# Modelling Photovoltaic Systems Using PSpice - 45271 06

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Power Conditioning and Inverter Modelling

Summary

Power-conditioning equipment for protection or control such as DC-DC converters, charge regulators or DC-AC converters are part of a PV system in most applications. This chapter is focused on the description of models for these elements in order to simplify the PSpice simulation for a wide range of PV system architectures incorporating power-conditioning equipment. The models explained in this chapter have different degrees of complexity and simulation quality.

6.1 Introduction

The energy supplied by the PV modules to the system is subject to variations depending on the operating conditions, especially the irradiance and temperature values, as well as the load demand profile. Power-conditioning devices and circuits are included in the PV system with the purpose either of protection or of losses reduction, allowing the PV generator to work as close as possible to the maximum power point, thereby optimizing the energy transfer, resulting in a more efficient system.

Charge regulator elements can also be introduced into the system to prevent undesired operation conditions and to protect the battery from entering overcharge and undercharge states.

This chapter describes some of these power-conditioning elements as well as charge regulation modules and the operation features in a PV system.

6.2 Blocking Diodes

In absence of irradiance the PV module short-circuit current, Zsc, is zero and the module Z(V) characteristic becomes similar to that of a diode as shown in Figure 6.1, where an ideal module with an equivalent infinite shunt resistance has been considered.

134 POWER CONDITIONING AND INVERTER MODELUNG ov 4v 8V 12v 1 6V 20v 24v I (vbias) vbias

Figure 6.1 PV generator characteristics in darkness

Under these circumstances, the PV generator acts as an additional load to the battery. Taking into account the PV systems current relationship in equation (5.35) in chaptek 5, pact! of the battery current, {bar. could be derived through the PV modules. This effecli I represented in Figure 6.1, where this part of the battery current can be identified as I(vbias).

The cursor shows the value of current that can be derived from the PV module eonsidaing the module connection to a 12V battery in the absence of irradiance. This effect can be prevented by including a blocking diode in the system. Figure 6.2 shows a PV system where a blocking diode has been connected in series with the PV array. Some considerations have to be taken into account:

0 The presence of a diode in series implies a diode voltage drop, which can be of khe order of 1 V depending on the diode model and power rating.

0 The magnitude of the current loss, I,,&, shown in Figure 6.1, depends strongly on the BV equivalent shunt resistance. Despite this dependence, considering typical equivalent shunt resistance values of real PV modules, the observed values of Id can be vq small.

According to the above considerations, the presence of a blocking diode in the PV system has to be carefully considered as a safety protection, however it may not be necessaq in some cases, especially if charge regulation elements are included. In this case the protection modules u vbatFw I , I Figure 6.2 Block diagram of a PV system with blocking diode

CHARGE REGULAITOff 135

I Ishunt

Vcontrol Vbat PV modules function of the blocking diode will be covered by the regulation elements described in the sections below.

1 lbat Battery

6.3 Charge Regulation

A key issue in any power conversion system is efficiency. As the energy flow from input to output strongly depends, not only on the irradiance and temperature, but also on the working conditions of all other system components, an efficient management of the flow becomes important in order to prevent the battery from entering overcharge and undercharge states.

Two basic types of electronic regulators are usually used for the battery charge manage- ment: parallel or shunt regulators and series regulators.

6.3.1 Parallel regulation

Shunt regulators are connected in parallel to the PV generator with the aim of diverting the excess energy generated, which occurs the battery reaches the overcharge mode. The input signal controlling the action of the regulator is the battery voltage, in particular when it reaches the onset of the overcharge mode, a shunt path for the current is activated. Figure 6.3 shows a typical configuration of this kind of regulator.

The transistor is OFF and Ishunt equals zero while Vcontrol remains smaller than the overcharge onset voltage, which must be known. When Vcontrol reaches this onset voltage the transistor turns ON and Ishunt becomes positive, diverting part of I,,,& across the shunt branch, limiting Ihat and therefore the battery voltage Vhat.

In Figure 6.4, Vcontrol is generated by a simple reference comparator, which takes Vb, and the onset reference value (in the figure V,,,) at the input and issues a signal with a negative value, close to the negative bias comparator supply, when the battery voltage is below the reference value, and a positive voltage, close to the positive bias comparator supply, when the battery voltage is greater than the reference value. The shunt regulation branch, composed of a transistor and a resistance, should be rated in order to dissipate the excess power generated by the PV modules and not delivered to the battery.

Figure 6.3 Typical shunt regulator configuration

136 POWER CONDITIONING AND INVERTER MODELLING

13 nz

10 ::' 1 O1 module ' I

:. 9

As an example of a PSpice simulation of shunt regulation, consider the PV system shown in Figure 6.4.

This system includes a PV module, a battery and a shunt regulator circuit composed of a voltage comparator and a npn bipolar transistor as the main components, and four resistors.

In particular, R1 and R2 are voltage dividers generating a voltage proportional to the battery voltage at the negative input of the comparator, R3 and R4 are power resistors in series with the collector circuit of the transistor and R3 is in series with the base of the transistor. These two resistors limit the current through the transistor terminals.

