CN100414799C - A kind of superconducting cable protection method - Google Patents

Abstract

The invention discloses a superconducting cable protection method, which adopts three different protection strategies according to the magnitude of the cable current. When the cable current is less than the minimum quench current, it is mainly protected by monitoring the temperature rise, flow and pressure difference at both ends of the cable; when the cable current is between the minimum quench current and the limit fault current, real-time calculation Inverse time protection is realized by heat accumulation; when the cable current is greater than the limit current, the protection trips without delay. The invention can effectively overcome the defects of the prior art. For severe short-circuit faults, the superconducting cable can be cut quickly; in the case of small fault currents, unnecessary frequent cuts can be avoided while ensuring safe operation, and the reliability of its power supply can be improved. In addition, the invention has fast detection speed and high reliability, mainly utilizes the port characteristics of superconducting cables, can simplify the number of sensors, has small heat leakage, and is convenient for engineering application.

Description

A kind of superconducting cable protection method

technical field

The invention belongs to the technical field of electric engineering, and in particular relates to a superconducting cable protection method, which is especially suitable for superconducting cable quench protection under short circuit conditions.

Background technique

After the superconducting cable is quenched, it will change from the superconducting state to the normal state, which will have an adverse effect on the superconducting cable body. Quench detection and protection of superconducting cables is one of the key technical issues that need to be solved in the practical application of superconducting cables. At present, the detection of superconducting cable quenching is mainly based on non-electrical quantities, including temperature, pressure, flow velocity, and ultrasonic detection (see Yu Xiaoyan, Li Jingdong, Tang Yuejin. Basic Research on Quench Detection of Superconducting Power Devices. Chinese Engineering Science, 2003, Vol. 5, No. 10, pp. 73, 77). The non-electric quantity detection method requires a large number of sensors, and it is difficult to timely reflect the sudden quench of the superconducting cable caused by the short-circuit current in the power grid. If the quench lasts for too long, it will seriously endanger the safe operation of the superconducting cable.

Contents of the invention

The object of the present invention is to overcome the above disadvantages and provide a new method for protecting superconducting cables. The method can overcome the shortcoming of slow response to short-circuit current in the simple non-electric quantity detection method, improve the reliability and accuracy of detection, and provide a reliable guarantee for the safe operation of superconducting cables.

A method for protecting a superconducting cable provided by the present invention comprises the following steps:

(1) Set the setting value, including: temperature rise overshoot setting value ΔT _set , low flow setting value L _set , inlet temperature overshoot setting value T _set , pressure difference overshoot setting value ΔP _set , definite limit protection delay setting Value t _set , current lower limit setting value I _low , limit current setting value I _set , heat accumulation setting value θ _set , heat accumulation delay counter setting value T _dclay ;

(2) Collect the three-phase current of the superconducting cable and calculate its true effective value I _a , I _b , I _c ; collect the instantaneous values of the inlet and outlet temperature, liquid nitrogen flow rate, liquid nitrogen pressure, and inlet and outlet pressure, and calculate and obtain the following average Values: inlet temperature T _aln , T _bln , T _cln of three-phase cable, outlet temperature T _aOut , T _bOut , T _cOut of three-phase cable, liquid nitrogen flow L _a , L _b , L _c of three-phase cable, three-phase Liquid nitrogen pressure P _a , P _b , P _c ; and then calculate the calculated temperature rise of the three-phase cable ΔT _a , ΔT _b , ΔT _c , the calculated value of the pressure difference between the inlet and outlet of the superconducting cable ΔP, the heat of the three-phase superconducting cable Cumulative quantities Q _a , Q _b , Q _c ;

(3) Judge the value of each variable and determine the protection mode:

(3.1) Superconducting cable temperature, pressure, flow signal detection:

(A) Judging whether the following formulas (a1)-(a4) are satisfied, if any of the conditions is satisfied, then trip immediately for protection, and then go to step (2); otherwise, continue to perform the following steps;

(ΔT _a ≥ΔT _set )∪(ΔT _b ≥ΔT _set )∪(ΔT _c ≥ΔT _set ) (a1);

