Experimental Assessment of a Centralised Controller for High-RES Active Distribution Networks
2. Proposed Centralised Control
- RES, which can regulate the reactive power injections and .
- Transformer OLTCs, which can adjust the tap position .
- A DC link, which is composed of two Voltage Source Converters (VSCs) in a back-to-back topology connecting two radial feeders. This device can regulate the active power flow between the feeders, , and two independent reactive power injections, . It is important to point out that the DC link is an interesting control asset with proven capability to reduce the network active power losses, maximize the penetration of RES, improve the network voltage profiles, and avoid branch saturations [20,26].
3. Laboratory Testing Platform
- First, this network is based on an actual MV German distribution system, fulfilling the proposed objective of the laboratory testing platform described above.
- Second, an important RES penetration is integrated into the network.
- Third, all the network data, including topology, parameters of lines and cables, loads, RES, and their corresponding daily load/generation curves are available and are well documented.
- Fourth, the benchmark network includes a DC link, a key component of the future active distribution system with high RES penetration.
3.1. MV Benchmark Distribution Network
3.2. Laboratory Scaled-Down Distribution Network
- Distribution network branches: The electrical lines of both scaled-down subsystems are represented by a lumped parameter model comprising the series resistor and reactor. The per unit values of these impedances are identical to those of the actual MV system. Therefore, the original line R/X ratios and equivalent lengths are maintained, leading to similar per unit voltage drops and power losses. Table 1 collects the exact values of the resistors and reactors used in the scaled-down network.
- Omnimode Load Emulators (OLEs): These are the building blocks that are responsible for representing any load, generator, or a combination of the two connected to any network node. Basically, each OLE is a VSC with a local controller (LC) whose AC and DC sides are connected to a scaled-down network node and a common DC bus, respectively, as shown in Figure 4. The VSC is a three-phase, three-wire, two-level insulated gate bipolar transistor (IGBT) VSC, rated at 400 V, 20 kVA with a switching frequency of 10 kHz. LCL coupling filters are used to connect the AC-side of the VSC to the scaled-down network. The inductors and the capacitor have the following ratings: L1 = L2 = 2.5 mH and C = 1 F. Note that all of the OLEs share a common DC bus which is regulated by an extra balancing VSC rated to 100 kVA. This is directly connected to the LV laboratory network by its AC side, providing the net active power required by OLEs: . In this way, each OLE may absorb/inject (load/generator) any active power into the AC scaled-down distribution system within the technical constraints imposed by the VSCs. The OLEs are connected to the following nodes: N3, N5, N6, N7, N8, N9, and N10 (subsytem 1), and N14 (subsystem 2). The active and reactive power references to the OLEs are set by a Signal Management System (SMS) which is detailed in the next subsection.
- Transformer with OLTC: The underlying idea of this feature is to represent the HV/MV transformers within the primary substations which are equipped with OLTCs to regulate the MV voltage. The transformer used for this purpose is a 400 V ± 5%/400 V, 100 kVA equipped with a thyristor-based tap changer, as shown in Figure 4.
- DC link: This DC link, originally included in the benchmark distribution system , is incorporated between N8 and N14 as a suitable device to maximise the RES penetration, as stated previously. Although several topologies can be used to create a flexible loop between radially operated feeders , the DC link is based on conventional back-to-back VSCs rated at 400 V and 10 kVA. Note that the DC bus of the DC link is totally independent of the one shared by the OLEs and the balancing VSC.
3.3. Control Scheme and Communication System
- Offline tasks: They are carried out by a host PC and mainly consist of the configuration of the setpoint profiles. The OLE active and reactive daily power curves () are defined through two tools developed in the host PC . Once these profiles have been determined, the daily setpoints of the DC link, and , the reactive power injected by the RES, , and the optimal OLTC tap position, , are automatically computed by the OPF described in Section 2. These setpoints and their computations are new features that are incorporated into the host PC with respect to . Finally, all these data are compiled and uploaded to the Real-Time Control System (RTCS) for real-time operation.
- Online tasks: These are executed by the RTCS which is responsible for two undertakings. On the one hand, the RTCS is in charge of sending the setpoints to the second control level composed of the LCs attached to each hardware controllable component during the online operation according to the profiles previously determined in the offline tasks. On the other hand, the RTCS receives measurements from each each LC attached to the OLEs (, and ), DC-link VSCs (, and ) and the tap position of the transformer OLTC (). After processing this information, it provides real-time monitoring of the system which is displayed in the host PC.
4. Experimental Assessment of the Proposed Centralised Control
4.1. Definitions of Test Cases
4.2. Definitions of KPIs
- Daily energy loss (/): This KPI measures the daily active energy loss in kWh/day, , and the percentage of loss reduction with respect to the base case, C1, .
- Voltage violation (): This KPI evaluates the percentage of time during the day that which the nodal voltages are outside the technical limits [0.95–1.05 pu].
- Variation of nodal voltages (): This index provides a global measurement of the daily voltage variations at the nodes of the network. It is computed as the average value of the difference between the maximum and minimum nodal voltages, measured in pu,
- OLTC operation (): This KPI shows the number of OLTC operations that occur during the 24-h testing period.
- RES reactive power injection (): This index provides a global measurement of the RES collaboration to the network reactive power support. It is computed by dividing the average value of the reactive power injected by the RES during the 24-h period by the total number of RES,
- DC link load (): This evaluates the daily average load of the DC link during the day, and it is computed as
- Transformer load (): This represents the daily average load of the transformer as a percentage of its rated power, which can be computed as
4.3. Experimental Results
Conflicts of Interest
|ADMS||Advanced Distribution Management System|
|CIGRE||International Council on Large Electric Systems|
|DSO||Distribution System Operator|
|IGBT||Insulated Gate Bipolar Transistor|
|KPI||Keys Performance Index|
|OLE||Omnimode Load Emulator|
|OPF||Optimal Power Flow|
|OLTC||On-Load Tap Changer|
|RES||Renewable Energy Sources|
|RTCS:||Real-Time Control System|
|RTU||Remote Terminal Unit|
|SMS||Signal Management System|
|TSO||Transmission System Operator|
|VSC||Voltage Source Converter|
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|Initial Node||End Node||Resistance (m)||Reactance (m)|
|RES reactive power||•||•|
|RES Connected to Bus||(kVA) Scaled down System||(MVA) MV System||(pu)||(pu)|
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García-López, F.d.P.; Barragán-Villarejo, M.; Marano-Marcolini, A.; Maza-Ortega, J.M.; Martínez-Ramos, J.L. Experimental Assessment of a Centralised Controller for High-RES Active Distribution Networks. Energies 2018, 11, 3364. https://doi.org/10.3390/en11123364
García-López FdP, Barragán-Villarejo M, Marano-Marcolini A, Maza-Ortega JM, Martínez-Ramos JL. Experimental Assessment of a Centralised Controller for High-RES Active Distribution Networks. Energies. 2018; 11(12):3364. https://doi.org/10.3390/en11123364Chicago/Turabian Style
García-López, Francisco de Paula, Manuel Barragán-Villarejo, Alejandro Marano-Marcolini, José María Maza-Ortega, and José Luis Martínez-Ramos. 2018. "Experimental Assessment of a Centralised Controller for High-RES Active Distribution Networks" Energies 11, no. 12: 3364. https://doi.org/10.3390/en11123364