American Institute of Aeronautics and Astronautics1

THE EXPERIENCE OF DEVELOPMENT AND UTILIZATION OF THE POWER SUPPLY AND CONTROL EQUIPMENT FOR THE STATIONARY PLASMA THRUSTERS OF THE ELECTRIC

PROPULSION PLANT OF RUSSIAN COMMUNICATION SATELLITE

M.F. Ganzburg*, L.A. Yemel’yanov†, V.A. Pitchulin‡, S.I.Yablochkin§

Public joint-stock company “AVEKS”, 17, 1-ya ul. Yamskogo Polya, 125124, Moscow, Russiatel.: +7(095)2570502; fax: +7(095)2577732

Lcard@lcard.ru

Y.V. Egorov¶

Research Institute of Applied Mechanics and Electrodinamics of Moscow Aviation Institute(RIAME MAI)

5, Leningradskoye shosse, p/b 43, 125080, Moscow, Russiasla@ex.ru

Abstract

According to the contracts with RCC “Energia” Moscow JSC “AVEKS” has been specialized in the area of elaboration, production and certification of the power supply and control equipment (PSCE) for the stationary plasma thrusters (SPT) from 1990 till nowadays. During this period the firm has acquired considerable experience in the area of creation of the PSCE in nonpressured implementation for operation of the electric propulsion plants of the spacecrafts, including up to eight SPT. In the process of development, optimization and certification of the PSCE “AVEKS” ran the considerable number of its tests both separately and in combination with SPT on the benches of JSC “AVEKS”, RCC “Energia” and OCB “Fakel”. In this connection “AVEKS” managed to acquire unique experience and knowledge of the features of designing the high-voltage, high-current contactless systems for operation in deep vacuum conditions as well as the electric interface between SPT and the spacecraft onboard control systems. The PSCE, developed under the contract with RCC “Energia” for the electric propulsion system (EPS) of “Yamal-100” spacecraft supplies eight SPT of M 70 type with power and controls them; two of them can operate simultaneously. The authors of the proposed article analyze the design features of the PSCE developed by “AVEKS” and the experience of its utilization in the structure of the EPS of “Yamal-100” during 44 months (about 3.7 years).

* Deputy director† Principal engineer‡ Principal engineer§ Chief engineer¶ Postgraduate student

Introduction

For the first time in the practice of utilization of communication satellites with SPT the PSCE, elaborated by JSC “AVEKS” for operation of eight SPT of M 70 type in the structure of the EPS of “Yamal-100” spacecraft7, has worked about 3.7 years under the vacuum conditions steady and reliably, meeting all the requirements of the customer. Nowadays the total operating time of the PSCE for its separate devices makes up from 600 to 32000 hours. According to the telemetry data the temperature mode does not exceed 18° C. The PSCE ensures the stability of thrust of all SPT according to the results of ballistic measurements ± 0.5 %; this fact testifies, that the discharge current stabilizing system works effectively.

JSC “AVEKS”, taking into account the results of utilization of the PSCE in the structure of the EPS of “Yamal-100” spacecraft, obtained new orders both for delivery of the modernized PSCE to RCC “Energia” for “Yamal 200” spacecraft (the expected launch of “Yamal 200” will take place in summer 2003) and for delivery of the PSCE for the EPS of “Yahta” spacecraft. The authors of the proposed article present the general concept of “AVEKS”, by which this enterprise was guided in the process of designing the PSCE for “Yamal-100”, “Yamal-200” and “Yahta” spacecraft, some technical characteristics of various types of the PSCE and also analyze some features of utilization of the PSCE of the EPS on the board of “Yamal-100” for 44 months.

39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit20-23 July 2003, Huntsville, Alabama

AIAA 2003-4877

Copyright © 2003 by V.A. Pitchulin, Y. V. Egorov. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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The general estimation of effectiveness of the projects of production the power supply and

Control equipment for electric propulsion systems, realized by “AVEKS”

The development of the PSCE of the EPS, containing up to eight SPT, may be considered as being successful, if minimum three tasks, interconnected between each other, are performed. They are as follows:- Commutation of 64 low-ohmic loads, such as

cathode heaters and gas distributor thermal throttles of eight SPT and consequently the choice of the principle of their power supply.

