CN110661238B - A multi-terminal flexible DC distribution network protection method based on current-limiting inductor voltage - Google Patents

Abstract

A multi-terminal flexible direct-current power distribution network protection method based on current-limiting inductance voltage comprises fault starting, fault detection, fault identification and fault pole selection; the fault starting is judged by du/dt and di/dt; the fault detection is to further detect whether the line has faults by adopting low-voltage and overcurrent protection; the fault identification is to reliably identify the internal and external faults of the area by using the current-limiting inductive voltage at two ends of a direct-current line; the fault pole selection is to determine whether the system has positive pole, negative pole or bipolar fault by using the current-limiting inductance voltage at two ends of a direct current line; and when a corresponding fault is detected, a tripping signal is sent out, and the corresponding direct current breaker acts to remove the fault. The invention solves the problem of low protection speed of the traditional direct-current power distribution network. The invention identifies the fault through the voltage at the two ends of the current-limiting inductor.

Description

A multi-terminal flexible DC distribution network protection method based on current-limiting inductor voltage

technical field

The invention relates to a direct current protection method, in particular to a multi-terminal flexible direct current distribution network protection method based on a voltage source converter (VSC).

Background technique

In recent years, with the development of power electronic technology and DC energy storage devices, the power supply and load in the power system have undergone great changes. The proportion of distributed power in the power supply is getting higher and higher. The load is connected to the grid through the converter. When using the traditional AC distribution network to realize the consumption of distributed power and DC load, it needs to be connected to the AC distribution network through a multi-level converter station, and the investment of a large number of power electronic equipment increases the investment cost. Compared with the AC power distribution system, the DC power distribution network has the advantages of large power transmission capacity, convenient access to distributed energy, low line loss, high reliability and high power supply quality.

The multi-terminal flexible DC distribution network can realize the connection with the AC distribution network through the DC distribution line, which is an important development direction of the DC distribution system. At present, the research on relay protection of multi-terminal flexible DC distribution network at home and abroad is still in the theoretical stage. Compared with AC distribution network, there is a lack of perfect protection methods. Therefore, the protection and fault identification of multi-terminal flexible DC distribution network is an urgent need. One of the core technologies to solve. Due to the small damping of the system and no natural zero-crossing point in the DC distribution network, once a fault occurs on the DC side, the fault current has a high peak value and a rapid rise, which will quickly spread to the entire power grid. For the multi-terminal flexible DC distribution network, the superposition of fault currents of multiple converter stations will bring further damage to the system after a fault occurs, which puts forward higher requirements for the quickness of the protection method. The method needs to identify the fault within 3ms. ABB and SIMENS proposed to use traveling wave protection and differential undervoltage protection as the main protection and differential protection as backup protection for HVDC transmission systems. , there is a large error, therefore, the traveling wave method is rarely used in the protection of the DC distribution network. The "handshake method" can use the AC side circuit breaker to remove the DC side fault when the DC circuit breaker has not been developed. It has important reference value for the early DC protection research, but this protection method needs to disconnect all the AC circuit breakers. This results in short-term power outages in non-faulty areas, reduces the reliability of power supply, and has a slow fault removal speed, which cannot meet the requirements of DC protection for selectivity and quickness. The DC fault protection methods based on the DC circuit breaker and the self-clearing of the converter can meet the requirements of DC protection for quickness, but because the core idea is still the "handshake method", it still cannot meet the protection requirements for selectivity. Aiming at the lack of selectivity of the DC grid, the inverse-time current variance protection method can identify the DC side fault and judge the severity of the fault, but this method is only suitable for the double-terminal DC distribution network and cannot identify the multi-terminal flexible DC distribution system area. Internal and external failures. For the multi-terminal DC grid, the fault current change rate on the DC side can be used to identify the fault, but in the multi-terminal flexible DC grid, the feed current of the adjacent grid will have a certain impact on the protection method.

To sum up, the main problem in the research of multi-terminal flexible DC distribution network is: due to the small system damping and no natural zero-crossing point in the DC distribution network, once the DC side fails, the peak value of the fault current is high and the rising speed is fast, which will spread rapidly. For the entire power grid, the protection method should be as simple as possible under the premise of meeting the requirements of quickness and selectivity, and the threshold setting should be as reasonable as possible rather than only obtained through simulation experiments.

