CN104852368A - Line differential protection method based on differential output of electronic current transformer - Google Patents

Abstract

The invention relates to a line differential protection method based on the differential output of an electronic current transformer, which belongs to the field of relay protection of electric power systems. The technical solution is: the integration link in the data processing of the high-voltage side of the traditional line differential protection is removed, and the differential signal output by the Rogowski coil sensor head is directly used to realize the longitudinal differential protection of the high-voltage transmission line, reducing the complexity of the device and reducing the data processing time. On the basis of ensuring the reliability and quick action, the adverse effect brought by the additional integral circuit is avoided, and it is compatible with the differential output of the Rogowski coil. The PSCAD/EMTDC simulation experiment shows that the differential protection scheme based on the current differential signal is effective and feasible. The differential signal output by the electronic current transformer is directly used to realize the fast differential protection of the line, and the integration link is removed, which is adapted to the interface of the electronic current transformer. If necessary, it can be used as the main protection of high-voltage transmission lines.

Description

Line Differential Protection Method Based on Differential Output of Electronic Current Transformer

technical field

The invention relates to a line differential protection method based on the differential output of an electronic current transformer, which belongs to the field of relay protection of electric power systems.

Background technique

With the development of the power industry, the required power transmission capacity continues to increase, and the operating voltage level is also getting higher and higher. Due to the inherent problems of iron core saturation, residual magnetism, and ferromagnetic resonance, traditional electromagnetic transformers are increasingly difficult to meet the growing needs of power systems. The application of electronic transformers will completely solve these problems. Electronic transformers have all the functions of traditional transformers, and have the benefits of not being affected by saturation and ferromagnetic resonance. They have good insulation performance, wide frequency bandwidth, large dynamic range, small size, Light weight, suitable for the development of digital protection and many other advantages, these features will undoubtedly greatly improve the performance of relay protection.

In recent years, electronic current transformers based on Rogowski coils have been gradually put into practical use, but since the output signal of the sensor head is the differential of the measured current, in order to restore the original signal, an additional integral module needs to be added, which will inevitably increase the high voltage The data processing time on the high-voltage side leads to a certain time delay and phase shift, which limits the action speed of the protection, reduces reliability, and increases the energy supply burden on the high-voltage side.

Contents of the invention

The invention provides a method for implementing fast differential protection of lines by applying differential signals output by electronic current transformers, making full use of effective information directly output by sensor heads, reducing device complexity, reducing data processing time, and avoiding integration The adverse effects brought by the link are beneficial to the rapid action of the differential protection. It does not need to use the channel to transmit the time information for synchronization processing, and does not need to calculate the channel delay. It meets the requirements of the IEC61850-9-2 standard and is compatible with the Rogowski coil. Compatible with the differential output to solve the above-mentioned problems in the background technology.

In order to achieve the above object, the present invention adopts following technical scheme:

A line differential protection method based on the differential output of an electronic current transformer, comprising the following steps:

Step 1: Through the data acquisition system on the high-voltage side of the electronic current transformer installed at both ends of the transmission line, directly obtain the current differential signal output by the Rogowski coil sensor head of the electronic current transformer and :

,in, , are the actual measured current (primary value) at the M terminal and N terminal of the line respectively, , are the measured current differential signals (secondary value) output by the electronic current transformer sensing head at the M-terminal and N-terminal of the line respectively, is the ratio coefficient of the electronic current transformer, and t is the time;

Step 2: Apply the full-wave Fourier algorithm to obtain and The phasor value of the corresponding fundamental component and , , and , The magnitude-phase relationship of :

,in, , respectively in step 1 , The phasor value of the corresponding fundamental component, , respectively in step 1 , The amplitude of the corresponding fundamental component, , respectively in step 1 , The phase angle of the corresponding fundamental component, is the fundamental angular frequency, ;

Step 3: Find the Differential Differential Current and differential braking current , , and , Relationship:

, ,in, , Respectively, the differential current and braking current of the traditional line differential protection added with the integral module, , , , , The meaning of is the same as that of step 2;

Step 4: The differential differential current obtained in step 3 and differential braking current Substitute into the action equation:

, when the action condition of the action equation is satisfied, the protection trips, otherwise the protection does not operate; among them, is the inflection point current of the traditional line differential protection added with the integral module, is the starting current of the traditional line differential protection added with the integral module, K is the slope of the braking line, , , The meaning of is the same as that of step 3.

