Maximum Power Point Estimation Techniques

Maximum Power Point Estimation (MPPE) techniques are the simplest max- imum power extraction strategy which can be applied to a PV system. These techniques rely on measured values and pre-defined relationships to estimate the likely MPP location under particular environmental conditions. Typically, this pre-defined relationship is designed for uniform operating conditions, which reduces the effectiveness of these techniques under PSC.

The simplest form of MPPE relies on relating the short-circuit current or open- circuit voltage to the MPP current or voltage, respectively, via a linear equa- tion. These techniques are referred to as fractional short-circuit current and fractional open-circuit voltage as they relate the measured quantity to the MPP value using a constant of proportionality, namely k1 and k2. In general, k1 takes

a value between 0.71 and 0.78, and k2 between 0.78 and 0.92 [16]. The fractional

open-circuit voltage relationship is given in (4.1) and the fractional short-circuit current relationship is given in (4.2).

Impp ≈k2Isc (4.2)

The fractional open-circuit voltage and fractional short-circuit current techniques provide a very basic approximation of the MPP location. Over time, as the cells degrade, the constants k1 and k2 will no longer represent the MPP location, and

under PSC these techniques will have poor performance [93].

In [94], the performance of the fractional open-circuit voltage technique under low power conditions is improved by allowing an adjustable sample time and sample period that depend on the amount of power being produced. Despite these enhancements, the technique will still fail to locate a global maxima under PSC. A MPPT method based on the fractional open-circuit voltage is applied in [95] for High Concentration PV (HCPV) modules. A HCPV system is one where a Fresnel lens, or similar, is used to concentrate the sunlight so that the modules receive between 300 to 2000 suns of equivalent irradiance. An important concern for HCPV systems is tracking the sunlight, so dual axis trackers are implemented and the fractional open-circuit voltage is used in this case to monitor and measure the MPP.

While the fractional short-circuit current and open-circuit voltage techniques rely on a single measurement to estimate the MPP location, other MPPE methods rely on sampling more points on the I-V curve to provide an estimation. In [96], six I-V pairs are sampled to provide an approximation of the I-V characteristic under the relevant uniform conditions. The MPP is then predicted by applying the IVMPPE technique. Other implementations include estimation of the I-V curves based on modelling of the cells under uniform conditions to find the MPP [97] and a single sensor approach relying on an empirical relationship to determine the MPP [98]. The Single Diode Model (SDM) is used to determine a robust analytical relationship using an approximation of the ideal section of the curve to accurately estimate the MPP voltage and current [99] and analytically verified in [100].

Other MPPE techniques include those that rely on the definition of the MPP locus, a relationship linking the locations of the MPPs under different irradiance levels on the I-V characteristics [101–104]. The MPP locus is shown on the uniform I-V characteristics in Fig. 4.1. For simplicity, the MPP locus is often

Figure 4.1: MPP locus.

approximated by a linear relationship, or in some cases by a piecewise linear approximation where a different relationship is defined for the low irradiance and high irradiance conditions [103]. In general, to use the MPP locus technique it is necessary to measure some system parameter, be it the short-circuit current or open-circuit voltage, to predict the MPP location. During this measurement, the system will be operating at a lower power than is optimal [101]. The MPP locus is defined based on PV cells experiencing uniform environmental conditions. As such, under PSC the MPP locus will no longer accurately represent the GMPP location.

A MPPE method with increased complexity involves using an Iterated Unscented Kalman Filter with six measured (I,V) points and information of the I-V and P-V curves from the manufacturers datasheet, to firstly approximate the parameters of the SDM and then estimate the MPP location of the characteristic [105]. This technique will only be applicable in the case of uniform environmental conditions as the method will be unable to fit the parameters to the model for non-uniform environmental conditions.

All MPPE techniques require some measurement of a system parameter (such as the short-circuit current or open-circuit voltage) and then use predefined relationships to approximate the MPP. These approximate relationships are developed on the basis of uniform environmental conditions which means that the estimation will most likely fail under PSC. MPPE can be executed quickly

to track to near the MPP, but cannot guarantee operation exactly at the MPP, and power losses occur during the tracking process. As the cells degrade over time and experience non-uniform environmental conditions, the accuracy of the estimation will degrade with time. These approaches exhibit no oscil- lations in steady-state and can be implemented with low complexity and low cost.