SUMMARY OF THE INVENTION

In order to solve the deficiencies that the prior art cannot meet the requirements for protection quickness and selectivity of the multi-terminal flexible DC distribution network, the present invention proposes a multi-terminal flexible DC distribution network protection method based on the current-limiting inductor voltage.

The present invention is realized by adopting the following technical solutions: a multi-terminal flexible DC distribution network protection method based on current-limiting inductor voltage, which is characterized in that the voltage and current of the current-limiting inductor of the DC line are used to determine fault startup, fault detection, and fault detection. Identification and fault selection.

S1: Failure characteristic analysis

(1) Bipolar short circuit fault

When a bipolar short-circuit fault or a unipolar grounding fault occurs in the system, the short-circuit current fed from the AC side is only the freewheeling current of the inductor, and the fault current is mainly the capacitor discharge current. The frequency domain i _l (s) expression of the DC line fault current for:

In the formula: u _c (0), i _L (0) represent the capacitor voltage and inductor current at the moment when the fault occurs, R, L, and C are the equivalent resistance, inductance and capacitance of the fault circuit on the DC side; s is the Laplace calculation son.

故障电流的时域 il₍t)表达式为：Under normal circumstances, the transition resistance of the bipolar short-circuit fault is 0, and the resistance of the DC line is also very small, and the fault loop is in an underdamped state, that is,

The time domain il ₍ t ) expression of fault current is:

In the formula: Re _eq , L _eq , and C _eq are the equivalent resistance, inductance and capacitance of the DC line, respectively; in order to facilitate the expression of the formula, in order to simplify the formula in the text, the intermediate variables α, β, ω that have _no practical significance are introduced. , ω _d ; e is the base of the exponential function In, and t represents time.

The differential equation of the fault current is:

Assuming that the initial time of the fault is t=0+ time, the differential value of the fault current at this time is:

For the DC distribution network, the difference between the DC current and the voltage at the initial moment of the fault is more than two orders of magnitude, so the differential value of the current at t=0+ is:

It can be seen from equation (5) that the current differential value is greater than 0 at the initial moment of the fault.

(2) Single pole ground fault

放电回路为幅值逐渐减小的RLC振荡电路，直流侧故障电流的表达式与双极短路故障相似；当过渡电阻较大时，电路处于过阻尼放电状态，即

故障电流的时域表达式为：When a single-pole grounding fault occurs in the DC distribution network, its transient process can be divided into two stages. When the transition resistance is small, the circuit is in an under-damped state, that is,

The discharge circuit is an RLC oscillator circuit with a gradually decreasing amplitude, and the expression of the DC side fault current is similar to that of a bipolar short-circuit fault; when the transition resistance is large, the circuit is in an overdamped discharge state, that is,

The time domain expression of fault current is:

In the formula: P ₁ and P ₂ are defined intermediate variables; R _f represents transition resistance; Re _eq , L _eq , and C _eq have the same definitions as in formula (2); e is the base of the exponential function In, and t represents time.

The differential equation of the fault current is:

Assuming that the initial time of the fault is t=0+ time, the differential value of the fault current at this time is:

For the DC distribution network, the difference between the DC current and the voltage at the initial moment of the fault is more than two orders of magnitude, so the differential value of the current at t=0 is:

From equations (5) and (9), it can be seen that at the initial time of the fault, no matter the circuit is in the under-damped state or the over-damped state, the current differential expressions of the single-pole grounding fault and the double-pole grounding fault are the same, and the current differential value at the initial time of the fault is greater than 0.

S2: Protection Criterion

In the DC system, the DC line includes positive and negative lines, and each line outlet is equipped with a protection device and a current-limiting inductance.

When a positive ground fault occurs on the DC side, the current increases rapidly. Therefore, for the current-limiting inductor, when a fault occurs in the positive direction, d _i /d _t > 0 at the moment of the fault, and the voltage across the positive line inductor is also greater than 0 , Similarly, when a unipolar grounding fault occurs in the negative pole, in the specified positive direction, the voltage across the inductance of the negative pole line is also greater than 0; on the contrary, when a fault occurs in the reverse direction of the protection device, the fault current increases in the opposite direction, and the protection device The current-limiting inductor voltage at is less than 0. Therefore, the longitudinal protection is formed according to the current-limiting inductor voltage at both ends of the line. The specific process is as follows:

(1) Fault start: collect the voltage and current of the lines Line1-Line4, and use d _u / d _t and d _i /d _t as the fault start criterion. When d _u /d _t and d _i /d _t exceed the threshold, Protection starts.

