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A typical SEPS is composed of a primary power source (solar array), an energy storage system (rechargeable batteries such as Ni-Cd, Ni-H2, or Li-Ion, or possibly new technologies such as flywheels), a shunt regulator, a power distribution and control unit (PDCU), and loads, as shown in Fig. 1.
Fig. 1. Block diagram of the spacecraft electrical power system.
A. Shunt Regulation
In order to investigate the operation and performance of the sequential shunt regulator and battery shunt, a direct energy transfer (DET) power system, as shown in Fig. 2, is studied. Four solar array strings: SA1 through SA4, are sequentially shunted array sections, which performs coarse regulation of the bus voltage. An active array section, SA0, which is directly connected to the main bus without shunt, is responsible for fine regulation of the bus voltage. Each section is individually switched in or out depending on the bus voltage and the switching reference voltages except the active section that is connected to the bus all the time. A sequential shunt regulator controls the switching of the four-shunt elements. The upper voltage setpoints of the regulator for solar array sections SA1 through SA4 are 41.50V, 41.48V, 41.45V, and 41.40V respectively. The lower setpoints are 41.4V, 41.2V, 41.1V, and 41.0V respectively.
Fig. 2. Schematic diagram of a simplified DET spacecraft electrical power system.
Each solar array section is an 88x2 array (series connections by parallel connections). The Ni-H2 battery is nominally an array of 30x8 cells, but in the simulation model two individual series strings are split out for more detailed study, and in one of those strings, an individual cell is revealed. Accordingly, the numbers of series and parallel cells are (24x1), (5x1), (1x1), (30x1), (30x6) respectively for B1 through B5. The initial state of charge (SOC) of B3 cell is 0.70 while those of all the others are 0.60. The load is a resistor of 10 ohms. A filter capacitor connected to the main bus is used to smooth the bus voltage. The resistance of the shunt resistor for a solar array section is chosen as 0.1 ohms, and then a shunt element that can bear more than 10 watts ohmic heat is needed since the short-circuit current of a solar array section is about 9.5A. To prevent an individual battery cell from overcharging, the turn-on voltage of the battery shunt is set to 1.4V and the re-activated voltage is 1.38V. The resistance of the shunt element is chosen as 2.5 ohms, through which about 550mA current will conduct when the battery is shunted. In this case, power of about 0.8 watt is dissipated in each battery shunt. For convenience, only one battery shunt is shown in Fig. 2.
Simulation is conducted for the first 40 minutes of the orbit cycle, and the results are given in Figs. 3 through 6. Fig. 3 shows the output current from four shunted solar array sections, which contribute to load requirement and battery charging (Note that it is not the actual current through each solar array, only the component of the current that is not shunted). Fig. 4 shows the bus voltage that increases initially from 40.0V. From Figs. 3 and 4, it can be seen that when the bus voltage arrives at the 41.40V setpoint of the shunt regulator, solar array SA4 is first shunted, and the output current from this section becomes zero. Then the remaining four solar array sections, including the active section, provide power for the batteries and load. There is a fast but small amplitude decrease in bus voltage and afterwards the bus voltage increases. The solar arrays SA3, SA2, and SA1 are sequentially shunted when the bus voltage arrives at their setpoints. When the bus voltage decreases below 41.4V after SA1 is shunted, section SA1 is activated again.
Due to the differences in initial SOC of the batteries, the battery cell voltages are different. The voltage of battery cell B3 increases faster than the others and it arrives at the upper limit setpoint first. Fig. 5 shows the voltage of cell B3. From Fig. 5, it is clear that when the voltage of cell B3 exceeds 1.4V, the shunt is activated. Then this battery discharges and the voltage decreases. When the battery voltage drops below 1.38V, the shunt is deactivated and the battery is charged again. The difference of the state of charge of batteries B2, B3, and B5 is shown in Fig. 6. The SOC of B5 is the highest, and the other two are close. From Fig. 6, it is seen that the increase of state of charge of B3 is slowed down due to the shunt at about 1100 seconds. From this study, it can be seen that the sequential shunt regulator can maintain the bus voltage at a relatively constant level and battery shunt contributes to charge equalization.
