file will reduce the active power generation capacity of the converter. This will not be desired by their owners. Alternatively, voltage regulation in a MG can be achieved using a DSTATCOM . DSTATCOMs have the same converter structure as the DERs. The DSTATCOM can be connected at the secondary side of the distribution transform- er to regulate its PCC voltage to a desired value. This can be carried out if a DSTATCOM exchanges reactive power with the network. The reactive power exchange can be controlled using the difference between the PCC RMS voltage and a desired value ( V T–DSTAT,desired ). This difference is utilized to generate the required voltage magnitude across AC filter ca- pacitor in DSTATCOM as ( ) DSTAT T desired DSTAT T I P ref DSTAT cf DSTAT cf V V s K K V V − − − − − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + + = , , (12) where V cf–DSTAT,ref is the assumed reference value for this voltage, K P and K I are PI controller parameters and the suffix DSTAT represents DSTATCOM. On the other hand, the DC capacitor voltage ( V dc ) in DERs is fixed by their DC sources; however, there is no such a DC source in DSTATCOMs. V dc in DSTATCOM can be kept
equal to its reference value ( V dc,ref ) when the AC system does not exchange any power with the DC capacitor . This can be reassured if the AC system replenishes the converter loss- es. For this, the angle of the voltage across the AC filter ca- pacitor ( δ cf ) must be varied versus the DC capacitor voltage variations as ( ) DSTAT dc ref DSTAT dc I P ref DSTAT cf V V s K K − − − − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ′ + ′ = , , δ (13) IV. S TUDY C ASE AND S IMULATION R ESULTS For investigating the performance of a network, with self– healing capability, which includes several MGs different simulation cases are considered. The network performance is investigated in various operating conditions, with different control algorithms for DERs and DSTATCOMs. Some of the simulation cases are discussed below while the network pa- rameters are given in the Appendix. 4.1. Case 1: Grid–Connected and Autonomous Mode Let us consider the simple structure of Fig. 1 to investigate the MG operation during grid–connected and autonomous modes. In grid–connected mode, each DER will generate its rated power and the extra load demand will be supplied by the grid or the extra generation will flow back into the grid. In autonomous mode, total power demand is shared among the DERs proportional to their rating. The operation of only one MG will be investigated in this section. For this, let us assume in the system of Fig. 1, CB G and CB M1 are closed while CB M2 , CB S1 and CB S2 are open. The system is assumed to be in steady state condition at t = 0 s and all the DERs are running in their rated condition. First, let us assume a 2–parameter control mode is de- ployed in DERs when the MG is in grid–connected operation which changes to 3–parameter control mode during autono- mous operation. At t = 1 s, the grid is disconnected (i.e. CB G is opened) and MG–1 will work in autonomous mode. There- fore, the DERs have to increase their output power to satisfy the load demand within the MG. At t = 2 s a load increase of
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