(T _aln ≥T _set )∪(T _bln ≥T _set )∪(T _cln ≥T _set ) (a2)

(L _a ≥ L _set )∪(L _b ≥ L _set )∪(L _c ≥ L _set ) (a3)

(ΔP≥ΔP _set ) (a4)

(B) Judging whether the following formulas (b1)-(b4) are satisfied, if any of the conditions is satisfied, then trip immediately for protection, and then go to step (2); otherwise, continue to perform the following steps;

((T _aln ≥T _Dset )∪(T _bln ≥T _Dset )∪(T _cln ≥T _Dset ))∩(t≥t _set ) (b1)

((ΔT _a ≥ΔT _Dset )∪(ΔT _b ≥ΔT _Dset )∪(ΔT _c ≥ΔT _Dset ))∩(t≥t _set ) (b2)

((L _a ≥ L _Dset )∪(L _b ≥L _Dset )∪(L _c ≥L _Dset ))∩(t≥t _set ) (b3)

((ΔP≥ΔP _Dset ))∩(t≥t _set ) (b4)

Among them, T _Dset is the fixed value of definite time protection temperature crossing range, ΔT _Dset is the definite time limit protection temperature rise definite value, L _Dset is the definite time protection low flow set value, ΔP _Dset is definite time protection pressure difference crossing range definite value value;

(3.2) When the three-phase current of the superconducting cable satisfies (I _a <I _low )∪(I _b <I _low )∪(I _c <I _low ), judge and process according to (A) and (B), and start Heat accumulation return delay counter; if the value of the heat accumulation return delay counter reaches the fixed value T _{dclay of} the heat accumulation return delay counter, the calculation results of the heat accumulation quantities Q _a , Q _b , Q _c and the heat accumulation delay counter are cleared;

(3.3) When (I _low ≤I _a <I _set )∪(I _low ≤I _b <I _set )∪(I _low ≤I _c <I _set ), according to the formula $Q=\underset{\mathrm{<figure-callout\; id="0"\; label="time"\; filenames="C20041006119600111.png"\; state="\{\{state\}\}">time</figure-callout>}0}{\overset{\mathrm{time}1}{\&Integral;}}{i}^2\left(t\right)\mathrm{dt}$ Calculate the heat accumulation inside the superconducting cable and make a detection judgment: if (Q _a ≥ θ _set ) ∪ (Q _b ≥ θ _set ) ∪ (Q _c ≥ θ _set ), the protection action trips and returns to step (2); if The heat accumulation Q does not reach the preset heat accumulation setting value θ _set , continue to accumulate, and perform the following steps; where, i(t) is the current of the superconducting cable at time t, and time0 takes the value that the cable current is greater than the lower limit setting value The starting time of , time1 takes the value of the current time;

(3.4) When (I _a ≥I _set )∪(I _b ≥I _set )∪(I _c ≥I _set ) is satisfied, the protection trips without delay action; otherwise, continue to perform the following steps;

(4) Go to step (2) and execute in a loop.

The advantages of the present invention are:

(1) Different protection strategies are adopted according to the magnitude of the current of the superconducting cable, and the respective advantages of the electrical quantity (that is, the three-phase current of the superconducting cable) and the non-electrical quantity in the quench detection can be used comprehensively, which can effectively overcome the problem based solely on the non-electrical quantity The quench detection method has the disadvantages of slow response speed and difficulty in effectively protecting the quench fault caused by short-circuit current.

(2) Different protection strategies are adopted according to the magnitude of the short-circuit current, and the superconducting cable can be quickly cut off in case of a serious fault to ensure the safety of the superconducting cable. In the case of a small fault current, on the premise of ensuring the safe operation of the superconducting cable, unnecessary frequent cutting of the superconducting cable can be avoided, and the reliability of its power supply can be improved.

(3) Superconducting current is used for quench detection, which has fast response speed, high reliability and strong practicability.

(4) The proposed quench detection scheme mainly utilizes the port characteristics of superconducting cables, which can simplify the number of sensors, facilitate installation, have small heat leakage, and facilitate engineering applications.

