Control and Design of Microgrid Components - Power Systems ...

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The direction of flow of the resulting current will be determined by the relative placement of these two voltages at the instant of closing. I_1 Z = R + j X E (grid) Z*I_1 I_2 Z*I_2 V (MG) E V V E Figure 2.1 Current Direction as the Switch is Closed. Figure 2.1 shows that with the convention of the angles growing in the anticlockwise direction, if E is ahead of V, then the resulting current I_1 will go towards the microgrid. Conversely, if V is ahead of E, then the resulting current I_2 will go towards the grid. The condition to be enforced is that when the switch is closed the resulting current must be flowing towards the microgrid. 2.2 Direction of Current at Steady State This section proves that the in steady state the active power flows always from the source that has higher frequency towards the other that has lower frequency before the connection takes place. The reason why the previous statement is true has to be sought into the power versus frequency droop. The case of one microsource connected to the grid is examined. The droop characteristic is reported in Figure 2.2 showing the requested power that is injected during the connection to grid, when operating at system frequency as well as the power injected during island mode, at a reduced frequency. In this chapter no frequency restoration during island mode is assumed, which implies that the frequency will stay at the lower value during the whole time the system operates disconnected from the grid. During island mode the power injected is also the power taken from the load, since no other sources are in the system. Notice that the power that the grid injects is the difference from the load power and what the unit injects. Obviously, the requested power must be smaller or equal than the load demand because if not power would be injected from the microgrid into the utility system. 8
ω ωo ω1 Preq Pgrid P Pload Figure 2.2 Power vs. Frequency Droop. In island mode the microgrid is operating at a lower frequency, ω1, than the utility system. The microsource injects the full quota of power to provide the load and the grid injects zero power since it is disconnected. After reconnection the microsource injects power according to the requested command, while the grid injects the remaining quota to meet the load demand. So, it is apparent that after synchronization, in steady state the power flows from the source that had high frequency (grid) during the island mode towards the source that had lower frequency (microgrid). It is less obvious to understand why the same principle rules the behavior of the steady state power when two microsources are connected while in island. Figure 2.3 shows the case when two microsources are connected in island. Each microsource was operating in island from the grid on beforehand, and after connection the two sources are interconnected, but still, in island from the utility system. ω ωo ω1a Ad Ac Bc ΔP ω1 ΔP Bd ω1b P Figure 2.3 Island Connection of Two Microsources, A has Higher Frequency than B. This is the situation that can be encountered during a reconnection procedure of microgrid sections after they all have been disconnected from each other and the grid because of a nearby 9
Page 1 and 2: PSERC Control and Design of Microgr
Page 3 and 4: Information about this project For
Page 5 and 6: Executive Summary Economic, technol
Page 7 and 8: Table of Contents Chapter 1. Introd
Page 9 and 10: List of Figures Figure 1.1 Microgri
Page 11 and 12: List of Figures (continued) Figure
Page 13 and 14: Chapter 1. Introduction Distributed
Page 15 and 16: 1.2.2 Operation and Investment The
Page 17 and 18: Figure 1.1 Microgrid Architecture D
Page 19: Chapter 2. Static Switch The static
Page 23 and 24: 2.3 Synchronization Conditions The
Page 25 and 26: droop. The issue is that it is poss
Page 27 and 28: This synchronizing behavior is unac
Page 29 and 30: Figure 2.12 shows on the upper plot
Page 31 and 32: Chapter 3. Microsource Details This
Page 33 and 34: with different size. This control i
Page 35 and 36: u + _ K F ω o + _ ω o s P u ω o
Page 37 and 38: Q versus E Droop E req Q m Q _ + E
Page 39 and 40: currents from the microsources incr
Page 41 and 42: inverter ratings limits (in kVA) ca
Page 43 and 44: A 16 DG B 22 DG Static Switch C 8 D
Page 45 and 46: ω Unit 2 Max Unit 2 ω o Unit 1 ω
Page 47 and 48: Unit 1 Min Unit 2 ω Unit 1 ΔPmin
Page 49 and 50: positive value. When this value has
Page 51 and 52: needs of power. This solution would
Page 53 and 54: This expression is very similar to
Page 55 and 56: Li-Pmax < Fi < Li for unit ‘i’
Page 57 and 58: ω ω ω o Fo ω o Fo F F Unit Powe
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Page 69 and 70: Figure 4.8 P and Q Plane Capability
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4.4 System Ratings The inverter per
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Switching Sequence 145 146 136 236
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generated by the inverter: the VA r
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% of voltage unbalance = 100 maximu
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Figure 5.2 shows the plots for P,Q
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Figure 5.5 shows the measures of P
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Figure 5.7 One Unit Connected to th
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Microsources are connected to the l
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The transformer impedance can be se
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o r 1 = 0.3277 Ω = 0.0547 Ω x x 1
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Table 6.3 Load Data Summary. Rated
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The worse case scenario is represen
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The filtering is achieved by a low
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works with an internal voltage leve
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terminals to achieve each of the th
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Table 6.5 Lookup Table for Switchin
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Chapter 7. Microgrid Tests There ar
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Table 7.1 Summary of Experiments fo
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Table 7.2 Series Configuration Cont
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Figure 7.6 Control of F1 and F2, Fe
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Figure 7.10 Control of P1 and F2, F
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pu, but when it is fed from a remot
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Unit 1: P, Q, F, Frequency, Voltage
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7.4.1 Unit 1 (F), Unit 2 (F), Impor
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Import From Grid, Setpoints are 10%
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Island, Setpoints are 10% and 90% o
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7.4.2 Unit 1 (F), Unit 2 (F), Expor
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Export to Grid, Setpoints are 10% a
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7.4.3 Unit 1 (F), Unit 2 (P), Impor
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7.4.4 Unit 1 (F), Unit 2 (P), Expor
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Chapter 8. Conclusions This work sh
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[14] Lasseter, R.,” Microgrids,
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