Reliability of spacecraft power system

Reliability of spacecraft power system

The calculation of the reliability of the spacecraft power system is strictly based on the reliability data of the included components. Reliability theory shows that component reliability is defined as the probability that a specific component will perform according to its performance specifications under specific operating conditions during each time interval. The determination of component reliability includes calculating component failure rate from partial failure rate data, applying this number rate to a mathematical model of failure, and performing failure mode analysis. For the evaluation of the power system, the following exponential failure method is sufficient to calculate the reliability of the components:
R(t)=[e-λt]
In the formula: λ is the combined failure rate of N parts or components included in the entire work cycle t.

The summed failure rate is defined as
λ=λ1+λ2+…+λn

1. Failure rate of various system components
This paragraph develops corresponding equations for the failure rate of various spacecraft or satellite power systems. The failure rate of the spacecraft power system presents a complex question. The power system must consider the failure rate of solar cells, energy storage batteries, and PC modules.

An expression for the failure rate of a solar panel can be written as
λsp=λ1+λ2+…+λn
In the formula: λSP is the failure rate of the solar panel.

This parameter includes all components included in panel manufacturing. The battery failure rate equation can be written as
λB=λ1+λ2+…+ηR
In the formula: λB is the failure rate of the battery composed of all R components.

The failure rate expression of the PC module can be written as
λPC=λ1+λ2+…+ηS
λpc is the failure rate of the PC module composed of all S components, equipment and circuits.

2. Failure rate estimation
The preliminary evaluation of the power system of the spacecraft shows that the failure rate of the solar cell array is moderate compared with the PC module. In theory, if a printed circuit is used, the solar cell has no failure mechanism. But as the battery ages, the battery output will decrease. Engineers and designers of solar power systems predict that the minimum working life of solar cells is 20 years and that of converters is 10 years. The failure rate of spacecraft batteries strictly depends on the type of electrodes used and the effects of aging [4]. As long as the battery is charged and discharged according to the instructions of the battery supplier, and the battery has not suffered serious physical damage, the battery’s working life can exceed 15 years. The higher probability failure mechanism of the basic spacecraft power system is shown in Figure 1. Two key system components are used in the spacecraft power system block diagram, namely the charge controller and the voltage regulator. The regulator can be a DSR or PWM regulator. In particular, the linearized battery charging characteristics and the temperature-dependent state-of-charge characteristics are strictly a function of the key circuit parameters used in the PWM regulator. The failure rate of these key parameters will seriously affect the failure rate of the spacecraft power supply.

Reliability of spacecraft power system
Figure 1 The key electrical components of the basic spacecraft power system

3. Reliability improvement of spacecraft power system using CC and PWM regulator technology
To ensure the reliability of the power supply system, it is very important to estimate the open circuit voltage and the remaining capacity of the battery with high accuracy. The reliability of the system usually requires the charging and discharging of the onboard battery. In addition, the actual measured open circuit voltage is strictly related to the state of charge of the battery. In short, when the ampere-hour efficiency of the battery is assumed to be 100% of the charge-discharge process, the open circuit voltage and the internal resistance of the battery are directly determined by the performance of the battery. If the accurate reliability data of the spacecraft power system components and the results of component failure analysis are ideal, then these statements must be carefully considered. According to the mission period, the reliability of the power system of the spacecraft using the CC redundant unit and the dissipative PWM stabilizer is improved.
The reliability of the spacecraft power system increases with the increase of redundant units, regardless of the type of regulator integrated in the power system. In addition, it is obvious from the reliability data that, compared with the pulse width modulation regulator PWM, the number of redundant units (2, 2) provides higher reliability for the dissipative regulator.
If the spacecraft or satellite goes through a period of darkness, the mode selection circuit will turn on the boost regulator and charge regulator to allow the least battery discharge, which is necessary to meet the power requirements during the darkness of the spacecraft. The boost regulator and mode selection circuit play a key role in ensuring that all spacecraft electrical systems, stabilization mechanisms, and monitoring sensors receive the required power during the dark period.

4. Reliability improvement of spacecraft power system using DET system, CC and battery supercharger technology
When all three redundant components, such as DET, CC and battery charger (BB), are integrated in the spacecraft power supply. However, the DET redundancy option provides the lowest system cost and system weight based on the mission deadline. The main reliability improvement is due to the use of the shunt regulator circuit in the DET system, as shown in Figure 2.

Reliability of spacecraft power system
Figure 2 Block diagram of the direct energy transfer power supply system on the spacecraft

5. Weight and cost associated with redundant systems
Redundant system components deployed in the spacecraft power supply system may result in weight and cost penalties. The total cost factor of the system and the increase in the total weight of the system strictly depend on the duration or length of the task (months) and redundant system elements, such as CC, BB, parallel boost regulator, PWM series regulator, DSR and DET sub system. When the reliability of the described system is critical, the energy storage and PC components used in redundant systems or subsystems are used by mission-critical spacecraft or geosynchronous orbiting satellites for secret military communications.

The conversion device occupies a considerable part of the total cost and total weight of the spacecraft or satellite. The reduction in weight and size of redundant elements or components deployed on a spacecraft or satellite is extremely important to the length or duration of the mission. Multifunctional secret military communications satellites provide critical services, including secure broadband communication lines and covert and uninterrupted communications between governments, military authorities, and important military officials deployed around the world. Such broadband communication satellites are equipped with an onboard power system, capable of providing 3.5kW of power and an estimated design life of more than 15 years. Airborne batteries play an important role in meeting the power requirements of electronic warfare systems, concealment and uninterrupted communications, and ground force support for unconventional warfare.

① The total weight and cost of the system according to the length of the task
Roughly estimate the total cost and weight increase of the system based on the task deadline. On the basis of the data given in these tables, it can be concluded that the reliability of a constant spacecraft power supply system can be estimated by linear interpolation between the numbers corresponding to the reliability of the system containing the entire redundant components.

② Reliability decreases as the mission period increases
So far, the focus has been on the increase in cost and weight based on redundant system options. Depending on the duration of the mission, the reliability of the spacecraft’s power supply system may be degraded, although various redundant system options have been used. The reliability of the power supply system will be minimized at the end of the spacecraft’s working life.

Although the reliability of the spacecraft’s power system decreases with the increase in mission duration, the power system will not experience catastrophic failure unless the spacecraft crashes or suffers severe structural damage. In the case of the redundant system option (CC, DSR) (2, 1), the reliability of the redundant system option (1, 1) is increased from 91.8% to 95.3% regardless of the length or duration of the task. However, the reliability of the power supply system will continue to decrease as the mission duration increases. Option (2, 1) means that two CCs have been integrated into the spacecraft power system, thereby improving reliability. Adding additional CC will increase the cost and weight of the power system.

③ Increase in weight and cost due to redundant systems
Redundant components are installed and activated in the spacecraft or satellite power system, which improves the reliability of the system, but the price paid is increased weight and component costs according to the length or duration of the mission. The expected increase in total power system weight and cost based on the mission period.

It can be seen from the list data that the cost coefficient of the DSR redundant system has the smallest increase. These conclusions are valid for spacecraft power systems, and these conclusions show that the use of various redundant systems has an increasing trend based on mission duration, weight and cost factors. In short, the performance data provided by the power systems deployed by various spacecraft and satellites shows that redundant systems tend to produce higher reliability based on mission duration or length. Moreover, regardless of the deployed Yuan Yu system, the reliability of the power system does decrease as the length of the task increases.