METHOD FOR PROTECTING LINES, AND PROTECTION ASSEMBLY

Abstract

In a method for protecting lines, in which a reactor device for reactive power compensation is provided on an electrical line, a resonant current is measured on the line side of the reactor device by a first measuring device after an opening of a circuit breaker. A voltage is measured by a second measuring device after the opening of the circuit breaker. A current in the reactor device is calculated by an evaluation device on a basis of the measured voltage, and the calculated current is subtracted from the measured resonant current by the evaluation device in order to obtain a corrected current.

Claims

1. A method for protecting lines, which comprises the steps of: providing a reactor device for reactive power compensation on an electrical line; measuring a resonant current on a line side of the reactor device by means of a first measuring device after an opening of a circuit breaker; measuring a voltage by means of a second measuring device after the opening of the circuit breaker; and calculating a current in the reactor device by means of an evaluation device on a basis of a measured voltage, and a calculated current is subtracted from the resonant current by means of the evaluation device in order to obtain a corrected current.

2. The method according to claim 1, wherein the corrected current is used by a protection device as an input variable for line protection functions.

3. The method according to claim 2, which further comprises using a threshold value for determining an open state of the circuit breaker for the protection device, wherein the threshold value is lower than in a case of use of the resonant current as the input variable.

4. The method according to claim 1, wherein a main reactor shunt and a neutral reactor shunt are used for the reactor device.

5. The method according to claim 1, which further comprises determining the current in the reactor device for each phase in each case by an analysis of symmetrical components.

6. A protection assembly for protecting lines including an electrical line, the protection assembly comprising: a reactor device for reactive power compensation; a circuit breaker for connecting or disconnecting the electrical line; a first measuring device, disposed on a line side of said reactor device, for measuring a resonant current flowing after an opening of said circuit breaker; a second measuring device configured to measure a voltage after the opening of said circuit breaker; and an evaluation device configured to calculate a current in said reactor device on a basis of a measured voltage, and to subtract a calculated current from a measured resonant current in order to obtain a corrected current.

7. The protection assembly according to claim 6, further comprising a protection device configured to use the corrected current as an input variable for line protection functions.

8. The protection assembly according to claim 7, wherein said protection device is configured to use a threshold value for determining an open state of said circuit breaker, wherein the threshold value is lower than in a case of a use of the measured resonant current as the input variable.

9. The protection assembly according to claim 6, wherein said reactor device has a main reactor shunt and a neutral reactor shunt.

10. The protection assembly according to claim 6, wherein the current in said reactor device is determined by means of said evaluation device for each phase in each case by an analysis of symmetrical components.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] FIG. 1 is an illustration showing a first power supply system;

[0030] FIG. 2 is an illustration showing a second power supply system;

[0031] FIG. 3 is an equivalent circuit diagram for showing symmetrical components; and

[0032] FIG. 4 is a graph showing a simulation for a three-phase current characteristic.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1, there is shown a so-called single-line illustration (single-phase illustration) 1 of an electrical line 2 with a length of 360 km. An injection of electrical power 3, for example a photovoltaic installation, is provided on a busbar 4. A reactor coil 7, 8 is arranged downstream of a circuit breaker 5 along the line 2. The reactor coil has a so-called main reactor shunt 7 and a so-called neutral reactor shunt 8. A current transformer, as a first measuring device, is arranged downstream of the reactor coil 7, 8 on the line side. The current transformer measures a resonant current which can assume, for a short period of time, a comparatively high value in the case of such long lines owing to resonance effects on opening of the circuit breaker 5. This current value may be above a tripping threshold of protective equipment (not illustrated), with the result that the protective equipment cannot perform certain protection functions such as, for example, breaker failure protection.

[0034] The invention proposes arranging a voltage transformer 10, which operates capacitively, for example, or a second measuring device on the line 2 (on the line side in relation to the circuit breaker, illustrated on the right in FIG. 2) in order to measure a voltage present in the reactor coil. On the basis of this voltage, a conclusion can be drawn in each case on the current in the reactor, and this current can be used for correcting the measured resonant current.