Description of drawings

FIG. 1 is a schematic structural diagram of a layered protection method adopted in the present invention.

Detailed ways

The present invention adopts three different protection strategies according to the magnitude of the superconducting cable current, that is, when the cable current is less than the quench current, the temperature, pressure and flow signals of the superconducting cable are mainly used to realize protection; when the current is greater than the quench current but less than the limit When the current is high, the inverse time overload protection is realized according to the heat accumulation inside the superconducting cable; when the cable current is greater than the limit current, the protection trips without delay.

The present invention involves the calculation of two key set values: limit current set value I _set and heat accumulation set value θ _set . There may be different calculation and setting methods according to the specific structure and cooling conditions of the superconducting cable. An example is given below to illustrate.

(1) Calculation of limit current setting value I _set

Due to the fast action speed of the non-time limit protection, the action setting "limiting current setting value" is determined according to adiabatic and current sharing. The maximum temperature that a superconducting conductor is allowed to withstand when determining the limiting current value is the minimum temperature among the temperature that the insulating material can withstand, the temperature that causes a fundamental change in the properties of the superconductor, and the temperature at which the superconducting strip is destructively damaged.

The heat balance process based on the calculation is that after the cable is quenched, it is mainly shunted by the silver base, the heat generation of the silver base is the heat source, and the silver base and the superconductor together act as the heat absorber. The corresponding differential formula is as follows:

${\mathrm{\&theta;}}^{\&prime;}=\frac{\frac{{\mathrm{\&rho;}}_{\mathrm{Ag}}\left(\mathrm{\&theta;}\right)i^2\left(t\right)}{A_{\mathrm{Ag}}}}{d_{\mathrm{Ag}}{A}_{\mathrm{Ag}}{C}_{\mathrm{Ag}}\left(\mathrm{\&theta;}\right)+d_{\mathrm{hts}}{A}_{\mathrm{hts}}{C}_{\mathrm{hts}}\left(\mathrm{\&theta;}\right)}.$ In the formula, ρ _Ag is the resistivity of silver, d _Ag is the density of silver, A _Ag is the cross-sectional area of silver base, C _Ag is the specific heat of silver, d _hts is the density of superconducting wire, A _hts is the superconducting wire Cross-sectional area, C _hts is the specific heat of the superconducting wire, and i is the cable current. Using the improved Euler method to solve this differential equation, the relationship curve of θ-i can be obtained. Taking θ as the maximum temperature allowed by the superconducting cable (determined by the design parameters of the superconducting cable body), the limiting current setting value I _set is obtained.

(2) Calculation of heat accumulation setting value θ _set

The calculation model is the same as that used in the calculation of the limit current setting value, and the calculation formula of the heat accumulation setting value is as follows: ${\mathrm{\&theta;}}_{\mathrm{sct}}=\underset{I_0}{\overset{{\mathrm{<figure-callout\; id="0"\; label="I"\; filenames="C20041006119600111.png"\; state="\{\{state\}\}">I</figure-callout>}}_0+\mathrm{\&Delta;I}}{\&Integral;}}\frac{(d_{\mathrm{Ag}}{A}_{\mathrm{Ag}}{C}_{\mathrm{Ag}}\left(\mathrm{\&theta;}\right)+d_{\mathrm{hts}}{A}_{\mathrm{hts}}{C}_{\mathrm{hts}}\left(\mathrm{\&theta;}\right))\&Center\; Dot;A_{\mathrm{Ag}}}{{\mathrm{\&rho;}}_{\mathrm{Ag}}}\&Center\; Dot;\mathrm{d\&theta;}.$ In the formula, ρ _Ag is the resistivity of silver, d _Ag is the density of silver, A _Ag is the cross-sectional area of silver base, C _Ag is the specific heat of silver, d _hts is the density of superconducting wire, A _hts is the superconducting wire Cross-sectional area, C _hts is the specific heat of the superconducting wire, I ₀ is the initial temperature of the superconducting cable (take the normal and stable operating temperature of the cable, such as 73K), Δ _T is the maximum allowable temperature rise of the superconducting cable (by the superconducting cable determined by the design parameters of the body).

