Multi-alternative power supply control
Changing operating conditions in space creates difficulties for supplying reliable power to spacecraft. A property of biological systems, homeostasis, can resolve challenges by using multi-alternative principles: Multi-level structure and control, functions diversity and partition, and structure modularity. For example, the power supply system (PSS) of an orbital station contains several alternative energy sources with a multi-level control structure. To embody the principle of modularity and partitioned functions, the general control zone is divided into non-overlapping active control ranges. Partitioning of functions, modularity, and the structure’s hierarchy excludes the possibility of a cascading increase in system failures and ensures uninterrupted station power supply in various flight modes.
Variability in space
Reliable power for an orbital spacecraft is difficult due to significantly varying luminous flux densities, variable temperature, and shade on solar batteries, even when a PSS design has multiple redundancies. Experience in designing and exploitation of a PSS shows the need for active reorganization of the operation modes and the interaction of its modules, depending on the current flight conditions. That is, better control is needed.
PSS design needs to form common principles, as well as build on successes achieved in creating autonomous PSSs using one methodological approach for resolving design problems.
The concept of a multi-alternative structure is proposed based on the functions of biological systems. Living communities can be sustained in a changing environment, a property of homeostasis.
The following principles comprise the concept: The principle of diversity, partitioned functions, the principle of multi-leveling, and modular design.
The PSS of the Russian segment of the International Space Station (ISS) is an example of the implementation of these principles. The results of applying mathematical and simulation models of the main system modules and examining the processes of applying the multi-alternative principles in critical functional modes are presented below.
General system structure
The subsystems of the main PSS are (Figure 1):
- Solar batteries, the primary source of energy on board
- A high voltage source located on the apparatus external to the space station and playing the role of a secondary energy source
- Electrochemical accumulator batteries, which accumulate energy if there is an excess in the system, and returning energy if there is a shortage.
All subsystems are equipped with automatic control circuits; interactions in various operating modes are discussed below.
Solar battery subsystems
A functional diagram of the voltage stabilization system for a solar battery is shown in Figure 2.
The adjustable value is the voltage UL across the load.
The master control is the constant reference voltage Ur.
The most significant external influences on the subsystem of solar batteries are:
- RL – load resistance
- W – luminous flux density
- T – temperature of the battery cells.
The total number of solar batteries, NSB, provides users with electric power due to parallel mode.
The control system for parallel-running solar batteries in a wide range of load current alteration is based on the evolutionary principle of multi-level control, according to which, as a load current increases, the required number of batteries, n ≤ NSB, is alternately enabled so that n-1 batteries give the maximum possible output current, ISB,max, determined by their volt-ampere characteristics, temperature T, and luminous flux density W. One battery with the conditional number n, connected last, operates in the mode of time-proportional control of the output current in accordance with the circuit in Figure 2, and the remaining NSB-n batteries remain unused.
As a result, at any value of the load current, not all the power given by the subsystem of solar batteries is regulated, but only the part of it that is the share of one battery. This simplifies the task of ensuring control system stability and quality. It also creates the possibility of unifying the control loops of each battery and the module structure of the subsystem of interchangeable modules. Together these provide reliable substation functioning in a wide range of alteration of parameters, loads, as well as in the event of an individual module failure. In particular, the failure or disconnection of any number of solar batteries does not change the dynamic properties of the control system.
For the technical implementation of the principle of solar battery subsystem operation described above, each battery controls a non-overlapping active range.
Figure 3 shows an example of such partition for NSB = 3, where u is the control signal at the input of the pulse width modulation converter, ISB1, …, ISB3 – output currents of solar batteries SB1, …, SB3. With the value of the control signal u > -0.5, all three batteries will tend to give their maximum output current. If the sum of these currents is excessive, and the power supply from one battery’s current is sufficient, for example, then, as a result of the feedback action, the control u will decrease, sequentially disconnecting SB3 and SB2 from the load until u enters the range -1.9 ≤ u ≤ -1.5 of active regulation of the battery SB1 current. The results of the simulation of the described processes when entering the shadow or emergency shutdown of one of two operating solar batteries are shown in Figure 4. Figure 4 illustrates the following critical modes of solar battery subsystem operation:
1. At the time t = 0.035 s, the load current jumps from 20 to 40 A. Since the maximum current of one battery ISB,max = 30 A, the battery SB1 begins to output its total current of 30 A. Additionally the battery SB2 comes into operation, giving an adjustable current of 10 A. The battery SB3 is not required to supply the users with power in this mode, hence its output current is equal to zero.
2. Over the time interval t = [0.06, 0.08] s, shading of the battery SB2 occurs (the luminous flux density decreases from W = 1000 W/m2 to W = 100 W/m2). The total current given by this battery falls from ISB2 = 10 А to the value ISB2 = 4 A and users get the missing current of 6 A from battery SB3.
3. At the time t = 0.1 s, emergency shutdown of the battery SB1 is simulated. The current of 40 A required for the consumers is provided by the total current of the battery SB2, together with the regulated current of 10 A from battery SB3.
