CN118352969A - Differential protection method based on variable transmission interval and variable length data frame - Google Patents

Abstract

The invention provides a differential protection method based on variable transmission interval and variable length data frame, comprising the following steps: the automatic control equipment in the power distribution area is calibrated through a GNSS module; the device samples according to the PPS signal provided by the GNSS; converting the alternating current signal of the voltage and the current into a synchronous phasor comprising an amplitude and a phase angle; sparse frame data is stored once every minute, dense frame data is stored at 1ms intervals for 200ms, and cyclic coverage is performed; the terminal devices on two sides perform local operation according to the synchronous phasor data, refresh and store the result so as to respond rapidly when a fault occurs; when a fault occurs, the terminal equipment is switched to a complete data frame sending mode through a frame structure, and differential protection logic operation is carried out. The invention effectively reduces the communication load and the cost and improves the overall efficiency and the economy of the system by intelligently adjusting the data exchange strategy under the condition of not sacrificing the protection performance.

Description

Differential protection method based on variable transmission interval and variable length data frame

Technical Field

The invention relates to the technical field of power system protection, and in particular to a differential protection method based on variable transmission interval and variable length data frame.

Background technique

In high-voltage transmission lines (110kV and above), differential protection is widely used because it uses electrical quantity information at both ends of the line to quickly, sensitively and accurately identify internal and external faults. This protection system usually relies on optical fiber as a communication medium, ensuring high-speed data transmission and reliability of the power supply system. However, in distribution networks, due to the large number of lines and complex topological structures, it is usually unrealistic to use optical fiber as the main communication medium from the perspective of economy and engineering implementation.

In recent years, with the rapid development of 5G communication technology, its application in distribution networks has begun to increase, especially as an information transmission channel. 5G technology has obvious advantages over 4G in terms of short communication delay and large bandwidth, which is crucial for the rapid isolation of distribution network line faults. In addition, 5G networks do not require additional cable laying, which is more feasible at the distribution network level.

Nevertheless, 5G technology also faces a series of challenges in the application of distribution network differential protection. First, 5G-based distribution network differential protection may occupy a large amount of 5G traffic and generate high traffic costs, while occupying valuable channel resources. Second, the download rate of commercial 5G networks is usually much higher than the upload rate, while in the application scenario of distribution network differential protection, the upload and download rate requirements are basically equal, which may lead to network efficiency problems.

According to the existing data exchange standards, distribution network differential protection may lead to huge data traffic. For example, based on 100Bytes per frame and 1ms transmission interval, the daily data volume may be close to 70G. If the data of the receiving terminal equipment is taken into account, the total daily data volume may exceed 200G. This large-scale data traffic demand may impose a significant burden on the entire 5G network. At present, the distribution network 5G differential protection of most power grids is still in the pilot stage, and the number of devices is limited. However, if differential protection based on 5G communication is widely adopted in the future, it may bring huge pressure to the communication network.

Summary of the invention

In order to overcome the shortcomings of the prior art, the present invention proposes a differential protection method based on variable transmission interval and variable length data frame. By intelligently adjusting the data exchange strategy, the communication load and cost are effectively reduced without sacrificing protection performance, thereby improving the overall efficiency and economy of the system.

To achieve the above object, the present invention provides a differential protection method based on variable transmission interval and variable length data frame, comprising:

Step S1: All automation control devices in the power distribution area are calibrated through the GNSS module to ensure the accuracy of synchronous measurement.

Step S2: The device performs sampling according to the PPS signal provided by the GNSS to ensure that the sampling moment and interval of the entire system are synchronized in time.

Step S3: Convert the AC signals of voltage and current into synchronous phasors including amplitude and phase angle, using a high-precision clock as a reference.

Step S4: The sparse frame data is stored once per minute, and the dense frame data is stored at 1 ms intervals for 200 ms, and is covered cyclically.

Step S5: The terminal devices on both sides perform local calculations based on the synchronized phasor data, and refresh and store the results so as to respond quickly when a fault occurs.

Step S6: When a fault occurs, the terminal device switches to a complete data frame sending mode through a frame structure, and performs a differential protection logic operation to determine whether the differential protection action conditions are met.

