A low loss mechanical HVDC breaker for HVDC Grid applications THOMAS ERIKSSON, MAGNUS BACKMAN, STEFAN HALÉN ABB AB, CORPORATE RESEARCH SWEDEN

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1 21, rue d Artois, F PARIS B4-303 CIGRE 2014 http : // A low loss mechanical HVDC breaker for HVDC Grid applications THOMAS ERIKSSON, MAGNUS BACKMAN, STEFAN HALÉN ABB AB, CORPORATE RESEARCH SWEDEN SUMMARY Recent years have seen an increasing interest in developing DC breakers for HVDC applications. The reason is that breakers open up the possibility to build multi-terminal HVDC Grids. The HVDC Grid is considered one of the most promising alternatives for transmitting large amounts of electrical power over long distances, as well as, connecting various AC grids together. The development of a HVDC Grid introduces very high demands on the protection devices, such as breakers. To be able to limit the consequences in both the AC and DC systems during a fault, the detection and interruption needs to be very fast compared to traditional AC breakers. Semiconductors offer a flexible HVDC breaker solution and easily meet the technical requirements of a HVDC Grid both regarding interruption time and current carrying capability. However, when considering the amount of losses produced by the semiconductors it is clearly of interest to also look at other solutions. Therefore, alternative concepts have been presented recently, combining semiconductors with a mechanical switch, often referred to as a hybrid. The hybrid concept largely reduces the losses in the system since in normal operation the current is by-passed and not flowing through the main semiconductor breaker branch. Of course it would be of interest to have a purely mechanical breaker. The conceptual difference in the mechanical alternative is the use of mechanically operated circuit breakers for interruption of the current instead of semiconductors. Recent development of breaker actuators has made it possible to realize mechanical alternatives for DC interruption even for the application of HVDC Grids. By introducing, for instance, electromagnetic actuators, the operation speed of the interrupters may be increased significantly. This paper presents the general design and testing of a mechanically based HVDC breaker with performance data relevant for an HVDC Grid implementation. The target data for the demonstration module were, 80 kv system voltage, 10 ka fault current interruption capability and current interruption within 5 ms. During the high power test several current levels were applied between 2 and 10.5 ka. The current was succesfylly interrupted in all test cases. In conclusion the mechanical HVDC breaker has very low losses and almost as fast interruption as the hybrid concepts. This performance makes it an attractive solution for certain applications in a future HVDC Grid. KEYWORDS HVDC circuit breaker; HVDC Grids; Mechanical circuit breaker; High power testing Thomas.R.Eriksson@se.abb.com

