The present disclosure relates to a method for limiting current rise in a high voltage DC network under fault conditions. It also relates to a high-voltage DC circuit breaker having a switching assembly for interrupting a high-voltage DC current and an inductive current rise limiter arranged in series to the switching assembly.
In high-voltage direct current (HVDC) systems (DC grids), known mechanical circuit breakers have to be able to switch off large currents very quickly because there are no natural current zero crossings, thus making it difficult to extinguish an arc in the circuit breaker. For example, in case of a ground fault, the current can rise quickly, and therefore a circuit breaker has to be fast, which makes it difficult to use mechanical circuit breakers at all.
To alleviate these issues, it has been known to align an inductive current rise limiting element in series to the switching assembly of the circuit breaker. Such a current rise limiting element may, for example, be an air coil with a constant inductance of about 100 mH. The inductance inherently limits the rise rate of the current in the event of a fault, thereby giving the switching assembly more time for switching off the current.
It can be advantageous to use current rise limiters of even higher inductance, but this can lead to system instabilities and can, for example, impair the system's ability to support fast (but regular) load changes. Also, inductive current limiters of large inductance can be bulky and expensive.
A method is disclosed for limiting a current rise in a high voltage DC network, the method comprising: selecting a current rise limiter which has an inductance that increases with a time-derivative dI/dt of a current I; and arranging the inductive current rise limiter in the network.
A high-voltage DC circuit breaker is also disclosed, comprising: a switching assembly for interrupting a high-voltage DC current I; and an inductive current rise limiter arranged in series to said switching assembly, wherein said current rise limiter has an inductance that will increase with a time-derivative dI/dt of said current I.
Exemplary embodiments and features thereof will become apparent from the following detailed description. The detailed description makes reference to the annexed drawings, wherein:
Exemplary methods and circuit breakers that can limit the current rise in a high voltage DC network effectively are disclosed herein.
For example, methods, circuit breakers, their use and high-voltage DC networks including such circuit breakers are disclosed.
The current rise can be limited by arranging an inductive current rise limiter in the network. The current rise limiter can have an inductance that increases with the current I that flows through the current rise limiter, and/or with the time-derivative dI/dt of the current I.
Hence, in an exemplary normal mode of operation, where the current I or its time derivative dI/dt is within a nominal range, the inductance of the current rise limiter is comparatively small and therefore has a comparatively weak influence on stability of the network. However, in the event of a fault, the current I and its time-derivative dI/dt increase, which leads to an increase of the inductance of the limiter and therefore can improve the limiter's ability to limit the rise of the current.
Exemplary methods as disclosed herein can be particularly useful in, for example, a high-voltage DC circuit breaker. Such a circuit breaker, which can be used to break a high-voltage DC current, can include a switching assembly for interrupting the high-voltage DC current as well as the inductive current rise limiter arranged in series to the switching assembly.
The use of limiters whose inductance rises with the current I or its time derivative dI/dt has been known for AC networks. However, in AC systems, these limiters have been used as current limiters, not as current rise limiters. When the AC current increases, their inductance increases, which in turn leads to a limitation of the AC current.
In a DC system, this AC based mechanism would not work and therefore these limiters could not be used as current limiters. As such, those skilled in the art would not have been motivated to use such limiters in a DC system. However, the present inventors have recognized that such limiters can be used as current rise limiters in a DC system.
Advantageously, the current rise limiter has an inductance that increases with the current I. Such a limiter generates an additional limiting effect on the rise rate of the current only when the current has reached a level above nominal, while its influence on current fluctuations at nominal current is low, thereby maintaining the system's capability to support sudden load changes.
Definition(s):
For purposes of describing exemplary embodiments in greater detail, the following exemplary definitions will be adopted.
The term “high voltage” encompasses voltages of 36 kV or more.
A current rise limiter having an “inductance that increases with a current” or “with a time-derivative dI/dt of said current” designates any device whose inductance increases automatically with the current or its time-derivative. In such a device, there may, for example, be a functional, bijective relationship between inductance and current (or time-derivative), or the relationship may not be bijective, but for example exhibit hysteresis effects. The change of inductance may for example also be triggered actively once the current or current rise exceeds a certain threshold. Also, the decrease of the inductance, when the current or its time-derivative drops back, may not be instantaneous, but rather may only occur after a certain delay, such as in embodiments where a superconductor has to regain its superconductivity.
Overview:
Those skilled in the art will appreciate that the network can be much more complex than that, with at least three voltage sources and/or loads on both sides of the circuit breaker. In addition, the current I may change direction when the distribution of loads and sources in the network changes dynamically.
A purpose of switching assembly 1 is to switch off the current I, for example in the event of a ground fault as indicated by 5. In the embodiment of
Arc gap 7 is arranged in a resonant circuit having a capacitor 8 and an inductance 9 (inductance 9 may for example be formed by a discrete inductor, or by the self inductance of the leads of the cables and the switch). In addition, an arrester (varistor) 10 is arranged parallel to switch 6.
As already mentioned, current rise limiter 2 can have an inductance that rises with the current I; for example, with the absolute value of the current I, or with the time-derivative dI/dt, such as with the absolute value of the time-derivative dI/dt.
