Short-circuit protection for electrical dc and ac networks of ships and offshore installations

Short-circuit current protection system for electrical DC and AC networks of ships and offshore systems

The invention relates to a short-circuit current protection system for electrical DC and AC networks of ships and offshore systems; such a system is known, for example, from WO 02/15361 A1.

DC and AC networks of ships designed as on-board networks and electrical driving systems (whereby ships are understood as both above and underwater ships) and offshore systems are island networks in character and are subject to special requirements in their operation.

DC networks or DC systems are widely used, for example, in underwater ships. With the DC systems supplied so far and planned new DC systems for underwater ships, prospective short-circuit currents of approx. 100 to max. 320 kA possible, which in the event of a short circuit leads to a high thermal load and, due to high mechanical forces from surge currents, to a considerable dynamic load on the entire DC system.

AC networks or three-phase systems are widespread as electrical systems in surface vessels. In such surface vessels, high short-circuit powers occur, in particular with large on-board electrical system powers.

WO 02/15361 A1 discloses a short-circuit current protection system with a high-temperature superconductor (HTS) current limiter through which the short-circuit current flows, a cryostat for receiving and cooling the HTS current limiter and a refrigerator for recondensing those present in the cryostat gaseous refrigerant. The HTS current limiter has the effect that the short-circuit current is limited in the first current rise. The short-circuit current can then be switched off, for example by a circuit breaker. After switching off the short circuit and a short cooling time, the HTS current limiter can be switched back into the affected circuit.

Short-circuit current protection systems of the aforementioned type have already been implemented and presented to the public. Due to the high short-circuit currents that occur particularly in DC and AC networks of ships and offshore platforms and the high requirements for small size and weight, operational and intrinsic safety, ease of installation and maintenance of these short-circuit current protection systems in ships and offshore Platforms have not yet been implemented in operational practice for such systems. The advantages over conventional short-circuit current protection systems, e.g. with a predetermined melting point, could therefore not yet come into play.

It is therefore an object of the present invention to provide a short-circuit current protection system of the type mentioned at the outset, which is designed to be practical and can be used in electrical DC and AC networks in ships and offshore systems.

This object is achieved according to the invention by the teaching of claim 1. Advantageous embodiments of the invention are the subject of the dependent claims.

Due to the design of the short-circuit current protection system that is suitable for ships and offshore platforms in terms of function and structure, the implementation of the short-circuit current protection system in the operational practice of ship and offshore platforms can be made possible and the advantages over conventional ones can be achieved System without HTS current limiter.

The means for electrical integration enable the protection system to be integrated into a potentially short-circuit in the AC or DC network of a ship or an offshore platform, as well as integration into control, automation, monitoring and diagnostic systems in these systems. The mechanical integration means that the protective system can be installed quickly and easily in the systems. The means for process engineering integration allow access to resources already existing in the system (eg electricity, water) from other systems, so that these resources do not have to be stored or generated and made available within the system. The protection system can thus be kept more cost-effective and smaller in size and weight.

The assembly and testing of the system can be carried out with little effort by designing the system as a preassembly unit that can be integrated electrically, procedurally and mechanically into ships and / or offshore platforms. The pre-assembly and testing can therefore be carried out outside the ship or the offshore platform and does not have to take place in the confined space available on board ships and offshore platforms. This avoids assembly and testing errors and high system availability.

A particularly simple and quick installation in the ship or in the offshore installation is possible if the pre-assembly unit comprises a control cabinet or a control panel in which the high-temperature superconductor (HTS) current limiter, the cryostat and the refrigerator are arranged are.

The protection system preferably has a current-time characteristic that is optimized for the short-circuit current limitation in electrical DC and AC networks of ships and offshore platforms.

According to a particularly advantageous embodiment of the invention, the high-temperature superconductor (HTS) current limiter limits the short-circuit current to a maximum of three times the nominal current or continuous rated current of the current limiter within one millisecond, regardless of the level of the unaffected short-circuit current. In this way, mechanical and thermal loads on the components of the ship's AC or DC network or offshore system can be kept low in the event of a short circuit. In addition, the operation of error-free subnetworks can be maintained, which means, for example, that the maneuverability of the ship can also be guaranteed in the event of a short circuit.