Furthermore, step S1 specifically includes:

Step S11: For all control devices in the power distribution area, a high-precision GNSS module is selected for configuration;

Step S12: Adjust and verify the GNSS modules on all control devices to ensure the accuracy of synchronization timing;

Step S13: Check and confirm that the synchronization operation of all devices is normal and the absolute time provided by GNSS is used as the reference standard.

Furthermore, step S2 specifically includes:

Step S21: according to system requirements, set the synchronous sampling rate (such as 12.8KHz) on all devices;

Step S22: Synchronize the internal clock of the device with the pulse per second signal of the GNSS module;

Step S23: under the triggering of the PPS signal of GNSS, voltage and current data are collected at a set sampling rate.

Furthermore, step S3 specifically includes:

Step S31: extracting and calculating the effective values of current and voltage from the synchronous sampling data;

Step S32: converting the effective value into a phasor, including a real part and an imaginary part, to meet the requirements of synchronized phasor;

Step S33: Ensure the accuracy of the phasor phase angle according to the GNSS clock, with the maximum value at zero phase angle and the second pulse time being synchronized as the standard.

Further, step S4 is specifically as follows:

Step S41: Setting a mechanism for storing data once per minute for the sparse frame;

Step S42: For dense frames, a storage interval of 1 ms is configured to quickly respond to failures;

Step S43: Ensure that the data storage depth is at least 200 ms, and set an overwrite mechanism to save storage space.

Furthermore, step S5 specifically includes:

Step S51: Perform local differential operation using synchronous sampling data to generate synchronous electrical quantities;

Step S52: Update the calculation results to the local memory in real time, and keep data records for at least 200ms;

Step S53: Continuously monitor during normal operation so as to quickly switch the data frame structure when a fault is detected.

Furthermore, step S6 specifically includes:

Step S61: continuously monitoring current and voltage data to identify any abnormal changes;

Step S61: when an abnormality is detected, immediately switch the data transmission mode from the sparse frame to the dense frame;

Step S61: start the differential protection algorithm and use the synchronous electrical quantity data to analyze and determine the fault;

Step S61: Determine whether the differential protection conditions are met according to the differential protection logic operation result, and take corresponding protection measures.

Furthermore, in step S3, based on the synchronous sampling data of the analog quantity, the software implements a synchronous phasor measurement algorithm to obtain a synchronous phasor with a time stamp. The specific algorithm is summarized as follows:

The voltage and current signals of the AC power system can be represented by phasors, which consist of two parts, namely the amplitude X (effective value) and the phase angle The real and imaginary parts are expressed in rectangular coordinates. The phase angle of the synchronized phasor is referenced by a high-precision synchronized clock (GPS, BeiDou). According to the definition of synchronized phasors in the IEEE C37.118.1-2011 standard, for AC signals The phase angle When the phase angle is 0 degrees, the maximum value of x(t) occurs at the second pulse moment; When it is -90 degrees, the positive zero crossing point is synchronized with the second pulse, where: x(t): represents the time-varying AC signal; X: represents the amplitude of the AC signal; f ₀ : represents the signal frequency; represents the initial phase angle of the AC signal; Represents the initial phase angle of the AC signal.

Furthermore, the differential protection is divided into phase current differential and zero-sequence overcurrent differential, and the algorithm is as follows:

Phase current differential element

Steady state stage I

Action equation:

in:

I _dΦ : phase differential current,

I _rΦ ：phase braking current,

It means the current phasor of the equipment on this side (M side);

It means the current phasor of the opposite side (N side) equipment;

Differential action current high threshold setting;

Steady-state II

Action equation:

in:

[Differential operating current rating] and 1.5ICap, whichever is greater.

When the action equation is satisfied, the steady-state phase current differential element of stage II will operate after a 25ms delay.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention provides a differential protection method based on variable transmission interval and variable length data frame, which significantly reduces the amount of data in the communication process by adopting a slow data exchange frequency (for example, exchanging one frame of data per minute, i.e., using sparse frames) under normal operating conditions. This not only reduces the network load, but also reduces the communication charges.

2. The present invention provides a differential protection method based on variable transmission interval and variable length data frame. Under normal operating conditions, only the most basic data is exchanged between terminal devices, and the length of the data frame is greatly reduced. This optimized data transmission method further reduces the communication burden of the network.