- The choice of the optimal structure of the PSCE and its component base.

- Ensuring the correspondence of the PSCE electrical parameters to the requirements of the SPT loads and at the same time optimization of the main SPT characteristics (thrust, and its stability, specific impulse and service life) by means of using the potential reserve capabilities of the SPT designs.

Solving the problem of the choice of current type in the power supply and control equipment for

supply the cathode heaters and thermal throttles with power

It is known1,2, that the structure of each SPT of M-70 or M-100 type and the other products, developed by OCB “Fakel”, includes:- Two cathodes (main and reserved) with the

corresponding heaters, needed for providing thermal emission of the cathodes during the process of SPT startup.

- Two thermal throttles (main and reserved); in the gas distributor unit of the SPT which are the adjusting regulators of the working medium xenon consumption for ensuring the thrust stability.

These SPT loads have the low-ohmic character, i.e. of the order of 0.2 - O.6 Ohm, and they are designed for releasing heat from some watts to 100 W. At the same time the character of electric current, flowing through them - direct, alternating or pulsating, is of no difference for this1. The developers of “AVEKS” (taking into account the fact, that the EPS of “Yamal-100” spacecraft uses eight SPT, i.e. 16 cathode heaters and 16 thermal throttles, and commutated currents attain 15 A, while their commutators must function in nonpressured equipment and at the same time produce a few thousands of commutations) paid special attention to the choice of current type - direct or alternating. It is known, that by supplying the cathode heaters and the thermal throttles of the SPT with direct current3,4,5 it is necessary to convert the rated voltage

of the spacecraft power supply system into the ones for the cathode heaters and the thermal throttles with their subsequent rectifying. In order to perform this task the additional mass is consumed, while the considerable quantity of heat is released by the rectifying diodes. In addition to these evident disadvantages:- For supplying the cathode heaters of SPT with direct current the following operations are required:

1. It is needed to realize the high-current commutation by the onboard computer commands of 16 heating chains of eight SPT by means of relay contacts or transistor keys reliably. Despite realization of commutation by the relay contacts3,4,5 is very simple, it is characterized by one serious disadvantage: if an accident fault takes place in the onboard computer, all the group of the relay majority contacts looses its commutating properties. Therefore they are commutated when the feeding converter is off.

2. It is necessary to stabilize direct cathode heater current (12 ± 0.3) A from real technological spread of resistance of the cathode heating chain.

3. It is needed to protect components from short circuits in the subsystems, supplying the cathode heaters with power.

All these conditions require additional mass and power consumption.- For supplying thermal throttles with direct current the powerful transistor regulator controlled by means of the discharge current sensor (DCS) is required besides the rectifier. Besides reliability and efficiency decrease all these shortcomings cause increasing the equipment mass and lead to the appearance of additional faults in the process of regulating the discharge current by the thermal throttle. All these disadvantages were avoided during development the PSCE by “AVEKS” by means of supplying the cathode heaters and the thermal throttles of SPT with alternating current and using magnetic amplifiers (as the discharge current stabilizers, manipulating the thermal throttle current) and magnetic throttles (as the power commutators of heater current). The advantages of the magnetic amplifiers and the magnetic throttles for controlling the low-ohmic loads of the SPT were demonstrated by M. Y. Glibitsky in 1980 yet6. However at that time because of the lack of effective magnetic cores, fabricated of amorphous steels, and also stringent requirements for the control equipment reliability and efficiency it was impossible to obtain any essential advantages of the magnetic amplifiers and magnetic throttles application in the control equipment. But in the process of improving the design solutions in the area of modern communication satellites creation, i.e. the ones, aimed at lowering the mass of these satellites and increasing their reliability during the

