave DG String#2 120V,1.5kVA 12 0V ±5 % ,5 0H z± 2% 0.04+0.4j 0.05+0.5j 0. 02 + 0. 2j 0. 02 + 0. 2j DG#11 120V,1.5kVA Master DG DG#22 120V,1.5kVA Slave DG DG#21 120V,1.5kVA Master DG DG#13 60V,0.75kVA Slave DG String#1 120V,1.5kVA 0. 02 + 0. 2j Pm g = 2. 5k W @ 0 .9 p f Pj(kW) ω 51 49 1.5 49.34 0.0013 p M = 1.25 Qj(kVAr) V 126 114 1.5 120.94 0.60 0.008 q M =

given in Table 4.1. A complete description of the hybrid microgrid simulated in this section is given in Chapter 5 The system power sharing, voltage and frequency regulation performance is validated for two different scenarios of hybrid microgrid. TABLE 4.1:PERFORMANCE VALIDATION PARAMETERS FOR HYBRID MICROGRID.

Description Label Value

Master DG

DC-link voltage V DC 200V

Filter Inductor Lf 2 [mH]

Filter Capacitor Cf 45 [uF]

Current controller proportional gain Kp c, 0.1 Current controller integral gain Ki c, 100 voltage controller proportional gain Kp v, 0.35 Voltage controller integral gain Ki v, 3.88

Slave DG

DC-link voltage V DC 100V

Filter Inductor Lf 2 [mH]

Filter Capacitor Cf 45 [uF]

Current controller proportional gain Kp c, 0.17 Current controller integral gain Ki c, 170 voltage controller proportional gain Kp v, 0.01 Voltage controller integral gain Ki v, 200

Power factor regulator

Proportional gain Kp pf, 0.1

Integral gain Ki pf, 0.001

Droop gains

Output voltage (RMS) V rms 120±5% [V]

Frequency f 50±2% [Hz]

Active power droop gain Mp 0.0013

Scenario A: Equal rating string units and unequal feeder impedance.

In this scenario, the microgrid comprises two string units, as shown in Figure 4.4. String#1 consists of one dispatchable master (DG#11) and two non-dispatchable slaves (DG#12 and DG#13), while String#2 consists of one dispatchable master (DG#21) and one non-dispatchable slave (DG#22).

The system performance is validated under various load and MPPT operating conditions. The microgrid load is set to 2.5 kW, 0.9 pf, and the MPPT power reference of String#1 slave DGs are set to 0.25 kW for t  [0, 1] sec, 0.5 kW for t  [1, 2] sec, and 0.75 kW for t  [2, 3] sec, while the MPPT power reference of String#2 slave DGs are set to 0.5 kW for t  [0, 1] sec, 1 kW for t  [1, 2] sec, and 1.5 kW for t  [2, 3] sec.

String Units Power Sharing: Simulation results for the active and reactive power of

the DGs, and microgrid voltage and frequency, are provided in Figure 4.5. The microgrid load is equally shared between both string units since they are equally rated. From Figure 4.5.a and Figure 4.5.b, String#1 and #2 deliver 1.25 kW and ~0.6 kVAr at the three different MPPT setpoints. Accordingly, the microgrid voltage and frequency deviates from its nominal value to 49.34 Hz and 120.94 Vrms, respectively, which mitigates the droop principle explained for parallel topology microgrid in section 2.2.1. The power sharing performance of the proposed hybrid microgrid is comparable the conventional droop-controlled parallel topology microgrid, where real and reactive power sharing are achieved by drooping the voltage and frequency of the DGs.

Series DG Active Power Output: The active power produced by the cascaded DGs in

String#1 are shown in Figure 4.5.b. From this figure, the slave DGs track the MPPT power reference of 0.25 kW for t  [0, 1] sec and 0.5 kW for t  [1, 2] sec. The increase in power produced by the slave DGs is followed by a reduction in active power output of the master DG, who is responsible for the overall power balance within the string and for ensuring proportional power sharing among the strings. Accordingly, the master DGs delivers 0.75 kW for t  [0, 1] sec and 0.25 kW for t  [1, 2] sec. From 2 sec onwards, the MPPT power reference for the slaves DG#12

and #13 is set to 0.75 kW/unit, exceeding the overall string power demand of 1.25 kW. Therefore, the active power for the slaves DG#12 and DG#13 are curtailed at 0.625 kW and the master DG#11 commanded active power drops down to zero as the entire power demand is fulfilled by the two solar PV slave DGs. Similar power sharing performance is achieved for String#2, as shown in Figure 4.5.c where the MPPT set points are changed in 0.5 kW steps. From this figure, slave DG#22 tracks the MPPT set points till 2 sec and power curtailment at 1.25 kW occurs from 2 sec onwards, while master DG#21 satisfies the overall power demand and produces 0.75 kW for t  [0, 1] sec, 0.25 kW for t  [1, 2] sec, and ~0 kW for 2 sec onwards.

