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7?DÂ<;7JKH;ÂÂÂ 9>7FJ;HÂÂÂÂ>;7:ÂE<ÂEKHÂJ?C; DC circuit breakers: an important role in the continuity and quality of service The development of DC circuit breakers boosts confidence in the evolving HVDC market. High voltage direct current (HVDC) links are of particular interest to large networks because they provide an efficient means of transmitting power MRTB SCHEME 30 Think T&D///Winter 2008-2009 over long distances without capacitive or inductive losses. They also make it possible to improve network stability by coupling with neighboring networks with simultaneous decoupling of frequency and phase. However, an important issue with HVDC is current interruption, a much more challenging task than with alternating current. Direct current interruption cannot be performed by a conventional circuit breaker. In AC networks the source drives the current periodically through zero (every one-hundredth of a second for a 50 Hz current), and current zero is the ideal instant to interrupt: while the arc power decreases, the circuit breaker can flush the contact area from conducting plasma and the current is permanently interrupted within 20 milliseconds. An analogy of the AC principle could be represented by a child’s swing: current zero corresponds to the highest point where the swing stops rising and starts to return in the opposite direction. At this instant there is no kinetic energy, which is equivalent to the absence of magnetic energy in the cables and lines of the AC network (only electrostatic energy remains). For DC networks the analogy is somewhat different, as there is no natural current zero. It is more like a train that runs at 9>7FJ;HÂ > MORE FOUR DIFFERENT TYPES OF DC BREAKER DEVICE As in any AC substation, commutating switching devices are needed on the DC side of HVDC stations. The main current commutating switches include up to four different types of DC circuit breaker device (see figure): t.FUBMMJD3FUVSO5SBOTGFS#SFBLFS.35# in case of monopolar operation, it is used to commutate direct current from the ground return in the metallic return (for corrosion and safety reasons, it is often not permitted to operate an HVDC scheme continuously at a high ground current); t(SPVOE3FUVSO5SBOTGFS4XJUDI(354 this is used to allow reconfiguration from monopolar to bipolar operation with metallic return to monopolar operation with ground return without interrupting power; this step is required before the scheme is allowed to return to bipolar operation; t/FVUSBM#VT4XJUDI/#4 UIJTJTSFRVJSFE to clear an earth fault in a pole converter by commutating the healthy pole current into the electrode line; t/FVUSBM#VT(SPVOEJOH4XJUDI/#(4 if there is an open-circuit fault on the electrode line, the NBGS is closed to provide a temporary earth connection; it allows the scheme to continue transmitting power. MRTB: Metallic Return Transfer Breaker; GRTS: Ground Return Transfer Switch; NBS: Neutral Bus Switch; NBGS: Neutral Bus Grounding Switch. constant speed: the only way to stop the train is to have it roll up a slope and then block it at still point. In the DC network, a voltage—opposing the current flow—has to build up in order to reach current zero. Another challenge DC current commutation is also challenging. This difficulty can be illustrated through another mechanical model: ramping up the DC current to 3,000 A in an overhead line that is, say, 1,000 km long, implies exerting 4.5 MJ. “This amount of energy is equivalent to lifting a 1 ton mass up to a height of 450 m,” explains Wolfgang Grieshaber, research engineer at AREVA T&D. “When dropped from this height, the mass impacts the ground at a speed of about 100 m/s. A switching action could be compared to catching this falling mass before it hits the ground.” In AC networks, typical arcing times are of the order of 20 milliseconds; this time elapses when decelerating the mass over the last meter of fall to deposit the mass gently onto the ground. Obviously a breaker alone cannot be used to commutate the direct current. So the absence of naturally occurring current zeros in DC networks means 3 “These devices are arranged in various ways to cope with every situation, depending on the customer’s requirements; they can perform a variety of tasks such as reconfiguration of the scheme and protection against faults, short circuits, etc.,” explains Bruno Kayibabu, Principal Engineer. The common duty for all these switches is to commutate up to full load direct current from one circuit into an existing parallel circuit; what differentiates them is the circuits involved. Though modest components, they play an important role in the continuity and quality of service. DC BREAKER TYPES Think T&D///Winter 2008-2009 31 7?DÂ<;7JKH;ÂÂÂ 9>7FJ;HÂÂÂÂ>;7:ÂE<ÂEKHÂJ?C; > MORE TESTING OF DC CIRCUIT BREAKERS: ARC VOLTAGE IS THE KEY The oscillations between circuit-breaker and LC circuit branches are crucial for ensuring direct current interruption. “The faster the current oscillation grows, the faster the oscillating current is able to reach the value of the DC current and create a current zero suitable for interruption,” says Grieshaber. These oscillations strongly depend on the arc voltage behavior, which is related to the current and the arc length. 3 32 the breaker has to be upgraded with elements that are able to create an artificial current zero. “This was achieved by adding an LC 1 oscillating circuit in parallel to the AC breaker,” says Bruno Kayibabu, Principal Engineer, AREVA T&D. The DC circuit breaker scheme on page 30 is just such an example with its two parallel branches, the first being a series LC circuit and the second a Surge Arrestor using a Metal Oxide Varistor (MOV). The switching scenario is a twostep process: s3TEP)NTHECLOSEDPOSITIONTHECURRENT flows through the breaker, LC and MOV branches playing no role. On a trip order, the breaker draws an arc of increasing length between its contacts, but the current flowing through the breaker (Iarc) remains almost unchanged from the initial value when the breaker was closed. As the arc voltage increases, the arc starts interacting with the LC circuit: the naturally occurring fluctuations of the arc voltage initiate current oscillations in the loop formed by the breaker and LC branch. The oscillating current (Ic) increases and finally exceeds the DC current (IDC) that is to be interrupted. Now Iarc has zero crossings and the breaker can interrupt the current. Arcing time is in the order of 20 milliseconds. Think T&D///Winter 2008-2009 s3 TEP 7HEN THE ARC CURRENT IS interrupted, the energy in the line (or cable) is still too high, and the line will continue charging up the capacitor. When the knee point voltage2 of the MOV is reached, it starts conducting, absorbs energy and clamps the voltage to UMOV. This voltage opposes the current flow through the MRTB and will slowly commutate the current into the metallic return. The duration for this action is at least an order of magnitude higher than for the arc extinction. Consequently, the energy dissipated in the MOV is the dimensioning factor. The DC circuit breaker concept that AREVA T&D has developed is simple and reliable as there are no active components included, only passive ones. “It will allow AREVA T&D to provide its customers with a complete set of solutions suitable for any HVDC transmission grid with high confidence and reliability,” concludes Kayibabu. r An LC circuit consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C, in series. 1 2 Knee point voltage: The voltage above which the surge arrester becomes highly conductive. TO KNOW MORE @ [email protected] Wolfgang Grieshaber Test campaigns were performed to clearly define the physical fundamentals of AREVA T&D’s DC circuit breaker solution, i.e., gain knowledge of the arc voltage and current as a function of time. “Then, in order to be able to correctly dimension our components, the goal was to configure the system and verify, on the whole range of current, that the arc’s dynamic resistance (dUarc /dIarc ) stays negative enough so that oscillation amplitudes are sufficient to rapidly interrupt the current under any circumstances,” explains Grieshaber.