Current differential protection method for self-adaptive half-wavelength line based on time-difference method

Abstract

A current differential protection method for a self-adaptive half-wavelength line based on a time-difference method. Since an electrical distance of half-wavelength power transmission is long, after a fault occurs, there is an obvious time difference between the actuation times for protecting starting elements at two sides of a line. According to the principles of wave propagation, the position of a fault point can be determined by means of a difference between the actuation times for protecting the starting elements at the two sides of the line. By means of taking the fault point as a differential point, a current value at the differential point can be obtained according to a long line equation by means of the voltage and current at protection-mounted positions at the two sides of the line, and a differential current is then calculated.

Claims

1. A self-adaptive current differential protection method for a half wavelength line based on a time difference method, comprising: determining, by a relay protection device, a location of a fault point according to action time T.sub.M and T.sub.N of protection starting elements on both sides of the half wavelength line; regarding the fault point as a differential point, and determining, by the relay protection device, currents at the differential point according to a long line equation; adaptively changing, by the relay protection device, a braking coefficient and a threshold value according to location of the differential point; and determining, by the relay protection device, whether to perform a current differential protection on the differential point on the basis of the currents at the differential point and the adaptively changed braking coefficient and threshold value; wherein determining the location of the fault point according to the action time T.sub.M and T.sub.N of the protection starting elements on both sides of the half wavelength line comprises: determining a distance of the fault point from a side M of the half wavelength line according to the following formula:
L.sub.FM=((T.sub.M−T.sub.N)v.sub.light+L)/2, in the formula, L.sub.FM being the distance of the fault point from the side M of the half wavelength line, L being a length of the half wavelength line and v.sub.light being a propagation velocity of light; and adaptively changing the braking coefficient and the threshold value according to the location of the differential point comprises: adaptively changing the braking coefficient k and a differential current threshold value I.sub.set according to the following formulae: $\begin{array}{c}k=\{\begin{array}{cc}-0.8L_{FM}/1000+0.8& L_{FM}<1000\mathrm{km}\\ 0& 1000\mathrm{km}\le L_{FM}\le 2000\mathrm{km}\\ 0.8L_{FM}/1000-1.6& L_{FM}<3000\mathrm{km}\end{array}\mathrm{and}\\ I_{\mathrm{set}}=\{\begin{array}{cc}-0.5L_{FM}/1000+0.8& L_{FM}<1000\mathrm{km}\\ 0.3& 1000\mathrm{km}\le L_{FM}\le 2000\mathrm{km}\\ 0.5L_{FM}/1000-0.7& L_{FM}<3000\mathrm{km}\end{array}.\end{array}$

2. The method according to claim 1, wherein actions of the protection starting elements meet the following criterion: $\{\begin{array}{c}\Delta f\left(t\right)=\Delta i_A^2\left(t\right)+\Delta i_B^2\left(t\right)+\Delta i_C^2\left(t\right)\\ .Math.d\Delta f\left(t\right).Math.>f_{\mathrm{set}}={\left(0.07\mathrm{kA}\right)}^2=0.005{\mathrm{kA}}^2\end{array}\hspace{1em},$ where Δi.sub.A(t)=i.sub.A(t)−i.sub.A(t−T), Δi.sub.B(t)=i.sub.B(t)−i.sub.B(t−T), and Δi.sub.C(t)=i.sub.C(t)−i.sub.C(t−T), i.sub.A(t), i.sub.B(t) and i.sub.C(t) are current sampling values of three phases A, B and C with relay protection devices of the half wavelength line, respectively, Δi.sub.A (t), Δi.sub.B (t) and Δi.sub.c (t) are mutations of the current sampling values of three phases A, B and C, T is a power frequency period, Δf(t) is a quadratic sum function of the current mutations, f.sub.set is a setting current value, d in |dΔf(t)| is a difference operator, and when |dΔf(t)| is larger than the setting current value f.sub.set, a protection starting criterion is met.

3. The method according to claim 1, wherein determining the currents at the differential point according to the long line equation comprises: determining the currents at the differential point according to the following long line equation: $\{\begin{array}{c}{I}_{x-}=I_M\cosh \left(\gamma x\right)-\frac{U_M}{Z_c}\sinh \left(\gamma x\right)\\ I_{x+}=I_N\cosh \left(\gamma \left(L-x\right)\right)-\frac{U_N}{Z_c}\sinh \left(\gamma \left(L-x\right)\right)\end{array},$ in the formula, x=L.sub.FM, I.sub.x+ and I.sub.x− being the currents at the differential point, U.sub.M and I.sub.M being phasor values of a voltage and current on the side M of the half wavelength line respectively, U.sub.N and I.sub.N, being phasor values of a voltage and current on a side N of the half wavelength line respectively, Z.sub.c being wave impedance of the half wavelength line and γ being a propagation constant of the half wavelength line.

