Excitation systems for high power synchronous generators with redundant configurations Zvonimir Jurin, Blaženka Brkljač, Marin Kolić KONČAR Elektronika i informatika Fallerovo šetalište 22, Zagreb, Croatia E-mail: uzbude@koncar-inem.hr tel. 00385 1 365 5768 Mladen Kajari, Vinko Ćesić, KONČAR Institut za elektrotehniku Fallerovo šetalište 22, Zagreb, Croatia E-mail: mkajari@koncar-institut.hr tel. 00385 1 365 5301 Abstract In the article the solutions of synchronous generator excitation system with thyristor converter, digital voltage regulator and field discharge equipment are presented with attention paid to achieve increased reliability and availability by applying redundant configurations. The excitation system with thyristor converter in three-phase fully controllable bridge connection and 1+1 configuration, twin channel digital voltage regulator and field discharge equipment for big and important generators in parallel operation are shown. Field results verify the project-foreseen characteristics and qualities of presented solution. I. INTRODUCTION To achieve high availability of synchronous generators for electric energy production, attention has to be paid to the increase of availability of all generator subsystems and among them of the synchronous generator excitation system during a very long-term service. Requirements on availability of excitation system, beside its technical functions, have been always high and improvements are still being implemented. The excitation system consists of exciter (the excitation power source) and voltage regulator. In the article, the solutions of thyristor rectifier exciter and digital voltage regulator are discussed with the aim of achieving reliable and redundant configurations with increased availability in synchronous generators operation. The actual solution of an excitation system consists of a big number of electrical, electronic and mechanical components and software products, working together under heavy technical conditions. The basic technical environmental requirements are extreme working and storage temperatures, their deviations and rate of change, combined with humidity, dust and oil vapour, vibrations and shock strains and electromagnetic interference. The requirements on excitation power source depend on machine characteristics and its role in power system. There are potential-source and controlled rectifier exciter (static excitation system), alternator supplied rectifier excitation system (brushless excitation system) that use mechanical power on generator shaft and auxiliary constant voltage shaft generator and static thyristor rectifier. The compound-source controlled-rectifier exciter that utilizes the potential and the current of synchronous generators, as an excitation power source, together with thyristorcontrolled rectifier is also used for special purposes. The actual and world most common solutions of high power synchronous generator excitation system, is potential source thyristor rectifier exciters with digital voltage regulator. The basic approach to achieve high reliability and availability is application the proper ratings for components and usage of high quality components. To avoid production failures comprehensive during and after production tests and commissioning have to be carried out. Beside that it is not possible to guarantee that no failure can occur. Fig. 1: Block diagram of a static excitation system with thyristor converter in 1+1 configuration, the twin channel digital voltage regulator and discharge equipment A1-55
To minimize risks of failure of synchronous generators excitation system and outage of production of the generator the hardware and software redundancy is applied: significant disturbance of generator operation. Converters have redundant cooling fans. 1. Thyristor converter configuration n-1 and 1+1, 2. Twin channel digital voltage regulator 3. Field discharge equipment and field overvoltage protection The block diagram of a static excitation system with thyristor converter in 1+1 configuration, the twin channel digital voltage regulator and discharge equipment is in Fig 1. II. THYRISTOR CONVERTER CONFIGURATIONS N-1 AND 1+1 The standard solution of excitation system thyristor converter is a three-phase bridge converter with 1 thyristor in series and n thyristor in parallel per bridge arm. Fast fuse is in series with each thyristor, Fig 2. The number of parallel thyristors per arm is n>3, to maintain selectivity of fuse protection. Current ratings of thyristor are so that n-1 thyristor per arm are capable to withstand all static and transient ratings and one additional thyristor per arm is for security reasons. The converter has redundant fans. Fig. 3. Thyristor converter in 1+1 configuration Behaviour of generator and excitation voltage after simulated failure on one thyristor in the