[0035] The following values result, for example:

TABLE-US-00001 i. Main ReactorNeutral Reactor Reactor inductance (Ω) 1600 1225 Reactor apparent power (Mvar) 100 100

[0036] When the circuit breaker is opened, the current which is measured in the first measuring device corresponds to the current flowing through the reactor coil. As a result, tripping of a protection device can be terminated with a delay if this measured current is greater than the threshold value for the determination of the disconnected state of the line 2 by the protection device (for example 50 mA).

[0037] For this reason, the invention proposes correcting the measured current by computation by virtue of a calculated current flow through the reactor coil being subtracted from the measured current. This current flow through the coil is calculated on the basis of a voltage measurement by means of a voltage transformer as second measuring device. The voltage transformer is arranged on the line side in relation to the circuit breaker. In addition, a calculated impedance of the reactor coil is used for the calculation.

[0038] An equivalent circuit diagram 20 to the single-line diagram in FIG. 1 is illustrated in FIG. 2 with three phases. Each phase L1, L2, L3 has a circuit breaker 21, 22, 23, wherein the sought actual current I.sub.CB is present for each phase in the circuit breaker (indicated by an arrow for phase L3). In addition, each phase has an ammeter 24, 25, 26, at which in each case the measured current I.sub.CT can be established. The voltage is measured using a voltmeter 31, 32, 33 for each phase. The reactor coil 27-30 has a first part for the so-called main reactor with one coil 27, 28, 29 per phase, wherein the current I.sub.SH to be calculated flows here. Furthermore, a so-called neutral reactor 30 is provided.

[0039] The current I.sub.CB at the circuit breaker can be calculated (for one of the phases) as follows:

I.sub.CB=I.sub.CT−I.sub.Sh.

[0040] The phase voltage is used for calculating I.sub.SH:

V.sub.Ph=I.sub.Sh_N.Math.ZN.sub.Sh+I.sub.Sh.Math.ZL.sub.Sh.

[0041] In order to solve the divided currents into ZN.sub.SH and ZL.sub.SH, so-called symmetrical components are analyzed for the reactor coil. The analysis of symmetrical components is a conventional method in electrical engineering and is known from, for example, Wikipedia (permanent link: https://de.wikipedia.org/w/index.php?title=Symmetrische_Komponenten&oldid=198714718).

I.sub.Sh_A=I1+I2+I0

[0042] The following results for phase A:

[00001]$I_{\mathrm{Sh\_A}}=\frac{V1}{Z_{\mathrm{sh}}}+\frac{V2}{Z_{\mathrm{sh}}}+\frac{V0}{3.Math.{\mathrm{ZN}}_{\mathrm{sh}}+Z_{\mathrm{sh}}}$$I_{\mathrm{CB\_A}}=I_{\mathrm{CT\_A}}-\left(\frac{V1}{Z_{\mathrm{sh}}}+\frac{V2}{Z_{\mathrm{sh}}}+\frac{V0}{3.Math.{\mathrm{ZN}}_{\mathrm{sh}}+Z_{\mathrm{sh}}}\right)$

[0043] Correspondingly, the currents at the circuit breaker can also be calculated for the other two phases B, C on the basis of an analysis based on symmetrical components.

[0044] FIG. 3 shows, by way of example, equivalent circuit diagrams 41-43 for the symmetrical components positive phase-sequence system, negative phase-sequence system, and zero phase-sequence system, wherein the positive phase-sequence system and the negative phase-sequence system have identical impedances Z.sub.SH, and the zero phase-sequence system is defined by Z.sub.SH+3*ZN.sub.SH. The circuit diagram 41 represents the so-called positive phase-sequence system, the circuit diagram 42 represents the negative phase-sequence system, and the circuit diagram 43 represents the zero phase-sequence system (cf. abovementioned Wikipedia article).