2 INTRODUCTION Recent years have seen an increasing interest in developing DC breakers for HVDC applications. The reason is that DC breakers open up the possibility to build multi-terminal HVDC Grids. The HVDC Grid is considered one of the most promising alternatives for transmitting large amounts of electrical power over long distances, as well as, connecting various AC grids together. The HVDC Grid also enables more efficient use of renewable energy sources such as wind, solar and hydro power in different geographical locations and thereby average out fluctuations in their output. The development of a HVDC Grid introduces very high demands on the protection devices, such as breakers. To be able to limit the consequences in both the AC and DC systems during a fault, the detection and interruption needs to be very fast compared to traditional AC breakers. There are two main alternatives for interrupting DC at high voltage. One is using semiconductors, such as IGBT s, which have the ability to turn off the current even though it does not have zero crossings. Another way is to create an artificial zero crossing inside the breaker. This is typically accomplished by a mechanical breaker/switch together with an auxiliary resonance circuit which generates an oscillation causing the current to go to zero in the interrupting unit. THE MECHANICAL HVDC BREAKER Semiconductors offer a flexible HVDC breaker solution and easily meet the technical requirements of a HVDC Grid both regarding interruption time and current carrying capability. However, when considering the amount of losses produced by the semiconductors it is clearly of interest to also look at other solutions. Therefore alternative concepts have been presented recently, combining semiconductors with a mechanical switch, often referred to as a hybrid [1]. The hybrid concept largely reduces the losses in the system since in normal operation the current is by-passed and not flowing through the main semiconductor breaker branch. The key components of the hybrid breaker are the fast mechanical switch and the switch assisting the commutation of the current to the main semiconductor breaker branch. Of course it would be of interest to have a purely mechanical breaker. The conceptual difference in the mechanical alternative is the use of mechanically operated circuit breakers for interruption of the current instead of semiconductors. There already exist a number of different mechanical HVDC switches such as neutral bus switches (NBS), neutral bus ground switches (NBGS) and metallic return transfer breakers (MRTB) to mention a few [2]. Some of these earlier breakers may interrupt up to 8 ka [3], but the main obstacle is that they are far too slow to meet the demands of the HVDC Grid during faults. The existing mechanical solutions have interruption times in the order of ms whereas the required interruption time for a HVDC Grid should be significantly lower [4]. However, recent development of breaker actuators has made it possible to realize mechanical alternatives for DC interruption even for the application of HVDC Grids. By introducing, for instance, electromagnetic actuators, [5], [6], the operation speed of the interrupters may be increased significantly. This is especially true when considering the use of interrupters where the moving mass and the stroke are limited. By using this type of actuator the reaction time and precision in the operation is greatly improved. This is of high importance when considering the fast detection and operation needed for a HVDC Grid application. By equipping a standard AC interrupter with such a drive, the demand of faster fault current interruption may be accomplished. 2

3 Figure 1. Relation between existing and suggested concepts for HVDC circuit breakers. There are two main alternatives for designing a mechanical HVDC breaker mentioned in the literature, passive and active resonance, [2]. Both concepts uses the generation of a current zero crossing by establishing an oscillating current superimposed on the DC current. These concepts typically consist of three parallel branches. The conduction path carries the nominal current during operation. This path typically only consists of an AC circuit breaker. Secondly an oscillation path is needed, to generate the current zero, consisting of at least a capacitor and in most cases an inductor, unless the stray inductance in the connecting leads are sufficient. Finally a third branch is used to dissipate the magnetically stored energy after interruption, this may, for instance be an arrester tuned to the appropriate voltage level. The passive resonance, or self-exited growing oscillation, concept takes advantage of the negative resistance and natural fluctuations of the arc generated in the circuit breaker when opened. In combination with the inductance and capacitance this forms a series resonance circuit causing a growing oscillation. The amplitude ultimately exceeds the amplitude of the nominal/fault current, hence, creating a current zero in the AC circuit breaker and the current is interrupted. The circuit analysis and testing presented here mainly focuses on the active oscillation concept. Some of the reasons for choosing this technique are: no time delay for generating the oscillation, compact resonance circuit and fast operation. These features are described in some more detail after discussing the principle of the active injection. The basic circuit is similar to the passive but with the difference that the capacitor is precharged and a closing switch is introduced to initiate the injection. Figure 2. Operation principle of the active injection circuit. The operation principle of the mechanical breaker with active injection is displayed in figure 2. In most cases it is necessary to introduce a series reactor, 1, to limit the rate of rise of the fault current. At fault detection, or normal disconnection, the AC circuit breaker is opened, 2. After fully opening the contacts, still with full current, the injection switch, 3, is closed. A 3

