**3. Modelling the Low Voltage grid**

The performance of microgenerators can be compared in this virtual lab using the designed low voltage grid model with six clusters of loads (Fig. 25). It is assumed that 85% of these loads are non-linear and 15% are linear. Also, on the transformer Medium Voltage side the 5th and 7th harmonics are considered. At the Low Voltage side it is assumed that the voltage RMS value is 2.5% above the nominal value.

Fig. 25. Topology of the simulated LV grid

The simulations are carried out assuming two different load scenarios:


Each one of these scenarios is tested:


It is assumed that the microgeneration total power never exceeds 25% of the transformer rated power SN.

Figure 26 presents the results obtained without μG, assuming that the transformer may be at 15 % or at 85 % of its rated power SN. The measurements of phase voltages and currents are carried out on the transformer LV side for each one of the groups of loads L1 to L6.

The performance of microgenerators can be compared in this virtual lab using the designed low voltage grid model with six clusters of loads (Fig. 25). It is assumed that 85% of these loads are non-linear and 15% are linear. Also, on the transformer Medium Voltage side the 5th and 7th harmonics are considered. At the Low Voltage side it is assumed that the voltage

50 m

125 m

200 m

L3

L1

80 kVA

72 kVA

68 kVA

64 kVA

60 kVA

56 kVA

L2

L4

L5

L6

275 m

350 m

425 m

**3. Modelling the Low Voltage grid** 

RMS value is 2.5% above the nominal value.

Fig. 25. Topology of the simulated LV grid

of values represented in Fig. 25). Each one of these scenarios is tested:

a. without μG;

rated power SN.

b. with conventional μG; c. with active μG.

The simulations are carried out assuming two different load scenarios:

assuming 15% of values represented in Fig. 25);

a. Distribution transformer at 15% of its nominal power (SN) (nearly no load scenario,

b. Distribution transformer at 85% of its nominal power (full load scenario, assuming 85%

It is assumed that the microgeneration total power never exceeds 25% of the transformer

Figure 26 presents the results obtained without μG, assuming that the transformer may be at 15 % or at 85 % of its rated power SN. The measurements of phase voltages and currents are

carried out on the transformer LV side for each one of the groups of loads L1 to L6.

30kV

MV 400kVA LV 30kV/400V

LV L1

L2

L3

L4

L5

Fig. 26 shows that the voltage THD increases more than 50% (as in load 6) from the no-load (15% SN) to the full load (85% SN) scenario. As the percentage of linear and non-linear loads is nearly equal for both scenarios, the current THD does not present significant changes (it even decreases slightly in the full load scenario). Also, the Power Factor results are similar for both scenarios, even though slightly lower for the no-load scenario. As for the load voltages RMS values, higher loads result in higher voltage drops. Also, as the transformer to load distance increases, the voltage drop increases as well.

Figure 27 presents the results obtained with μG assuming that the transformer is at 15 % of its rated power SN (no load scenario). The measurements of phase voltages and currents are carried out on the transformer LV side for each one of the groups of loads L1 to L6.

Design of a Virtual Lab to Evaluate and Mitigate

reduces voltage THD.

LV L1

LV L1

**4. Conclusions** 

similar.

power filter.

L2

L2 L3

Current THD; c) Power Factor

L4 L5

L3 L4

Power Quality Problems Introduced by Microgeneration 205

conventional microgeneration slightly increases voltage THD, while active microgeneration

Active μG - phase A Active μG - phase B Active μG - phase C Conventional μG - phase A Conventional μG - phase B Conventional μG - phase C

**Voltage THD Current THD** 

L5 L6 a) LV L1

**Power Factor Voltage RMS value** 

L6 c) LV L1

In this paper a virtual lab was designed to evaluate and mitigate some power quality problems introduced by μG. The virtual lab includes the Medium/Low voltage (MV/LV) transformer, the distribution lines, linear and non-linear loads, conventional μG and active μG. To validate the designed models, the current waveforms and distortion obtained for each one of the virtual lab loads were compared to those measured with the most used electric and electronic equipment, showing that the obtained results are

The μG model is simulated based on its final stage converter, a single phase inverter, while the active μG also includes high order harmonics compensation, to perform as an active

Fig. 28. Results obtained for 85% SN of conventional μG or active μG; a) Voltage THD; b)

L2 L3

L2

L3

L4

L5

L6 d)

L4 L5 L6 b)

**Power Factor Voltage RMS value** 

Fig. 27. Results obtained for 15% SN of conventional μG or active μG: a) Voltage THD; b) Current THD; c) Power Factor; d) Value of RMS voltage

From the results obtained for the first scenario (15% SN) (Fig. 27), the use of active μG guarantees a clear improvement of voltage and current THD, when compared to the conventional μG. Also, the use of active μG guarantees near unity power factor, even though it is negative. This results from the fact that the power flows from the microgenerators to the transformer, instead of flowing from the transformer to the loads.

Figure 28 presents the results obtained with μG assuming that the transformer is at 85 % of its rated power SN (full load scenario). The measurements of phase voltages and currents are carried out on the transformer LV side for each one of the groups of loads L1 to L6.

The results obtained for the full load scenario (85% SN) (Fig. 28) show the improvement introduced by active μG in voltage THD (Fig 28a), as well as current THD (Fig 28b) and power factor (Fig 28c). From Fig. 28 active microgeneration allows a voltage THD reduction up to 30%, when compared to conventional microgeneration. Also, comparing with the values obtained without microgeneration (Fig. 26) it is possible to conclude that

Active μG - phase A Active μG - phase B Active μG - phase C Conventional μG - phase A Conventional μG - phase B Conventional μG - phase C

**Voltage THD Current THD** 

a) LV L1

**Power Factor Voltage RMS value** 

L6 c) LV L1

From the results obtained for the first scenario (15% SN) (Fig. 27), the use of active μG guarantees a clear improvement of voltage and current THD, when compared to the conventional μG. Also, the use of active μG guarantees near unity power factor, even though it is negative. This results from the fact that the power flows from the microgenerators to the

Figure 28 presents the results obtained with μG assuming that the transformer is at 85 % of its rated power SN (full load scenario). The measurements of phase voltages and currents are

The results obtained for the full load scenario (85% SN) (Fig. 28) show the improvement introduced by active μG in voltage THD (Fig 28a), as well as current THD (Fig 28b) and power factor (Fig 28c). From Fig. 28 active microgeneration allows a voltage THD reduction up to 30%, when compared to conventional microgeneration. Also, comparing with the values obtained without microgeneration (Fig. 26) it is possible to conclude that

carried out on the transformer LV side for each one of the groups of loads L1 to L6.

Fig. 27. Results obtained for 15% SN of conventional μG or active μG: a) Voltage THD; b)

L2

L2 L3

L3

L4

L5

L4 L5

L6 b)

L6 d)

LV L1 L2

LV L1

L2

L3

L4

Current THD; c) Power Factor; d) Value of RMS voltage

L5

transformer, instead of flowing from the transformer to the loads.

L3 L4

L5

L6

conventional microgeneration slightly increases voltage THD, while active microgeneration reduces voltage THD.

**Voltage THD Current THD** 

L3 L4

**Power Factor Voltage RMS value** 

Fig. 28. Results obtained for 85% SN of conventional μG or active μG; a) Voltage THD; b) Current THD; c) Power Factor