4. The method according to claim 1, wherein determining whether to perform the current differential protection on the differential point on the basis of the currents at the differential point and the adaptively changed braking coefficient and threshold value comprises: forming following criterion for the current differential protection to determine whether to perform the current differential protection on the differential point on the basis of the currents at the differential point and the adaptively changed braking coefficient and threshold value: $\{\begin{array}{c}.Math.I_{x-}+I_{x+}.Math.\ge k.Math.I_{x-}-I_{x+}.Math.\\ .Math.I_{x-}+I_{x+}.Math.\ge I_{\mathrm{set}}\end{array}\hspace{1em},$ and if such criterion for the current differential protection is met, performing the current differential protection.

5. A non-transitory computer storage medium stored therein instructions that, when executed by a processor, cause the processor to execute a self-adaptive current differential protection method for a half wavelength line based on a time difference method, the method comprising: determining a location of a fault point according to action time T.sub.M and T.sub.N of protection starting elements on both sides of the half wavelength line; regarding the fault point as a differential point, and determining currents at the differential point according to a long line equation; adaptively changing a braking coefficient and a threshold value according to location of the differential point; and determining whether to perform a current differential protection on the differential point on the basis of the currents at the differential point and the adaptively changed braking coefficient and threshold value; wherein determining the location of the fault point according to the action time T.sub.M and T.sub.N of the protection starting elements on both sides of the half wavelength line comprises: determining a distance of the fault point from a side M of the half wavelength line according to the following formula:
L.sub.FM=((T.sub.M−T.sub.N)v.sub.light+L)/2 in the formula, L.sub.FM being the distance of the fault point from the side M of the half wavelength line, L being a length of the half wavelength line and v.sub.light being a propagation velocity of light; and adaptively changing the braking coefficient and the threshold value according to location of the differential point comprises: adaptively changing the braking coefficient k and a differential current threshold value I.sub.set according to the following formulae: $\begin{array}{c}k=\{\begin{array}{cc}-0.8L_{FM}/1000+0.8& L_{FM}<1000\mathrm{km}\\ 0& 1000\mathrm{km}\le L_{FM}\le 2000\mathrm{km}\\ 0.8L_{FM}/1000-1.6& L_{FM}<3000\mathrm{km}\end{array}\mathrm{and}\\ I_{\mathrm{set}}=\{\begin{array}{cc}-0.5L_{FM}/1000+0.8& L_{FM}<1000\mathrm{km}\\ 0.3& 1000\mathrm{km}\le L_{FM}\le 2000\mathrm{km}\\ 0.5L_{FM}/1000-0.7& L_{FM}<3000\mathrm{km}\end{array}.\end{array}$

6. The non-transitory computer storage medium according to claim 5, wherein actions of the protection starting elements meet the following criterion: $\{\begin{array}{c}\Delta f\left(t\right)=\Delta i_A^2\left(t\right)+\Delta i_B^2\left(t\right)+\Delta i_C^2\left(t\right)\\ .Math.d\Delta f\left(t\right).Math.>f_{\mathrm{set}}={\left(0.07\mathrm{kA}\right)}^2=0.005{\mathrm{kA}}^2\end{array}\hspace{1em},$ where Δi.sub.A (t)=i.sub.A (t)−i.sub.A (t−T), Δi.sub.B (t)=i.sub.B (t)−i.sub.B (t−T), and Δi.sub.c (t)=i.sub.c(t)−i.sub.C(t−T), i.sub.A (t), i.sub.B (t) and i.sub.C (t) are current sampling values of three phases A, B and C with relay protection devices of the half wavelength line, respectively, Δi.sub.A (t), Δi.sub.B (t) and Δi.sub.C (t) are mutations of the current sampling values of three phases A, B and C, T is a power frequency period, Δf(t) is a quadratic sum function of the current mutations, f.sub.set is a setting current value, d in |dΔf(t)| is a difference operator, and when |dΔf(t)| is larger than the setting current value f.sub.set, a protection starting criterion is met.

7. The non-transitory computer storage medium according to claim 5, wherein determining the currents at the differential point according to the long line equation comprises: determining the currents at the differential point according to the following long line equation: $\{\begin{array}{c}{I}_{x-}=I_M\cosh \left(\gamma x\right)-\frac{U_M}{Z_c}\sinh \left(\gamma x\right)\\ I_{x+}=I_N\cosh \left(\gamma \left(L-x\right)\right)-\frac{U_N}{Z_c}\sinh \left(\gamma \left(L-x\right)\right)\end{array},$ in the formula, x=L.sub.FM, I.sub.x+ and I.sub.x− being the currents at the differential point, U.sub.M and I.sub.M being phasor values of a voltage and current on the side M of the half wavelength line respectively, U.sub.N and I.sub.N being phasor values of a voltage and current on a side N of the half wavelength line respectively, Z.sub.c being wave impedance of the half wavelength line and γ being a propagation constant of the half wavelength line.