3. The present invention provides a differential protection method based on variable transmission interval and variable length data frame. Even when the amount of data is reduced and the communication frequency is lowered, this scheme still maintains the core performance of differential protection, including fast response (speed), high sensitivity and reliability, thereby ensuring the stable operation of the power system.

4. The present invention provides a differential protection method based on variable transmission interval and variable length data frame, especially for application in distribution network differential protection. It makes full use of time-stamped synchronous phasor data and adapts to different operating conditions by changing the frame structure and transmission interval, which is uncommon in traditional differential protection implementation methods.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

Fig. 1 is a schematic diagram of the steps of the present invention;

FIG2 is a schematic diagram of the action logic of frame switching of the present invention;

Fig. 3 is a schematic diagram of sampling of the present invention;

FIG4 is a schematic diagram of synchronous sampling logic of the present invention;

FIG5 is a schematic diagram showing the definition of the synchronous phasor phase angle and second pulse of the present invention;

FIG6 is a schematic diagram of a synchronized phasor calculation model of the present invention;

FIG7 is a schematic diagram of the transceiver processing logic of the present invention;

FIG8 is a schematic diagram of the power grid layout of the present invention.

Detailed ways

The technical solution of the present invention will be more clearly and completely explained below through description of preferred embodiments of the present invention in combination with the accompanying drawings.

Definition of noun:

The Global Navigation Satellite System (GNSS) is an air-based radio navigation and positioning system that can provide users with all-weather three-dimensional coordinates, speed and time information at any location on the Earth's surface or in near-Earth space.

The Global Navigation Satellite System (GNSS) consists of one or more satellite constellations and their augmentation systems required to support specific operations.

The International Committee on Global Navigation Satellite Systems announced the world's four largest satellite navigation system suppliers, including China's BeiDou Navigation Satellite System (BDS), the United States' Global Positioning System (GPS), Russia's GLONASS and the European Union's Galileo Navigation Satellite System (GALILEO). GPS is the world's first global system established and used for navigation and positioning. After a rapid recovery, GLONASS has become the world's second largest satellite navigation system. Both are in the process of modernization and updating; GALILEO is the first fully civilian satellite navigation system and is in the trial stage; BDS is a global satellite navigation system independently built and operated by China, providing global users with all-weather, all-day, high-precision positioning, navigation and timing services.

The full name of PPS in English is Pulse Per Second, which means pulse per second, pulse number per second. PPS is the abbreviation of pulse number per second.

The GPS receiver module (GPS Receiver) can generate a second pulse signal, which indicates the time interval of the whole second, and marks the specific time corresponding to the UTC (Coordinated Universal Time) given by GPS through the rising edge of PPS. The accuracy of indicating the whole second time can reach tens of nanoseconds, without cumulative error. The PPS generated every second and the whole second time it gives can be used for precise frequency modulation of the crystal oscillator - clock synchronization.

FPGA is a semi-customized circuit in the field of application-specific integrated circuits. The internal structure, function, and external interface of the chip can be changed through programming.

SBAS - Satellite-Based Augmentation System. A system that enhances the accuracy of GNSS signals.

QZSS - Quasi-Zenith Satellite System. A Japanese regional satellite positioning system.

DGNSS-Differential Global Navigation Satellite System. A technology that improves the positioning accuracy of the GNSS system.

AGNSS-Assisted Global Navigation Satellite System. Provides assistance data through the network or other means to improve the performance of the GNSS system.

SoC-System on Chip. Refers to a single chip that integrates multiple electronic system functions.

PVT-Position, Velocity, and Time. The three basic information provided by the GNSS system.

From the operation of the distribution network, we can see that the distribution network operates normally most of the time without any faults. However, in order to ensure rapid detection and isolation of faults after they occur, even during the normal operation of the power grid, high-speed and large-capacity exchange of equipment data is required to achieve rapid response in the event of a sudden fault.

As shown in Figure 1, the present invention is:

Step S1: All automation control devices in the power distribution area are calibrated through the GNSS module to ensure the accuracy of synchronous measurement.

Step S2: The device performs sampling according to the PPS signal provided by the GNSS to ensure that the sampling moment and interval of the entire system are synchronized in time.

Step S3: Convert the AC signals of voltage and current into synchronous phasors including amplitude and phase angle, using a high-precision clock as a reference.