4 resonance circuit is formed superimposing an AC current on the DC current through the interrupter. If the oscillation is chosen correctly a zero crossing is generated and the current is interrupted. After interruption the current is commutated to the arresters, 4, and the energy stored in the reactor and system inductances are dissipated while the current decays to zero. When reaching zero current the residual current disconnecting circuit breaker, 5, is opened and the system is disconnected. As mentioned, the active injection scheme has a number of advantages. In contrast to the passive solution the maximum resonance amplitude is reached immediately, i.e. no time delay needed for generating the oscillation makes the operation faster. If, on top of this, an AC circuit breaker is used which have the ability to interrupt high current derivatives the circuit may be designed using a high injection frequency. There are two major benefits with the high injection frequency. The time delay for injection becomes negligible but also as important the capacitance of the injection capacitor can be kept low which reduces both cost and size of the breaker. However, there are also a number of challenges with this concept. A solution has to be implemented for charging the injection capacitor and a reliable closing switch must be introduced. Since the time between fault detection and current injection needs to be kept low, fast contact separation of the AC interrupter is required. One possible solution for doing this is to use a number of smaller interruption units instead of one large to accomplish the dielectric clearance needed. If this solution is selected it also introduces the challenge of controlling the recovery voltage sharing across multiple contacts, [7], [8]. DESIGNING A HVDC BREAKER WITH ACTIVE INJECTION When designing the HVDC breaker circuit, as well as deciding component values, there are a number of factors that need to be considered. If a prospective fault current of approx. 10 ka is assumed the injection current amplitude needs to be high enough to secure safe interruption but still in a range that limits the size and stress of the other components. Other dimensioning factors are: rate of rise and fall time of the nominal/fault current, maximum permitted current derivative at interruption, derivative of the recovery voltage and the amount of energy to dissipate. The rate of rise of the fault current may be controlled by introducing a current limiting reactor and even though the rector is necessary in many cases it is of interest to limit the size and cost. This may to a large extent be influenced by the performance of the breaker. High current interruption capability and fast operation will reduce the value of the inductance. In the end this will of course create an optimization exercise with system parameters, reactor value, breaker performance and total cost. The rate with which the current needs to decay after interruption is determined by the circuit inductance together with the counter voltage established after interruption. The counter voltage is decided by the protection level of the surge arresters intended for energy dissipation. This also creates an optimization process which includes: the fault clearing time and insulation design level of the surge arresters and the injection capacitors. Fast current decay is at the expense of higher arrester and capacitor insulation requirements. As mentioned earlier high injection frequency makes it possible to limit component values and decrease interruption time. The frequency, and amplitude, used is limited by the allowed current derivative, di/dt, at interruption. This value is very crucial for reliable operation of the breaker and should be investigated thoroughly during the design process. The allowed di/dt mainly depends on the type of AC interrupter used, the actual fault current and the derivative of the recovery voltage after current interruption, [9]. The later may, at least to some extent, be influenced by a snubber capacitor across the interrupter. Given the allowed di/dt the frequency and amplitude of the injection current is calculated from the capacitor value and the 4

5 charging voltage of the capacitor. An illustration of the relation between di/dt and du/dt and an example of a safe operation area is displayed in figure 3. Figure 3. An illustration of the relation between di/dt and du/dt and the safe operation area. The surge arresters serve two purposes, limiting the recovery voltage and dissipating the stored magnetic energy. When determining the energy specification of the arresters not only the energy stored at interruption needs to be considered but also the operation sequence decided by the system requirements, such as, number of auto reclosing operations. In the case where a multiple interrupter scheme is selected also the voltage sharing is an important factor. Since there is a statistical distribution regarding interruption performance and different post arc currents after interruption, [8], it might be necessary to introduce voltage grading components in-order to secure the overall performance. Since the HVDC breaker is a key component for realizing a HVDC Grid the performance and reliability is crucial. As described it is important to consider numerous aspects when designing the complete breaker. To accomplish sufficient reliability the statistical influence of all the designing factors needs to be understood, not only on the component level but also for the whole set-up. It should be mentioned that the optimization of the component values may look quite different depending on the system requirements for a specific installation. However, if a thorough investigation is performed on the various parameters it is, in most cases, possible to find an optimum implementation. DEMONSTRATOR SET-UP AND TESTING To be able to verify the performance during realistic conditions a mechanically based HVDC breaker with performance data relevant for a HVDC Grid implementation was designed, built and tested. The target data for the demonstration module were, 80 kv system voltage, 10 ka fault current interruption capability and current interruption within 5 ms. The 80 kv level is chosen since it is suitable for realization, transportation and testing, not because it is a limit of the concept. It is also considered as a reasonable building block for reaching higher voltages, such as, 320 kv or beyond. Below is a schematic of the circuit used for verification testing, figure 4. 5

