Peak Power Tracking

The solar array generates more power at higher voltage at the beginning of life and when cold coming out of an eclipse. The maximum power can be extracted only if the bus voltage is varied with years in service and with temperature. However, the load must be supplied power at the same voltage, generally lower than the maximum power producing voltage of the solar array. A suitable switching regulator between the solar array and the load, as shown in Figure 4.11, remedies this disparity between the maximum power producing voltage and the constant load voltage. The series regulator input voltage is then maintained at the maximum power producing level by the peak power tracker, and the output voltage is stepped down to the constant load voltage by varying the duty ratio as required. The peak power tracking is activated only when the battery needs



FIGURE 4.11 Peak power tracking architecture for a mission with wide variations in solar flux and temperature.

charging or the load demand exceeds the solar array output. Otherwise, the excess power is left on the array raising the array temperature. The battery relay is opened up when the battery is fully charged.

The peak power tracking (PPT) electronic controller senses the maximum power point in one of the following ways:

• The solar array output power — the product of voltage and current — is continually computed and fed to the peak power tracker. The array operating voltage is changed until the peak is detected.

• As seen in Chapter 8, the bus dynamic and static (a.c. and d.c.) impedances are equal in magnitude at the peak power point. A ripple is injected into the solar array bus, and the dynamic impedance dV/dl and the static impedance V/I are continuously measured. The bus voltage is adjusted such that both impedances are equal.

• The ratio of the Vmp to the Voc for any solar array is approximately constant, say K (typically 0.70 to 0.75). The Voc of a solar cell coupon maintained in the same environment as the main array is continuously monitored. The operating voltage of the main array is then adjusted to K-Voc to extract the maximum power.

• The inner voltage control loop regulates the solar array output voltage to the reference value from the PPT controller. By changing this reference value at regular intervals, the PPT controller moves the operating point of the solar array. In each time interval, the PPT controller calculates the solar array power slope by multiplying the sensed solar array voltage and current. If this power slope is positive, the PPT controller increases the reference value until the sensed power slope is negative, and vice versa. Thus, the operating point of the solar array is located near the peak power point where the power slope is zero. The algorithm can be written as

where K is a suitable constant. The peak power voltage thus derived is fed to the series switching regulator, which then converts the array voltage to the load voltage. Figure 4.12 shows a complete PPT assembly.

High Performance SA Power Regulation

* Peak Power Tracking at <1 % of Pmax

* Regulation Efficiency > 96 %

* Power Accomodation > 1,500 W

• Bus Load Variation 300 to 2,200 W

Charge Control for Extended Battery Life

• Autonomous by V/T-Method

♦ 16 Commandable EOC Voltage Limits (Temperature & Current Compensated)

Comprehensive Redundancy

* 4 for 3 Power Regulators

♦ Power & Charge Control

• Pyro Electronics

* RS 485 Serial Data Bus

FIGURE 4.12 Power regulator unit with peak power tracker.

(Source: Dornier Satellitensysteme Gmbh, Daimler-Benz Aerospace. With permission.)

The peak power tracking architecture is particularly useful in the following applications, where the additional weight, power loss, and cost of adding such an assembly can be justified:

• Small satellites having no pointing gimbals, such that the solar array is not always oriented towards the sun

• Satellites having the solar radiation and array temperature varying over a wide range, indirectly varying the array voltage

In low Earth orbit, where the battery must be charged in a short period. The PPT allows maximum power to be captured for several minutes after each eclipse when the array is cold. Architecture without the PPT feature, such as a DET bus, would waste a significant amount of power as shown in Figure 4.13. If a DET system were designed to deliver the required power at one-half the illumination at EOL, the power waste would be CD watts at EOL full sun, BD watts at BOL full sun, and AD watts at EOL full sun on a cold array. The PPT design eliminates this waste by utilizing all the power that can be generated.

FIGURE 4.13 Power wasted in direct energy transfer architecture in certain conditions.

The main advantages of the peak power tracking are that it maximizes the solar array output power all the time, and it does not require the shunt regulator and the battery charge regulator. On the other hand, it results in poor system efficiency due to power loss in the peak power tracking converter. Moreover, since this loss is dissipated inside the spacecraft body, it negatively impacts the thermal system.

The PPT can have three configurations: series, parallel, and seriesparallel, as shown in Figure 4.14.1 The series-parallel configuration yields better system efficiency because the input and the output power conversion is processed by a single converter for all operating modes, as seen in Table 4.1.

A PPT algorithm has been developed2 and tested using only the solar array voltage information, giving the tracking control without using current sensors. This results in low ripple current, hence lighter bus filter capacitors.



— Battery

(a) Series configuration

(a) Series configuration

^T Battery (b) Parallel configuration

^T Battery (b) Parallel configuration

(c) Series-parallel configuration FIGURE 4.14 Battery charge and discharge options in peak power tracking architecture.
Table 4.1 Operating modes of series-parallel battery discharge regulators in the PPT system


Series regulator

Battery charger

Battery discharger




Regulated bus

PPT discharge



Regulates bus

(partial sun)

PPT charge

Regulates bus



(full sun)

Trickle charge

Regulated bus

Trickle charge


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