OPTIMIZATION OF DC PLANT TOPOLOGY USING AE 1000NX INVERTERS

Transcription

1 OPTIMIZATION OF DC PLANT TOPOLOGY USING AE NX INVERTERS The NX, a bipolar inverter, is specifically engineered to reduce wiring and labor costs by accepting DC inputs from a dual separable array. The DC architecture associated with the NX inverter, when implemented correctly, can save significant conductor material costs compared to a traditional monopolar design. In order to maximize material and labor savings, it is important Table of Contents Monopolar vs. Bipolar Architecture 2 Electrical Specifications: NX Input Specifications 3 Circuit Combiners and the Combiner PV Tie 4 String Sizing Considerations 5 to understand the differences in DC electrical design for a bipolar architecture compared with traditional monopolar architecture. This paper differentiates between bipolar and monopolar DC architectures and provides highlevel array diagrams to illustrate the differences. The paper also will walk through the inverter input specifications for the NX inverter and a typical monopolar inverter and discuss how to apply conductor sizing considering the circuit ampacity, length, and voltage drop. DC Conductor Size: Ampacity and Voltage Drop Calculations 7 Conductor Ampacity 7 DC Voltage Drop 7 Conclusion 8

2 Monopolar vs. Bipolar Architecture Traditional monopolar architecture has multiple arrays all with a single polarity, typically all less than VDC to ground. Each array is combined with a combiner box, and often re-combined using a re-combiner before terminating at the inverter. Overcurrent protection is installed at each a combiner box and re-combiner, and often where the conductors terminate at the inverter, as well. See Figure 1, below. Bipolar architecture utilizes a similar approach but has multiple arrays with opposite polarities. Half of the array is positive and operates between positive VDC to ground, and the other half of the array is negative and operates between negative VDC to ground. Similar to the monopolar architecture, bipolar architecture strings terminate in combiner boxes and also in re-combiners. String and re-combiners are designated as positively grounded or negatively grounded. Once they terminate inside the inverter, they are directly connected to a contactor that connects each bipolar array together when in operation and isolates them when not in operation. The NX inverter utilizes a combiner PV tie (CPT), a separate device, which moves the contactor from inside the inverter and places it within the array. Placing the CPT within the array results in a reduction of conductor home run length of close to 5%. In addition, there is a reduction in conductor size, and a reduction of combiner equipment. See Figure 2, below. Figure 1. Monopolar diagram Figure 2. Bipolar diagram 2

3 Electrical Specifications: NX Input Specifications The NX has terminals for ten input pairs, ten positive and ten negative. Each input pair has a circuit comprised of three components, two combiner boxes (one positively grounded, one negatively grounded) and a CPT between them. Each combiner box typically ranges from ten combined circuits to 24 in extreme cases. The CPT located between each pair of combiner boxes accepts the neutral input from each combiner box. The function of this device, when in operation, is to connect each pair of combiner boxes in series with each other and disconnect the ground, resulting in an array grounded through alternate means via the ground fault detector interrupter (GFDI) circuit. See Figure 3. When not in operation, it grounds the arrays individually through a 5 A fuse at the CPT and through a 3 A fuse at the inverter. See Figure 4. Positive and Negative in Series CB+ CPT Figure 3. Operational mode single array Positive Sub-Array Negative Sub-Array CB+ CPT CB- CB- Figure 4. Sleep mode two separate arrays 3

4 Circuit Combiners and the Combiner PV Tie Each input pair has a maximum rating of A Imp, and 3 A Isc. The A Imp limit is governed by the DC contactors. The actual rating of the contactor is higher than A, however; should there be a fault and the contactor needs to open under load, the contactor can reliably open. This should be treated as a target output circuit current and should not be exceeded. The inverter has been designed to accept a maximum of 3 A Isc, which should also be treated as a hard limit. Circuit currents that exceed this may cause damage to the inverter. The neutral conductors of each sub-array should not be fused in each combiner box, since each array is grounded when not in operation, and grounded by alternate means when in operation and referenced to ground through the ground fault detector interrupter (GFDI) circuit. The two figures below illustrate how the module current, Isc Imp, increases with Watt class and how this increasing effect dictates the maximum number of strings Isc Impp Linear (Isc) Linear (Impp) Current (Amps) Watt Class (W) Figure 5. Module current vs. module Watt class 25. Max Number Strings Amp Combined String Current Within Specifications Combined String Current Exceeds Specifications Imp Limit Module Imp Figure 6. Maximum string limits: combined Imp < A 4

5 String Sizing Considerations String length is a primary step in the DC electrical design. PV engineers must consider the electrical characteristics of the module, specifically the max power voltage (Vmp), open circuit voltage (Voc), annual weather extremes for the location, and the max power point tracking (MPPT) window of the inverter. See Figure 8, below. It is important to consider annual weather conditions for a proposed site, as the string voltages are influenced by the extreme temperature for a given location. The average high temperature impacts the minimum Vmp, and the extreme low or ASHRAE low temp impacts the maximum Voc. Locations that have fairly moderate temperatures year round will have a much narrower energy band, while locations that have large temperature swings will have a wide energy band. The scatter plot below (Figure 7) of DC input voltages has been created to illustrate this condition. Consider a climate like Puerto Rico vs. Central California vs. North Dakota. Pmp (kw) Mild Climate Low 16 C / High 37 C PV Plot Vmp (VDC) 18 Series PR 1 V Delta Pmp (kw) Moderate Climate Low -5 C / High 47 C PV Plot Vmp (VDC) 18 Series CA 15 V Delta Pmp (kw) Extreme Climate Low -2 C / High 46 C PV Plot Vmp (VDC) 18 Series ND 275 V Delta Figure 7. Energy band graphs: DC power as a function of operating voltage for different climates 5