Step S4: The sparse frame data is stored once per minute, and the dense frame data is stored at 1 ms intervals for 200 ms, and is covered cyclically.

Step S5: The terminal devices on both sides perform local calculations based on the synchronized phasor data, and refresh and store the results so as to respond quickly when a fault occurs.

Step S6: When a fault occurs, the terminal device switches to a complete data frame sending mode through a frame structure, and performs a differential protection logic operation to determine whether the differential protection action conditions are met.

The innovation of this solution is to propose a new data transmission method in the distribution network differential protection. On the one hand, under normal system operation, a slow data exchange frequency is adopted. In the implementation scheme, one frame of data is exchanged every 1 minute (sparse frame). At the same time, under normal operating conditions, only the most basic data is exchanged between terminals, and the frame length is greatly shortened, further reducing the communication load.

The data format of the sparse frame is shown in Table 1:

Table 1

In normal operation, the "sparse frame" only transmits the message content required for normal operation status verification. In addition to the conventional frame header and frame tail (including CRC check code), the meaning of the channel data is explained as follows:

Device address information, ID information of the terminal device, and the master station identifies the location of the terminal device in the power topology based on this ID information;

Data time information depends on the timing function of GNSS. The equipment can perform sampling and data processing according to absolute time, and the obtained data also has accurate time stamp information. Therefore, the time stamp information of the electrical data must be included in the information transmission message. The receiving device performs phasor calculation and state recognition for the electrical data matching the same time stamp of the terminal according to the time stamp information of the received message;

Voltage phasor: Based on the voltage phasor of the absolute time scale, the synchronous voltage phasor in the area under the jurisdiction can be obtained at the master station, which can effectively identify system instability and voltage anomaly;

Power values, including active power and reactive power, need to store the load conditions of electrical nodes under normal operation during the closing operation after a distribution network fault, and identify the load conditions of relevant nodes before closing the loop after a fault, to avoid overload instead of expected tripping after the system is closed, which reduces power supply reliability;

The switch status information helps the system identify the switch status of the electrical node where the terminal is located, as well as related operation information, to provide data support for further optimizing the system operation mode.

When the system is in normal condition, the voltage is close to the rated voltage for normal operation, and the current is the current corresponding to normal power flow transmission, which is usually not higher than the rated current. Under normal operating conditions of this system, some steady-state data are exchanged on both sides of the distribution network terminal. These data are used as status monitoring data of the distribution network, and are transmitted to the other end through the master station. The other end completes data verification under steady-state conditions.

When a system fault occurs, such as a phase-to-phase fault, the terminal will experience electrical characteristics that are different from normal operating conditions, which can be divided into the following four situations:

Out-of-area fault

In the case of an out-of-zone fault, the terminals on both sides of the line feel a through fault current, which is much larger than the load current of the system in a normal state. At the same time, the voltage between the fault phases/phases will also be significantly reduced, showing typical fault electrical characteristics.

Intra-area faults of single-ended radial lines

When a single-ended radial line has an in-area fault, the power supply terminal also shows a decrease in the fault phase/phase-to-phase voltage and an increase in current, showing obvious fault electrical characteristics;

Because there is no power supply behind the load side, the load side terminal of the fault line cannot feel obvious fault current, and even because the power transmission channel is blocked, the fault phase current may be less than the normal load current. However, the fault phase/phase-to-phase voltage will drop significantly.

Intra-zone faults with double-ended power supply

The distribution network will be in dual power supply operation mode under a few working conditions, or partially show the electrical characteristics of the power supply side because of the configuration of new energy (photovoltaic, energy storage) on the load side. In this case, if an intra-area fault occurs, both sides of the line will also show the situation of fault phase/phase voltage reduction and current increase, showing obvious fault electrical characteristics;

Therefore, based on the above characteristics of the normal state and fault state of the system, the criterion for triggering the frame structure switching is designed in the distribution network terminal. According to the electrical characteristics of the distribution network fault, the following electrical criteria are designed as the triggering conditions:

Phase overcurrent starting element

Imax>Iqd_set

Note: Imax is the maximum value of the three-phase circuit; Iqd_set is the phase overcurrent starting setting;

