Design and Application of Power Supply and Distribution System of Chang'e 4 Relay Satellite

XIE Meng, DUAN Baogang, YAO Yuying, JIAO Yusheng, ZHAO Shuo

1 DFH Satellite Co. Ltd, Beijing 100094

2 Shanghai Institute of Space Power-Sources, Shanghai 200245

Abstract: A 28 V-half-regulated power bus topology and an integrated PCDU (Power Conditioning and Distribution Unit) were adopted to meet the energy demand for the Chang'e 4 relay satellite. This paper first introduces the mission features and composition of the PSDS (Power Supply and Distribution System) for the Chang'e 4 relay satellite.Due to this satellite's unusual orbit, operational mode and project restrictions, special analysis and design was conducted on the PSDS from the perspective of weight-reduction, power management, and reliability and so on. Extreme low temperature storage of SA (Solar Array) was considered and how the antenna affects the SA was analyzed. A new kind of high-specific-energy 45 Ah (Ampere-hour) battery cell was used for the first time. To make sure that the satellite would successfully pass the long shadow zones, a 100% DOD (Depth of Discharge) experiment was carried out on the battery. Since the sunlight is almost always available and there are very few times for the battery to charge or discharge, battery care to extend its lifetime is also discussed. PCDU is a device that integrates power conditioning and power distribution in one unit. The PCDU on Chang'e 4 relay satellite can output more power with less weight because of the adoption of a 28 V-half-regulated power bus topology which was also used for the first time and used lighter material for its mechanical framework. Experiment under low temperature on PCDU was conducted as well and a hot backup equalizing charge technique which is beneficial to keep performance of the battery is illustrated. The power distribution module, which is a module of PCDU, enhances the power utilization security by utilizing a static impedance measurement and build-in-test to avoid possible short circuits. As for EED (Electrical Explosive Device) module, a protection plug was specially designed and three switches with different functions were connected in series to prevent the EED from exploding by error. In addition, the allowable minimum EED bus voltage for each EED was evaluated in case of low battery voltage caused by the possible postponement of the launching time. Complete verification experiments on the ground were conducted to confirm the correctness of the design and on-orbit test data conformed to the expected results and theoretical calculation. The power supply and distribution system has been working normally since the day the Chang'e 4 relay satellite was launched into space.

Key words: Chang'e 4 relay satellite, power supply and distribution system, reliability, low temperature

1 INTRODUCTION

The Chang'e 4 relay satellite was launched successfully on May 21, 2018 and is now operating at the Earth-moon second Lagrange Point which is almost 460 thousands kilometers away from the Earth[1]. It is now providing reliable long-distance data transmission between the TT&C stations on Earth and Chang'e 4 lunar probes, including the lander and the rover, which were launched together on December 8, 2018 and landed as planned on the far side of the moon on January 3, 2019.

Chang'e 4 relay satellite was designed, assembled, integrated and tested by DFH Satellite Co. Ltd based on the CAST 100 satellite platform. The PSDS (Power Supply and Distribution System) is composed of a SA (Solar Array), Li-ion battery,PCDU (Power Conditioning and Distribution Unit) and low-frequency cables. A 28 V-half-regulated power bus topology and a specific high-specific-energy 45 Ah (Ampere-hour) NCA battery cell which were both used for the first time were introduced for this satellite. The PCDU is a device that integrates a S4R (Sequential Switching Shunt Serial Regulator) module, power distribution module, EED (Electrical Explosive Device) module and energy management module. Every part of PSDS was specially analyzed and designed to strike a balance between weight-reduction and high dependability.

2 MISSION FEATURES

Due to the Chang'e 4 relay satellite's unusual orbit, operation mode and project restrictions, the PSDS had to take into account the following features and requirements.

1) Higher power with less weight

The total weight of the Chang'e 4 relay satellite is about 433 kg and the power consumption is about 700 W. However,the weight of PSDS was strictly controlled within 23 kg (without the low-frequency cables). This means only 5.3% of the total weight was allowed while the typical weight proportion of CAST 100 platform PSDS is nearly 10% with typical power consumption of 400 W.