6 Figure 4. Schematic of the high power test circuit used for verification testing. TEST RESULTS During the high power test several fault current levels were applied between 2 and 10.5 ka. It is of interest to cover the operation spann from normal load current to full prospective fault current since it is not obvious that the most difficult interruption is the one with the highest current. Below are a number of test results displayed, figures 5-6. It was decided that for these tests it would be most sufficient to use a short circuit generator to apply the test current. The generator may supply both the required current and voltage, as well as, provide a time constant that resembles the case of a DC fault with a current limiting reactor. 1.5 x I Breaker U Breaker HVDC Breaker x Breaker Current Breaker Voltage x 10-3 Figure 5. Breaker current, incl. arrester, and recovery voltage during fault current test 10kA. 6

7 5000 HVDC Breaker x Breaker Current I Breaker U Breaker Breaker Voltage x 10-3 Figure 6. Current through the interrupter and recovery voltage during load current test 2kA. Figure 7. Setting up the high power tesing of the mechanical HVDC circuit breaker. CONCLUSIONS To be able to design a mechanical circuit breaker for HVDC Grid applications it is of great importance to thoroughly investigate all the parameters influencing the performance and optimization. To show that it is possible to build a mechanically operated HVDC breaker, which has the possibility to fulfill HVDC Grid requirements, an 80 kv demonstrator was built and tested. The results verify functionality and all fault current levels, up to 10.5 ka, were interrupted successfully within 5 ms. In conclusion the mechanical HVDC breaker has very low losses and almost as fast interruption as the hybrid concepts. This performance makes it an attractive solution for certain applications in a future HVDC Grid. 7

8 BIBLIOGRAPHY [1] J. Häfner, B. Jacobson Proactive Hybrid HVDC Breakers-A key innovation for reliable HVDC grids (Cigré Bologna, Paper 0264, 2011). [2] D. Andersson, A. Henriksson Passive and active DC breakers in the three Gorges-Changzhou HVDC project (Proc. Int. Conf. Power Systems, 2001, pp ). [3] S. Tokuyama, K. Arimatsu, Y. Yoshioka, Y. Kato Development and interrupting tests on 250kV 8kA HVDC circuit breaker (IEEE Trans. Power App. Syst., vol. PAS-104, no.9, pp , Sep [4] C. M. Franck HVDC Circuit Breakers: A Review Identifying Future Research Needs (IEEE Transactions on power delivery, Vol. 26, No. 2, April 2011 [5] T. Genji, O. Nakamura, M. Isozaki, M. Yamada, T. Motita, M. Kaneda 400 V class high-speed current limiting circuit breaker for electric power system (IEEE Trans. Power Del., vol. 9, no. 3, pp , Jul [6] W. Holaus, K. Frohlich Ultra-fast switches-a new element for medium voltage fault current limiting switchgear (Proc. IEEE Power Eng. Soc. Winter meeting, 2002, vol. 1, pp [7] A. Lee, P. Slade, K. Yoon, J. Porter The development of a HVDC SF6 breaker (IEEE Trans. Power Apps. Syst. Vol. 104, No. 9, pp , Sept. 1985). [8] M. Sugati, T. Igarashi, H. Kasuya, S. Okabe, Y. Matsui, E, van Lanen, S. Yanabu Relationship Between the Voltage Distribution Ratio and the Post Arc Current in Double-Break Vacuum Circuit Breakers (IEEE Trans. On Plasma Science, Vol. 37, No. 8, pp , Aug. 2009). [9] W. J. Premerlani Forced commutation performance of vacuum switches for HVDC breaker application (IEEE Trans. On Power App. and Syst., Vol. PAS-101, No. 8, pp , Aug. 1982). 8