And (4) doubling. Meanwhile, zero sequence voltage and zero sequence current on the line are increased, wherein the amplitude of the zero sequence voltage is equal to the phase voltage in normal operation. Therefore, zero-sequence voltage or zero-sequence current exceeding a certain threshold can be used as a starting condition of the single-phase earth fault:

X₀＞X_0d

the zero sequence voltage and the zero sequence current can be directly measured or calculated by three-phase voltage or current:

X_{0} (t) = \frac{1}{3} (X_{A} (t) + X_{B} (t) + X_{C} (t))

in the above two formulae, X represents a voltage or a current.

When the grounding point has transition resistance, the zero sequence voltage is reduced along with the increase of the transition resistance. Meanwhile, in order to overcome the influence of unbalanced voltage during normal operation, the threshold of the zero sequence voltage is generally 20% of the phase voltage.

The power frequency effective value or instantaneous value of the zero sequence voltage current can be used as the basis of the fault starting condition.

2) Selection of specific frequency bands

The fault transient signal includes a broad frequency spectrum, and a frequency band, which includes a main energy and has a uniform relationship between voltage and current, is defined as a characteristic frequency band. The definition of the characteristic frequency band is different according to different fault types and different fault conditions.

For a low-current ground fault, it can be proved that the main resonance frequency of the transient signal is contained in the characteristic frequency band, and therefore, the cutoff frequency of the characteristic frequency band is determined according to the main resonance frequency. It is impossible to have a pass band of zero width due to the digital filter actually used. Therefore, a certain safety margin must be added to the calculated primary frequencyAs the high-end cutoff frequency, the following equation is specifically used:

ω_r＝max(200，0.1ω_h)

wherein:is the dominant resonant frequency. And the low-end cutoff frequency of the characteristic frequency band can be selected to be 3 times of the power frequency. Namely, the cutoff frequency of the characteristic frequency band is: (3. omega₀，ω₁+ω_r)。

3) Obtaining the specific frequency band component of the transient voltage and current signal

Since the transient voltage and current signals outside the characteristic frequency band do not completely satisfy the uniformly set relationship, the transient voltage and current signals cannot be used for determining the fault direction according to a uniform method. Therefore, the transient zero sequence voltage and current signals must be filtered before calculating the fault direction. The filtering functions are mainly as follows: filtering out unnecessary transient signals and high-frequency interference signals.

And designing a digital filter according to the calculated characteristic frequency range, so that the characteristic frequency range component of the transient voltage and current signal can be obtained. The IIR filter has good amplitude-frequency characteristics, but nonlinear phase shift can be brought, so that transient signals are distorted, and the detection effect is influenced. Therefore, after the IIR filter is used to forward filter the transient signal, it is necessary to perform backward filtering from the end to the head end to make the phase shift of the final filtered signal at each frequency zero. FIR filters have poor amplitude-frequency characteristics and require a longer filter window to achieve the same effect. But the phase shift brought by the method is linear, the transient signal is not distorted, and inverse filtering is not needed.

4) Calculating transient reactive power and instantaneous reactive power

Let the corresponding voltage of the non-sinusoidal circuit port be u_T(t) current is i_T(T) with a period of T.

First, Hilbert transform is performed on the voltage signal:

<math> <mrow> <msub> <mover> <mi>u</mi> <mo>^</mo> </mover> <mi>T</mi> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>=</mo> <msub> <mi>u</mi> <mi>T</mi> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>*</mo> <mi>h</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>=</mo> <mfrac> <mn>1</mn> <mi>π</mi> </mfrac> <msubsup> <mo>&Integral;</mo> <mrow> <mo>-</mo> <mo>∞</mo> </mrow> <mrow> <mo>+</mo> <mo>∞</mo> </mrow> </msubsup> <mfrac> <mrow> <msub> <mi>u</mi> <mi>T</mi> </msub> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> </mrow> <mrow> <mi>t</mi> <mo>-</mo> <mi>τ</mi> </mrow> </mfrac> <mi>dτ</mi> </mrow> </math>

then the non-sinusoidal circuit reactive power is defined to be equal to the average power of the port voltage Hilbert transform and the incoming port current:

<math> <mrow> <msub> <mi>Q</mi> <mi>T</mi> </msub> <mo>=</mo> <mfrac> <mrow> <mo><</mo> <msub> <mover> <mi>u</mi> <mo>^</mo> </mover> <mi>T</mi> </msub> <mo>,</mo> <msub> <mi>i</mi> <mi>T</mi> </msub> <mo>></mo> </mrow> <mi>T</mi> </mfrac> <mo>=</mo> <mfrac> <mn>1</mn> <mi>πT</mi> </mfrac> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>T</mi> </msubsup> <msub> <mi>i</mi> <mi>T</mi> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <msubsup> <mo>&Integral;</mo> <mrow> <mo>-</mo> <mo>∞</mo> </mrow> <mo>∞</mo> </msubsup> <mfrac> <mrow> <msub> <mi>u</mi> <mi>T</mi> </msub> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> </mrow> <mrow> <mi>t</mi> <mo>-</mo> <mi>τ</mi> </mrow> </mfrac> <mi>dτdt</mi> </mrow> </math>

for current i_T(t) further decomposing to obtain a component in which reactive power is generated, namely a reactive current:

i_{T q} (t) = G_{q} {\hat{u}}_{T} (t)

wherein: g_qThe proportional constant is set to be equal in value to the average power of the current and its reactive component in a cycle. The following can be obtained:

G_{q} = \frac{< {\hat{u}}_{T}, i_{T} >}{{| | {\hat{u}}_{T} | |}^{2}} = \frac{< {\hat{u}}_{T}, i_{T} >}{{| | u_{T} | |}^{2}}

then, the instantaneous reactive power of the non-sinusoidal circuit is defined to be equal to the product of the Hilbert transform of the port voltage and the instantaneous value of the reactive component of the current flowing into the port:

q_{T} (t) = {\hat{u}}_{T} (t) i_{Tq} (t)

5) fault direction determination using instantaneous reactive power and load characteristics

When the detection point detection impedance is capacitive in a specific frequency component, if the reactive power or the instantaneous reactive power between the transient voltage and the transient current of the detection point is greater than zero, the fault is positioned at the downstream of the detection point, otherwise, the fault is positioned at the upstream of the detection point.

When the detection point detection impedance is inductive in a specific frequency component, if the reactive power or the instantaneous reactive power between the transient voltage and the transient current of the detection point is larger than zero, the fault is positioned at the upstream of the detection point, otherwise, the fault is positioned at the downstream of the detection point.

For short circuit faults and large current grounding system ground faults, if the fault is located in a protected section, a trip command should be immediately output to isolate the fault line.

And for a low-current ground fault, if the detected line is determined to be a fault line, a trip instruction can be immediately output to isolate the fault line. Or firstly sending alarm information, continuing to run for a period of time according to regulation, and sending a tripping instruction again by manual intervention at a proper time.

For example, as shown in fig. 1, the transient waveform of the two actual small-current ground faults is that the voltage of the fault phase is reduced during the fault, and the fault line can be determined by calculating the transient reactive power or the instantaneous reactive power of the zero-sequence current specific frequency component of the line.

As shown in fig. 2, the transient reactive power and direction at two ends of the low-current ground fault point are shown in the diagram, where the sections D3 and D4 are fault sections.

2. Power station single-phase earth fault line selection method

For small current grounding faults, the fault line selection can be realized by comparing fault information of all outgoing lines by using the method of the invention like the traditional method. The method comprises the following concrete steps:

1) the change of zero sequence voltage and zero sequence current is used as the starting condition of the fault,

2) determining a characteristic frequency range according to the main resonance frequency of the transient zero sequence current,

3) filtering the transient zero sequence voltage and current signals to obtain a characteristic frequency band component,

4) calculating the magnitude and direction of the transient voltage and current signal reactive power and the instantaneous reactive power,

5) comparing the transient reactive power directions of all outgoing lines to determine a fault line,

the transient reactive power of the faulty line is negative and the healthy line is positive. Because the current signal of the sound line is easy to be interfered by noise when being weaker, a plurality of outgoing lines with larger transient current amplitude or transient reactive power can be selected and then the polarities of the outgoing lines are compared.