[0045] FIG. 4 shows in each case a characteristic for a measured resonant current 80, 81, 82 for the three phases L1, L2 and L3. In this case, at t=0 ms in the simulation, a remote short circuit has been applied on the 350 km-long line. The amplitude of the short-circuit current I (A) is in each case specified as being between 1 A and −1 A (on the secondary side, i.e. at the current transformer), wherein given a transformation ratio of the current transformer of 1000, for example, an amplitude of between 1000 A and −1000 A results on the line on the primary side.

[0046] At time 460 ms, the circuit breaker in the simulation is opened. This is characterized by the perpendicular line 90.

[0047] For a first “measurement” after 450 ms, i.e. prior to opening of the circuit breaker at approximately 460 ms, the following values result (phasor measured variables, specified in each case with absolute value (length) and angle):

TABLE-US-00002 Measured value Absolute value Angle (°) Current phase A 417 mA 135 Current phase B 629 mA 89 Current phase C 519 mA −108 Voltage phase A 60.75 V −49 Voltage phase B 48.41 V −169 Voltage phase C 58.07 V 80

[0048] Since at this time the breaker is still closed, the measured current is not referred to as resonant current. In the following examples, after opening of the breaker, the measured current is referred to as resonant current.

[0049] The following calculated values result from the measured values:

TABLE-US-00003 Calculated variable Absolute value Angle(°) Positive phase-sequence system 55.61 V −46 Negative phase-sequence system 6.37 V −86 Zero phase-sequence system 1.36 V 36 Corrected current phase A 424 mA 147 Corrected current phase B 696 mA 91 Corrected current phase C 505 mA −99

[0050] It can clearly be seen that the corrected current in each case deviates only a little from the measured current in terms of absolute value and phase, prior to opening of the circuit breaker.

[0051] If the measurement is repeated after opening of the circuit breaker at 500 ms, i.e. after opening of the circuit breaker, the following measured values result:

TABLE-US-00004 Measured value Absolute value Angle (°) Resonant current phase A 89 mA 62 Resonant current phase B 30 mA −55 Resonant current phase C 88 mA −170 Voltage phase A 77.98 V −12 Voltage phase B 4.87 V 27 Voltage phase C 76.72 V 81

[0052] The following calculated values result from this:

TABLE-US-00005 Calculated variable Absolute value Angle (°) Positive phase-sequence system 48.44 V −25 Negative phase-sequence system 15.85 V −85 Zero phase-sequence system 37.41 V 34 Corrected current phase A 7 mA −27 Corrected current phase B 3 mA −149 Corrected current phase C 7 mA 104

[0053] By virtue of the correction of the measured resonant current, very low corrected currents (below 10 mA) result which can be considered to be substantially zero (this is the expected value when the breaker is open). The corrected currents are below a typical threshold value of 50 mA which, in the case of protection devices, is considered to be a still existing current flow when the circuit breaker is closed.

[0054] Thereby, the invention enables improved identification of an open breaker for downstream protection devices.

[0055] At 550 ms, the following results:

TABLE-US-00006 Measured value Absolute value Angle (°) Resonant current phase A 91 mA 65 Resonant current phase B 91 mA −54 Resonant current phase C 92 mA −173 Voltage phase A 63.14 V −24 Voltage phase B 60.95 V −149 Voltage phase C 65.59 V 96

[0056] The following calculated values result from this:

TABLE-US-00007 Calculated variable Absolute value Angle (°) Positive phase-sequence system 63.17 V −26 Negative phase-sequence system 1.74 V −43 Zero phase-sequence system 2.85 V 105 Corrected current phase A 7 mA −22 Corrected current phase B 5 mA −98 Corrected current phase C 8 mA 113

[0057] By virtue of the correction of the measured resonant current, in turn very low corrected currents (below 10 mA) result which enable improved fault identification in the protective equipment.