The present invention proposes a new method of directly utilizing the differential output of an electronic current transformer to realize line differential protection, and its basic principles are described as follows:

(1) Differential output signal based on Rogowski coil ECT

The equivalent circuit of the Rogowski coil is shown in Figure 1, R ₀ is the internal resistance of the coil, L is the self- _inductance of the coil, RL is the load resistance, C is the interturn capacitance of the coil, e ( t ) is the coil induced potential. The induced electromotive force of the coil satisfies the following formula:

(1)

Among them, M is the mutual inductance coefficient of the coil, and i is the measured current.

The equivalent circuit of the Rogowski coil shown in Figure 1 can be listed as:

(2)

Taking the Laplace transform and simplifying the input-output relationship is:

(3)

when ,and When the Rogowski coil is in an open circuit state, then , the inverse Laplace transform gives:

(4)

At this time, the output voltage is proportional to the differential of the measured current to time, and this state is the external integral working state of the Rogowski coil ECT. In practical applications, the external integral working method can realize the measurement of pulse current, power frequency current and harmonic current, which is the main working method of Rogowski coil ECT.

(2) Basic principle of differential current protection

As shown in Figure 2, it is the basic principle diagram of the traditional longitudinal current differential protection. When the line MN is operating normally and the protected line is short-circuited externally (k2), the currents on both sides are equal in magnitude and opposite in direction, and the phasor and modulus It is approximately zero; when the line is short-circuited (k1), the fault current flowing through both sides of the line is in the positive direction, and its phasor and modulus are large. The differential relay (KD) is based on the phasor sum of the secondary currents on both sides ( ) modulus to determine whether an internal fault occurs.

(3) Full-wave Fourier algorithm

It is assumed that when a fault occurs in the power system, the obtained current is a periodic function, which contains various harmonics and non-attenuated DC components in addition to the fundamental component. Let's set this function to , expanded into Fourier series as follows:

(5)

In the formula , are the amplitudes of the sine and cosine phases of the DC, fundamental and harmonic components, respectively, is the fundamental angular frequency, is the harmonic order, Represented as a DC component. According to the principle of Fourier series, it can be found in the Fourier algorithm of the whole cycle:

(6)

where T is the fundamental period, so The subharmonic components in are:

(7)

(8)

When this algorithm is implemented on a computer, it calculates discrete sampled values. Calculate with discrete values and For values of , the two integrals can be found using the trapezoidal rule:

(9)

where N is the number of sampling points in one period, is the kth sampling value. This algorithm uses all the sampling values of one period for calculation, so the length of the data window is one sampling period T. It can be seen that as long as the , Find out, the amplitude of the nth harmonic component of the current , initial phase angle can be calculated. When n = 1, the calculated result is the amplitude and phase angle of the current fundamental component.

The beneficial effects of the present invention are: the present invention proposes a new method for realizing line differential protection by utilizing the differential output signal of the electronic current transformer. This method makes full use of the effective information output by the sensor head of the electronic current transformer, reduces the complexity of the device, reduces the data processing time, and avoids the adverse effects of the additional integral circuit on the basis of ensuring reliability and quick movement. . In addition, the differential signal output by the electronic current transformer is directly used to realize the fast differential protection of the line, and the integral link is removed, which meets the needs of the interface of the electronic current transformer and has a wide application prospect. It can be used as the main protection of the high-voltage transmission line.

Description of drawings

Fig. 1 is an equivalent schematic diagram based on Rogowski coil ECT;

Figure 2 is the basic principle diagram of traditional longitudinal current differential protection;

Fig. 3 is a comparison diagram of the present invention and the traditional differential protection ratio braking characteristic curve;

Figure 4 is a diagram of the design scheme of line differential protection based on Rogowski coil differential output;

Fig. 5 is a simulation model diagram of differential protection based on current differential signal;

Fig. 6 is a time series diagram of differential differential and braking current modulus when a fault occurs in the line;

Fig. 7 is a time series diagram of differential differential and braking current modulus when an out-of-area fault occurs on the line;

Fig. 8 is a curve diagram of the differential protection ratio braking characteristic based on the current differential signal when an internal fault occurs on the line;

Fig. 9 is a curve diagram of the differential protection ratio braking characteristic based on the current differential signal when an out-of-area fault occurs on the line;

Figure 10 is a flowchart of a line differential protection method based on the differential output of an electronic current transformer.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing.