As shown in fig. 3, the frequency of the transient signal is high, and there are many lines to be monitored, and the conventional acquisition method directly controlled by the CPU of the microprocessor cannot meet the requirements. The whole set of equipment mainly comprises a plurality of acquisition units DAU1-DAUn and a microprocessor CPU unit, and can monitor a plurality of even all outgoing lines. The dotted line portion in the figure is an optional portion. One of the acquisition units DAU is responsible for monitoring voltage signals, and each of the other acquisition units DAU1-DAUn is responsible for monitoring zero sequence currents of a certain number of outgoing lines. The whole device can select different acquisition unit numbers according to the outgoing line number of the power station. After a fault occurs, all the acquisition units are started by the zero-sequence voltage or the zero-sequence current at the same time, and the acquisition units record data before and after the fault with a certain length and transmit the data to the CPU unit of the microprocessor.

The CPU unit of the microprocessor is responsible for the management and coordination work of the whole set of the device, receives a fixed value set by a user, stores and analyzes fault data recorded by each acquisition unit after a fault, calculates a fault line by using the method, and provides a line selection result and an alarm signal in different forms after the fault line is determined. And can upload the fault data or analysis results to the superior master station or the remote system as required.

3. Fault segmentation method

Fault segmentation can also be achieved for low current ground faults.

Referring to fig. 4, a typical system configuration is shown, wherein S1-S5 are disconnecting switches, CB1 and CB2 are circuit breakers, and RTUs and FTUs are terminal devices. Compared with the method using transient zero-sequence current, the method needs zero-sequence voltage and zero-sequence current signals at the same time.

The whole system consists of three parts: and the terminal equipment RTU and FTU distributed on each detection point of the line are positioned in the central processing unit and the communication system of the transformer substation. And the terminal equipment RTU and FTU are responsible for monitoring the change of zero-sequence voltage and zero-sequence current of the line in real time, calculating the magnitude and direction of transient reactive power after a fault occurs, and reporting the calculation result to the central processing unit. And the central processing unit determines a fault section and sends out an alarm signal after receiving the information reported by each terminal device. And then the corresponding switch is manually or automatically controlled remotely to isolate the fault and recover the power supply of a sound line.

The method comprises the following concrete steps:

line terminal equipment:

2) determining a characteristic frequency range according to the main resonance frequency of the transient zero-sequence current,

3) obtaining characteristic frequency band component by filtering zero sequence voltage and current

4) Calculating and reporting the magnitude and direction of the transient reactive power and the transient reactive power, wherein the main station comprises:

1) determining a fault section according to fault information of different detection points on a fault line,

the fault is located in the section with opposite reactive power directions on two sides. And if the transient reactive power directions of all the detection points are larger than zero, the fault point is positioned between the bus and the first detection point. Conversely, if the transient reactive power direction of all detection points is less than zero, the fault point is located between the last detection point and the line end.

According to the method for detecting and protecting the line fault direction of the power system, the fault direction is judged by calculating the reactive power or instantaneous reactive power of the specific frequency component of the transient voltage and current generated by the fault, for the low-current ground fault of the indirect grounding system, the method is not influenced by an arc suppression coil and an unstable arc, and the detection reliability and sensitivity are high because the fault transient current is far larger than the steady-state current. For the large-current fault caused by short circuit, single-phase grounding of a large-current grounding system and the like, the detection speed of the method is high, and the fault direction can be given within 5mS generally.

Claims

1. The method for detecting and protecting the line fault direction of the power system judges the fault direction by calculating the reactive power or instantaneous reactive power of a transient voltage current specific frequency component generated by a fault, and comprises the following steps:

a using the change of voltage or current as the fault starting condition,

c searching the fault starting moment and the transient signal delay length,

the method is characterized in that:

h, filtering out the interference components in the transient voltage and current of the fault, reserving the specific frequency band components with concentrated energy and definite voltage and current relationship,

2. The power system line fault direction detection and protection method of claim 1, wherein:

a, establishing a fault model based on line distribution parameters, analyzing and detecting phase difference and frequency relation between voltage and current of impedance under different frequency bands, requiring that most energy of transient signals is reserved in the selected frequency band, and definite and uniform capacitive or inductive relation is presented between the voltage and the current,

b defining the average reactive power of the transient voltage-current signal as the average power of the Hilbert transform of the voltage signal and the current signal in the transient period,

c, defining the transient voltage and current signal instantaneous reactive power as the product of the voltage signal Hilbert conversion and the current signal reactive component instantaneous value.