The present invention needs to set the setting value first, including: temperature rise setting value ΔT _set , low flow setting value L _set , inlet temperature setting value T _set , pressure difference setting value ΔP _set , definite protection delay The fixed value t _set , the current lower limit setting value I _low , the limit current setting value I _set , the heat accumulation amount setting value θ _set , and the heat accumulation delay counter setting value T _dclay .

The present invention needs to collect the three-phase current of the superconducting cable and calculate its true effective values I _a , I _b , and I _c . It is also necessary to collect the instantaneous values of the inlet and outlet temperature, liquid nitrogen flow rate, liquid nitrogen pressure, and inlet and outlet pressure, and calculate the following average values: the inlet temperature T _aln , T _bln , T _cln of the three-phase cable and the outlet temperature T of the three-phase cable _aOut , T _bOut , T _cOut , liquid nitrogen flow rate La , L _b , L _c of the three-phase cable, three-phase liquid nitrogen _pressure Pa , P _{b ,} P _c . According to the above average calculation, the calculated temperature rise of the three-phase cable ΔT _a , ΔT _b , ΔT _c , the calculated value of the pressure difference between the inlet and outlet of the superconducting cable ΔP, and the heat accumulation of the three-phase superconducting cable Q _a , Q _b , Q _c .

The above variable values that need to be collected can all be obtained through sampling.

According to the magnitude of the current of the superconducting cable, the present invention divides the protection method into three layers as shown in FIG. 1 .

1. Level I:

This level of protection corresponds to the self-fault protection of the cable, including high AC loss and cooling system failure. When the superconducting cable current i is less than the current lower limit setting value I _low i.e. i<I _low (wherein i is the cable current, I _low is the lower limit setting value of the current, where the value is the critical current of the superconducting cable), the first I Layers of protection. If the heat accumulation calculation of the superconducting cable is being processed before, the calculation is stopped, and the relevant return delay counter is started; if the delay time is up, the heat accumulation calculation result and the heat accumulation delay return counter are cleared.

This layer of protection is mainly protected by monitoring the temperature rise at both ends of the cable, inlet temperature, flow, and pressure difference changes, including various non-electrical quantity quick-break protection and definite time protection. The specific criteria are as follows:

(1) Quick break protection

(1) The action criterion for over-the-top over-the-top protection against temperature rise is (ΔT _a ≥ ΔT _set ) ∪ ( ΔT _b ≥ ΔT _set ) ∪ ( ΔT _c ≥ ΔT _set ), where ΔT _a , ΔT _b , and ΔT _c are three-phase The calculated value of the cable temperature rise, ΔT _set is the setting value of the temperature rise beyond the range.

(2) The action criterion of the inlet temperature cross-regional quick-break protection is (T _a ≥ T _set )∪(T _b ≥T _set )∪(T _c ≥T _set ), where T _a , T _b , and T _c are three-phase The measured value of the cable inlet temperature, T _set is the setting value of the temperature cross-range.

(3) The action criterion of the low-flow quick-break protection is (L _a ≥ L _set ) ∪ (L _b ≥ L _set ) ∪ (L _c ≥ L _set ), where L _a , L _b , and L _c are three-phase cable Flow measurement value, L _set is the low flow setting value.

(4) The action criterion of the pressure difference cross-region quick-break protection is (ΔP≥ΔP _set ), where ΔP is the calculated value of the pressure difference between the inlet and outlet of the superconducting cable, and ΔP _set is the setting value of the pressure difference cross-region.

If one of the above conditions is met, it will trip immediately for protection; if none of the above conditions are met, the following definite time protection judgment will be made.

(2) Definite time protection

(1) The action criterion of the inlet temperature cross-range definite time protection is ((T _a ≥T _Dset )∪(T _b ≥T _Dset )∪(T _c ≥T _Dset ))∩(t≥t _set ), where T _a , T _b , and T _c are the measured values of the three-phase cable inlet temperature, T _Dset is the fixed value of the definite time protection temperature crossing the domain, and t _set is the definite value of the definite time protection delay.

