This is the first in what should be a series of blog posts on photovoltaic (PV) simulators, also known as solar array simulators (SAS). In this introductory blog post, the topics covered will be:
- Why you would choose a PV Simulator for your application
- How a PV simulator differs from a standard DC Power Supply
- What solutions Keysight offers for testing solar products
There will be future blog posts and application notes that will go more in depth on PV simulators including how they generate their output and how to program them. Be on the lookout for those!
Why Choose a PV Simulator?
The first topic that we are going to discuss is why you would want to use a PV Simulator instead of an actual PV Array. The short answer is: a PV Simulator is a whole lot more practical than a PV Array.
The longer answer is that a PV Array will be large, very expensive, and the output power is uncontrollable in that the output power will depend on variable environmental conditions such as temperature and sun exposure (also known as irradiance). To put the size into perspective, a 15 Kw PV array would contain 50 300 W solar panels (300 W is a common size for a solar panel). This would take up almost 1000 ft2!
A PV simulator is a much more flexible solution: it is much more efficiently sized, less expensive, has a programmable output, and it is backed by a warranty in case anything happens to it. You can program the output to simulate any weather condition that you’d like to test. For instance, you can program it to output a characteristic that simulates 50% cloud coverage over your array. If you were using your actual PV array you would need to wait until the weather conditions were perfect to do this. A PV Simulator also is made to be a test instrument. In the case of the Keysight PV Simulator: it is efficiently sized to be installed into a standard test rack (up to 15 kW in 3 rack units) and it has a full suite of SCPI (Standard Commands for Programmable Instruments) to control and measure its output.
What makes a PV Simulator different from a standard DC power supply?
The second thing that we want to discuss is why would you use a PV simulator instead of a standard DC power supply. The short answer for this topic is that a PV simulator’s output is a bit different than the output of a standard power supply.
A standard DC power supply typically comes with one of two output characteristics: rectangular or auto-ranging.
Figure 1 - Rectangular Characteristic Figure 2 - Autoranging Characteristic
A rectangular output characteristic is what you would see on most standard power supplies. There is a rated voltage (VRATED) and a rated current (IRATED). This is illustrated in Figure 1. You calculate the maximum power (PMAX) by multiplying VRATED and IRATED. The power supply can output any voltage and current combination as long as it is within the specified limits. In Figure 1, that would be anywhere under the rectangle formed by the limits (hence the name).
Less commonly, there are DC power supplies with an autoranging output characteristic. The difference is that the limits are determined by a PMAX that is not the product of VRATED and IRATED. It is probably easiest to look at an example. Let’s take a look at the Keysight N6752A DC power supply module. It is rated for 50 V (V1 in fig. 2) and 10 A (I2). If PMAX was equal to VRATED multiplied by IRATED, this would make the N6752A a 500 W power supply but it is rated for 100 W. The N6752A is capable of outputting any voltage or current combination equal to 100 W from 50 V, 2 A (V1, I1) to 10 V, 10 A (V2, I2). Autoranging power supplies are very flexible since you do not need to worry about having multiple power supplies to cover different voltage and current combinations for the same power levels.
A PV Simulator actually has another type of output characteristic, commonly referred to as the I-V curve.
Figure 3 – PV Array I-V curve
The output curve in Figure 3 represents the output characteristic of a solar array more accurately than either of the other two output characteristics. The first thing to notice is that the y axis is current in this figure, and not voltage as it is in the other two graphs. The second thing to notice is that the shape is different from the other two characteristics. This shape is representative of the natural output characteristic of a PV array.
There are two ways that you can generate an I-V curve with a PV Array Simulator. The first way is referred to as SAS mode. In this mode, the user inputs four parameters that are shown in Figure 3: the open circuit voltage (VOC), the maximum power voltage (VMP), the short circuit current (ISC), and the maximum power current (IMP). PV Simulator firmware then uses these parameters to generate the curve based on a mathematical model. This is the easiest way to generate a curve since you only need to enter four parameters. The tradeoff is that you are limited to the mathematical model that the instrument is using.
The second way, a more complex method that generates PV curves, is referred to as table mode. In table mode, the user sends a list of voltage and current pairs to the unit. The PV Simulator firmware parses all the entered points and generates a PV curve. The PV Simulator firmware will interpolate between the points to draw a complete curve. In fact, you can input a table with as little as three points! This is a much more flexible approach than SAS mode but it is also more difficult to do. There are rules that must be followed in order to generate the curve and if any rules are broken, you will get an error. The programming is also more complicated because you need to enter tables of values. Table mode is very important though since it lets you generate any table you want to. SAS mode limits you to the particular mathematical model that the PV Simulator follows.
Most of the power supplies that Keysight sells are optimized to work in constant voltage (CV) mode. Keysight’s power supplies do work in constant current (CC) mode as well. However, they are optimized to be voltage sources rather than current sources. One of the biggest differences from our standard DC power supplies is that our PV Simulators are optimized to work in CC mode since PV arrays are often modeled in circuitry as a CC device (unlike something like a battery that is a CV device).
What Does Keysight Offer?
There are two families of PV Simulators available from Keysight.
The first is the E4360A family of modular Solar Array Simulators.
Figure 4 - The Keysight E4360A Solar Array Simulator
The second family released is the N8900APV family.
Figure 5 - The Keysight N8900APV PV Simulator
This is great for high power terrestrial applications such as PV string inverters. We also have a software package to make controlling them a bit easier. The N8900APV family also doubles as an autoranging DC power supply.
As you can see, Keysight offers quite a few different ways to test devices that are powered by PV Arrays. Please feel free to contact us with any questions that you have about PV Simulators and stay tuned to this blog for some more posts highlighting some of the great features of the PV simulator.
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