The high-temperature superconductor (HTS) current limiter preferably has a thin layer of yttrium barium copper oxide (YBCO) as the high-temperature superconductor. The thin layer can be applied, for example, to a strip conductor or a plate conductor.

On the basis of the thin-film material, the short-circuit current can be extremely quickly limited (e.g. within three milliseconds to a maximum of three times the rated current or continuous rated current), which makes it particularly suitable for use in ship and of-shore systems. Such high limiting speeds are not possible with so-called bulk superconductors, which are used not as a thin layer but as a solid material.

In addition, due to the extremely low thermal capacity of the thin layers, rapid recooling can be achieved in less than 2s. Also because of the quick reconnectability in terms of time, an HTS current limiter on a thin-film basis is particularly suitable for achieving a high level of supply reliability on ships and offshore platforms.

Further advantages of the invention, further details and advantageous refinements of the invention can be found in the drawings and the description of the drawings.

In detail show in exemplary execution:<dl id="dl0001"><dt>FIG 1:</dt><dd>the switching state of a HTS current limiter in normal operation</dd><dt>FIG 2:</dt><dd>the switching status of a HTS current limiter in the event of a short circuit</dd><dt>FIG 3:</dt><dd>a system structure and interfaces of a short-circuit current protection system</dd><dt>FIG 4:</dt><dd>a schematic representation of a test field structure for testing a HTS current limiter</dd><dt>FIG 5:</dt><dd>Current setting measurements for different short-circuit currents</dd><dt>FIG 6:</dt><dd>Current, voltage and resistance curve on the HTS current limiter of a short-circuit current protection system during a typical limiting process</dd><dt>FIG 7:</dt><dd>Current, voltage and resistance curve at the HTS current limiter of a short-circuit current protection system at the beginning of the limitation process</dd><dt>FIG 8:</dt><dd>Current and voltage curve when addressing a target melting point according to the prior art</dd></dl>

Superconductivity opens up completely new applications, for example as a high-temperature superconductor current limiter (HTS current limiter). If the current exceeds a design value, a superconducting element reaches the normal conducting state within significantly less than 1 ms. The property of the material to tip from the superconducting to the normal conducting state is called the "quench" and is only of interest for use in a current limiter. In the superconducting state, the HTS current limiter has almost no measurable resistance. The current is limited by increasing the resistance and is caused by the (over) current flowing through, which must be above the so-called critical current of the HTS current limiter. The HTS current limiter works as a primary release and does not require an external trigger signal to respond (increase resistance) or trigger (quench). Triggering by an external magnetic field is practically irrelevant for the application, since the magnetic fields to be used would have to be much too large.

HTS current limiters are used in switchgear when possible short-circuit currents come close to the permissible breaking currents of the circuit breakers or can even go beyond them.

If an HTS current limiter is installed in the circuit of the short circuit, it changes its resistance from zero to the (design) value when its type-dependent response current is exceeded, which enables the short circuit to be safely controlled by the switchgear. The normally conductive resistance element is only designed for short-term operation; Therefore, after the HTS current limiter responds, the short-circuit current from the switchgear must be interrupted in order to protect the resistance element against thermal overload.

The HTS current limiter can preferably be used in island networks, in particular in electrical AC and DC networks of ships and offshore platforms. In the DC voltage network of submarines in particular, increases in performance led to the switchgear structure being adapted for some conceivable short-circuit cases and so-called target melting points being provided. A short circuit becomes manageable when a target melting point responds, but leads to its failure (similar to a fuse) and the switchgear is only fully available again after a new target melting point has been installed.

When using an HTS current limiter, there is the advantage that the switchgear is fully operational again after eliminating the short circuit and a short cooling time (approx. 2 seconds) to achieve superconductivity.

A superconducting current limiter significantly improves the electrical grid conditions, especially in island grids, such as those found on board ships and offshore platforms. However, the current limiter must be adapted to its surroundings. This is about system integration<ul id="ul0001" list-style="bullet" compact="compact"><li>Electrical</li><li>Process engineering</li><li>Mechanically</li></ul>