operating thyristor bridge is shown in Fig. 4. Transfer to the second bridge is carried out in app. 50 ms. Fig. 2. Thyristor converter in n-1 configuration The nowadays-applied configuration of thyristor converter is 1+1 Fig. 3, because high rating thyristors are now available. This configuration consists of two equal three-phase bridge converters with 1 thyristor in series and 1 thyristor in parallel per bridge arm. Fast fuse is in series with each thyristor. Each bridge is rated to withstand all static and transient generator excitation ratings. One bridge converter is in operation while the other is hot stand-by. AC current of bridges are monitored and in case of a thyristor failure the changeover is taking place using free wheeling thyristor without any Fig. 4. Transfer of excitation Converter Bridge 1 to Bridge 2 in the 1+1 configuration with the generator in parallel operation to the grid A1-56
III. TWIN CHANNEL DIGITAL VOLTAGE REGULATOR The single channel digital voltage regulator in comparison to the analogue voltage regulator offers a lot of benefits such as: Performing of sophisticated control, protection, supervision and logical and sequential control functions, Short response time, Long term stability, Self monitoring functions, Reduced diversity of hardware modules, Digital standardized communication to the superimposed control system. Twin channel digital voltage regulator is a solution with two identical digital voltage regulators working in parallel in hot stand-by configuration. Each of two channels has the same input signals and is executing the same application program. Each channel has a complete structure with automatic and manual voltage control, reactive power regulation, limiters protecting the generator of going-out the generator capability diagram, compound function, V/Hz limiter and power system stabilizer (PSS) functions. Every channel controls both thyristor bridges Comprehensive monitoring program monitors the function of both regulators and in case of detection of an irregularity or exception the changeover is performed to the healthy channel without interfering normal generator operation. Fig. 5. Changeover of Channel 1 to Channel 2 with generator in parallel operation to the grid The changeover between channels of two-channel digital voltage regulator after simulated disturbance in operating channel is shown in Figure 5. IV. FIELD DISCHARGE EQUIPMENT AND FIELD OVERVOLTAGE PROTECTION There are two basic solutions of generator field discharge equipment, one, which include DC field circuit breaker with discharge contact, non-linear discharge resistor and separate circuit for overvoltage protection and second which includes AC circuit breaker with common circuit for discharge and overvoltage protection. Besides, there is normal operative procedure for generator deexcitation, the transfer of thyristor exciter to inverter operation mode. In classic solutions of discharge equipment the thyristor converter is connected to synchronous machine field winding via de- excitation D.C. breaker with a discharge contact and a discharge resistor, Fig. 6. When instantaneous electrical fault arises in the synchronous machine or in outside facilities, the short-circuit current will appear and the field should be suppressed as quickly as possible in order to limit fault current damage. It is also necessary to ensure safe de-excitation in event of a fault in the exciter control circuit causing severe over-excitation. The voltage dependent or linear discharge resistor is used to achieve rapid field suppression with regard to the highest permissible voltage across the field winding and the field breaker. The purpose of the rotor overvoltage protection is to prevent the machine field and thyristor converter from being exposed to an excessive overvoltages induced in the machine field with rectifier excitation, which could arise during certain transient condition such as improper synchronizing, loss of synchronism and asynchronous operation. The main overvoltage protection circuit comprises the antiparalell connected thyristor (crow bar) and resistor in series. The thyristors are controlled by the over-voltage sensing triggering unit with BOD elements. When the voltage exceeds the preset protection level the thyristor switch-on and conduct the current caused by over voltage. The series resistor is intended to limit the current in the protection circuits. On the other hand, the resistor must be so designed that the voltage drop caused by the highest induced field current flowing through the resistor does not exceed the protection level. The overcurrent relay supervises of the protection and gives a tripping impulse when the protection operates. When thyristor converter is applied in configuration 1+1, freewheeling thyristor is used to short-circuit field winding during emergency change over between bridges under high currents. A1-57