2) Operation mode

The Chang'e 4 relay satellite has to work at its peak power to provide all-weather, real-time data transmission for to support the Chang'e 4 lunar probes during the whole exploration mission. Unlike most LEO (Low Earth Orbit) small satellites running from shadow zone to sunlight zone 14 to 15 times a day, the minimum time between two shadow zones for the Chang'e 4 relay satellite is 29 days. Thus, the battery has very few chances to charge or discharge. Since the relay satellite is expected to work 5 years or longer, how to store and maintain the Li-ion battery during the long-sunlight zone should be fully considered.

3) Large temperature variation

The outer space environment that Chang'e 4 relay satellite has to operate in is much harsher than that for most LEO small satellites which run at a relatively small temperature variation.The relay satellite has to suffer from a torrid environment higher than 100℃ during sunlight zones and low temperature greater than minus 200℃ in shadow zones which may last longer than 5 hours. This proposes high demands on the PSDS.

3 OVERALL INTRODUCTION FOR PSDS

The schematic diagram of PSDS of Chang'e 4 relay satellite is shown in Figure 1. The 28 V-half-regulated power bus topology was adopted to reduce the weight and volume of PCDU effectively. The power from SA is regulated by the PCDU via S4R circuits in the sunlight zone so that a steady bus voltage is established and the battery could be charged[2]. This topology which is much simpler than an all-regulated one would let the battery supply all the devices on this satellite directly through the discharging switches at a very high efficiency without BDRs (Battery Discharge Regulator). The configuration of PSDS is listed in Table 1.

The Chang'e 4 relay satellite adopted a single bus and distributed power supply configuration. The primary bus provides power for all subsystem equipment via cables. There are three long-term power utilization sections: platform section, payload section 1 and payload section 2. The platform section includes OBDH (On Board Data Handling), TT&C (Tracking Telemetry and Command) and AOCS (Altitude and Orbit Control System). Payload section 1 includes relay data transmission,solid state power amplifiers and modulators. Payload section 2 includes a scientific payload made in the Netherland. There are two short-term power utilization sections: the solar panel unfolding EEDs and the antenna unfolding EED. The EED bus is derived directly from the battery without passing the discharge switches.

Figure 1 Schematic diagram of power supply and distribution system

Table 1 Configuration of power supply and distribution system

4 SPECIFIC DESIGN AND VERIFICATION

4.1 Solar Array

Although the SA design could draw lessons from some of the LEO satellites in terms of mechanics design, EMC and anti-irradiation, there are still two points that need to be considered: extreme low temperature in shadow zones and the antenna shadow effect.

1) Extreme low temperature storage

The SA can generate electricity normally from -145 to 90 degree centigrade but the minimum temperature that Chang'e 4 relay satellite has to tolerate is about -206 degree centigrade (calculated value), which will occur in one of the shadow zones. Although no electricity is generated at that time, it is still necessary to make sure that the SA could work normally again after the satellite returns back to the sunlight zone. Fortunately,the SAs installed on Chang'e 4 relay satellite shared the same thermal design and manufacturing process with its predecessor Chang'e 3 probe's SAs which have successfully survived down to minus 216-degree-centigrade during ground testing. The test results also verified that the silver connectors, which are vulnerable to space environment and temperature, between solar cells are designed and manufactured robustly to tolerate the extreme low temperature.

To better understand the low temperature the SAs have to suffer, a different type of thermistor which is useful only when the satellite is passing a shadow zone was specially installed on one of the four SA panels while the other three panels share a common type of thermistor.

2) Antenna shadow effect

The Chang'e 4 lunar probes have to take a break during the moon night to dodge the extremely low temperature and the Chang'e 4 relay satellite orients itself to the sun. The SA o the relay satellite cannot output its maximum power during the sun-orientation period because the ribs of the large-diameter umbrella-shaped antenna shadow a part of the solar panels and the metallic wire mesh decreases the intensity of sunlight. The worst condition occurs when the angle between sunlight and+Z axis of the satellite is 30°which is shown in Figure 2. Calculation shows that nearly one third of the SA current is lost which was also verified during on-orbit testing.