8. The non-transitory computer storage medium according to claim 5, wherein determining whether to perform the current differential protection on the differential point on the basis of the currents at the differential point and the adaptively changed braking coefficient and threshold value comprises: forming following criterion for the current differential protection to determine whether to perform the current differential protection on the differential point on the basis of the currents at the differential point and the adaptively changed braking coefficient and threshold value: $\{\begin{array}{c}.Math.I_{x-}+I_{x+}.Math.\ge k.Math.I_{x-}-I_{x+}.Math.\\ .Math.I_{x-}+I_{x+}.Math.\ge I_{\mathrm{set}}\end{array}\hspace{1em},\mathrm{and}$ if such criterion for the current differential protection is met, performing the current differential protection.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a flowchart of a current differential protection method for a half wavelength line according to an embodiment of the disclosure.

(2) FIG. 2 is a schematic diagram of adaptively changing a braking coefficient and threshold value for the current differential protection according to an embodiment of the disclosure.

DETAILED DESCRIPTION

(3) The disclosure will be further described below in combination with the drawings and embodiments in detail. It should be understood that the embodiments provided herein are only adopted to explain the disclosure and not intended to limit the disclosure. In addition, the embodiments provided below are adopted not to provide all embodiments of the disclosure but to implement part of embodiments of the disclosure. The technical solutions recorded in the embodiments of the disclosure may be implemented in any combination thereof without conflict.

(4) Before the disclosure is further described in detail, nouns and terms involved in the embodiments of the disclosure will be described, and the nouns and terms involved in the embodiments of the disclosure are applied to the following explanations.

(5) 1) HWACT refers to ultralong-distance three-phase AC transmission of which an electrical distance is approximate to a power frequency half wave, i.e. 3,000 km (50 Hz) or 2,600 km (60 Hz).

(6) 2) A protection starting element (also called as a relay protection starting element) is configured to start current differential protection elements in relay protection devices mounted on both sides of a half wavelength line, and action of the protection starting element indicates the start of a fault in the half wavelength line.

(7) 3) The current differential protection element is a protection element calculating a differential current according to the currents on both sides of the half wavelength line and acting for protection when the differential current reaches a set action value.

(8) 4) The relay protection device is an automatic measure and equipment capable of timely sending an alarming signal to a duty officer or directly sending a tripping command to a controlled circuit breaker to stop development of these events when the failure of electric equipment (such as a generator and a line) in a power system or the power system itself endangers safe operation of the power system. A complete set of equipment implementing this automatic measure is collectively called as a relay protection device under a normal circumstance.

(9) The relay protection device is formed of a measurement part, a logic part and an execution part.

(10) The measurement part acquires sampling values of the currents and voltages on both sides of the line.

(11) According to magnitudes, properties, logic states of output quantities of the measurement part, sequence of appearance or their combination, the logic part enables the relay protection device to work (determining whether a criterion for the protection starting element and a criterion for the current differential protection element are true or not) according to a certain logic relationship, and finally determines whether to execute tripping or send the signal, and transmits the related command to the execution part.

(12) The execution part finally completes a task of the relay protection device according the signal transmitted by the logic part.

(13) Taking a microcomputer relay protection device as an example, it is formed of highly integrated built-in bus single-chip microcomputer, a high-accuracy current and voltage transformer, a high-insulating-intensity exit intermediate relay, a high-reliability switching power module and the like.

(14) 5) A braking coefficient is a ratio of operating current to braking current. When the relay protection device implements current differential protection, a sum of current phasor on both sides of the half wavelength line is the operating current (also called the differential current), a difference between the current phasor on both sides is the braking current, a product of the braking coefficient and the braking current is a braking amount, and protection action starts when the action current is higher than the braking amount.

(15) 6) A threshold value: the threshold value is a starting current of the current differential protection element, and when the action current is higher than the threshold value, the current differential protection element is started, and a protection judgment is made.

(16) 7) A setting current refers to a maximum current that can pass through a heating element for a long time without causing the differential relay protection device to act. A value of the setting current is specified based on bearing capabilities of the half wavelength transmission line and a power grid.

(17) An embodiment provides a self-adaptive differential protection method for a half wavelength line based on a time difference method. Exemplarily, steps of the method are shown in FIG. 1.

(18) (1) Determine the action time of protection starting elements.

