In our NEW series, we unpack the importance of solar system design, and what it involves. In part one, we looked at Mechanical Design, outlining the key design attributes, and their relationships with performance modelling calculations.
In part two, we give you a basic understanding of the electrical portion of solar system design…
Understanding Electrical Design
The electrical design of the array determines the electrical behaviour of the array: the powers and voltages that the system will be running at. This can have a major impact on the system's energy production.
1. String Size and Voltage
The number of modules in series (known as the “string size”) will determine the voltage of the strings. Voltages add in series – so the longer the string, the higher the string voltage. Any inverter has an input voltage range for the DC voltages that it can accept from the array. As long as the string voltages are within that inverter’s operating voltage range, the inverter will operate normally.
Beware of thinking just in terms of STC: the string voltage will drift up and down depending on the temperature of the modules. The module’s temperature coefficient will indicate exactly how the values for Vmp and Voc will move based on the module temperature. Note that the value for the temperature coefficient is always negative: higher temperatures will reduce voltages, while lower temperatures will raise voltages.
If a design results in under-voltage or over-voltage at the inverter, this can result in production losses, sometimes significant. If the array is moderately outside of the inverter’s voltage range (typically on the low end), then the inverter will peg at its minimum voltage, and will end up running the modules at a higher voltage than their MPP voltage. So this is technically mismatch loss, since the modules are not running at their peak power. If the string voltage is further outside of the inverter’s operating range, then the string may collapse completely, and will not be able to deliver any power at all at the inverter’s minimum voltage.
Codes often mandate a maximum system voltage, and so the string voltage must always be below this value, even on the coldest day. Values for max system voltage are typically 600V, 1000V, or potentially higher – but consult with your local inspector for guidance here.
2. Conductor and Combiner Design
Each string of modules must be connected to their corresponding inverter by a set of wires, and often a combiner box. As a result, each module string has a specific conductor path from the modules to the inverter connection. Based on the wire used, and the distance that must be covered by that wire, each string home run will have a specific resistance value.
Wire losses are based on the standard equation (I2R), and are modelled based on the specific resistance of each home run, and the hourly current of the module string. The conductors will also lead to voltage drop between the modules and the inverter: the modules will have to run at a slightly higher voltage than the inverter’s draw voltage. In extreme cases, this can potentially cause under-voltage problems at the inverter, and/or parallel mismatch between strings.
3. Inverter Load Ratio
A solar array’s Inverter Load Ratio (ILR) is the ratio between the DC nameplate power (defined as the sum of the module DC power at STC) and the AC power (defined as the inverter maximum AC power production).
ILR values are usually greater than 1.0 (meaning that the DC system power is greater than the AC power), often between 1.1-1.3, and sometimes higher. This is because modules rarely generate at their nameplate power level, mainly because the sun is rarely at full strength, and when it is, it is typically hotter than 25 degrees C. This is why the Normal Operating Conditions (NOCT) values are used. Additionally, there are resistive losses between the surface of the module and the inverter – specifically, wire losses, mismatch losses, and converter losses if there are any DC-DC electronics installed in the array. So in order to have the DC system and AC system appropriately matched, the design will call for the DC power to be 10-20% higher than the AC power.
More recently, higher Inverter Load Ratios have begun to increase in popularity, with some designed values approaching or surpassing 1.5. These designs will clip power at the peak of the day, but will result in more generation at the “shoulders” of the day (late morning and late afternoon). These designs are generally driven by very inexpensive modules, systems with limits to their AC power rating, or time-of-use pricing regimes.
4. Inverter Choice
The inverter choice also has a major impact on a system’s performance. An inverter’s efficiency losses (typically 2-5%) are one of the larger efficiency losses in the system. Additionally, the DC input voltage window for the inverter will determine the potential string sizes that can be designed. Finally, the size and location of the inverter will determine the distances between the modules/combiner boxes and the inverter – and will therefore determine the wire content and wiring losses.
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