Zero sequence overcurrent starting element

3I0>I0qd_set

Note: 3I0 is the zero-sequence current amplitude; I0qd_set is the zero-sequence overcurrent starting setting;

Zero sequence voltage starting element

3u0>U0qd_set

Note: 3u0 is the zero-sequence voltage amplitude; U0qd_set is the zero-sequence overvoltage starting setting;

Low voltage starting element

Umin<Ul_set

Note: Umin is the minimum value of three-phase voltage; Ulset is the low voltage starting setting value;

In the power distribution terminal, the action logic of frame switching is shown in Figure 2:

When the terminal senses a system abnormality (such as a sudden change in current, a voltage drop, or a rise in zero-sequence voltage), the frame structure of the data sent by the terminal is restored to a complete frame, and the sending method is also changed to intensive sending (intensive frame) to ensure that the terminal equipment has sufficient information to determine the system operating conditions, quickly identify and form judgment criteria, and create conditions for quickly eliminating the fault.

Because the frame length is variable and the transmission interval is dynamically adjusted, new requirements are put forward for the data characteristics of the terminal. In the implementation of this solution, GNSS satellite timing is introduced, and the terminal realizes synchronous sampling based on the absolutely precise time of GNSS, and the electrical data has a precise time stamp. The data calculated and generated by the terminal and sent to the outside are all with absolute time stamps, and the received data are also with time stamps. Differential protection can directly use such data for differential calculations, abandoning the synchronous sampling and adjustment processing of the data on both sides of the traditional differential protection, and also making the data transmission method with variable transmission interval and variable frame structure of this solution feasible.

Table 2

As shown in table 2:

In the case of system abnormalities, the meaning of the "dense frame" channel data is explained as follows:

Device address information, the meaning is the same as the interpretation of "sparse frame";

Data time information, the meaning is the same as the interpretation of "sparse frame"

Electrical phasor information, based on the synchronous electrical phasor of the absolute time scale, including the phase-by-phase synchronous current phasor and the phase-by-phase synchronous voltage phasor; the terminal uses the synchronous vectors at both ends of the line to form a differential current for differential protection action discrimination; the synchronous voltage phasor can assist in completing the capacitive current compensation when necessary;

Switch status information, the meaning is the same as the definition of "sparse frame".

GNSS Implementation Solution

GNSS module - positioning and timing module, which obtains accurate absolute time by receiving signals from multiple satellite systems (the GNNS module model selected for this system is UM220-IVL, which is a GNSS multi-system, high-precision timing module. It is based on the multi-system, low-power, high-performance SoC chip-UFirebird design with completely independent intellectual property rights, supports BDS, GPS, GLONASS and Galileo systems, can receive and process two or three of them at the same time, or work independently as a single system. It supports SBAS, QZSS system and DGNSS data input functions, has advanced AGNSS functions, and can improve positioning speed through the auxiliary data service of Unicore Starlink when connected to the Internet).

The RF signal of the UM220-IVL Beidou/GPS dual-mode antenna receiver is first input into the RF chip through a low noise amplifier and a surface acoustic wave filter. The RF chip converts the RF signal into a baseband signal through frequency synthesis and frequency conversion, and despreads and demodulates the baseband signal, using pseudo code measurement and carrier phase measurement technology to measure position, speed, time and other parameters. The module receives the PVT original value of the RF signal, performs navigation processing and timing processing, and provides users with navigation positioning, timing and other services.

The key technical indicators of UM220-IVL are positioning accuracy and timing accuracy. The static horizontal positioning accuracy of UM220-IVL is better than 10m; the static elevation positioning accuracy is better than 10m. The timing accuracy of UM220-IVL, 1PPS output uncertainty should be better than 50ns.

UM220-IVL supports three timing modes: fixed point timing, optimized position timing, and positioning timing. The timing mode can be switched and queried through internal commands. UM220-IVL can track GPS, BDS Beidou, GLONASS, Galileo and other systems, and the system can be switched through internal commands.

Fixed time service

Fixed-point timing mode is only for timing applications in static scenarios. In this mode, the user needs to enter the exact center position of the receiver antenna through the CFGTM command. UM220-IVL uses this accurate position to calculate the distance between the antenna and the satellite, and calculate the time for timing.