6 As the number of modules in a string increase, the scatter plot shifts from left to right. If the string voltage is too low (too few modules in a string), certain points throughout the year may not fall within the MPPT window, and therefore could result in lower performance. In comparison, string voltages that are too high (too many modules in a string) will also fall outside this window on the high side, resulting in the same negative effect, potentially causing damage to the inverter. The NX inverter has a wide MPPT window between ±55 VDC and ± VDC. DC MPPT Window (c-si String Options) 21 Module String 2 Module String Voc (Temp adi) MPPT Maximum 19 Module String 18 Module String 17 Module String 16 Module String Voltage (V) MPPT Minimum Vmp (Temp adi) Low High Temperature Range Extreme Low Extreme High Temperature Range Module Temperature ( C) Figure 8. String length and MPPT graph 6

7 DC Conductor Size: Ampacity and Voltage Drop Calculations Once the string length and combiner box size have been selected, the output conductors can be sized accordingly, first for ampacity based on the circuit ampacity and appropriate over-current protection, and then for voltage drop. Conductor Ampacity The DC conductors are sized according to maximum circuit current, calculated from the Isc of the PV output circuits and adjusted 125% per 69.8(A)(1), extreme irradiance, and an additional 125% per 69.8(B)(1), continuous use on over-current protection device (OCPD). This circuit current adjustment together is 156%. The NX inverter is designed with current transducers (CTs) on each of the input pairs that monitor the output circuit currents at all times, and act as circuit isolation devices. If the system senses a reverse current greater than 1 A, it will stop switching, open the DC contactors, and engage the ground fault fuses. In a reverse current scenario, once the system separates the arrays and engages the grounds, the 3 A ground fault fuses at the inverter open and the inverter remains in a latching ground fault mode. The 1 A reverse current limit should be treated as a device that is 1% rated and therefore the exception in 69.8(B)(1)(a) is applied. By applying this exception, the maximum circuit current adjustment is reduced to 125%, versus a typical system with over-current protection rated at 8%. This reduction in ampacity adjustment often results in a smaller gauge conductor; however, since the CTs limit the reverse current to 1 A, the conductors must be sized with a minimum ampacity of 1 A, regardless of a lower circuit ampacity. DC Voltage Drop Once the conductor has been sized for the appropriate ampacity, the conductor must be checked for maximum allowable voltage drop using conductor properties, Table 8 of NEC, and the round trip length of the circuit. Voltage drop consideration is only important during an operating condition; therefore, Vmp and Imp are used in the calculation. Acceptable voltage drop between the combiner boxes and the inverter ranges between 1. and 1.5%. To calculate the voltage drop for a monopolar design, the total round trip length between each combiner box and the inverter is used. For a bipolar design, the same round trip circuit is used; however, the distance is roughly 5% of the monopolar design, as it captures two combiner boxes in the circuit, twice the number of combined strings. The operating voltage and current for each circuit are identical, assuming that the combiner boxes in each scenario has the same number of strings; however, since the round trip distance is roughly 5% in length, the percent voltage drop decreases proportionally. This lower-percent voltage drop typically results in decreased conductor size, often driving the conductor to be sized by ampacity calculation. Decreased sensitivity to voltage drop makes inverter placement and geographic constraints more manageable. Further savings in other areas of the plant can be recognized by enabling designs that were initially infeasible due to higher conductor costs. In addition, standard block sizes that were once limited to 1 MW or less can now be increased up to sizes ranging from 3 to 4 MW. The increased standard block sizes recognize savings, but after considerably higher DC distances, the savings can start to diminish due to the increased conductor size related to voltage drop. Figure 9, below, illustrates the savings in conductor length possible with a bipolar system. 1 Distance (Feet) CB1 & CB2 CB3 & CB4 CB5 & CB6 CB7 & CB8 CB9 & CB1 CB11 & CB12 Combiner Box Monopolar Bipolar Figure 9. Circuit Length: Monopolar vs. Bipolar 7

8 Conclusion This fundamental understanding of the inverter and bipolar DC architecture can result in significant savings when compared to a traditional monopolar design. The NX inverter and associated CPTs enable PV engineers to design the DC output circuits with roughly 5% of the conductor length and often a decrease in conductor size. The bipolar architecture also reduces constraints on inverter placement and plant shape and enables standard block sizes to increase. Advanced Energy Industries, Inc Sharp Point Drive Fort Collins, Colorado 8525 U.S.A. T: F: To view AE's complete product portfolio, visit: Specifications are subject to change without notice. 215 Advanced Energy Industries, Inc. All rights reserved. Advanced Energy is a U.S. trademark of Advanced Energy Industries, Inc. ENG-NXOptDCPlant-26-2 M 2.15