(2) Fault detection: use low-voltage and over-current protection to achieve fault detection, and its role is to further detect whether the line is faulty on the basis of the fault start criterion.

(3) Fault identification: measure the voltage value of the current-limiting inductance at both ends of each line in the specified positive direction, calculate the voltage value of the current-limiting inductance at both ends of the line, and use the current-limiting inductance voltage at both ends of the DC line to form fault identification and fault selection. The function of fault identification criterion is to reliably identify faults inside and outside the zone.

(4) Fault pole selection: The function of fault pole selection criterion is to determine whether the system has a positive, negative or bipolar fault. When the corresponding fault is detected, a trip signal will be sent out, and the corresponding DC circuit breaker will act to remove the fault.

The invention solves the problem that the protection quickness of the traditional direct current distribution network is not strong. The boundary element is constructed by installing current-limiting reactors at both ends of the DC line, and fault identification is performed by the voltage at both ends of the current-limiting inductor. The invention is not affected by transition resistance, fault distance and inverter power reversal, can accurately identify faults inside and outside the zone and select fault poles after a DC side fault, and can meet the requirements of DC protection on selectivity, quickness, sensitivity performance and reliability requirements.

Description of drawings

(1) Figure 1 is a schematic diagram of a multi-terminal flexible DC distribution network.

(2) Figure 2 is a calibration diagram of the positive and negative line directions.

(3) FIG. 3 is a flow chart of the protection method.

Detailed ways

The invention includes a multi-terminal annular DC power distribution system, the multi-terminal annular DC power distribution system includes a converter, a DC circuit breaker and a DC line, and the converter adopts a two-level voltage source converter (VSC), As shown in Figure 1.

In Figure 1: S1~S4 represent the AC system; VSC1~VSC4 are the inverters; Bus1~Bus4 are the DC bus bars; 12, 14, 21, 23, 32, 34, 41, 43 represent the DC circuit breaker numbers; Line4 is the DC transmission line; F1~F4 are the fault points, among which: F1 is on the Line1 line, F2 is on the Line2 line, F3 is on the Line4 line, and F4 is on the Bus2 DC bus. The DC circuit breaker is installed at the outlet of the DC bus, and the current limiting inductor is installed between the DC circuit breaker and the DC transmission line. Each transmission line includes two lines, positive and negative.

For the four-terminal flexible DC power distribution system shown in Figure 1, the master-slave control method is adopted, and the converter VSC1 is controlled by a constant DC voltage to support the voltage of the entire DC system as the balance node of the system; the converters VSC2 ~ VSC4 Constant power control is adopted as the power node of the system.

A multi-terminal flexible DC distribution network protection method based on current-limiting inductor voltage, comprising the following steps:

S1: fault start

After a fault occurs on the DC side, the DC bus voltage decreases and the current increases at the moment of the fault. Therefore, the fault start criterion is formed according to the variation of the DC voltage and current after the fault, as shown in formula (1), when the conditions of formula (1) are satisfied at the same time, the protection scheme starts.

In the formula: u and _i represent the positive or negative DC line voltage and current, _Un and In represent the rated value of the DC voltage and current; t represents the time.

S2: Failure Detection

When the system is disturbed or disturbed, it may cause malfunction of the fault-starting element. Therefore, it is necessary to construct fault detection criteria to identify disturbances and faults. Compared with the fault transient process, the system disturbance takes a long time and will not cause serious system undervoltage and overcurrent. Therefore, the fault detection element is formed according to the low voltage and overcurrent protection. A fault can be determined only when each sampling point satisfies the low-voltage and over-current conditions. The fault detection criterion is shown in formula (2). When the conditions of formula (2) are met at the same time, a fault is detected.

In the formula: _i represents the positive or negative DC line current, _Un represents the DC voltage, In represents the rated value of the current, and U _dc represents the voltage between the poles of the DC side.

S3: Fault identification

(1) Positive and negative line direction calibration

As shown in Figure 2, taking converter station 1 as an example, each line outlet is equipped with a protection device and a current limiting inductance, 12P and 12N are positive and negative protection devices respectively, Lr is a current limiting inductance, and the positive line of the positive line is specified. The direction is that the bus bar points to the line, and the positive direction of the negative line is that the line points to the bus bar.