A simulation illumination experiment on SA was conducted on ground and the result showed that only 70 percent of the sunlight could pass the metallic wire to reach the SA to generate electricity. However the SA of the Chang'e 4 relay satellite should meet the energy demand for the whole satellite under this special condition. The good news is that not all the payloads have to work during this period so it is still possible to maintain the battery neither in charge nor discharge.

Figure 2 Antenna shadow effect when the angle between sunlight and +Z axis of the satellite is 30°

4.2 Li-ion Battery

1) High specific energy

The Li-ion battery on the Chang'e 4 relay satellite consists of 7 high-specific-energy 45 Ah battery cells which are connected in series within a metal frame. Thanks to the lithium nickel cobalt aluminum oxide, also known as NCA, the core material of the cells, the battery can store more energy with lighter weight.The specific energy of the Li-ion battery on the Chang'e 4 relay satellite is higher than 140 Wh/kg which is about 20 percent lighter than normal batteries.

Of particular note is that it is the first time for the 45 Ah-NCA battery cell to be sent into the outer space.

2) 100% DOD cycles experiment

A 100% DOD (Depth of Discharge) cycles experiment was conducted on the ground as the satellite will fly through several shadow zones which may last 5 hours or longer. The battery is the only power source within the shadow zones.

The temperature was set 20 ± 3 centigrade before the experiment began. The charging current was 18 A during the constant-current charging period and did not decrease until the cell voltage ascended to 4.1 V which was the start of the constant-voltage charging period. The charging process came to an end when the charging current decreased from 18 A to 2.25 A. After being put aside for 10 minutes without any operation,the battery was discharged at 45 A until the battery cell voltage dropped to 3.0 V. The charge-discharge operation was repeated and the capacity of the battery was recorded every time.The relationship between the capacity maintenance ratio and cycle times is shown in Figure 3. It can be concluded that 99.4 percent of the capacity remained after 326 cycles.

3) On-orbit storage and management

Unlike most LEO remote sensing satellites, the Chang'e 4 relay satellite works in the sunlight zone almost all the time,except during some special circumstances like during the orbit maintenance period which happens once every two weeks or when passing through a long shadow zone which happens three times a year at most. Thus, seldom does the battery have a chance to charge or discharge which is not helpful for extending the lifetime of the battery.

To take better care of the battery, two measures were taken as follows:

a) Lower the battery voltage. Low voltage is beneficial to keep the battery in an optimum state when the battery neither charges nor discharges for a long time. There is no need to worry about the continuous decreasing battery voltage due to the effects of self-discharge. The PCDU sets a battery voltage threshold under which the battery can be charged automatically to its expected level again.

b) Lower the charging current. Charging is still necessary when the satellite completes its orbit maintenance or goes back to the sunlight zone after flying through a shadow zone. Since there is enough sunlight time, it's a good choice to charge the battery with a low current which can be set by simply changing the level of a controlling signal produced by PCDU.

Figure 3 Relationship between capacity maintenance ratio and cycle times

4.3 PCDU

The PCDU, which is the core of the PSDS, is an aggregation of two devices: the PCU (Power Conditioning Unit) which is used to generate steady power for the primary bus and the PDU (Power Distribution Unit) which is used to distribute power to the whole satellite. Three aspects are discussed in this section: higher power with less weight, experiments under low temperatures and the hot backup equalizing charge technique.

1) Higher power with less weight

The PCDU on Chang'e 4 relay satellite adopts a 28 V-half-regulated power bus topology which charges the battery through S4R circuits without BCRs and enables the battery to discharge directly to the payloads without BDRs. No BCRs or BDRs is the main reason why the PCDU can work more efficiently with less weight. The SA can output more than 800 watts and it is possible to support a system with a same power consumption level. This PCDU doubles the power capacity of CAST100 platform from which the Chang'e 4 relay satellite is derived.

