What is a PV Simulator and What Does it Do?
2019-09-19 | 12 min read
This is the first in what will 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 blog posts in the coming weeks that will go more in depth on how to program PV simulators. 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. The output power will depend on variable environmental conditions such as temperature and sun exposure (also known as irradiance) whcih is very hard to control in a lab enironmant. 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 for a 10 kW, 1 kV, 10 A Power Supply
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).
Figure 2 - Autoranging Characteristic for a 10 kW, 1 kV, 30 A Power Supply
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 example above. It is rated for 1000 V and 30 A maximum. If PMAX was equal to VRATED multiplied by IRATED, this would make this a 30 kW power supply but it is rated for 10 kW. This supply is capable of outputting any voltage or current combination equal to 10 kW from 1000 V, 10 A to 333.3 V, 30 A. 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 (or curve) 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).
When the PV Simulator is operating in SAS or table mode, the output is adjusted via a feedback loop. The PV simulator monitors the value of the output voltage and adjusts the output current to the value dictated by the mathematical model. This feedback loop is constantly running at a high speed and will continually adjust to the demands of the load.
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.
The E4360A Solar Array Simulator is intended for low power, space-based applications.
Figure 4 - The Keysight E4360A Solar Array Simulator
The second family released is the N8900APV family.
This is great for testing high power terrestrial applications such as PV string inverters. The N8900APV family also doubles as an autoranging DC power supply.
We also have two software offerings for the N8900APV products. The first one is the SAS Control Software. This software is meant to control a single PV Simulator output. You can control the output state, monitor the operating point, and send PV curves to the instrument. All this functionality is included with the software for no charge. The software also has a licensed feature to do some automated EN50530 testing.
The second software offering is the Keysight DG9000A Advanced Photovoltaic Test Software. This is software that was designed to help test multiple MPPT input solar inverters. There are three different options available: for four, eight, or twelve MPPT input inverters. With this software, you can look at how the inverter is operating as a whole and get the big picture of what each input is doing.
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.