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service life up to 15 years, the developers of the control equipment had to solve new problems: the equipment had to operate in the nonpressured spacecraft compartment and also in combination with the considerable number of SPT - up to eight or more. In this connection the problem of ensuring the reliable commutation with low power losses of the high-current loads of the large group of SPT became important. The PSCE of the EPS systems for “Yamal-100” 7

and also “Yamal-200” and “Yahta” spacecrafts, elaborated by “AVEKS” and equipped with the magnetic amplifiers and the magnetic throttles based on the up-to-date magnetic cores with operation frequencies up to 20 kHz, allowed to obtain their mass characteristics, close to the characteristics of the contactless direct current commutators when reliability and efficiency of this equipment was much higher. It’s also necessary to notice that the process of production of the magnetic amplifiers and the magnetic throttles is simple and their cost is much lower. The magnetic amplifiers are proof against the factors of different space environment influence in the geostationary orbit operation conditions. Using the magnetic amplifiers and the magnetic throttles allows essential simplifying the process of realizing the necessary control functions of the PSCE; they are as follows:- Stabilizing the cathode heater current from

destabilizing factors of spread in output power source resistance and technological spread of the heater resistance.

- Protection from short-circuits.- Galvanic separation of power and control

circuits.- Circuits integrity check.- Minimization of the number of current

sensors of loads for the SPT, which are simultaneously inoperative.

It should be especially noticed, that the means of stabilizing the discharge current by thermal throttle and limiting its start and anomalous mode values based on the magnetic amplifiers are highly effective and simple. The best example of successful utilizing the magnetic amplifiers and magnetic throttles for commutation of the loads of eight SPT and for control over these loads is their application in the PSCE of the EPS of “Yamal-100” spacecraft (Fig. 2). The specific characteristics of the commutating and regulating section of the PSCE of the EPS are the following: mass - 2.5 kg per one SPT, the heat release value is 2 W per one SPT. For comparing: the experimental commutation equipment, developed by “Polus” Scientific Production Center in nonpressured implementation and for the first time delivered in the geostationary orbit in summer 2002 on the board of

“Express” spacecraft has the following specific mass characteristic: 4.8 kg per one SPT; “Polyus” uses no magnetic amplifiers and magnetic throttles. Thus method of supplying the SPT cathode heaters and thermal throttles with alternating current (especially if there are a lot of them in the structure of the EPS) by comparable mass allows gaining considerable advantages in comparison with the method of supplying them with direct current; these factors of advantage are related to reliability, efficiency and cost. Supplying the cathode heaters and the thermal throttles with alternating current in the PSCE, elaborated by “AVEKS”, to a considerable extent determined the structure of its high-current commutating section.

Estimation of the effectiveness of the structure solutions, concerning the power supply and control

equipment

The effectiveness of the development of the PSCE, as well as any other type of the onboard spacecraft equipment, is determined by the criteria of satisfying all the requirements of the customer’s performance specifications (including the reliability requirements) with minimal parameters of cost, mass and heat release.

The specialists, taking into account the PSCE service life requirements, which are from 10 to 15 years, and the stringent conditions of its utilization in the geostationary orbit, in the process of designing this equipment paid special attention to reliability; it is ensured by all the known methods. They are as follows:- Realization of the contactless commutation

methods of the high-current and high-voltage circuits.

- Utilization of reliable electronic components and their electrical underloading; the loading factor is 0.5.

- Structure redundancy in the form of the so-called “cold” reserve of the main assemblies of the PSCE.

- The so-called “hot” reservation of all the circuit-forming electronic components; majority reservation of interface channels (two from three) which control the high-voltage and high-current commutators.

- Providing the PSCE operation and also its reserve both in the automatic mode from the onboard computer and in the operator mode from the Space Flight Control Center and in this connection installing sufficient number of current and voltage sensors.

- Realization of the vast program of certification tests of the PSCE in the autonomous mode and in combination with SPT.

- Automation of all the processes of testing the PSCE.

