This section describes the analysis of the performance of the centralised control on the scaled-down system under different test cases. These are evaluated through KPIs to quantify the influence of the considered control assets in high-RES active distribution networks.
4.3. Experimental Results
The objective function proposed for the operation of high-RES active distribution networks is based on an operation with minimal technical losses. This section describes the evaluation of the previously described test cases, which involved the analysis of the following electrical magnitudes: power losses, nodal voltages, and current circulating at the primary substation transformer. In addition, the previously defined KPIs allowed the key magnitudes to be quantified in a comprehensive manner to assess the performance of the proposed control.
for the studied test cases and the loss reduction with respect to the base case, C1,
, when the load and generation daily profiles presented in Section 3
were implemented into the testing platform. In the laboratory testbed, the 24-h profiles were scaled to the last 48 min and the duration of the tests was reduced.
C1 presented the greatest daily power losses as no control assets were operating to act on the voltages and power flows to reduce the system losses. The introduction of the OLTC operation in C2 reduced energy losses by almost 5%. The OLTC setpoint was computed in the OPF whose objective function was to reduce the total power losses in the network. Therefore, the tap was established in the −5% position to increase the nodal voltages and to achieve the intended objective.
In test case C3, the RES reactive power capability was also included in the control. This caused the daily energy losses to be reduced by more than 15% with respect to C1. This occurred because the RES were able to provide reactive power to the system. Figure 6
shows the RES reactive power injected at nodes N3 and N8 with respect to their rated power levels for test cases C3 and C4. This is represented using violin plots which allow the distribution of any magnitude as well as its range of variation and frequency of occurrence to be visualized. Note that most of the time, which corresponds to the wider part of the violin plot, the RES were injecting reactive power corresponding to 20% of their rated power levels. This high RES reactive power injection was used to provide part of the reactive power demanded by the loads, thus avoiding the need to supply it from the primary substation, as shown in Figure 7
. Note that the reactive power supplied from the primary substation in C3 was lower than 0.05 pu during the 24-h period, helping to reduce the energy losses.
The DC link integration in C4 further reduced the energy losses by up to 25% with respect to C1, as shown in Table 3
. This device injected reactive power at the interconnected nodes N8 and N14 by means of VSC1 and VSC2 respectively during the 24-h period, as depicted in Figure 8
. This power, added to the RES reactive power, led to almost zero reactive power being supplied from the primary substation, as shown in Figure 7
. In this way, the energy losses reduced with respect to C3. An additional effect on the RES reactive power injections was observed. In C4, the RES did not to have to inject as much reactive power as in C3, as can be observed in Figure 6
, even becoming zero in some nodes, like N8. This effect was quantified in a global manner with
collected in Table 3
, where lower values for this KPI in C4 with respect to those in C3 can be appreciated. Table 4
summarises the rated power and the reactive power injections of the RES units in C3 and C4. The second and third columns indicate the rated power of the RES used in the scaled-down system and the MV system respectively. The two last columns depict the maximum reactive power injected by the VSCs interfacing the RES units during the day in cases C3 and C4. These values refer to the rated power of each device. The RES connected to N5 injected the maximum amount of reactive power, reaching 31.45% of its rated power. With the current technology, these reactive power values are easily reachable due to the combined effect of two actions: (i) the VSC coupling reactance is becoming smaller by using LCL filters, and (ii) the VSC DC voltage is continuously increasing. This extends the VSC reactive power range.
Notice that the DC link also controlled the active power transferred from subsystem 1 to subsystem 2, as shown in Figure 8
. Outside the period of high injection of RES active power (0–10 h and 13–0 h), the DC link absorbed active power from N14 and injected it into N8. This meant that part of the load from subsystem 1 was powered by subsystem 2 which is less loaded and has shorter branches, helping to reduce the total power losses of the system. Conversely, within the hours of high RES active power injection, the active power flow was inverted in the DC link: VSC1 absorbed active power from subsystem 1 and it was injected by the VSC2 into subsystem 2. In this way, part of the power generated by RES in subsystem 1 was transferred to feed the loads in subsystem 2. Therefore, this active power was not supplied by the primary substation, thus reducing the current in this system and the energy losses.
Finally, note that the DC-link loading, , during the day was 49.4%. This means that the DC link was used at half load and there is therefore still a wide margin to take advantage of its flexibility of operation. For example, the RES penetration in subsystem 1 could increase and still be managed by the current DC link.
shows the 24-h nodal voltages at nodes N3, N6, N8, and N14 for the different test cases. These buses were selected to represent the behaviour of nodes nearby (N3) and far from (N6) the primary substation. In addition, nodes N8 and N14 were also included because they are the connection points of the DC link. The analysis of Figure 9
reveals that undervoltage situations—voltages below 0.95 pu—exclusively occurred in the base case, C1, due to the lack of control assets operating in the network. This situation led to a very high
value in C1, as shown in Table 3
. These voltage violations were more severe at nodes N6 and N8 corresponding to subsystem 1 because of two reasons. First, subsystem 1 was more loaded than subsystem 2, as depicted in Figure 3
, especially during the hours without RES generation. This caused greater current flows and, consequently, greater voltage drops along the lines. This effect was especially significant around 8 and 20 h when the RES generation was almost zero and the demand was peaking.
The introduction of the OLTC in C2 pushed the voltages within the ±0.05 pu regulatory band around the rated voltage and, consequently, voltage violations were eliminated, as illustrated by its
. In C2, the tap was established in the −5% position for most of the day. However, according to the information provided in Table 3
, two OLTC operations
(from −5% to 0% position) over the 24-h period were required to maintain the voltages within the limits. These changes occurred at around 11 h and 13 h when RES generation was maximum, as shown in Figure 3
, and the network voltages were excessively high. The range of variation of nodal voltages
was significantly reduced with respect to C1, as shown in Table 3
. This effect can also be observed in Figure 9
where the violin plots are shortened, concentrating the nodal voltages within a narrower band. This trend was maintained in C3 due to the contribution of RES to the regulation of voltage with reactive power injections. In addition, it can be seen that the average voltage of nodes N3, N6, and N8 from subsystem 1 increased due to the local effect of the reactive power injections. As a consequence, additional OLTC changes
(from −5% to 0% position) were required to maintain the voltages within the technical limits. This longer time of the tap within the 0% position caused lower voltages within subsystem 2, as can be observed for the node N14 in Figure 9
C4 incorporated the operation of the DC link between nodes N8 and N14 allowing the injection of additional reactive power into these nodes and active power transfer between both subsystems. This led to a minimum range of variation in the nodal voltages and maximum values of these in all the test cases. In fact, in C4, the voltages oscillated in a range between 1 and 1.05 pu over the 24-h period.
shows the daily evolution of the current circulating through the primary substation transformer for the studied test cases. This current reduced as the number of control assets increased. The analysis of C4 revealed that during some periods, the current was almost zero. This means that the generation of RES with adequate management by the control assets is enough to operate the system without the need of supplementary power from the primary substation. Finally, it is worth noting that the state of load of the transformer
also progressively reduced in the subsequent test cases, as shown in Table 3
. As a consequence, the benefits for the distribution system are clear in this respect: reduction of transformer losses, increment of useful life, and increase of the system loadability, which allows new investment in power assets to be deferred.