The HTS current limiter realizes its function with the help of one or more HTS elements. An HTS element is, for example, a plate conductor or ribbon conductor with an applied thin layer of YBCO material. Such HTS elements can be interconnected to form HTS modules in order to meet the requirements for the HTS current limiter from an electrical point of view (eg rated voltage, rated current). The HTS elements are therefore the key components of the system. Other components for fulfilling the task are:<ul id="ul0002" list-style="bullet" compact="compact"><li>Cryostat with lid</li><li>Built-in components in the cryostat with constructive mounting of the module and sensors</li><li>Power supply to the HTS module</li><li>Refrigerator consisting of cold head and compressor<ul id="ul0003" list-style="none"><li>a) Electrical function of the protection system FIG. 1 shows a current limiter 1 through which an operating current I flows during normal operation. A voltage drop U_FCL over the superconductor of the current limiter 1 is not measurable. A slight voltage drop, which occurs as a result of the Cu connections and the contact resistances, can be measured across the current leads 32. The switching state of the current limiter 1 in the event of a short circuit KS in the circuit 6 and an equivalent circuit diagram are shown in FIG. 2. When the operating current through the HTS module rises above the value of the tripping current, the resistance of the superconductor (quench) increases suddenly. A voltage UFCL drops suddenly across the current limiter 1. This rise in the voltage UFCL across the current limiter 1 is given to an adjustable input 3 of a pulse trigger (capacitor trigger) 2 by means of a voltage divider, not shown. The signal processing in the pulse trigger 2 takes place via an optical interface and lasts less than 0.5 ms. The pulse release output 4 is actuated and controls the (connected in parallel) release magnets 7 (voltage release and locking magnet) of the circuit breaker 5. The circuit breaker 5 opens the circuit 6 after the switch's own time.</li><li>b) Selection and function of the auxiliary facilities<ul id="ul0004" list-style="none"><li>ba) cryostat A cryostat is used. The cryostat is a double cylindrical non-magnetic container. A vacuum (10-6 bar) is created between the two cylinders, which are bolted to the cylinder head like a flange. The vacuum is created by a vacuum pump. The container must be vacuum-tight. Therefore the vacuum, once created, is permanent. In the event that the vacuum still has to be pumped in, a corner valve is provided on the outer wall of the cryostat, to which a mobile vacuum pumping station can be connected. Maintaining the negative pressure is irrelevant to the actual function of the current limiter. Pumping can therefore also take place when the current limiter is live. Maintaining the vacuum is important so that the losses in the cold remain sufficiently low so that the refrigerator can recondense the evaporated amount of nitrogen in the closed cryostat at any time.</li><li>bb) cryostat cover The cryostat cover contains all the necessary sealed bushings for the following components:<ul id="ul0005" list-style="bullet" compact="compact"><li>Power supply Power supplies are used which can permanently carry a current equal to the continuous rated current. These bushings are thermally optimized.</li><li>Lifter For filling the cryostat with liquid nitrogen.</li><li>Pressure relief The cryostat is operated closed. If the refrigerator fails, an overpressure will build up in the cryostat due to the evaporating nitrogen. For this purpose, a pressure relief valve and a rupture disc are provided as security (back-up protection). The pressure relief valve opens at 0.5 bar pressure. The rupture disc made of sheet steel is installed on the cover using a flange. The permissible overpressure at which the rupture disc will tear open is 0.6 bar.</li><li>Bathroom heater The purpose of the bath heater is that the cryostat can be warmed up more quickly, e.g. B. for repairs inside the cryostat. This is particularly important in ships and offshore plants, in which downtimes due to repair work must be kept as short as possible. For this purpose, a heating foil is glued to a steel sheet, which is operated with mains voltage. The current is determined by the resistance of the film in order to be able to use fuses that are normally present. Power is supplied via a cable that can be plugged into the controller located in the control cabinet. The plug-in connection is only inserted if necessary.</li><li>Level probe (level) The purpose of the level probe is to monitor the level of liquid nitrogen in the cryostat. At the same time, the supply of liquid nitrogen can be stopped when filling via a solenoid valve.</li><li>Cable entry The implementation serves for additional cables, which, for. B. for temperature sensors and cold head counter heating are necessary.