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The structure of the high-voltage section of the power supply and control equipment

The structure of the PSCE for the EPS of “Yamal-100” naturally corresponds to the one of the EPS itself. As this system includes four main SPT, which produce the thrust in the directions “north-south”, “west-east”, and four reserve SPT, as in the EPS of “Yamal-100” spacecraft, the PSCE has one static converter for four main SPT and another for reserve thrusters; each converter forms uncontrollable rated voltages for all the loads of four SPT. Such functions as switching on and off the converter, needed for operation of the selected group of SPT, selection of some particular SPT from this group, startup of its launch procedure are performed by the onboard computer according to the corresponding program-algorithmic software of the PSCE; in this case the functions of the prelauch control based on the parameters of the PSCE sensors

and (if it is necessary) of the automatic control over its structure reserve are performed. Each group consisting of four SPT provided with the discharge current, ignition and cathode heaters supply buses galvanicaly separated from the primary buses of the spacecraft power supply system, the spacecraft airframe and the other group of SPT; inside each group all SPT have the nonswitched main discharge current supply common bus (“minus” of discharge voltage), and the required SPT is chosen by means of commutating the SPT anode on the “plus” bus of the group. Thus the PSCE consists of two monoblocks. The first monoblock is the electric power supply unit (EPSU), containing two static converters 28.5/315V (SC1, SC2). The second one is the commutation unit (CU), comprising two contactless power commutators of loads (PC1, PC2); each of these commutators is provided for its own group of the SPT loads (Fig. 1).

Fig. 1. Structure of the high-voltage section of the power supply and control equipment

Each of the SPT anodes is commutated by means of the majority key, controlled by the onboard computer commands and by the signals, generated by the PSCE (when the mode of limitation of the anomalous SPT discharge current fluctuations is formed). In order to limit the discharge current brief increase in the

anomalous mode the PSCE uses the technical solution, patented by the specialists of OCB “Fakel”1. The principle of limitation of the discharge current anomalous inrushes is realized by the method of its pulse-width modulation (PWM) by means of the transistor key, controlled by DCS (Fig. 2); when in

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the case of the anomalous overshoot of the discharge current its value 1.5-fold exceeds the rated one, the transistor key is switched off and then on its subsequent lowering to 0.6 of the rated value it is switched on again. A DCS is designed as a direst current measuring transformer, included in each SPT group minus bus (only one SPT in the group can operate at the same time), which converts discharge current being measured into proportional output voltage with less then 2%

error. The current flows from the anode to the cathode uninterruptedly due to the diode (Fig. 2), through which the current caused by electromotive force of self-induction of the SPT magnetic system passes. This current supports plasma discharge of the SPT and, when the anomaly fluctuation continues not more then 1.5 – 2 s, the discharge current becomes rated again.

Fig. 2. Diagram of discharge current commutation, its pulse-width limitation and cathode heater currentcommutation

If the anomalous mode duration is more than 2 s, the onboard computer switches the SPT off with its subsequent immediate startup using time-compressed procedure. In the process of using SPT in the EPS of “Yamal-100” spacecraft during each combination of maneuvers (3 - 5) separate short processes of PWM (<2 s) in some SPT happen. During the period of utilization of “Yamal-100” SPT till 2001 the processes of PWM with duration more then 2s with thruster restart took place approximately one time per 25 maneuvers. Taking into account that the transistor key temperature increase during the long-drawn process of PWM comes slow enough (the PWM mode period, maximally permitted by the transistor key, is 45 s according to the results of the experimental works), the specialists of the Space Flight Control Center prolonged the SPT restart delay due to PWM mode from 2 to 4 s by program method in the onboard program-algorithmic software. After that till nowadays there have no been any SPT restart detected, though sometimes the PWM processes take place.