Optimize position timing

The optimized position timing mode is also a timing application in a static scenario. In this mode, the receiver collects a certain number of positioning points (observation time) and calculates these positioning points to obtain the exact position of the antenna. After that, this position is locked and switched to the fixed-point timing mode, and fixed-point timing is performed based on the locked position.

The observation time and observation accuracy are set through internal commands. Both conditions must be met at the same time to enter the fixed-point timing mode. The observation status can be queried.

The position estimation process only needs to be performed once after the UM220-IVL is installed. After the position optimization is completed, the timing mode in the receiver settings will automatically switch to fixed-point timing mode.

As one of the innovative points of this solution, a GNSS module is added to the hardware system of the automatic control equipment. Its core functions are as follows:

The GNSS module inputs the pulse per second signal (PPS) into the FPGA. The ADC sampling module of the FPGA performs synchronous sampling calculations based on the PPS signal, combines it with the timing information of the GNSS, forms synchronous vector data (voltage and current) with time stamps, and sends this electrical data to the master station.

Synchronous Sampling Implementation Method

In the series of terminal devices, the electrical quantity synchronous phasor acquisition technology based on high-precision clock is innovatively applied. According to the PPS signal (second pulse signal) sent by GNSS, the whole second is divided into equal parts according to the sampling rate within a strict whole second interval. As shown in Figure 3, taking the sampling rate of 12800 points/second as an example, the FPGA divides the whole second into equal parts according to this sampling rate, and the equal division interval is 78.125us. FPGA adjusts the sampling time of the analog sampling conversion chip (ADC, such as AD7606) so that the sampling interval and sampling time of the ADC chip are strictly in accordance with the equal division interval.

Because the automation control devices in the distribution network area are all based on the same GNSS benchmark, the sampling time and sampling interval of each device are synchronized in the absolute time dimension, providing a synchronized measurement data source for the calculation of the synchronous vector.

Figure 4 is a synchronous sampling logic block diagram (taking 12.8K sampling rate as an example)

After receiving a new PPS signal, the whole second TPPS(N) is calibrated according to the PPS reception time, and the difference is subtracted from the previous whole second TPPS(N-1) to obtain the latest second interval Tinterval, and the second interval is divided according to the sampling rate (taking 12800/second as an example) to obtain the refreshed sampling interval Tsample.

Furthermore, this interrupt also sets the next sampling time t_smp(n) according to the latest sampling interval and calibrates the correct sampling sequence number (0 to 12799 cycles).

In the distribution network control terminal, based on the synchronous sampling data of the analog quantity, the software implements the synchronous phasor measurement algorithm to obtain the synchronous phasor with time stamp. The specific algorithm is summarized as follows:

The voltage and current signals of the AC power system can be represented by phasors, which consist of two parts, namely the amplitude X (effective value) and the phase angle The synchrophasor phase angle is expressed as real and imaginary parts in rectangular coordinates. The synchrophasor phase angle uses a high-precision synchronous clock (GPS, BeiDou) as a reference. According to the IEEE C37.118.1-2011 standard, for AC signals The phase angle When the phase angle is 0 degrees, the maximum value of x(t) occurs at the second pulse moment; When it is -90 degrees, the positive zero crossing point is synchronized with the second pulse, as shown in Figure 5.

Figure 6 shows the basic algorithm model of synchronized phasor measurement, which includes analog low-pass filter, synchronous sampling, DFT calculation, digital filtering and other links to complete phasor acquisition.

The device uses dynamic phasor fast software algorithm to calculate the AC voltage, AC current, amplitude of sequence components, phase angle, frequency and frequency change rate of the channel, and has high measurement accuracy and fast dynamic response performance.

Local data storage and message, receiving and sending processing, as shown in Figure 7, is the switching process for sparse frames and dense frames

When the device is in normal operation (non-fault disturbance state), it sends messages with sparse frame structure at long period intervals (1 frame/minute); when the device is in abnormal operation (fault disturbance state), it sends messages with dense frame structure at short period intervals (1 frame/millisecond).