Fig. 3. Output current from four solar array sections: SA1 (line), SA2 (dot), SA3 (dash), and SA4 (dash-dot).
Fig. 4. Simulation result of the bus voltage.
Fig. 5. Voltage of battery cell B3.
Fig. 6. State of charge of three battery sets: B2 (dash), B3 (line), and B5 (dash-dot).
B. One-Orbit Cycle Simulation
Based on the native and imported models available in the VTB, a wide variety of systems can be easily configured and simulated in the VTB. A representative SEPS is described in this section. Different systems can be easily obtained by changing the system topology and the parameters of components. The example system, as shown in Fig. 7, comprises a solar irradiance model to illuminate the solar cell, a solar array to convert the solar illumination into electrical power, a Ni-H2 battery array, and a resistive load. Several auxiliary components in the system are responsible for appropriate and efficient operation of the entire system.
Fig. 7. Schematic diagram of example satellite electrical power system.
The primary energy conversion device is an 88x19 (series connections by parallel connections) array of single junction silicon cells. Each cell has an active area of 2.4 X 6.6cm2, and a responsivity of 0.35 A/W. The battery is a 30x10 array of Ni-H2 cells, each having a nominal voltage of 1.24 V and a capacity of 1.25 A-h. The initial state of discharge of the battery is 0.5. All the solar array cells and all the battery cells are lumped into a single model for this particular orbital simulation, as shown in Fig. 7. An overcurrent relaying protect system is used to isolate the fault on the load side by sensing the load current when it exceeds the reference current. The starting current of the relay is 14 amperes, and then the corresponding relaying time is about 2 seconds. The charging current reference and floating voltage reference for the battery charge controller are 5A and 42V respectively. The LVD setpoint is 38.5V. An OR logic gate is responsible for handling the tripping signal to the power switch. This system is simulated for the first 2 hours of the mission, and Fig. 8 shows a snapshot of the 3D visualization of the simulation.
Fig. 8 A snapshot of the 3D visualization of the simulation.
The calculated results in this system are shown in Figs. 9 through 15. The time axis in these figures is scaled in seconds and the time step for simulation is 100 milliseconds. Fig.9 shows the power profiles of the solar array, battery and load. It can be seen from Fig.9 that when the solar array is ON, it provides power for the load, charging the battery simultaneously. During eclipse, the battery provides power for the load, and the bus voltage decreases from a value approximately equal to solar array voltage to the battery voltage, which is shown in Fig.12. The power dissipated by the resistive load decreases with the bus voltage since it is proportional to the square of bus voltage. After a cycle, the solar array powers the load and charges the battery again.
Figs. 10 and 11 respectively show the sampled data for the voltage and current of Ni-H2 battery. The battery is initially charged at 5A current, and after about 1700 seconds of constant current charging the battery voltage arrives at the 42V set point and thereafter it floats at that voltage. The charging current tapers immediately. When the solar array is in eclipse, the battery begins to discharge. As a result, the battery current reverses and its voltage decreases. The discharge current is not controlled and depends on the loads. It is seen from Fig. 11 that the discharging current is about 5A. At the end of the discharge cycle, the battery voltage decreases to about 39V. It is clear from these two figures that the charge controller performs the constant current charging/constant voltage floating scheme correctly.
Fig. 13 shows the energy conversion efficiency of the solar array, which is about 15%. The efficiency declines as the output current decreases. Fig.14 shows the state of discharge (SOD) of the battery, which decreases when it is charged and increases when discharged. Fig. 15 shows the temperature of the solar array, which increases rapidly at the beginning of the cycle and then maintains at a relatively stable level. The temperature of solar array decreases during eclipse.
Fig. 9. Power profiles of
solar array (solid line), battery (dash), and load (dash-dot).
Fig. 10. The battery voltage increases from 41.5V. After about 1700 seconds of 5A charging it arrives at the 42V set point and thereafter floats at that value.
Fig. 11. The battery is
initially charged at 5A current. After the battery voltage arrives at 42V, the
battery current tapers immediately.
Fig. 12. Curve of the bus voltage.
Fig. 13. Energy conversion efficiency of solar array.
Fig. 14. State of charge of the battery
Fig. 15.Temperature of solar array