</li><li>light The inside of the cryostat can be observed with an external camera. Sufficient light is required for this, which is optically guided into the interior of the cryostat via an optical waveguide (external light source). In this way, the operating status of the HTS current limiter can be monitored, countermeasures taken immediately in the event of abnormalities, and thus high availability of the HTS current limiter can be guaranteed.</li><li>camera In order to be able to carry out a vertical diagnosis, a b / w finger camera (CCD) is provided, which is supplied externally via the control cabinet. A commercially available monitor with video input can be connected. Additional bushings are available as a reserve.</li><li>Refrigerator The refrigerator works on the GiffordMc-Mahon principle and has a cooling capacity of 300W @ 77K (commercially available).</li></ul> In principle, other known methods are possible, such as. B. pulse tube, according to the Stirling principle or mixture refrigeration system. The electrical connection power is 7kW 400V 3∼AC and the need for water to recool the oil compressor is 270 l / h at a flow temperature of 25 ° C. The refrigerator consists of a cold head and a compressor. The compressor generates the necessary pressure of the working medium (helium). The working medium (supply and return) is fed to the cold head via flexible pressure lines. The refrigerator itself cannot be regulated. The refrigerator is regulated indirectly by means of counter heating, which is attached to the condenser of the cold head. The counter heating is switched on and off by a temperature control device so that the temperature of the liquid nitrogen is 77K at ambient pressure. The required temperature sensor is a Pt100, which is in the liquid nitrogen. Another Pt100 is attached to the capacitor to check the temperature of the cold head. Alternatively, pressure should also be included as a control criterion. However, the necessary compressor is preferably an oil-free linear compressor which requires little maintenance, which is particularly important for ships and offshore systems.</li></ul></li><li>c) Interfaces The interfaces and integration means relevant for integrating the system into a ship or an offshore platform are listed below:<ul id="ul0006" list-style="none" compact="compact"><li>ca) Electrical interfaces<ul id="ul0007" list-style="bullet" compact="compact"><li>Busbar The power supply lines on the cryostat cover are connected to the control cabinet via cables.</li><li>Connection auxiliary voltage 400V 3∼</li><li>For connection signals see table 1</li></ul><img file="EP1526625A2_D0001.tif" /></li><li>cb) Mechanical and procedural interfaces Mechanical and procedural interfaces and associated integration means are listed in Tab. 2.<img file="EP1526625A2_D0002.tif" /><img file="EP1526625A2_D0003.tif" /></li></ul>3 shows a basic illustration of a system structure and interfaces of a short-circuit current protection system 40. The HTS current limiter 1 with its cryostat, the circuit breaker 5, the refrigerator consisting of a cold head 17 and a compressor 10, and a device 19 for level measurement of the liquid Refrigerant in the cryostat and a bath heater 18 are arranged in a control panel or switch cabinet 11. In addition, in the control panel or the control cabinet 11 is a temperature controller 14 for the cold head 17, a temperature controller 16 for the bath heater 18, a controller 15 for the level of the refrigerant (e.g. liquid nitrogen) and 5 necessary components (e.g. a pulse trigger 2) for controlling the circuit breaker. The cold head 17 and the bath heater 18 have suitable temperature sensors (for example pt100 elements) for measuring the temperature of the refrigerant. The control panel or the control cabinet 11 with the components mounted therein represents a pre-assembly unit that can be integrated electrically, procedurally and mechanically into ships and / or offshore platforms. The circuit breaker 5 and the HTS current limiter 1 are in this case in a circuit 13 of the ship or the offshore platform integrable. The cold head 17 of the current limiter 1 is by the also in the control panel or the control cabinet 11 arranged compressor 10, which in turn can be connected to a water intermediate cooling system 12 of the ship or the offshore platform. Since both the compressor 10 and the cold head 17 are arranged in the control panel or the control cabinet 11, the connecting lines between the two components can be kept short. Pressure losses on these connecting lines can thus be kept low and a good degree of effectiveness of the short-circuit current protection system 40 can be achieved.</li><li>d) Switching behavior of the HTS current limiter in short-circuit current protection systems for electrical DC and AC networks of surface and underwater vessels and offshore systems.</li></ul></li></ul>