The mechanical framework of PCDU is made of magnesium aluminum alloy and it is much lighter than pure aluminum which is the material normally used. This is another factor that contributes a lot to weight reduction.

What should be also particularly mentioned is that it is the first time for a 28 V-half-regulated PCDU to be sent into the outer space.

2) Experiments under low temperatures

Low temperature experiments on PCDU were conducted on the ground. The temperature was set -40℃ and all the necessary operations including load current setting were completed before the experiment. As the only power source, the battery kept discharging for 6.5 hours during the whole experiment. To ensure the security of the power supply during shadow zones,two discharging switches, which could be controlled by respective instructions, were connected in parallel. Each switch was examined by disconnecting the other. The two switches both disconnected themselves automatically which led to the failure of power supply of the whole satellite to protect the battery from over-discharging after the battery voltage dropped lower than 21 V. The S4R circuits came back to work after the shadow zone was exited and the SA current started to rise again. The PCDU restarted normally with a steady bus voltage and all reasonable telemetries.

3) Hot backup equalizing charge technique

Chang'e 4 relay satellite is expected to work normally at the Earth-moon second Lagrange Point for more than 5 years.The battery performance is a key factor that determines the life of the satellite. The uniformity of the 7 battery cells is a direct reflection of the battery performance. The difference between any two battery cells has been no larger than 20 mV at any certain time since the day the satellite was sent into space. Usually the uniformity is considered unacceptable if the difference exceeds 40 mV. The difference may become larger with degradation of the battery and the battery will continue to deteriorate as a result of the vicious circle.

A hot backup equalizing charge technique is adopted to avoid this phenomenon. Each battery cell is equipped with an equalizer which actually consists of an electronic switch and a number of power resistors. The equalizer will be turned on to let the obtrusive cells discharge so that the voltage uniformity can be regained. Two equalizers are connected in parallel for each battery cell to enhance the dependability to make sure the equalizing charge will work normally all the time.

The control logic is implemented by the energy management module which samples and compares the voltage of the 7 battery cells every minute. The equalizers turn on if the corresponding cell voltage exceeds 40 mV than the minimum cell.The equalizers turn off if the cell voltage falls to 20 mV higher than the minimum cell.

4.4 Power Distribution Module

The power distribution module which is responsible for distributing the power of SA or Li-ion battery to all devices of the satellite is a module of PCDU. As mentioned before, there are three long-term power utilization sections: platform section,payload section 1 and payload section 2. Two methods were adopted to ensure the security of the power supply on the satellite or during ground test: static impedance measurement and build-in-test realized by energy management module, which is also a module of PCDU.

The results of the static impedance measurement on platform section, payload section 1 and payload section 2 are shown in Table 2. The test results show that there were no short circuits each time when new devices were installed or removed. The static impedance measurement reflects the state of devices on satellite is correct.

Every time the PCDU is powered on, the energy management module issues the build-in-test-on instruction automatically to apply the test voltage on the three power utilization sections. The voltage of corresponding section will be found to be very low if there is a short circuit. The test result is reported back to the engineers through telemetry.

4.5 EED Module

The EED module which is responsible for providing the driving circuits for the EEDs to unfold the SA panels and the umbrella-shaped antenna is also a module of PCDU.

A protection plug is specially designed and three switches with different functions are connected in series to prevent EED from exploding accidently: the satellite-rocket separation switch which controls the EED-bus-on instruction, the EED bus switch and the trigger switch. Meanwhile, each of the three switches is equipped with its own backup and respective redundant trigger instructions.

Table 2 Results of static impedance measurement on power utilization sections

After separation of rocket and satellite, the satellite-rocket separation switch which consists of two parallel-connected switches and 8 pairs of contactors closes and the EED-bus-on instruction can be issued. Only after the trigger switch is closed can the corresponding EED explode.