Example 6.1 shows a simulation corresponding to the circuit shown in Figure 6.4. PSpice

models for operational amplifiers and for transistors can be easily found in the PSpice library.

Example 6.1

Consider the circuit shown in Figure 6.4, write a netlist of the circuit and simulate the operation considering that the PV module is composed of 36 solar cells connected in series, with a short-circuit current of 5 A, an open voltage of 2.3 Vand a total power of 85 W under standard AM1.5G 1 kWlm2 irradiance and 25 "C operating temperature. The battery shown in the circuit is formed by an association of six 2 V elements in series with a total capacity of 1840 Wh and the initial value of the SOC,, at the beginning of the simulation is 0.75.

The battery has a nominal voltage value Vbnt = 12 V, and the battery overcharge onset is assumed to happen when Vbar = 14.8 V. Consider as input to the system an irradiance profile given by the file 'irrad.st1'. To simplify the problem let the temperature to be constant at 25 "C ambient temperature. Consider the PV module model described in Chapter 4, 'module-l.lib'.

Solution We start writing the netlist

CHARGE REGULATION 137

********Example 6.lnetlist ****Module, shunt regulator andbattery connection xmodule 0 3 1 module-1 params:ta=25, iscmr-5, tr=25, +vocmr = 2.3, ns = 36, np = 1, nd = 1 ,pmaxmr = 85 .inc module-l.lib xbatl 3 0 7 bat pararns: ns=6, SOCm=1240, k= .8, D=le-5, SOC1=0.75 . inc bat. cir R1312 100000 R2 12 0 100000 R3392 Q-Q1 9 1 0 Q40240 .model Q40240 npn vref8Odc 7.4 x741128131410ad741 . inc opamp. lib R410111250 vcc 13 0 dc 15 vee14Odc -15 . inc irrad.st1 vmesur 10 stimulusVgg . tran Is 130000s .probe . end

The irradiance profile for this example has been taken from real monitored data and has been included in the netlist by means of ‘irrad.st1’. Figure 6.5 show the irradiance profile corresponding to two days of April in Barcelona (Spain).

Figure 6.6 shows the resulting battery voltage waveform. As can be seen, the battery voltage V(3), is limited to the value of 14.8 V by the shunt regulator as set by the circuit. The PV module output voltage shown, Vl(xmodule.dl), is the voltage at node before the series resistance module Rsm (see Figure 6.4).

As can be seen in Figure 6.4 the operational amplifier compares Vbat/2 to a constant reference fixed voltage, v,f = 7.4 V. When Vbat reaches the value of 14.8 V, i.e. entering into overcharge, the output of the comparator becomes positive enabling the shunt branch

0s 20Ks 40Ks 60Ks 80Ks 100Ks 120Ks 14OKs

Time Figure 6.5 Irradiance profile in Example 6.1

138 POWER CONDITIONING AND INVERTER MODEWMG 18V

16V 14V

0s 20Ks 40Ks 60Ks 80Ks lWKs 12OKs 11W)<s

Time 12v V(3) Vl(xmodule.dl)

Figure 6.6 V l(xrnodule.dl), evolution Simulation results for battery voltage, V(3) (bottom), and PV module voflage (top),

1.2A

0.8A 0.4A

0s 20Ks 40Ks 60Ks 80Ks lOOKs 12OKs TLTOICs v I(R3) Time

Figure 6.7 Current evolution across the shunt circuit branch, I(=)

6.OA

4.OA 2.OA loOKs 12OKs 14obcs 0s 20Ks 40Ks 60Ks 80Ks v I (xbatl .vcurrent) A (xmodule.rs)

Time

Figure 6.8 Currents at the battery (bottom), I(xbatl.vcurrent), and at the output of the PV generator (top), I(xmodule.rs) formed by the transistor and R3. Figure 6.7 shows the current I(R3) evolution ~CTOSS this shunt circuit branch.

The current entering the battery is limited by the action of the shunt regulation. Figure 6-8 shows the PV module output current, I(xmodule.rs), and the current at the battery,

I(xbat1.vcurrent). The di€ference between these two currents is the current derived by the regulation branch, formed by the transistor and R3, shown in figure 6.8.

6.3.2 Series regulation

The second regulation approach is known as series regulation. T&y tlEosc OF lb~ charge regulators in photovoltaic applications use this kind of regulation [6.1]. Figure 69 shows a block diagram of a standalone PV system including a series charge regulator ekmcmL

As described above, the battery has a recommended voltage window bet- tbe low

(V,,,) and high (V,,,) where it operates at rated capacity and efficiency. If th Wery is forced to work outside of this window, it may be irreversibly damaged or upexate hmmctly. The series charge regulators prevent the battery from working out of this voltage wn0do.w.