(2) The action criterion of definite time-limited protection for temperature rise is ((ΔT _a ≥ΔT _Dset )∪(ΔT _b ≥ΔT _Dset )∪(ΔT _c ≥ΔT _Dset ))∩(t≥t _set ), where ΔT _a , ΔT _b , ΔT _c are respectively

The calculated value of the temperature rise of the three-phase cable, ΔT _Dset is the fixed value of the temperature rise of the definite time protection, and t _set is the definite value of the definite time protection delay.

(3) The action criterion for low flow definite time protection is ((L _a ≥ L _Dset )∪(L _b ≥L _Dset )∪(L _c ≥L _Dset ))∩(t≥t _set ), where L _a , L _b , L _c are three-phase cable flow measurement values respectively, L _Dset is the fixed value of low flow protection for definite time limit protection, and t _set is the definite value for definite time limit protection delay.

(4) The action criterion of definite time-limited protection of differential pressure is ((ΔP≥ΔP _Dset ))∩(t≥t _set ), where ΔP is the calculated value of the pressure difference between the inlet and outlet of the superconducting cable, and ΔP _Dset is the definite time limit The protection pressure difference exceeds the threshold setting value, and t _set is the fixed-time protection delay setting value.

If one of the above conditions is met, trip for protection; if not, proceed to the following steps.

The quick-break protection and the definite-time protection generally need to be put into use at the same time in the device, and the setting value of the quick-break protection must be greater than the setting value of the corresponding definite time protection to ensure the selectivity of the two protections. This is because, when the non-electric signal deviates from the normal value, the superconducting cable is allowed to continue running for a period of time t _set , and the cable does not need to be cut off immediately, so at this time, the cable is protected by a definite time limit; the non-electric signal deviates from the normal value. When it is large, it will seriously affect the safety of the superconducting cable, so the cable must be cut off immediately by means of quick-break protection.

Second, the second level:

Level II protection corresponds to the protection of the cable current between the minimum quench current and the larger fault current, that is, the protection when I _low ≤ i<I _set . The basic criterion is (Q≥Q _set ), where Q is the calculated value of the heat accumulation of the superconducting cable, and Q _set is the setting value of the heat accumulation. The tripping time of this layer protection has inverse time-limit characteristics, that is, the more severe the heat, the shorter the tripping time.

When the cable current exceeds the lower limit setting value of the current and is less than the limit current setting value (ie I _low ≤ i<I _set ), the root mean square value of the cable current is obtained from the real-time sampling data, according to the formula $Q=\underset{\mathrm{<figure-callout\; id="0"\; label="time"\; filenames="C20041006119600111.png"\; state="\{\{state\}\}">time</figure-callout>}0}{\overset{\mathrm{time}1}{\&Integral;}}{i}^2\left(t\right)\mathrm{dt}$ The heat accumulation inside the superconducting cable is calculated for detection and judgment. The value of time0 is generally the initial moment when the cable current is greater than the lower limit setting value, and the value of time1 is the current moment.

If the calculated heat accumulation Q reaches the preset heat accumulation setting value θ _set , the protection action trips; if the heat accumulation Q does not reach the preset heat accumulation setting value θ _set , continue to accumulate and continue to perform the following steps.

3. Level III:

Level III protection corresponds to severe short-circuit faults. At this time, due to the large short-circuit current, the time for the cable to withstand overcurrent is short, and the cable needs to be cut off as soon as possible. The protection strategy adopted is that when the current of any phase superconducting cable exceeds the limit current setting value I _set (ie i≥I _set ), the protection will trip without delay. The action criterion is: (I _a ≥I _set )∪(I _b ≥I _set )∪(I _c ≥I _set ). Among them, I _a , I _b , and I _c are the three-phase currents of the superconducting cable respectively, and I _set is the setting value of the limiting current.