Implementation of the PWM method of the anomalous mode discharge current limitation by means of the separate transistor key from the structure point of view allowed the specialists not to place demands of performing the current generator functions for the static converter, forming discharge voltage; as a result the high-effective circuit engineering solutions giving the static converter high efficiency up to 0.96, the high level of reliability (estimated probability of no-failure operation is 0.9999 for 10 years) and 10-fold current overload capability have been realized. The other structure problems of creating the PSCE include two important additional questions:

1. The necessity to stabilize the SPT discharge current and voltage3,4;

2. The necessity to supply the SPT magnetic system with power from an autonomous power supply unit8.

The first problem is conditioned by the SPD M-70 BR technical specification containing the requirement of providing both discharge current stabilization from

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changing the input pressure of xenon (1.75 ± 0.1 atm) and discharge voltage stabilization from changing the onboard rated values of the electrical supply system (28.5 ± 0.5) V in order to obtain the rated ± 3% thrust stability. The developers of the PSCE met the first requirement, having created the high-effective system of stabilizing of discharge current by means of control the thermal throttle current. This system with corresponding tolerance effectively meets the requirement for discharge current stabilization. According to the results of the combined tests of the PSCE and SPT the effectiveness of the stabilizer was checked in such way that with input xenon pressure rate 1.75 ± 0.2 atm and input voltage rate 28-29V the rated value of discharge current (2.23A) was kept within ±0.01A error; this result exceeds the corresponding PSCE specification requirements twice. “AVEKS” had the additional opportunities for ensuring the discharge voltage stability, because it had also developed the regulation and control equipment (ARC) of the power supply system for “Yamal-100” spacecraft. ARC is elaborated in such way that in the process of 3.7-year utilization it actually keeps rated main bus voltage value (which feeds the static converters) with fare better accuracy (± 0.05 V) than it is required by the ARC specification. Therefore the problem of discharge voltage stabilization in the PSCE was solved at a higher system level. The results of ballistic measurements of spacecraft position after maneuvers performed during all 3.7 years allowed to determine the thrust stability value better then 0.5%.

The second problem connected to the design of PSCE structure is providing optimal amplitude and frequency characteristics of oscillations in discharge current (which are conditioned by the natural instability of the plasma discharge). It also involves the necessity of choice the method of the SPT magnetic system power supply (with an autonomous source3 or without it7). Existing of nonoptimal oscillation mode leads to deterioration almost all the operating characteristics of SPT9. Therefore the PSCE performance specification included the requirement for ensuring maximal oscillation amplitude level Uosc< 10V. To a certain extent this requirement may be met by the combination of three methods or by one of them:- Installing LC-filter in the main discharge

cirquit3,4 (Fig. 2).- Choosing the optimal discharge voltage value.

It is known10 that that the discharge voltage increase up to 350V for the SPT of M-70 and M-100 type allows lowering the amplitudes of electric discharge current oscillations considerably and also increases the thrust, the specific impulse and SPT efficiency. Taking in consideration SNECMA experience, accumulated in the process of development of

PPS-1350 SPT, the modification of M-100 SPT, where the value of discharge voltage was 350V, “AVEKS” in the considered PSCE increased the rated discharge value from 300V to 315V, i.e. for operation withthe upper limit, approved by the corresponding performance specification.

- Autonomous supplying the SPT magnetic system9. This solution was not applied in the PSCE considered in order to minimalize it mass and maximize reliability. At the same time relatively low capacity of 1.5 mcF ensured Uosc<10V with three-fold tolerance.

Thus nowadays M-70BR SPT simultaneously performs its functions on the board of “Yamal-100” spacecraft and passes the life tests as the new modification of the corresponding SPT class. According to the results of ballistic measurements the SPT thrust value of the “Yamal-100” EPS is higher then the rated value for about 3%, that directly testifies the effectiveness of the discharge voltage increase. Also the thrust decrease tendency has not been detected even after long time of a separate SPT operation. The discharge current anomalous modes leading to the SPT restart have not been detected for the last two years. All these data testify that the solutions accepted are correct.