When a positive ground fault occurs at the DC side F1, the current increases rapidly. Therefore, for the current-limiting inductor, when a fault occurs in the positive direction, d _i /d _t > 0 at the moment of the fault, so there are:

In the formula: U _L12P is the voltage across the current limiting inductor of the positive line.

Similarly, when a unipolar ground fault occurs at the negative pole, the voltage across the inductance of the negative pole line is also greater than 0 in the specified positive direction.

On the contrary, when the positive earth fault occurs at the back side F4 of the protection device 12P in FIG. 1, the fault current increases in the opposite direction, so there are:

Therefore, the protection criterion for failure of the protection device m in the positive direction is:

In the formula: U _Lmt represents the current-limiting inductor voltage; m is the number of the DC circuit breaker, m = 12, 21, 14, 23, 41, 32, 43, 34; t represents the positive and negative poles of the DC circuit breaker, t = P , N, P means positive pole, N means negative pole; R _mt is the positive direction fault identification signal, R _mP = 1 means that the positive direction fault of the protection device occurs, R _mN = 1 means that the positive direction fault of the protection device occurs, R _mt = 0 Indicates that a reverse fault has occurred; U _set is the protection criterion threshold.

(2) Threshold setting

U _set is the protection criterion threshold. According to theoretical analysis, the voltage across the current-limiting inductor is 0 under normal circumstances, and U _set can be set to 0, but considering the unbalanced voltage of the voltage transformer, the threshold should be greater than 0. According to the enterprise standard of State Grid Corporation of China (Q/GDW10531-2016), the amplitude error of the DC electronic voltage transformer is ±0.2%, and the error of one transformer is set to be -0.2% and the other to be +0.2%. The unbalanced voltage is not greater than 0.4%, considering that the reliability factor is 2, the protection criterion threshold U _set can be taken as:

U _set = 2 × 0.4% × U _n (6)

(3) Fault identification

According to the protection devices at both ends of the DC line, the faults inside and outside the zone can be judged. The specific criteria are:

In the formula: n represents the DC circuit breaker at the opposite end of the line where m is located; R _mt is the fault identification signal in the positive direction of the protection device m, R _nt is the fault identification signal in the positive direction of the protection device n; S _t is the fault identification signal inside and outside the area, S _t = 1 indicates that an in-zone fault has occurred, otherwise, it is an out-of-zone fault.

S4: Fault selection

The purpose of setting the fault pole selection criterion is to further identify whether the fault occurs in the positive pole or the negative pole. If a positive ground fault occurs, the DC circuit breaker at both ends of the positive line will operate; if a negative ground fault occurs, the DC circuit breaker at both ends of the negative line will operate. Action; if a bipolar short-circuit fault occurs, the DC circuit breakers at both ends of the positive and negative lines will act. The fault selection criterion is:

In the formula: S _P is the positive fault identification signal, S _N is the negative fault identification signal; when S _P = 1, it is determined as a positive fault; when S _N = 1, it is determined as a negative fault; when S _P *S _N =1, Determining a bipolar fault.

The process flow of the multi-terminal flexible DC distribution network protection method of the present invention is shown in FIG. 3 .

Failure to start:

When the system is running, it begins to enter the protection link, and the DC circuit breaker is used to read the voltage value and current value; and the voltage and current values are differentiated by time t to calculate d _u /d _t and d _i /d _t respectively. When all the conditions of formula (1) are satisfied, the protection starts, and the next fault detection is carried out;

In the formula: u and _i represent the positive or negative DC line voltage and current respectively, U _n and In represent the rated value of the DC voltage and current respectively; t represents the time.

Troubleshooting:

The fault detection criterion is:

In the formula: i represents the positive or negative DC line current, and _Un and In represent the rated values of the DC voltage and current, respectively.

When the above conditions are met, the converter is blocked and the next fault identification is performed; otherwise, the voltage value and current value are read cyclically;

Fault identification:

Measure the voltage value of the current-limiting inductance at both ends of each line in the specified positive direction, calculate the voltage value of the current-limiting inductance at both ends of the line, and judge the fault inside and outside the zone according to the protection devices at both ends of the DC line. The specific criteria are:

In the formula: n represents the DC circuit breaker at the opposite end of the line where m is located; R _mt is the forward direction fault identification signal of the protection device m, R _n is the forward direction fault identification signal of the protection device n; S _t is the internal and external fault identification signal, S _t = 1 indicates that an in-zone fault has occurred, otherwise, it is an out-of-zone fault.

