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

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Chapter 5. Unbalanced Systems This chapter addresses the fact that unbalanced conditions of small magnitude are a relatively common operation of the system and the microsources need to be able to operate in this environment. The reasons that determine unbalance are first given. An active approach at eliminating unbalance determines higher requirements in the ratings of the source. The microsource needs to operate in the unbalanced environment without attempting to restore balanced conditions. 5.1 Reasons that Determine Unbalance Microgrids need to operate under unbalanced conditions due to the fact that electrical systems naturally have some degree of unbalance. There are many reasons why the supply voltage may have unbalance, for instance the geometrical configuration of conductors in overhead transmission lines. The conductors on the towers are swapped every so many miles to minimize the effect of the asymmetric mutual coupling between the conductors and the ground, but nonetheless there is always some residual asymmetry left that determines unbalance. Another cause of unbalance is the insertion of three phase loads with unbalanced impedances: a delta load that draws unbalanced currents will determine positive as well as negative sequence components in the voltage across the system. An unsymmetrical placement of single phase loads between phases and neutral determines not only negative, but also zero sequence component due to the fact that the currents in the neutral do not sum up to zero anymore. Single phase loads across feeders are already distributed evenly on the three phases as much as possible to avoid problems of unbalance, but complete symmetry is nearly impossible to guarantee, more so in the face of the uncertainty of what loads are inserted and when. A distant fault outside the microgrid somewhere in the utility system is also responsible for unbalanced voltages at point of common coupling. Transformer connectivity can help to mitigate the effects of unbalance. Delta-wye transformers, with the delta side on the utility side are common practice in the distribution system. An advantage of this transformer is that it traps the zero sequence currents in the delta side allowing only the positive and negative to pass through. Another advantage is that a given voltage sag on the primary propagates through the windings of the transformer as a smaller voltage sag on the secondary. For instance, a single line to ground fault on the primary of a delta transformer would create a 100 percent sag on the faulted phase, but on the wye connected secondary of the transformer the voltage will sag at most at 33 percent on the line to line while sagging at most 58 percent on the line to neutral [7]. There are several effects to unbalance, for instance induction motors will have an induced torque component at twice the system frequency. This is because the negative sequence will generate a torque in the opposite direction of the one generated by the positive sequence. Unbalance has detrimental effects on the machine, for instance it increases the heat generated in the resistive coils of the rotor that sees a field rotating at a relative speed that is twice the rotor speed, inducing high currents. Also, there are mechanical stresses on the bearings due to the pulsating term in the torque. The voltage unbalance is defined in Appendix D of ANSI Standard C84.1-1989 [8] as: 64
% of voltage unbalance = 100 maximum deviation from average voltage average voltage This standard requires that electrical supply systems should be designed and operated to limit the maximum voltage unbalance to 3 percent when measured at the point of common coupling under no load conditions. ANSI states that approximately 98 percent of the utilities surveyed are within this limit, with 66 percent within 1 percent unbalance. To comply with the ANSI standard, National Electrical Manufacturers Association (NEMA) [9] requires that a motor needs to be derated by a factor of 0.9 to be able to withstand the currents at the negative sequence without exceeding the nameplate ratings. 5.2 Unbalance Correction When a voltage unbalance exceeds the nominal value that can be tolerated within a system as stated by the ANSI standard, some corrective action must be taken to prevent this unbalance from creating problems in the operation of the equipment inside the microgrid. One approach could be to inject sequence currents from the inverter to rebalance the system. The inverter can only generate voltages on a line to line basis, so the scope of the correction would only be limited to the negative sequence. The measures of voltages and currents that are passed to the control need to be conditioned to identify the positive and negative sequence components. Each of the components is then separately controlled: the positive sequence quantities are regulated to the externally requested values while the negative sequence quantities are regulated to zero. The structure of the control needs to be more complex and the ratings of the inverter need to be augmented to be able to carry the negative sequence currents. Another approach is to transfer to intentional islanding mode by separating the microgrid from the utility in the case that the unbalance exceeds the tolerable levels. This solution does not require any modifications in the control and the microsources only need to be able to correctly operate under unbalanced conditions as long as the level of unbalance is below a determined threshold. In this case the sensing equipment at the point of interconnection with the grid needs to be able to evaluate the unbalance and send a tripping signal to the static switch when limits are exceeded. This solution does not require any extra rating of the inverter to allow for the negative sequence current injection. 5.3 Operation Under Unbalance The inverter ratings need to be increased by a large factor to correct for sustained voltage unbalances. Avoiding the active cancellation of negative sequence implies that the static switch needs to be able to selectively disconnect the microgrid when unbalances exceed the tolerance level of 3 percent as recommended by the ANSI standard. This approach implies that the basic control of the microsource needs to be able to correctly operate in an environment that may contain unbalances up to that tolerance. Unbalances affect the basic control because the opposite rotating negative sequence introduces 120Hz oscillations in the measure of power and voltage magnitude. The measurement system assumes quantities with positive sequence component only: when unbalances are present some calculations are affected. For instance, the d-q stationary components will no longer be represented by two vectors of same magnitude and phase. When plotted on the d-q plane these components will not describe a circle but an ellipse. The 65
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PSERC Control and Design of Microgr
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Information about this project For
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Executive Summary Economic, technol
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Table of Contents Chapter 1. Introd
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List of Figures Figure 1.1 Microgri
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List of Figures (continued) Figure
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Chapter 1. Introduction Distributed
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1.2.2 Operation and Investment The
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Figure 1.1 Microgrid Architecture D
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Chapter 2. Static Switch The static
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ω ωo ω1 Preq Pgrid P Pload Figur
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2.3 Synchronization Conditions The
Page 25 and 26: droop. The issue is that it is poss
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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
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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
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Page 51 and 52: needs of power. This solution would
Page 53 and 54: This expression is very similar to
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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
Page 71 and 72: 4.4 System Ratings The inverter per
Page 73 and 74: Switching Sequence 145 146 136 236
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Page 79 and 80: Figure 5.2 shows the plots for P,Q
Page 81 and 82: Figure 5.5 shows the measures of P
Page 83 and 84: Figure 5.7 One Unit Connected to th
Page 85 and 86: Microsources are connected to the l
Page 87 and 88: The transformer impedance can be se
Page 89 and 90: o r 1 = 0.3277 Ω = 0.0547 Ω x x 1
Page 91 and 92: Table 6.3 Load Data Summary. Rated
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Page 101 and 102: Table 6.5 Lookup Table for Switchin
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Page 105 and 106: Table 7.1 Summary of Experiments fo
Page 107 and 108: Table 7.2 Series Configuration Cont
Page 109 and 110: Figure 7.6 Control of F1 and F2, Fe
Page 111 and 112: Figure 7.10 Control of P1 and F2, F
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Page 115 and 116: Unit 1: P, Q, F, Frequency, Voltage
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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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