Secondary high-voltage subsystem
The subsystem functional scheme with an external secondary high voltage source is shown in Figure 5. This subsystem under consideration is designed to connect users to the external high voltage power source, whose voltage, UEX, is consistent with the solar battery subsystem via a controlled converter (see Figure 5).
To ensure high reliability of this subsystem, a multi-alternative principle of selectivity is used. That is, the division and specialization of its functions ensure stable operation of the subsystem in various modes. Since the high-voltage power source is a voltage source, the over-current mode is the most common critical mode of its operation. If the converter currents IC do not exceed the specified IC,max, the voltage UL control loop operates in the subsystem (see Figure 5).
However, during an emergency or nominal increase in power consumption the converter current reaches the critical value, IC,max, and the control passes to the current control loop, ensuring the equality IC = IC,max, which is safe for the equipment. The voltage control (stabilization) loop is then blocked parametrically due to a higher transmission coefficient of the current control channel. When the load is removed, the subsystem control function is transferred automatically from the current loop to the voltage loop.
An illustration of the critical operating modes of the subsystem of the secondary voltage source is shown in Figure 6:
IL (t) – changing of the load current at the time t = 0.09 s from 15 A to 30 A. These values do not exceed the maximum current of the converter, IC,max = 50 A. Over the time interval t = [0.12, 0.15] s, the required load current IL = 60 А overcomes IC,max.
UL (t) – changing of the voltage across the load. In the range of permissible currents IL ≤ IC,max, the voltage control loop stabilizes the value UL at UL = 28.5 ± 0.5 V; at overcurrent, to maintain the equality IC = IC,max, the voltage UL reduces.
IC (t) – changing of the output current. Over the interval t = [0.12, 0.15] s, the converter control passes to the current loop, which limits the current of the converter to values of 50 to 52 A.
Uin (t) – changing of the level of the input voltage applied to the converter. Over the time interval t = [0.04, 0.06] s, a short-term drop (depression) of this voltage is reproduced from 180 V to 100 V. The voltage control loop in this mode maintains the value UL (t) at the given level.
The joint operation of the subsystem of the high-voltage power source with other subsystems is discussed below.
Accumulator battery subsystem
The subsystem under consideration performs two functions:
1. Accumulates excess energy in the station
2. Uses power supply when there is a shortage (e.g., shading of solar panels or absence of an external secondary power source).
A functional diagram, Figure 7, illustrates the implementation of the principle of multi-leveling and functions partition in the subsystem. It shows that the reversible voltage converter is controlled by two independent channels of the regulator (charge channel and discharge channel), each containing a two-level control system: one for voltage (at currents not exceeding critical values) and another for current (at currents that tend to exceed critical values).
Switching of the operation of these channels is automatic as a result of the separation of the individual levels (zones) of all its subsystems operation in the general range of voltage control in the PSS (Figure 8).
Transitions from one zone to another take place in accordance with the present value of the load current and the state of the accumulator batteries such that, with a small capacity of operating consumers, electricity is provided from solar batteries (which are connected alternately as the power consumption increases; see Figure 3). However, accumulator batteries operating in charge mode also can refer electricity to the consumers.
If the power of the consumers exceeds the total capacity of the solar battery subsystem, the control signal u will pass into the zone of active regulation of the secondary power source, transferring all the solar batteries to the full current output mode (see Figure 8).
Further increase in the power usage of consumers (not supplied by the subsystems of solar batteries due to the transition of the station to the shady part of orbit, or in the case of external secondary source disabling) will cause batteries to go into discharging mode (see Figure 8).
The reverse converter of the accumulator battery subsystem controls the charging and discharging modes with the help of the adjustable booster voltage UB, which is selected automatically depending on the present difference between voltage across the load and on accumulator batteries’ clamps. The control u is the same for all the power supply subsystems and ensures coordinated interaction.
Figure 9 illustrates the process of such interaction with significant change in the load current and substantial shading of the solar batteries. In all situations in the system, users are uninterruptedly supplied with power, the excess of which (for IL <60 A, see Figure 9) is directed to the charge of the accumulator batteries. Thus, multi-level control is carried out in separate subsystems and in the PSS of the station as a whole.
Analysis of the critical operating modes of the autonomous PSS of the space station shows that the survivability of the system under consideration is achieved as a result of application of the evolutionary principles of multi-alternative control:
- Multi-levelness, which creates the diversity of behavioral strategies of the system by transferring control and distributing the power supply functions between the subsystems and within each of them, depending on the present situation
- Modularity, which reduces the possibility of cascade (technologically connected) development of an emergency situation and failure
- Functions partition, ensuring high efficiency of organized control channels with narrow functional purposes.
On the basis of these principles, active redirection of the energy and information flows of the PSS and change in the strategy of how subsystems function in extreme situations are realized.
Semen L. Podvalny is head of the Department of Automated and Computing Systems, and Eugeny M. Vasiljev is associate professor of the Department of Automated and Computing Systems, at Voronezh State Technical University; edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
KEYWORDS Power supplies, power control, multi-alternative control
- Multi-alternative control helps with spacecraft power supply design.
- Multi-levelness, modularity, and partitioning functions all help.
Could multi-alternative design help your automation implementations?
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