Fig. 6 Discharge equipment with DC field circuit breaker and separate circuit for rotor over voltage protection In the Fig. 6 the usual and most adequate solution for the field circuit breaker and over voltage protection is shown. The merits of this solution are the completely independent over voltage protection circuit and field circuit breaker that separates generator excitation on both poles from thyristor rectifier. In case of failure on field circuit breaker or non-linear deexcitation resistor, the rotor over voltage protection could act as back-up deexcitation. Usually DC field circuit breaker has two coils for switching-off, which, together with monitoring of switching-off circuits, realize very reliable solution. The alternative solution of discharge equipment is implementation thyristors from thyristors over-voltage protection (crow bar) and AC circuit breaker at primary of thytistor rectifier Fig. 7. Deexcitation is performed by switching-off of the AC circuit breaker what causes the over voltage on the generator excitation winding. This over-voltage will activate over-voltage protection (crow bar), reduce the overvoltage and deexcite the generator. In this case the same resistor is used for the de-excitation and for the field circuit over voltage protection, and in a case of its failure, there is no redundant deexcitation circuit to prevent damage of rotor winding. Fig. 7 Discharge equipment with AC field circuit breaker and common discharge resistor wit rotor over voltage protection Fig. 8 Discharging of generator by DC field circuit breaker and nonlinear discharge resistor Site tests are carried out on excitation system type SEM 11E, delivered and putted into operation for two units on Hydro Power Plant Varaždin. Generator and excitation system data are given in Table 1. It is Potential source thyristor rectifier exciter (static excitation system) supplied from the generator terminals. It is controlled by twin (two) channel digital voltage controller with separate hardware for each channel. Each channel of digital voltage regulator comprises Generator voltage regulator (AVR), Excitation current regulator (SR), Trigger pulse system and Follow-up control between main and stand-by controller, and between AVR and SR mode for each channel. Excitation power is derived by: three-phase fully-controlled (6-pulse), air forced cooled, thyristor converter in configuration 1+1, with one thyristor per branch and two converter bridges in parallel. One thyristor bridge is in stand-by and blocked. TABLE I GENERATOR AND EXCITATION SYSTEM DATA Unit A Unit B Rated power S n MVA 53.5 50 Nominal voltage U n KV 10.5 10.5 Power factor cosϕ n 0.85 0.85 Speed n n min -1 125 125 Frequency f n Hz 50 50 Nominal current In A 2920 2740 Nominal excitation current I fn A 890 940 Nominal excitation voltage U fn V 210 220 Nominal exciter current I EN A 1250 1250 Ceiling current I p A 1780 1880 A1-58
V. CONCLUSION The presented excitation system with redundant configuration with a thyristor converter in consisting 1+1 three phase thyristor bridges, classic discharge equipment with field circuit breaker and separate circuit for over-voltage protection, and microprocessor based twin-channel voltage regulator is suitable for big and important synchronous machines in parallel operation. The excitation system is characterized by increased safety, reliability and availability because of build-in redundancy. Field results have verified characteristics in exploitation and qualities of presented strategy. REFERENCE [1] Z. Jurin, M. Kolić, M. Kajari, V. Ćesić: Redundant configuration in synchronous generator excitation system Hydro conference, Dubrovnik 2003. [2] S. Marijan, M. Kajari, N. Perić: Microprocessorbased Control System for Electrical Machines, Proceedings of the 8th International Conference on Electrical Drives and Power Electronics, EDPE 94, Pula, Croatia, pp 203-209, September 1994. [3] Mladen Kajari, Siniša Marijan, Vinko Ćesić, Nedjeljko Perić: Digital Control of a Synchronous Generator Excitation System, Proceedings of the 8th International Conference on Electrical Drives and Power Electronics, EDPE 94, Pula, Croatia, pp 203-209, September 1994. [4] N. Perić, S. Marijan, M. Kajari: Microprocessorbased control system for the control of electrical machines and processes. 6th European Conference on Power Electronics and Applications, EPE 95, Sevilla, pp (2546-2550) [5] S. Marijan, M. Kajari, N. Perić: Microprocessorbased control system for electrical machines, Automatic 1-2/1995, Zagreb, pp (19-26). [6] V. Ćesić, M. Kajari, S. Marijan, Z. Jurin, M. Kolić: Excitation system with microprocessor based twin-channel voltage regulator for synchronous machines, EPE-PEMC 2002. Dubrovnik & Cavtat, A1-59
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