Basically, the way the series charge controller works, is by opening the load wh the battery reaches V,,, and connecting the load circuit when the battery has been reehiwged enough so that its output voltage recovers. On the other hand, the charge rqpfatar disconnects the battery from the PV array when full charge is achieved, this mea~.~ when the battery voltage reaches V,,,, and resets the connection as soon as the battery &.a been discharged enough.

controlled switches or relays as shown in Figure 6.10. Table 6.1 illustrates an example fa This series regulation can be easily implemented using standard elm- 0

I Ibat I 1

Figure 6.9 PV system including a series charge regulator

Figure 6.10 Schematic representation of charge series regulation

140 POWER CONDIl/ONING AND INVERTER MODELLlNG

Relay 1 Battery-PVpanel

PV modules connected PV modules disconnected v,,, = 13.9 v Vbat = 12.8 V

Table 6.1 State of switches

OPEN CLOSED Load disconnected Load connected v~n= 1 v v,,, = 12 v

Relay 1 on

7r Relay 1 off

1- .r * 12.8 13.9 a battery with a nominal voltage of 12 V, showing the desired battery voltages for switching relays 1 and 2; in this case the battery voltage window operation is limited by V,, = 13.9 V and Vmin = 1 V. The signal controlling the relay switching can be easily made using two comparator circuits with the hysteresis loops shown by Figure 6.1.

Relay 2 on

Relay 2 off 1r v F 1- 1 12

By the same nature as series regulation, the only power lost at the regulator circuit is the losses at the relays themselves, which are small compared to the losses at the resistor and transistor used in parallel regulation. The balance of energy suffers to some extent because part of the energy generated by the PV modules is lost when the input to the battery switch is open.

Example 6.2 illustrates this kind of charge regulation strategy. The system simulated is the same system shown in Figure 6.9 but concentrates on the action of switch 1, which governs the battery-PV modules connectioddisconnection.

Example 6.2

Consider a lead-acid battery with the following characteristics:

0 Initial state of charge: SOCl = 0.45 0 Maximum state of charge: SOC, = 1000 Wh

CHARGE REGULAnON l441

Number of 2 V series cells: n, = 6 K = 0.8, chargeldischarge battery efficiency

D = 1 x lo-’, battery self-discharge rate Battery voltage for PV module’s disconnection: 13.9 V

0 Battery voltage for PV module’s reconnection: 13.4 V

The battery is connected to a Pv module formed by 36 solar cells in series. The mdule characteristics are:

Short-circuit current, = 5 A Open-circuit voltage, vQcmr = 2.3 V Maximum output power, pmr = 85 W

Finally an 800 R DC load is connected permanently to the battery. Consider the irradiance profile shown in Figure 6.12 as input data for the simulation of the PV system behaviour.

1.2KV

0.8KV 0.4KV ov 0s 20Ks 40Ks 60Ks 80Ks lOOKs 120Ks 140Ks

Time 0 V(1)

(a) Write the PSpice netlist for the simulation of the above PV system including a series charge regulator to control the PV modulehattery switching. Use a non-inverting Schmitt trigger to implement the switching control. Include the irradiance profile as ‘1rrad.stl file’. The netlist is the following:

**Example6.2.cir *****module, charge requlatorandbatteryconnection xmodule 0 3 1 module-1 params: iscmr=5, tr=25, +vocmr - 2.3 ,ns= 36,np- l,nd= 1 ,pmaxmr =85, ta= 25 . inc module-1. lib .inc irrad.st1 vmesur 10 stimulus Virrad

142 POWER CONDITIONING AND INVERTER MODELLING

xbatl 4 0 7 batstdparams: ns=6, SOCm=1000, k= .8, D=le-5, SOC1= .45 .inc batstd.cir Rbat 4 0 800 R14 8 10000 R2 8 0 10000 x741 12 8 13 14 10 ad741

. inc opamp. lib Vcc13 0 dc15 Vee 14 0 dc -15 vref 16 0 dc 6.8 R6 12 16 4100 R7 12 100 20 Wch34vcurrentswlmod .model swlmod iswitch (ioff=-lOe-5, ion=lOe-6, Roff =1.0e+8, Ron=O.Ok) vcurrent10100dcO .tranls140000s .probe .end

 I,, I,, -..................................... I
 ,,, .., ,,, , ,.......,..... ................,...,............,...,.... i.-.......,.....i.......................... : I I--.. I I,, I,, ,,, *I, ................,.......,... ,,, ,,, I,, I,, .. .... I i...j t...i...i.............i...i...... . ................,............................-..............

(b) Plot the PV modules output voltage evolution

The result is reached by ploting node V(3) which is the output node of the PV array 8s shown in Figure 6.13.

(c) Plot the battery voltage evolution, showing the connection and disconnection voltage levels.

12vr " I, :-: : I '. I ' I ! : .,

The result is shown in Figure 6.14.

disconnected from the battery until the reconnection voltage is reached. As can be seen the voltage drops once V,,, is reached and the PV array remains

Figure 6.13 PV generator output voltage (note that the minimum voltage corresponds to the baicery voltage at night)

MAXIMUM POWER POINT TRACKERS 143 14.0V

13.5V

13.0V

12.5V 0s 20Ks 40Ks 60Ks 80Ks lOOKs 120Ks 14OKs

Time Q V(4) Figure 6.14 Battery voltage

6.4 Maximum Power Point Trackers (MPPTs)

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