A line differential protection method based on the differential output of an electronic current transformer, the implementation steps of the method are as follows:

Step 1: Through the data acquisition system on the high-voltage side of the electronic current transformer installed at both ends of the transmission line, directly obtain the current differential signal output by the Rogowski coil sensor head of the electronic current transformer and :

,in, , are the actual measured current (primary value) at the M terminal and N terminal of the line respectively, , are the measured current differential signals (secondary value) output by the electronic current transformer sensing head at the M-terminal and N-terminal of the line respectively, is the ratio coefficient of the electronic current transformer, and t is the time;

Step 2: Apply the full-wave Fourier algorithm to obtain and The phasor value of the corresponding fundamental component and , , and , The magnitude-phase relationship of :

,in, , respectively in step 1 , The phasor value of the corresponding fundamental component, , respectively in step 1 , The amplitude of the corresponding fundamental component, , respectively in step 1 , The phase angle of the corresponding fundamental component, is the fundamental angular frequency, ;

Step 3: Find the Differential Differential Current and differential braking current , , and , Relationship:

, ,in, , Respectively, the differential current and braking current of the traditional line differential protection added with the integral module, , , , , The meaning of is the same as that of step 2;

Step 4: The differential differential current obtained in step 3 and differential braking current Substitute into the action equation:

, when the action condition of the action equation is satisfied, the protection trips, otherwise the protection does not operate; among them, is the inflection point current of the traditional line differential protection added with the integral module, is the starting current of the traditional line differential protection added with the integral module, K is the slope of the braking line, , , The meaning of is the same as that of step 3.

Figure 2 shows the basic principle diagram of the longitudinal current differential protection. When the line MN is operating normally and the protected line is short-circuited (k2), the currents on both sides are equal in magnitude and opposite in direction, and the phasor and modulus are approximately zero; when the line is short-circuited inside (k1), the faults flowing through both sides of the line The current is in the positive direction, and its phasor and modulus are very large. The differential relay (KD) judges whether an internal fault occurs according to the phasor and modulus of the secondary current on both sides.

It can be known from formula (4) that the output signal of the Rogowski coil sensing head is the differential of the measured current. Therefore, a differential protection scheme based on the differential output principle of the electronic current transformer is designed.

It may be advisable to set the instantaneous value of the current at both ends of the line as:

(10)

Among them, I _m and I _n are the effective values of the fundamental current at both ends of the line, is the fundamental angular frequency, , is the initial phase angle, and t is the time.

The corresponding current phasor is:

(11)

The differential of the original current is:

(12)

The corresponding current phasor is:

(13)

Similarly, we can get:

(14)

Then the differential current can be obtained as:

(15)

It can be seen from formula (15) that the operating current modulus of differential protection based on differential current becomes the original times.

In order to improve the sensitivity of internal short circuit and the reliability of non-action during external short circuit, differential relays with braking characteristics are often used.

Domestic protection often uses phasor difference braking current:

(16-a)

(16-b)

In formula (16-a), , Respectively, the current phasor at both ends of the line, the braking amount is the modulus of the phasor difference between the two sides of the current. In formula (16-b), is the phasor difference braking current based on the differential output of the electronic current transformer, its size is times.

From the above derivation, the action equation of the ratio braking differential protection based on the current differential signal is:

(17)

In the formula: is the differential differential current, is the differential braking current, is the differential knee current, is the differential starting current, K is the slope of the braking line.

According to formula (17), the differential protection ratio braking characteristic curve based on the differential signal can be drawn, as shown in Figure 3.