4 shows a schematic representation of a test field structure for testing and comparing the switching behavior of an HTS current limiter 30 that can be used in short-circuit current protection systems for ships and offshore platforms and a target melting point SSS known from the prior art.

A test stand 20 with a circuit breaker Q1 and a setpoint melting point SSS is used for safety if the current limiter 30 should not respond as desired. In addition, the function of the target melting point SSS is to be demonstrated for comparison (see FIG. 8).

A switch cabinet 22 essentially contains two circuit breakers Q2 and Q3, a controller 23 and a pulse release (capacitor release) KA for quickly opening the circuit breaker Q3. The test is controlled by the controller 23. Circuit breakers Q2 and Q3 are closed and opened at specified times, thus also determining the short-circuit duration.

Apart from the preliminary attempts to set the current (see FIG. 5), the circuit breaker Q2 is always closed and the circuit breaker Q3 is switched off via the capacitor release KA if the voltage UFCL at the current limiter 30 exceeds a voltage threshold. In all experiments, the voltage at the output of the capacitor release KA is measured and the correct response of the capacitor release KA is checked.

The current I is measured via a current transformer installed in the control cabinet 22 as well as the voltages at the HTS current limiter UFCL, at the circuit breaker Q3 UQ3, and at the output of the capacitor release Uimp, in each case via 100x or 1000x probes. A transient recorder with 12-bit resolution and a maximum sampling rate of 1 million measuring points per second is used for this.

The feed 21 into the test field according to FIG. 4 takes place from a 3-phase transformer with downstream mercury vapor rectifiers. The open circuit voltage therefore shows a ripple of approx. 15% with a period of 3.333 ms. In the following, the effective value (rms value) of the open circuit voltage is used as the DC voltage.

The maximum short-circuit current that can be achieved at DC 900 V is approx. 150 kA. 5 shows current setting measurements (calibration curves) for three different short-circuit currents Ik (150 kA, 75 kA, 25 kA). The maximum achievable current increase di / dt for Ik = 150 kA is 15.7 kA / ms. These values ​​are measured with a full short circuit at the entrance to the test field. The structure to be tested increases the resistance and inductance of the circuit, so that Ik and di / dt decrease.

The typical switching behavior of the HTS current limiter 30 is based on a measurement with Urms = 925 V and a prospective short-circuit current Ik, prosp. = 150 kA shown. 6 shows the entire period from the beginning of the short circuit (t = 0 ms) to the opening of the circuit breaker (t = 22.5 ms) until the arc extinguishes (t = 41 ms). The voltage UFCL at the HTS current limiter 30 clearly shows the residual ripple of the rectifier. In the current course Ik, this ripple is overlaid by a falling tendency: after approx. 5 ms, the current falls below 1000 A and the current to be switched off by the circuit breaker Q3 is just under 700 A. The resistance R of the HTS current limiter 30 increases suddenly, then afterwards steadily on until switch Q3 opens and remains almost constant at approx. 1.25 Ω, while an arc burns in circuit breaker Q3. After the arc is extinguished, no more energy is converted in the HTS current limiter 30 and it cools back to operating temperature within a few seconds. The HTS current limiter 30 and thus the switchgear are now ready for use again.

The beginning of the limitation process is shown in FIG. 7 shown. From t = 0, the current begins to rise at approx. 12 kA / ms. The voltage UFCL and the resistance R only begin to rise shortly before the forward current of 2730 A is reached. Resistance R increases to approx. 0.6 Ω within less than 0.1 ms; this value corresponds to the resistance of the HTS elements just above the critical temperature of 88 K. The peak voltage reaches 1.5 times the rated voltage and is below 1000V again after 0.5 s. The short-circuit current Ik is limited to less than three times the rated current or continuous rated current of the HTS current limiter in 0.25 ms and then drops significantly. Due to the quick limitation to values ​​below the nominal current, the voltage drop at the source is also very short. As the voltage UPE at the test field input shows, it only takes approx. 0.25 ms in the example above.

Comparison measurement with a target melting point

For comparison with the HTS current limiter 30, a target melting point SSS at DC 600 V and Ik, prosp. tested approx. 140 kA. For this purpose, the output of the test stand 20 is bridged with a short-circuit bar 31, see also FIG. 4th The test is carried out with a target melting point rated for DC 300 V at DC 600Vrms, since no target melting points for higher voltages are available.

FIG. 8 shows the current profile without (Ik, prosp, dotted) and with the target melting point (I, solid line). After the target melting point has melted (approx. 95 kA), the voltage Usss at the target melting point increases and after a further approx. 8 ms, the circuit breaker opens (see voltage ULS at the circuit breaker). The voltage peak at the circuit breaker is 2.5 times the DC voltage.

The limitation process results from the arc which arises with the melting of the target melting point, with the associated noise and gases from the melting process.