Claims (1)

1. a hyperconductive cable guard method comprises the steps:

(1) setting value is set, comprises: territory setting value Δ T is got in temperature rise _Set, flow is on the low side setting value L _Set, inlet temperature is got over territory setting value T _Set, pressure differential is got over territory setting value Δ P _Set, definite time protection time-delay definite value t _Set, lower current limit setting value I _Low, limiting current setting value I _Set, heat history amount setting value θ _Set

(2) gather hyperconductive cable three-phase current and calculate its real effective I _a, I _b, I _cGather the instantaneous value of gateway temperature, liquid nitrogen flow, liquid nitrogen pressure, inlet and outlet pressure, and calculate the following mean value of acquisition: the inlet temperature T of threephase cable _Aln, T _Bln, T _Cln, the outlet temperature T of threephase cable _AOut, T _BOut, T _COut, the liquid nitrogen flow L of threephase cable _a, L _b, L _c, three-phase liquid nitrogen pressure P _a, P _b, P _cCalculate the temperature rise calculated value Δ T of threephase cable again _a, Δ T _b, Δ T _c, hyperconductive cable import and export pressure differential calculating value Δ P, three-phase hyperconductive cable heat history amount Q _a, Q _b, Q _c

(3) each variate-value is judged, and definite protected mode:

(3.1) hyperconductive cable temperature, pressure and flow signal detect:

(A) judge whether following formula (a1)-(a4) satisfies, if wherein there is arbitrary condition to satisfy, then tripping operation is protected at once, forwards step (2) to; Otherwise, continue to carry out following step;

(ΔT _a≥ΔT _set)∪(ΔT _b≥ΔT _set)∪(ΔT _c≥ΔT _set) (a1)；

(T _aln≥T _set)∪(T _bln≥T _set)∪(T _cln≥T _set) (a2)

(L _a≥L _set)∪(L _b≥L _set)∪(L _c≥L _set) (a3)

(ΔP≥ΔP _set) (a4)

(B) judge whether following formula (b1)-(b4) satisfies, if wherein there is arbitrary condition to satisfy, then tripping operation is protected at once, forwards step (2) to; Otherwise, continue to carry out following step;

((T _aln≥T _Dset)∪(T _bln≥T _Dset)∪(T _cln≥T _Dset))∩(t≥t _set) (b1)

((ΔT _a≥ΔT _Dset)∪(ΔT _b≥ΔT _Dset)∪(ΔT _c≥ΔT _Dset))∩(t≥t _set)(b2)

((L _a≥L _Dset)∪(L _b≥L _Dset)∪(L _c≥L _Dset))∩(t≥t _set) (b3)

((ΔP≥ΔP _Dset))∩(t≥t _set) (b4)

Wherein, T _DsetFor the definite time protection temperature is got over the territory definite value, Δ T _DsetFor territory definite value, L are got in the definite time protection temperature rise _DsetBe definite time protection flow definite value on the low side, Δ P _DsetFor the definite time protection pressure differential is got over the territory definite value;

(3.2) as the satisfied (I of hyperconductive cable three-phase current _c＜I _Low) ∪ (I _b＜I _Low) ∪ (I _c＜I _Low) time, according to (A) and (B) judge and handle; Start delay counter, if delay time arrives, with heat history amount Q _a, Q _b, Q _cResult of calculation and the counter O reset of heat history delay time;

(3.3) as (I _Low≤ I _a＜I _Set) ∪ (I _Low≤ I _b＜I _Set) ∪ (I _Low≤ I _c＜I _Set) time, according to formula $Q=\underset{\mathrm{time}0}{\overset{\mathrm{time}1}{\&Integral;}}{i}^2\left(t\right)\mathrm{dt}$ Calculate the heat accumulation of hyperconductive cable inside, detect judgement: if (Q _a〉=θ _Set) ∪ (Q _b〉=θ _Set) ∪ (Q _c〉=θ _Set), the protection would trip goes back to step (2); If heat accumulation Q does not reach default heat history amount setting value θ _Set, continue accumulation, carry out following step; Wherein, the electric current of hyperconductive cable when i (t) is moment t, the time0 value is the initial moment of cable current greater than the lower limit setting value, the time1 value is a current time;

(3.4) as satisfied (I _a〉=I _Set) ∪ (I _b〉=I _Set) ∪ (I _c〉=I _Set) time, protect no deferred action tripping operation; Otherwise, continue to carry out following step;

(4) forward step (2) to, circulation is carried out.

Images