The structure of the high current commutating section of the power supply and control equipment

As it was stated earlier, the fact that the SPT cathode heaters and thermal throttles are supplied with alternating current completely defines the structure of the PSCE commutation section. Thus the autonomous alternating current channel (28V, 10kHz) runs from each static converter into the unit commutating each group of the SPT loads (Fig. 3). When all the keys (K1- K16) are off, residual current of the magnetic throttles (MT1 – MT8) of the order of 40 mA flows through the heaters of the cathodes that is useful for circuit integrity checking. When it is necessary to heat some cathode the onboard computer controls the relay contacts and the electric power up to 100W is applied from the static converter through the corresponding magnetic throttle to the selected cathode heating circuit. The thermal throttles control subsystem operates in the similar way, the difference is only that when one of them is chosen it controls the thermal throttle current not discretely but following the changes of the discharge current value. Also application of alternating current in the cathode and thermal throttle circuits simplifies design of telemetrical sensors of the corresponding parameters.

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Fig. 3. Structure of the high current commutating section of the power supply and control equipment

The low-current relays controlling the power keys of the discharge, heating and thermal throttle circuits are controlled by digital commands of onboard computer, which are sent to the PSCE through the relay matrix 6X6 (36 commands). It should be especially noticed that the mentioned PSCE structure solutions are realized on the domestic electronic component base that significantly reduces the PSCE cost in comparison with based on imported components. For all the 3.7 years of utilizing the PSCE on the board of “Yamal-100” spacecraft high effectiveness of all the structure solutions has been confirmed.

Conclusions

By the present moment for the first time in the practice of utilization of commercial communication satellites with SPT the PSCE has worked about four years in the conditions of the geostationary orbit stably and reliably, meeting all the consumer requirements. During this operation effectiveness of the following features have been conformed:- Engineering solutions on providing long

term operation of high current and high voltage equipment in the conditions of vacuum.

- Schematic and constructive solutions on providing electromagnetic compatibility of PSCE and other onboard equipment

(galvanic separation, twisting, grounding). There were no detected any onboard computer and retransmitter faults during SPT operation.

- Discharge stabilization circuits design (therewere no detected any discharge self-quenching).

- Design of circuits of contactless discharge current commutation and pulse-width limitation and also of cathode heaters and thermal throttles current commutation and control when they are supplied with alternating current.

- System of stabilization discharge current from input xenon pressure and voltage fluctuation.

The described solutions can be considered as valuable for future design of satellite power processing and control equipment for thrust subsystems with SPT.

References

1. V. Grauer, K. Kozubsky, V.Zhasan. Stationary Plasma Thruster. Patent of Russian Federation No 2009374, Int. Cl. H0 5H 1/54, 1991.

2. M. Day, N. Maslennikov, T. Randolph, W. Rogers. SPT-100 subsystem qualification status. AIAA Paper 96-2713, July 1996.

3. G. Fisher, T. Colbert, M. Day. Design of a high efficiency power processor for the Russian Stationary Plasma Thruster. 23rd International Electric Propulsion Conference, 1993, IEPC-93-043.

4. A. Chernishev, N. Katasonov. Power Processing Unit for Electrical Propulsion Thruster Systems. 24th

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International Electric Propulsion Conference, 1995, IEPC-95-124.

5. H. Declercq, E. Bourguignon, T. Scalais. Power Processing Unit for Stationary Plasma Thruster. 26th International Electric Propulsion Conference, 1999, IEPC-99-059.

6. M. Glibitsky. Systems of Power Supply and Control for Electric Propulsion Plants. Moscow, Mashinostroenie, 1981.

7. V. Kim, M. Ganzburg, S. Yablochkin, V. Pitchulin. Power Supply and Control Equipment for Russian Stationary Plasma Thrusters for Propulsion Subsystems of Geostationary Communication Satellites. Paper at 3-rd International Spacecraft Propulsion, Cannes, 2000.

8. A. Petrenko, J Hamley, J. Sankovich. The Problem of Power Processing and Telemetry Control Unit Design for Stationary Plasma Thruster. 24th International Electric Propulsion Conference, 1995, IEPC-95-126

9. C. Garner, J. Brophy, J. Polk, L. Pless. Performance Evaluation and Life Testing of the SPT-100. AIAA Paper 94-2856, July 1994.

10. J. P. Boeuf, L. Garrigues. Low Frequency Oscillations in a Stationary Plasma Thruster. Journal of Applied Physics, Vol 84, No 7, 1998.