Accompanying drawing 1 is the equivalent circuit diagram of Rogowski coil. R ₀ is the internal resistance of the coil, L is the self- _inductance of the coil, RL is the load resistance, C is the interturn capacitance of the coil, e ( t ) is the induced potential of the coil, and U _o is the output voltage. Rogowski coils are often made of enameled wire evenly wound on a ring-shaped skeleton. The skeleton is made of non-ferromagnetic materials such as plastic or ceramics. Its relative magnetic permeability is the same as that in air, and there will be no iron core saturation. It is a distinctive feature that is different from traditional current transformers with iron cores.

Attached Figure 2 is the basic schematic diagram of traditional longitudinal current differential protection. When the line MN is in normal operation and the protected line is short-circuited externally (k2), the currents on both sides are equal in magnitude and opposite in direction, and the phasor and modulus are approximately zero; When the line is internally short-circuited (k1), the fault current flowing through both sides of the line is in the positive direction, and its phasor and modulus are large. in, , The phasor value of the primary current at the protective installation on both sides of the line respectively, , The phasor value of the secondary current at the protection installation on both sides of the line respectively, and the differential relay (KD) according to the phasor sum of the secondary current on both sides The size of the modulus is used to judge whether an internal fault occurs.

Accompanying drawing 3 is the comparative diagram of the braking characteristic curve of the new and old differential protection ratios. in, is the differential current, is the braking current, is the knee current, is the starting current, is the differential differential current, is the differential braking current, is the differential knee current, is the differential starting current.

Attached Figure 4 is the design scheme of line differential protection based on Rogowski coil differential output, which consists of Rogowski coil sensing head, high-voltage side data acquisition system, optical fiber transmission and interface, merging unit, clock synchronization module, protection module, and power supply module and other parts.

The differential signal output by the Rogowski coil is filtered by the anti-aliasing filter, and the integral restoration is not performed. The A/D module directly performs synchronous sampling, converts the analog differential signal into a digital signal, and finally converts it into an optical signal through E/O conversion. Transfer to the low-voltage side for use by protective devices. On the low-voltage side, the merging unit receives the sampled signal after O/E conversion, and after processing by modules such as resampling and phase compensation, it encodes and outputs according to the IEC61850-9-2 format. At the same time, the merging unit uploads the received GPS second pulse signal and time signal to the A/D sampling module on the high voltage side. The current differential signal processed by the merging unit is transmitted to the protection device through the process layer switch to realize the fast differential protection scheme based on the differential output of the electronic current transformer.

Figure 5 shows a differential protection simulation model based on current differential signals, which is built on the PSCAD/EMTDC platform. The rated voltage of the power supply is 500kV, the rated power is 300MVA, and the frequency is 50Hz; the line model adopts the Berrylon distribution parameter model, the total length of the line is 300km, and the parameters are: z ₁ =0.035+j0.42Ω/km , z ₀ =0.30+j1.14Ω/km. The total simulation time is 0.5s, the fault start time is 0.22s, and the fault duration is 0.1s. During the simulation process, the current data obtained at both ends of the line is processed by the differential module in PSCAD to simulate the differential output of the Rogowski coil.

Attached drawings 6 and 7 show the differential differential and braking current modulus time series diagrams when faults occur in the line and outside the area. The fault in the area is set as a short circuit of phase A to ground at a distance of 100km from the busbar at the M end. The fault is a short-circuit of phase A to ground at the outlet of the M-side busbar. Among them, the solid line represents the differential differential current, and the dotted line represents the differential braking current.

It can be seen from accompanying drawing 6 that the differential differential current modulus value is much larger than the differential braking current modulus value when there is a short circuit in the area, and the protection can give a trip command within half a cycle and has high sensitivity; it can be seen from accompanying drawing 7 It can be seen that the differential braking current increases greatly when the external short circuit occurs, while the differential differential current decreases. This is because when the external short circuit fault occurs, the line voltage decreases, and the line capacitive current decreases greatly compared with that when no fault occurs. Small, the magnitude of the fault current flowing through the protection at both ends is almost equal, and the direction is opposite, the braking current of the phasor difference is very small, and the protection is reliable and does not operate.

Accompanying drawings 8 and 9 show the differential protection ratio braking characteristic curves based on the current differential signal corresponding to the accompanying drawings 6 and 7. During the simulation, the braking current takes the phasor difference braking current, the braking coefficient K takes 0.5, and the differential starting current , differential inflection point current .