(19) The action time of the protection starting elements is a time when the protection starting elements detect a fault, and the action time of the protection starting elements on both sides of the half-wavelength line are set to T.sub.M and T.sub.N.

(20) Here, the both sides refer to relay protection devices on both sides relative to a spatial range of the half wavelength line, and the relay protection devices are mounted on both sides of the half wavelength line respectively.

(21) A criterion of judging the protection starting elements on the basis of a time difference method is shown in a formula (1):

(22) $\begin{array}{cc}\{\begin{array}{c}\Delta f\left(t\right)=\Delta i_A^2\left(t\right)+\Delta i_B^2\left(t\right)+\Delta i_C^2\left(t\right)\\ .Math.d\Delta f\left(t\right).Math.>f_{\mathrm{set}}={\left(0.07\mathrm{kA}\right)}^2=0.005{\mathrm{kA}}^2\end{array}& \left(1\right)\end{array}$

(23) where Δi.sub.A(t)=i.sub.A(t)−i.sub.A(t−T), Δi.sub.B(t)=i.sub.B(t)−i.sub.B(t−T), and Δi.sub.C(t)=i.sub.C(t)−i.sub.C(t−T), i.sub.A(t), i.sub.B(t) and i.sub.C(t) are current sampling values of three phases A, B and C with relay protection devices of the half wavelength line, respectively, Δi.sub.A(t), Δi.sub.B(t) and Δi.sub.C(t) are mutations of the current sampling values of three phases A, B and C, T is a power frequency period, Δf(t) is a quadratic sum function of the current mutations, f.sub.set is a setting current value, d in |dΔf(t)| is a difference operator, and when |dΔf(t)| is larger than the setting current value f.sub.set, a protection starting criterion (i.e. a condition under which the protection starting elements act) is met.

(24) (2) A location of a fault point is determined by virtue of the time difference method, and is represented by a distance L.sub.FM of the fault point from a side M of the half wavelength line, L.sub.FM being shown as follows:
L.sub.FM=((T.sub.M−T.sub.N)v.sub.light+L)/2  (2)

(25) where v.sub.light is a light velocity, and L is a length of the half wavelength line.

(26) (3) The fault point L.sub.FM is regarded as a differential point, and currents I.sub.x+ and I.sub.x− at the differential point are obtained by virtue of a long line equation:

(27) $\begin{array}{cc}\{\begin{array}{c}{I}_{x-}=I_M\cosh \left(\gamma x\right)-\frac{U_M}{Z_c}\sinh \left(\gamma x\right)\\ I_{x+}=I_N\cosh \left(\gamma \left(L-x\right)\right)-\frac{U_N}{Z_c}\sinh \left(\gamma \left(L-x\right)\right)\end{array}& \left(3\right)\end{array}$

(28) (4) A braking coefficient and threshold value in a criterion for current differential protection are adaptively determined as follows:

(29) $\begin{array}{cc}k=\{\begin{array}{cc}-0.8L_{FM}/1000+0.8& L_{FM}<1000\mathrm{km}\\ 0& 1000\mathrm{km}\le L_{FM}\le 2000\mathrm{km}\\ 0.8L_{FM}/1000-1.6& L_{FM}<3000\mathrm{km}\end{array}\mathrm{and}& \left(4\right)\\ I_{\mathrm{set}}=\{\begin{array}{cc}-0.5L_{FM}/1000+0.8& L_{FM}<1000\mathrm{km}\\ 0.3& 1000\mathrm{km}\le L_{FM}\le 2000\mathrm{km}\\ 0.5L_{FM}/1000-0.7& L_{FM}<3000\mathrm{km}\end{array}& \left(5\right)\end{array}$

(30) (5) Whether to perform a current differential protection on the differential point is determined on the basis of I.sub.x+, I.sub.x− and the adaptively changed braking coefficient and threshold value, the criterion of the current differential protection is:

(31) $\begin{array}{cc}\{\begin{array}{c}.Math.I_{x-}+I_{x+}.Math.\ge k.Math.I_{x-}-I_{x+}.Math.\\ .Math.I_{x-}+I_{x+}.Math.\ge I_{\mathrm{set}}\end{array}\hspace{1em},& \left(6\right)\end{array}$
and

(32) if the criterion of the current differential protection shown in the formula (6) is met, current differential protection is performed.

(33) Finally, it should be noted that: the above embodiments are merely intended for describing the technical solutions of the disclosure rather than limiting it. Those skilled in the art, although referring to the above embodiments, should know that modifications or equivalent replacements may still be made to specific implementation modes of the disclosure, and any modifications or equivalent replacements made without departing from the spirit and scope of the disclosure shall fall within the scope of protection of the claims of the disclosure applying for approval.