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34000A Series Operation Manual 155
5-16. The MPPT algorithm for 34000A series DC Load
The MPPT algorithm for 34000A series DC Load
Many photovoltaic (PV) devices, such as PV panels and concentrated photovoltaic (CPV)
modules, require outdoor testing for design verification, durability, and safety. A low cost means
of testing the output power of PV devices outdoors is to use DC electronic load; it provides high
power-handling capability at a low cost.
Often one of the main functions of outdoor PV testing is maximum power-point tracking (MPPT).
But because E-loads are general-purpose instruments, it is up to the PV test engineer to
implement an algorithm in test software to perform MPPT.
Fortunately, there are a lot of MPPT algorithm you can choose from, with more than 19
published papers on the implementation and performance of different MPPT algorithm. However,
these algorithms were designed for solar inverters. Inverter are different fro test systems, so a
MPPT algorithm that performs well in an inverter may not necessarily perform well in a PV test
system. This article introduces a MPPT algorithm that is a good fit for PV testing with an E-load.
We discuss how the algorithm is implemented and why it's a good fit for MPPT in outdoor PV
test systems.
The main difference between implementing a given MPPT algorithm in an inverter and an E-load
is in the I/O latency. In inverters, the MPPT algorithm runs on an internal microprocessor that
can measure and compute and make load adjustments in microseconds. To perform the same
set of operations with custom software and an E-load could easily take tens of milliseconds due
to the unavoidable I/O latency between the computer and the E-load. This I/O latency is the
main bottleneck affecting tracking speed. With that in mind, we chose and modified the MPPT
algorithm discussed in this article to meet the needs of a PV test system using the 34000A
series E-load.
An E-load is an instrument that can sink and measure the output power of a power source, such
as a power supply or a PV device. Like a variable resistor, an E-load can be adjusted to control
the amount of power it is sinking. E-loads can measure the voltage drop across them and the
current they are sinking. E-loads typically have four modes of operation: constant current,
constant resistance, constant voltage and constant power. The E-load will maintain its mode
setting even when the power output of the source it is connected to changes. For instance, if the
E-load is connected to the output of a PV panel and has a constant-voltage (CV) mode setting of
30V, it will adjust its internal resistance to remain at 30V as the I-V curve of the panel varies. If
the maximum voltage of the panel (Voc) drop below 30V, the E-load will act like an open and the
voltage across it will be whatever Voc is. In photovoltaic test, E-loads are typically used in CV
mode, so we will use this mode for defining the algorithm.
34000A series E-LOAD MPPT algorithm
The algorithm that will be used for performing MPPT with the 34000A series E-load is a modified
version of the Perturbation and Observation (P&O) algorithm, which we will refer to as the
Perturbation and Observation for 34000A series load MPPT algorithm. The 34000A series load
MPPT algorithm works by comparing the
small perturbation values and sampling of
instantaneous values of a PV device's output. These values give us the measuring change in the
V-P curve to tell us if we are already at the MPP. If we are not at the MPP, it tells us which way
to go on the curve to find it.
The mathematical relationships of voltage and power that 34000A series uses to track the MPP
can be expressed as:
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