It can be easily seen from Figures 8 and 9 that when an internal fault occurs, the differential protection based on the current differential signal operates reliably, and the operating time mainly depends on the value of the differential starting current; when an external fault occurs, the differential protection -The differential curve is always in the non-action area of the ratio braking characteristic curve, and the protection is reliable and non-action.

In order to make further quantitative analysis, the simulation results are summarized in Table 1~Table 3. In Table 1~Table 3, except for the variables in each table, other parameters take default values: 1) Metal short circuit; 2) Phase A ground short circuit at 100km away from the M-side busbar; 3) Phase angle difference of power supply at both ends 20°; 4) Braking current is taken as phasor difference braking current. The action amount and braking amount in each table take the modulus value after one cycle of failure for easy comparison. In order to increase the reliability of the protection action, the holding time of 4ms is taken, that is, when the differential differential and braking current meet the conditions of the action equation and keep for 4ms, the protection action trips.

Table 1 shows the differential protection action based on the current differential signal under different transition resistance conditions. When the fault occurs in the zone, with the increase of the transition resistance, the action amount decreases rapidly, the braking amount changes little, and the action time of the protection is prolonged. This is because with the increase of the transition resistance of the fault point, the current value of the fault component at both ends of the line becomes smaller, so that the magnitude of the differential current can be compared with the load current, and it is less than the value set according to a certain proportion of the load current for a long time. The starting current value leads to an increase in the protection operating time.

Table 2 shows the action of differential protection under different fault location conditions. The action time of the protection does not change much, both are around 7~9ms. This is because when a short circuit occurs at different locations in the area, although the amplitude ratio of the measured current at both ends of the line will change, the phase relationship will change little. When the action amount decreases, the braking amount also decreases accordingly, so it has little influence on the time domain range in which the action amount is greater than the braking amount after a fault.

Table 3 shows the operation of differential protection under the condition of different phase angle differences between the power supplies at both ends. With the increase of the voltage difference between the two ends of the power supply, the amount of action decreases, the amount of differential increases, and the operating time increases slightly. The change rule is in line with the operating principle of differential protection.

The above simulation results show that the differential protection scheme based on the current differential signal meets the requirements of the high-voltage line for the relay protection device, and is compatible with the differential output of the Rogowski coil sensor head, avoiding the adverse effects of the integration circuit, and protecting The plan is effective and feasible.

Accompanying drawing 10 shows the flowchart of the line differential protection method based on the differential output of the electronic current transformer.

Claims (1)

1. A line differential protection method based on the differential output of an electronic current transformer, characterized in that it comprises the following steps: Step 1: Through the data acquisition system on the high-voltage side of the electronic current transformer installed at both ends of the transmission line, directly obtain the current differential signal output by the Rogowski coil sensor head of the electronic current transformer and : ,in, , are the actual measured current (primary value) at the M terminal and N terminal of the line respectively, , are the measured current differential signals (secondary value) output by the electronic current transformer sensing head at the M-terminal and N-terminal of the line respectively, is the ratio coefficient of the electronic current transformer, and t is the time; Step 2: Apply the full-wave Fourier algorithm to obtain and The phasor value of the corresponding fundamental component and , , and , The magnitude-phase relationship of : ,in, , respectively in step 1 , The phasor value of the corresponding fundamental component, , respectively in step 1 , The amplitude of the corresponding fundamental component, , respectively in step 1 , The phase angle of the corresponding fundamental component, is the fundamental angular frequency, ; Step 3: Find the Differential Differential Current and differential braking current , , and , Relationship: , ,in, , Respectively, the differential current and braking current of the traditional line differential protection added with the integral module, , , , , The meaning of is the same as that of step 2; Step 4: The differential differential current obtained in step 3 and differential braking current Substitute into the action equation: , when the action condition of the action equation is satisfied, the protection trips, otherwise the protection does not operate; among them, is the inflection point current of the traditional line differential protection added with the integral module, is the starting current of the traditional line differential protection added with the integral module, K is the slope of the braking line, , , The meaning of is the same as that of step 3.