An exemplary operation of the circuit breaker of
In
As can be seen, the current I begins to rise quickly. However, this leads to an increase of the inductance of current rise limiter 2, which in turn increasingly limits the rise rate of current I. In the example of
Also, and as can be seen in
At a time t2, the oscillations reach an amplitude where they are sufficient to compensate the current I and therefore to generate a current zero crossing in the lower branch, for example, in the arc, at which time the arc is extinguished and the current I1 in the lower branch is cut off. Another exemplary possibility is to use an inverse current injection in order to actively create a zero current in the lower branch. Current I will continue to flow through the upper branch and can be interrupted by a switch 10b at time t3. Hence, the current zero crossing generated by one of these features allows for use of known AC breaker technology, such as the switch 6 or mechanical switch 6 or circuit breaker 6 or puffer circuit breaker 6 or even self-blast circuit breaker 6.
Due to the rise limitation induced by current rise limiter 2, more time remains for the creation of a current zero condition before the current reaches a level where it can not be compensated by these oscillations or the injected current.
Current Rise Limiter:
In the following, some exemplary advantageous embodiments of current rise limiter 2 are discussed.
In the exemplary embodiment of
A first coil 12 is wound around each core 11, with the two coils 12 being arranged in series and carrying the current I; for example, the first coils 12 are in series to switching assembly 1.
In addition, a second coil 13 is wound around both cores 11. An auxiliary DC current Iaux is generated by a current source 14 and fed through second coil 13.
The winding sense of the various coils can be chosen such that one of the coils 12 increases its inductance for large positive currents I while the other one increases its inductance for large negative currents I. This is discussed in more detail for the left hand core 11 of
The auxiliary current Iaux in the second coil 13 generates a magnetic field Haux which drives the iron core 11 into saturation above the saturation flux density Bsat. The permeability of the iron core 11 and thus the inductance of the current rise limiter 2 is low. The current I in the first coil 12 generates in at least one core 11 an additional magnetic field H1 in the opposite direction of Haux causing a reduction of the total magnetic flux density B in core 11.
In the absence of current I, core 11 is saturated by flux B; for example, B1 is above B sat. When a current I starts to flow in coil 12, it partially compensates in at least one of the cores 11, the magnetic field Haux of the auxiliary current Iaux. When the resulting magnetic flux density B1 in the iron core 11 remains higher than the saturation flux density Bsat, the inductance experienced by first coil 12 is low. However, when current I increases during a fault situation, H1 will increase as well and will start to lower the resulting total magnetic flux density B1 below Bsat. Thus, core 11 becomes unsaturated. The permeability of the unsaturated core 11 is increased, and therefore also the inductance of current rise limiter 2 increases.
The exemplary current rise limiter 2 of
Another exemplary embodiment of current rise limiter 2 is shown in
In the embodiment of
Another exemplary embodiment of current rise limiter 2 is a shielded iron core limiter as shown in
A superconducting shield 17, including (e.g., consisting of) a coil of superconducting material, is arranged between coil 12 and core 11, thereby shielding coil 12 magnetically from core 11 while the current I is low. As soon as the current I is high enough to induce a current of sufficient amplitude in shield 17, shield 17 looses its superconductive properties, the field of coil 12 penetrates into core 11, and the effective permeability of core 11 increases the inductance of coil 12. The resistivity of the no longer superconducting coil 17 acts like a resistance in the primary coil 12.
Another exemplary embodiment of a current rise limiter 2 is shown in
When the current sensor detects a rise of the current, a small charge is used as a stored energy mechanism to interrupt the switch (main conductor). When the main conductor has been opened, the current flows through the parallel fuse, where it is limited to, for example, within less than one millisecond and is then shut down. The current then flows through the parallel inductance 20, which has an impedance value that is higher than that of the closed Is-limiter 21.
Several Is-limiters can be arranged in series if a single Is-limiter is unable to carry the full voltage over inductance 20.
The current sensor of the Is-limiter can be designed to be triggered, if current I exceeds a given threshold. Alternatively, or in addition thereto, it can be triggered if the time-derivative dI/dt exceeds a given threshold or a combination of both thresholds.
Notes:
Those skilled in the art will appreciate that
Some possible embodiments of current rise limiter 2 are described herein. However those skilled in the art will appreciate that any other type of current rise limiter can be used, if for example its inductance increases with I or dI/dt. For example, any inductive AC fault current limiter technology with an inductance increasing with the AC current can be used as a DC current rise limiter in accordance with the present disclosure.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
1: switching assembly
2: current rise limiter
3: voltage source
4: load
5: fault
6: switch
7: arc gap
8: capacitor
9: inductance
10: arrester (varistor)
10
b: switch
11: iron core
12, 13: first and second coils
14: current source
17: superconducting shield
20: inductance
21: Is-limiter
Number | Date | Country | Kind |
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11001813.2 | Mar 2011 | EP | regional |
This application claims priority as a continuation application under 35 U.S.C. §120 to PCT/EP2012/053525, which was filed as an International Application on Mar. 1, 2012 designating the U.S., and which claims priority to European Application 11001813.2 filed in Europe on Mar. 4, 2011. The entire contents of these applications are hereby incorporated by reference in their entireties.
Number | Date | Country | |
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Parent | PCT/EP2012/053525 | Mar 2012 | US |
Child | 14017876 | US |