Series DGs Reactive Power Output: The reactive power produced by the DGs in

String #1 and #2 are based on the principle of equal power factor (pf) operation. As shown in Figure 4.6.a the slave DGs #12 and #13 share 0.115 kVAr while produce 0.25 kW for t  [0, 1] sec, and 0.22 kVAr while produce 0.5 kW for t  [1, 2) sec, so as to maintain constant P/Q ratio, i.e. pf synchronisation. The master DG#11 compensates for the reactive power shortfall by producing 0.37 kVAr for t  [0, 1] sec and 0.15 kVAr for t  [1, 2] sec. Similar reactive power sharing performance is achieved for the DGs in String#2, as shown in Figure 4.6.b.

As discussed in section 4.3, the power output of the cascaded DGs is controlled by regulating their operating voltage instead of operating current as for the parallel topology microgrid. This performance is illustrated in Figure 4.6.c for String#1. Up to 1 sec, the operating voltage of master DG#11 is 78.76 Vrms and of slave DG#12 and #13 is 21.09 Vrms and the overall string voltage is 120.94 Vrms. From 1 sec onwards, the operating voltage of DGs change with their output power, while the overall microgrid voltage is maintained within a narrow range (120 Vrms ± 5%), following the droop principle for parallel topology microgrid.

Further evaluation of the system performance is conducted by considering the MPPT set points rescaled from the actual irradiance profile. Figure 8 shows the power output of the DGs in String#1, where the slave DGs follow the MPPT reference power and the master DG compensates for the shortfall. The power curtailment of slave DGs is activated for a short time period, i.e. t  [2.1, 2.4] sec, while the power

demand is solely met by the slave DGs. Note that while the power output of the string DGs changes continuously, the overall string power is considerably constant and shared proportionally according to the respective DG power rating, as shown in Figure 4.7.a and Figure 4.7.b. The corresponding output voltage of DGs in String #1 is also shown in Figure 4.7.c and Figure 4.7.d, where it can be seen that the overall string voltage is constant for the entire operating range. Note that there is a slight error in reactive power sharing and voltage sharing performance at string level due to the effect of unequal line impedances.

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Figure 4.5: Simulation results for hybrid microgrid Scenario A showing (a) microgrid frequency, DGs and string units active power output for (b) String#1 (c) String#2

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Figure 4.6: Simulation results for hybrid Scenario A, showing, DGs and string units reactive power output for (a) String#1and (b) String#2, and DGs and string units operating voltage for (c) String#1 and (d) String#2

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Figure 4.7: Simulation results for hybrid microgrid Scenario A showing, DGs and string units active power output for (a) String#1 and (b) String#2 (c) DGs and string units operating voltage for (c) String#1 and (d) String#2, with actual irradiance profile.

DG#12 60V,0.75kVA Slave DG String#2 120V,0.75kVA 12 0V ±5 % ,5 0H z± 2% 0.04+0.4j 0.05+0.5j 0. 02 + 0. 2j 0. 02 + 0. 2j DG#11 120V,1.5kVA Master DG DG#22 60V,0.75kVA Slave DG DG#21 120V,0.75kVA Master DG String#1 120V,1.5kVA Pm g = 1. 9k W @ 0 .7 p f

Scenario B: Unequal rating string units and unequal line impedance

The system architecture for the second scenario is shown in Figure 4.8.a where the microgrid is formed by string units of unequal ratings. The system parameters are the same as given above in TABLE 4.1, except the droop gain for DG#21 which is now

twice the prior value. microgrid load is set at 1.9 kW, 0.707 pf and the MPPT is kept constant at 0.5 kW and the corresponding simulation results are shown on Figure 4.9.

The active power sharing performance with the proposed master-slave control is shown in Figure 4.9.b where it can be seen that the active power shared by the string units is proportional to their power ratings. Specifically, string#1 unit with twice the power capacity than string#2 unit produces 1.35 kW and string#2 produces 0.65 kW. The corresponding microgrid frequency according to the droop curves is 49.25Hz and this result is shown in Figure 4.9.a. The reactive power sharing performance is illustrated in Figure 4.9.d where string#1 share 1.34kVAr and string#2 share 0.6 kVAr. At these reactive power loadings, the expected microgrid voltage is 115.28V, as shown in Figure 4.9.c.

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Figure 4.8: Simulation setup for hybrid microgrid Scenario B (a) Simulation setup (b) Active power droop curves (c) Reactive power droop curves

Pj(kW) ω 51 49 49.25 1.3 Mp=0.00133 for string#1 Mp=0.00266 for string#2 0.65 Qj(kVAr V 126 114 115.28 1.34 Mq=0.008 for string#1 Mq=0.016 for string#2 0.6

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Figure 4.9: Simulation results for hybrid microgrid Scenario B, showing (a) microgrid frequency (b) String and microgrid active power, (c) microgrid voltage, and (d) String and microgrid reactive power

4.5. Summary

This chapter has presented a new hybrid topology microgrid that exploits the benefits of both series and conventional parallel topologies. The proposed microgrid provides flexibility to integrate non-dispatchable DGs with lower rated dc-link voltages, while also provides the redundancy and reliability offered by the parallel topology. A fully decentralised master-slave control strategy has been developed that effectively coordinates the power sharing between the paralleled string units and achieves power balance among the cascaded DGs within the strings. The proposed control strategy ensures proportional power sharing among the string units, and satisfies the operational constraints set by the non-dispatchable DGs such as MPPT tracking, PV curtailment, and reactive power support. The performance of the proposed microgrid and power management scheme, under various operating conditions, have been extensively verified using PSIM simulations.