The EED bus is directly derived from the Li-ion battery without passing the discharging switches to minimize the possibility of any potential error in the circuit path which may lead to total failure. Also, it helps to keep the primary bus voltage steady and minimize possible interference with other devices to let the battery, instead of the SA, provide the impulse current caused by the explosion of EED.

Since it is possible that the launch time might be postponed and the battery discharges for a longer time which it may lead to the decrease of EED bus voltage, it is necessary to evaluate the allowable minimum EED bus voltage. The explosion of an EED requires a current of no less than 5 A. From Table 3 which shows the allowable minimum EED bus voltage for each EED,we can see that the minimum battery voltage is 24 V. According to the fault countermeasure strategy during the powered-flight phase, the ultimate battery voltage is higher than 25 V.

5 ON-ORBIT TEST

Chang'e 4 relay satellite was launched successfully on May 21, 2018. 55 seconds after the satellite was separated from the rocket, the SA panels unfolded successfully. 24 minutes later, the umbrella-shaped antenna unfolded to its expected position.

Table 3 Allowable minimum EED bus voltage

The Chang'e 4 relay satellite has been working normally for more than one year. The largest SA current was 28.4 A which occurred several days after the Chang'e 4 lunar probes were launched and 855 W was transmitted to the bus. The SA current was about 21 A which conformed to the theoretical calculation when part of the SA was shadowed by the antenna.

As mentioned before, the battery voltage usually stays at a low level. The energy it carries is enough for orbital maintenance which is carried out once every two weeks. However,special actions have to be taken when the satellite is going to pass through a long-time shadow zone. On June 17, 2019, the satellite passed through a 5-hour shadow zone (14:13-19:11)and there was no real light for two and a half hours (15:28-17:56). The variation of sunlight intensity is shown in Figure 4.

Four days before this shadow zone, the battery temperature was raised from 17℃ to 25℃. One day later, the battery voltage was charged to its allowable highest level to store as much energy as possible. All devices on board were turned on to raise temperature of the satellite cabin to reduce battery discharging caused by thermal control in shadow zone. During the period in the shadow zone, the devices were turned off or turned on according to respective priorities and the load current was about 10 A. The battery voltage dropped from 28.8 V to 24.95 V after 35.2 Ah was discharged with a 78.2% DOD.Meanwhile, the battery cell voltage dropped from 4.1 V to 3.56 V with a difference which was no more than 20 mV at any specific time. The battery temperature was kept above 22℃ which was much more comfortable than expected. The minimum temperature the SA endured was -160℃ which occurred at the time when the satellite came out of the darkness and the SA current began to emerge again (17:56). The key parameters of PSDS during this shadow zone are shown in Figure 5.

Figure 4 The variation of sunlight intensity during the shadow zone on June 17, 2019

Like other satellites, the SA current of Chang'e 4 relay satellite tends to decrease a little due to ultraviolet irradiation and charged particle irradiation. The SA current was about 27.2 A when the satellite reached its mission position. The SA current decreased to 26.3 A one year later equivalent to 3.3% of power loss. This phenomenon conforms to the normal behaviors of on-orbit SAs. Usually, the SA current decreases most in the first year and 1% to 2% current loss is expected every year in the future. So, it can be predicted that at least 24.2 A will still be available and the power issue will not be a problem to Chang'e 4 relay satellite 4 years later. As to the battery, the discharging voltage of 7 cells is still higher than 3.7 V which is quite high above the alarm threshold with good uniformity after tens of times of orbital maintenance. Even if the battery decays 4 years later, we have alternative methods such as raising the battery voltage to store more energy, simply by issuing several commands via PCDU. The battery can rest us assured.

6 CONCLUSIONS

Chang'e 4 relay satellite has been working normally for more than one year. Reliability measures and targeted design performance have been confirmed on orbit. The successful application of PSDS on Chang'e 4 relay satellite, offers possible choice and reference to other satellites with similar power consumption levels.

Figure 5 Key parameters of PSDS during the shadow zone on June 17, 2019