Some utilities in US have already started implementing self– healing projects . It is to be reminded that following as- sumptions are required for the self–healing networks: • Sufficient DER capability in MG • Availability of communication interfaces among DERs, protection devices, circuit breakers and switches. III. N ETWOK AND M ICROGRID S TRUCTURE Let us consider a MV feeder connected to two MGs as shown in Fig. 1. MG–1 has 3 DERs and 5 loads while MG–2 has 2 DERs and 4 loads, connected to Low Voltage (LV) feeders. A Distribution Static Compensator (DSTATCOM) is installed at the secondary side of the distribution transformer in each MG to regulate the voltage at its Point of Common Coupling (PCC). The loads are assumed as residential loads and all DERs are assumed to be converter–interfaced DERs. During grid connected mode, all DERs will be operating in their rated conditions or based on the outcomes for economic analyses. In autonomous mode, when CB M1 and CB M2 are open, the DERs inside each MG will be sharing the loads within each MG separately. In autonomous mode, if the pow- er demand in each MG is higher than the power generation capacity of the DERs in that MG, load shedding must be ap- plied to some of the (non–critical) loads in that MG. Now, let us assume that a fault occurs on the upstream of CB G . Based on the network protection coordination, CB G , CB M1 and CB M2 open under such conditions. Now, each MG can work in autonomous mode. However, let us assume a situation where the power generation in MG–2 is less than its power demand; while the power generation capacity in MG–1 is higher than its power demand. Now, let us assume the net- work has self– healing capability and CB M1 and CB M2 close the circuit while CB G remains open. In this way, the two MGs are now connected together. Hence, the DERs in MG–1 can share some of the loads in MG–2 and prevent/reduce load shedding in MG–2. This paper focuses on the power sharing and voltage regulation issues of the network in these situa- tions. The protection devices, their coordination, communica- tion and resynchronization of interconnecting MGs are be- yond the scope of this paper. 3.1 Power Sharing in Microgrid In this section, the power sharing method within the MG is discussed. For a converter–interfaced DER, with the structure shown in Fig. 2(a), the instantaneous active and reactive power flows from converter output to its PCC are ( ) ( ) ( ) T T cf cf T T T cf T cf T V V L V q L V V p − − = − × = δ δ ω δ δ ω cos sin (1) where V T = | V T | ∠ δ T and V cf = | V cf | ∠ δ cf are respectively the RMS value of PCC voltage and AC filter capacitor voltage of Fig. 1. Schematic diagram of the network under consideration. Fig. 2. (a) Schematic diagram of VSC for DERs and DSTATCOMs, (b) Single–phase equivalent circuit of VSC.
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- Fall '10