Limits for the Photovoltaic Module

3. INTEGRATING AN ENERGY STORAGE TO

DC

AC PV

Array

io-abc

v_pv io-abc

DC

DC ibatt

(a) Single-stage PV inverter and EES connected with a bi- directional dc-dc converter at the PV generator terminals

DC

AC

io-abc

vdc io-abc

DC

DC i batt

PV Array

DC

DC vpv

vpv

(b) Double-stage PV inverter and EES connected with a bi- directional dc-dc converter at the dc-link

DC

AC

io-abc

io-abc

DC

AC PV

Array v_pv vpv

i_o-abc

io-abc

(c) Single-stage PV inverter and EES connected with its own inverter

Figure 3.1 The dierent topologies

a PV array can t the limited operating voltage range, which can prevent deviation from the MPPs and reduce mismatching losses. The converter or inverter controlling the voltage across the PV modules should have high eciency over a wide range of input voltage and input power since these variables are dened in very wide ranges as functions of solar irradiance and ambient temperature. Thus, there is a trade-o

Especially for the case of the single-stage inverter, decreasing the minimum pos- sible MPPT voltage results in more energy extracted from the PV panels during low irradiance levels, but a less ecient inverter design [3]. For a single-phase sys- tem, the dc-side voltage of the inverter needs to be higher than the peak value of its ac-side voltage and for a three-phase system, higher than the square root of three times the peak value of its ac-side voltage, i.e.

for single-phase: v_dc> v_{ac, peak} for three-phase: v_dc>√

3v_ac,peak (3.1)

If the dc-side voltage is lower than this limit, the shape of the inverter ac-currents gets distorted due to overmodulation. Similar behavior does not occur when the dc-dc converter is used to control the voltage instead of the inverter. Thus, in order to reach the MPP voltages lower than this limit, the dc-dc converter is required to control the voltage across the PV modules instead of the inverter. Another possibil- ity is to use a diode to bypass the boost converter when the voltage is high enough, as done, for example, in [26]. However, in this case, the diode increases the losses.

As a result, the eciency of the system decreases.

Therefore, during low irradiance levels, the double-stage PV inverter scheme is more ecient, assuming that the increase of MPPT mismatch losses with the single-stage topology would be greater than the losses caused by the additional power conver- sion stage. However, the best conguration represents an optimal trade-o between the converter losses and MPPT mismatch losses, and is highly dependent on the variation of the solar irradiance levels that are typical for the chosen location.

Since utility-scale PV systems require numerous PV modules, it is possible to connect them in series in order to achieve a high enough voltage level to utilize single-stage PV inverters. According to [5, 16], in order to achieve a high enough in- put voltage for a single-phase inverter in the European systems (230 V_rms) to control the voltage across the PV modules with the inverter instead of the additional dc-dc conversion stage, minimum 16 PV modules in series are required. The minimum required amount of series-connected PV modules of a certain system can be identi- ed by comparing the lowest desired MPP voltage of the modules with the voltage requirement given in Eq. 3.1. For European systems with single-phase inverter, this requirement is 325 V. In Fig. 3.2 are presented the MPP voltage and power as a function of irradiance for dierent amounts of series-connected PV modules at 25°C.

200 400 600 800 1000 1200 1400 1600 1800 2000 0

100 200 300 400 500 600 700 800

0 1000 2000 3000 4000 5000 6000 7000 8000

Irradiance (W/m²)

MPPvoltageat25◦ C(V) MPPpowerat25◦ C(W)

N= 1 N= 5 N= 10 N= 15 N= 20 N= 25

Figure 3.2 The MPP voltage and the produced power as a function of irradiance for N PV modules connected in series at 25°C

200 400 600 800 1000 1200 1400 1600 1800 2000 0

100 200 300 400 500 600 700

0 1000 2000 3000 4000 5000 6000 7000

Irradiance (W/m²)

MPPvoltageat75◦ C(V) MPPpowerat75◦ C(W)

N= 1 N= 5 N= 10 N= 15 N= 20 N= 25

Figure 3.3 The MPP voltage and the produced power as a function of irradiance for N PV modules connected in series at 75°C

Based on this gure, 16 PV modules seems feasible when the irradiation stays higher than 500 W/m².

However, if the temperature increases, the MPP voltages decrease. In Fig. 3.3 are presented the MPP voltage and power at 75°C. At this high temperatures, 16

5 10 15 20 25

Number of PV modules in series

0 200 400 600 800

Expectable maximum open-circuit voltage (V)

Figure 3.4 The maximum open-circuit voltage as a function of the number of series- connected PV modules assuming open-circuit voltage of 45 V for one module

series-connected PV modules does not meet the required minimum voltage but a minimum of 19 modules are needed in order to reach the required voltage around the same minimum irradiance level. Thus, when deciding the operating range of the MPP voltage, it is necessary to take the eect of both the temperature and the irradiance on the MPP voltage into account when choosing the amount of series- connected PV modules.

Fig. 3.4 presents the maximum open-circuit voltage for dierent amounts of series- connected PV modules. The total open-circuit voltage for 16 PV modules may raise as high as 720 V, which has been too high for MOSFET and IGBT semiconductors in the past, since their normal operating voltage has been as low as 450510 V [16].

Note, that the operating voltage is typically selected much lower than the breakdown voltage, and this de-rating is based on the need to tolerate transients and to guaran- tee the long term reliability of the converter [13]. Therefore, in the single-stage PV inverter, multiple semiconductors of this type need to be placed in series in order to reach the required voltage rating. This introduces new problems since it must be guaranteed that these series connected semiconductors function synchronously and switch state at the same time. In order to avoid this and use only single switches in series, semiconductors with higher ratings need to be used or a topology with the additional dc-dc converter and dc-link can be utilized.

However, nowadays in medium power industry applications, such as PV, 6001700 V

IGBTs are dominant and they typically switch from 10 to 20 kHz [13], thus enabling a more ecient single-stage topology. Furthermore, in the last two decades, newer generations of power semiconductor devices based on wide band-gap materials, such as silicon carbide (SiC) and gallium nitride (GaN), have been developed. The SiC MOSFET switching frequency is signicantly higher than 50 kHz and they have already been introduced commercially rating from 600 to 1700 V [13]. Even 15 kV prototypes have been introduced [39]. These new technologies would enable the usage of more ecient single-stage PV inverters.