3. The power system line fault direction detection and protection method of claim 2, wherein: the method for analyzing the phase-frequency characteristics of the line impedance comprises the following steps:

is provided with L₀、R₀、C₀、G₀Are respectively unit lengthInductance, resistance, distributed capacitance and conductance of the corresponding mode component of the dimensional circuit, Z₂For the load of the corresponding module component at the end of the line, i is the length from the detection point to the end of the line, the impedance Z of the detection point₁Comprises the following steps:

wherein:

referred to as the line propagation constant,

referred to as the line wave impedance,

and analyzing the phase-frequency characteristic of the line impedance according to the phase and frequency relation of the detection point impedance Z1.

4. The power system line fault direction detection and protection method according to claim 1 or 2, characterized in that: the transient signal reactive power and the transient reactive power are defined in the following modes:

the time window T is taken to be a fixed period of time after the fault,

the transient voltage-to-current reactive power Q is defined as the Hilbert transformation of the voltage signal u (T) and the average power of the current signal i (T) in the transient period T, i.e. the average power

<math> <mrow> <mi>Q</mi> <mo>=</mo> <mfrac> <mn>1</mn> <mi>T</mi> </mfrac> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>T</mi> </msubsup> <mi>i</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mover> <mi>u</mi> <mo>^</mo> </mover> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mi>dt</mi> <mo>=</mo> <mfrac> <mn>1</mn> <mi>πT</mi> </mfrac> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>T</mi> </msubsup> <mi>i</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <msubsup> <mo>&Integral;</mo> <mrow> <mo>-</mo> <mo>∞</mo> </mrow> <mo>∞</mo> </msubsup> <mfrac> <mrow> <mi>u</mi> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> </mrow> <mrow> <mi>t</mi> <mo>-</mo> <mi>τ</mi> </mrow> </mfrac> <mi>dτdt</mi> <mo>,</mo> </mrow> </math>

Wherein,

<math> <mrow> <mover> <mi>u</mi> <mo>^</mo> </mover> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>=</mo> <mfrac> <mn>1</mn> <mi>π</mi> </mfrac> <msubsup> <mo>&Integral;</mo> <mrow> <mo>-</mo> <mo>∞</mo> </mrow> <mo>∞</mo> </msubsup> <mfrac> <mrow> <mi>u</mi> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> </mrow> <mrow> <mi>t</mi> <mo>-</mo> <mi>τ</mi> </mrow> </mfrac> <mi>dτ</mi> </mrow> </math>

for the Hilbert transform of the voltage signal,

the transient voltage-current transient reactive power q (t) is defined as the Hilbert transformation of the voltage signal u (t) and the reactive component i (t) of the current signal i (t)_q(t) multiplication of instantaneous values, i.e.

q_{T} (t) = {\hat{u}}_{T} (t) i_{T_{q}} (t),

Wherein the reactive component i_q(t) can be represented as

<math> <mrow> <msub> <mi>i</mi> <mi>q</mi> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>=</mo> <mfrac> <mrow> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>T</mi> </msubsup> <mover> <mi>u</mi> <mo>^</mo> </mover> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> <mi>i</mi> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> <mi>dτ</mi> </mrow> <mrow> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>T</mi> </msubsup> <msup> <mi>u</mi> <mn>2</mn> </msup> <mrow> <mo>(</mo> <mi>τ</mi> <mo>)</mo> </mrow> <mi>dτ</mi> </mrow> </mfrac> <mover> <mi>u</mi> <mo>^</mo> </mover> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>.</mo> </mrow> </math>

5. The power system line fault direction detection and protection method according to claim 1 or 2, characterized in that: the fault direction is determined as follows:

when the detection point detection impedance is capacitive in a specific frequency component, if the reactive power or instantaneous reactive power between transient voltage and current of the detection point is greater than zero, the fault is positioned at the downstream of the detection point, otherwise, the fault is positioned at the upstream of the detection point,