If a sparse frame is received from the opposite device, the data of the corresponding time stamp on this side is matched according to the time stamp of the sparse frame to complete the data corresponding work (calculation of differential momentum and braking amount); if a dense frame is received from the opposite device, the data of the corresponding time stamp on this side is matched according to the time stamp of the dense frame to complete the data corresponding work (calculation of differential momentum and braking amount);

Local data storage is divided into two levels:

First, for sparse frames (taking 1-minute data interaction as an example), local storage is based on the interval of whole minutes, and one frame of local synchronous electrical data message is stored every whole minute;

Second, for dense frames (taking 1mS data interaction as an example), the local data is stored at a 1mS data interval. The data storage depth is not less than 200mS, and it is covered in a loop.

Differential protection implementation scheme

For the terminal devices on both sides that constitute the differential, because the local sampling is a synchronous phasor measurement mechanism based on an absolute clock, the local sampling data is complete and fully calculated. The synchronous electrical measurement results of the local calculation are refreshed and stored in the local memory, and the storage depth is not less than 200mS.

In the event of a system failure, whether it is an internal or external fault, both sides will usually trigger frame structure switching and send complete data frames instead. At that time, the protection on both sides can perform logical operations, judgment criteria and output of differential protection.

Differential protection is divided into phase current differential and zero-sequence overcurrent differential. The algorithm is as follows:

Phase current differential element

Steady state stage I

Action equation:

in:

IdΦ: phase differential current,

IrΦ: Phase braking current,

Same meaning as above.

Steady-state II

Action equation:

in:

[Differential operating current rating] and 1.5ICap, whichever is greater.

When the action equation is satisfied, the steady-state phase current differential element of stage II will operate after a 25ms delay.

Based on synchronized phasor data with time stamps, this scheme proposes a differential protection implementation method with a variable frame length structure and a variable transmission interval. It can significantly reduce the amount of interactive information in external communications without reducing the performance of differential protection (including speed, sensitivity and reliability), and effectively reduce communication charges.

This solution has been piloted in the Jiangsu Huai'an Ecological Cultural Tourism Intelligent Connection Pilot Zone Demonstration Project, and its specific implementation is shown in Figure 8:

Ring network 1 consists of 10kV Qiushi D15 line and 10kV Jingyuan D14 line (powered by 10kV I-section busbar of Banzha transformer, which can be operated in a ring). D15 outgoing line—D1501 ring network cabinet—D1509 ring network cabinet—D1512 ring network cabinet—D1412 ring network cabinet—D1411 ring network cabinet—D1410 ring network cabinet—D1403 ring network cabinet—D1401 ring network cabinet—D14 outgoing line, forming a ring network. New cables need to be laid between D1512 ring network cabinet and D1412 ring network cabinet to form a ring network.

Ring network 2 consists of 10kV Yunlin D41 line and 10kV Jinghui D42 line (powered by 10kV IV busbar of Banzha transformer, which can be operated in a closed ring). D41 outgoing line - D4101 ring network cabinet - D4105 ring network cabinet - D4106 ring network cabinet - D4205 ring network cabinet - D4204 ring network cabinet - D4201 ring network cabinet - D42 outgoing line constitutes a ring network.

New cables need to be laid between the D4106 ring main unit and the D4205 ring main unit to form a ring network.

New cables need to be laid between ring network 1 and ring network 2 to form a connecting line. (Between D1501 ring network cabinet and D4101 ring network cabinet)

The modified line framework is as follows:

The ring main units involved in this project are: D1501 ring main unit, D1509 ring main unit, D1512 ring main unit, D1412 ring main unit, D1410 ring main unit, D4101 ring main unit, D4106 ring main unit, D4205 ring main unit, D4204 ring main unit, and D4201 ring main unit.

The above specific implementations are only descriptions of the preferred implementations of the present invention, and do not limit the protection scope of the present invention. Without departing from the design concept and spirit of the present invention, various modifications, substitutions and improvements made by ordinary technicians in this field to the technical solution of the present invention based on the text description and drawings provided by the present invention should all fall within the protection scope of the present invention. The protection scope of the present invention is determined by the claims.

