• Fig 36 shows the diagram in which the DG power generation is calculated using its current and adjusted in order to maintain a constant E . • Here, the DG current I and the utility feeder voltage E are measured and fed back to the LC control loop.
90 • The steady state P ω characteristics for two DGs are shown in Fig 37, which allow power to change between P=0 and P=Pmax as ω changes. • If power is imported from the grid before islanding, the frequency in island mode, ω imp will drop to be smaller than ω n . Accordingly, both DG1 and DG2 operating points will shift downward on droops . • In this case, DG2 reaches its maximum power and the steady state characteristic slope switches to vertical, and the operating point moves downward vertically (right side of the figure). Regulating the DG Power in Island Fig 37. P Z characteristics for two DG units
91 • Fig 38 shows the revised version, if Pmax was not enforced, in which the two DGs settle in ω1 (shown with circles). Here, P 1 (at ω 1 ) = P1_1 P 2 (at ω 1 ) = P max + Δ P max • With Pmax enforced, the frequency is shifted to ω2 as P1 takes on the additional load so that the final operating point (shown in squares) would satisfy the limits: P 1 (at ω 2 ) = P1_1 + Δ P max P 2 (at ω 2 ) = P max Regulating the DG Power in Island Fig 38. Island mode operation with a limiting Pmax for output power
92 • To prevent a power injection that exceeds the maximum value, we generate a power error errP max with the reference set to P max , as shown in Fig 39. • This error is passed on to an integral block that has a dynamic limiter which prevents its output to ever become positive. • As long as power is less than P max , errP max is positive and the output of the integral is dynamically limited to zero. • If power is larger than P max , errP max is negative and the integral would start generating a negative value for the offset. It will keep on translating down until the offset exactly matches the value for which errP max = 0 Regulating the DG Power in Island Fig 39. Offset generation to limit max power with integral block
93 • The opposite consideration in Fig 37 takes place with power initially exported to the grid • Now the attention in this case is to find the changes that need to be done when zero power limit is exceeded • The operation points are compared in Fig.40. Regulating the DG Power in Island Fig 40. Limiting P min =0 on output power control Without P min considered, the DGs operate at ω 1 (the dots): P 1 (at ω 1 ) = P min – ΔP min P 2 (at ω 1 ) = P2_1 With P min considered, the DGs operate at ω 2 (the squares): P1(at ω 2 ) = P min (=0) P2(at ω 2 ) = P2_1 ΔP min
94 • Fig 41 shows that the first step is to calculate errP min , using the minimum power setpoint as zero.
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- Trigraph, Electric power, Electricity distribution