Claims (9)

1. A differential protection method based on a variable transmission interval and a variable length data frame, comprising:

step S1: the automatic control equipment in the power distribution area is calibrated through a GNSS module, so that the accuracy of synchronous measurement is ensured;

step S2: the device samples according to the PPS signal provided by the GNSS, so as to ensure the synchronization of the sampling time and the interval of the whole system in time;

step S3: converting the alternating current signal of the voltage and the current into a synchronous phasor comprising an amplitude and a phase angle;

Step S4: sparse frame data is stored once every minute, dense frame data is stored at 1ms intervals for 200ms, and cyclic coverage is performed;

step S5: the terminal devices on two sides perform local operation according to the synchronous phasor data, refresh and store the result so as to respond rapidly when a fault occurs;

step S6: when a fault occurs, the terminal equipment is switched to a complete data frame sending mode through a frame structure, and performs differential protection logic operation to determine whether the action condition of differential protection is met.

2. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein step S1 specifically comprises:

Step S11: selecting a high-precision GNSS module for configuration aiming at all control equipment in a power distribution area;

Step S12: adjusting and verifying GNSS modules on all control equipment to ensure the precision of synchronous time service;

Step S13: checking and confirming that the synchronous operation of all devices is normal, and taking absolute time provided by GNSS as a reference standard.

3. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein step S2 specifically comprises:

step S21: setting synchronous sampling rate on all devices according to system requirements;

step S22: synchronizing an internal clock of the device with a second pulse signal of the GNSS module;

step S23: and under the triggering of the PPS signal of the GNSS, acquiring voltage and current data according to a set sampling rate.

4. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein step S3 specifically comprises:

step S31: extracting and calculating effective values of current and voltage from synchronous sampling data;

step S32: converting the effective value into phasors, including a real part and an imaginary part, to meet synchronous phasor requirements;

step S33: and ensuring accurate phasor phase angles according to the GNSS clock, and synchronizing the maximum value at the zero-degree phase angle with the second pulse time as a standard.

5. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein step S4 is specifically as follows:

step S41: a mechanism for storing data once every minute is set for the sparse frame;

Step S42: for dense frames, configuring a storage interval of 1ms to cope with fault quick response;

step S43: ensure a data storage depth of at least 200ms and set up an overlay mechanism to save storage space.

6. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein step S5 specifically comprises:

step S51: carrying out local differential operation by using synchronous sampling data to generate synchronous electric quantity;

Step S52: updating the calculation result to a local memory in real time, and keeping a data record of at least 200 ms;

Step S53: monitoring is continued during normal operation to quickly switch data frame structures when a failure is detected.

7. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein step S6 specifically comprises:

Step S61: continuously monitoring the current and voltage data to identify any abnormal changes;

Step S61: when an abnormality is detected, immediately switching a data transmission mode from a sparse frame to a dense frame;

step S61: starting a differential protection algorithm, and analyzing and judging faults by using synchronous electric quantity data;

Step S61: and determining whether the differential protection condition is met or not according to the differential protection logic operation result, and taking corresponding protection measures.

8. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein in step S3, based on analog synchronous sampling data, software implements a synchrophasor measurement algorithm to obtain a synchrophasor with a time stamp, and the specific algorithm is summarized as follows:

The voltage and current signals of the alternating current power system are represented by phasors which are composed of two parts, namely amplitude X and phase angle The rectangular coordinates are used as a real part and an imaginary part, and the synchronous phasor phase angle takes a high-precision synchronous clock as a reference; for ac signals, according to the synchrophasor definition of the IEEE C37.118.1-2011 standard When the phase angle isAt 0 degrees, the x (t) maximum occurs at the pulse per second instant; when phase angleAt-90 degrees, the positive zero crossing is synchronized with the second pulse, wherein: x (t): representing a time-varying alternating current signal; x: representing the amplitude of the ac signal; f ₀: representing the signal frequency; representing the phase angle of the alternating current signal; Representing the phase angle of the ac signal.

9. The differential protection method based on variable transmission interval and variable length data frame according to claim 1, wherein the differential protection is divided into phase current differential and zero sequence overcurrent differential, and the algorithm is as follows:

Phase current differential element

Steady state segment I

Equation of motion:

wherein:

i _dΦ: the phase differential current is used to determine the phase difference,

I _rΦ: the phase braking current is applied to the motor,

Meaning the current phasors of the equipment at the side;

meaning contralateral device current phasors;

The differential action current is high in threshold value;

Steady state section II

Equation of motion:

wherein:

The larger of [ differential operation current constant ] and 1.5 ICap;

When the motion equation is satisfied, the steady-state II-phase current differential element is subjected to 25ms delay motion.