Fault selection:

The purpose of setting the fault pole selection criterion is to further identify whether the fault occurs in the positive pole or the negative pole. If a positive ground fault occurs, the DC circuit breaker at both ends of the positive line will operate; if a negative ground fault occurs, the DC circuit breaker at both ends of the negative line will operate. Action; if a bipolar short-circuit fault occurs, the DC circuit breakers at both ends of the positive and negative lines will act. The fault selection criterion is:

In the formula: S _P is the positive fault identification signal, S _N is the negative fault identification signal; when S _P = 1, it is determined as a positive fault; when S _N = 1, it is determined as a negative fault; when S _P *S _N =1, Determining a bipolar fault.

The protection scheme includes fault startup, fault detection, fault identification, and fault selection criteria. Among them, d _u /d _t and d _i /d _t are used as fault start criteria; low-voltage and over-current protections are used to realize fault detection. The current-limiting inductor voltage at both ends of the line constitutes the fault identification and fault pole selection criteria. The function of the fault identification criterion is to reliably identify the faults inside and outside the area; the function of the fault pole selection criterion is to determine whether a positive, negative or bipolar fault has occurred in the system. When the corresponding fault is detected, a trip signal will be sent out, and the corresponding DC circuit breaker will act to remove the fault.

Claims (1)

1. A multi-terminal flexible DC distribution network protection method based on current-limiting inductance, including fault start, fault detection, fault identification, and fault pole selection; the fault start is based on du/dT, di/dT criteria; fault Detection is to use low-voltage and over-current protection to further detect whether the line is faulty; fault identification is to use the current limiting inductor voltage at both ends of the DC line to reliably identify internal and external faults; fault pole selection is to use the current limiting at both ends of the DC line. The inductance voltage is used to determine whether the system has a positive, negative or bipolar fault; when the corresponding fault is detected, a trip signal is sent, and the corresponding DC circuit breaker operates to remove the fault; it is characterized by the following steps: (1) Fault start: When the system starts to run, it starts to enter the protection link, and the DC circuit breaker is used to read the voltage value and current value; When all the conditions of formula (1) are satisfied, the protection starts, and the next fault detection is carried out;

In the formula: u represents the positive or negative DC line voltage, _i represents the positive or negative DC line current, U _n represents the rated value of the DC voltage, In represents the rated value of the DC current; T represents the time; (2) Fault detection: The fault detection criterion is:

In the formula: U _dc represents the voltage between the poles of the DC side; When the above conditions are met, the converter is blocked and the next fault identification is performed; otherwise, the voltage value and current value are read cyclically; (3) Fault identification: It is stipulated that the positive direction of the positive line is the bus pointing to the line, and the positive direction of the negative line is the line pointing to the bus; The protection criterion for failure of the protection device m in the positive direction is:

In the formula: U _Lmt represents the current-limiting inductor voltage; m is the number of the DC circuit breaker, m=12, 21, 14, 23, 41, 32, 43, 34; t represents the positive and negative poles of the DC circuit breaker, t=P , N, P represent the positive pole, N represents the negative pole; R _mt is the fault identification signal in the positive direction, R _mp = 1 means that the positive pole of the protection device has a positive fault, R _mN = 1 means that the negative pole of the protection device has a positive fault, and R _mt = 0 Indicates that a reverse fault has occurred; U _set is the protection criterion threshold; The criterion threshold U _set can be taken as: U _set = 2×0.4%×U _n (4) Measure the voltage value of the current-limiting inductance at both ends of each line in the specified positive direction, calculate the voltage value of the current-limiting inductance at both ends of the line, and judge the fault inside and outside the zone according to the protection devices at both ends of the DC line. The specific criteria are:

In the formula: n represents the DC circuit breaker at the opposite end of the line where m is located; R _mt is the fault identification signal in the positive direction of the protection device m, R _nt is the fault identification signal in the positive direction of the protection device n; S _t is the fault identification signal inside and outside the area, S _t = 1 indicates that an intra-area fault has occurred, otherwise, it is an out-of-area fault; (4) Fault pole selection: The purpose of setting fault pole selection criteria is to further identify whether the fault occurs in the positive or negative pole. If a positive ground fault occurs, the DC circuit breaker at both ends of the positive line will operate; if a negative ground fault occurs, then The DC circuit breakers at both ends of the negative line act; if a bipolar short-circuit fault occurs, the DC circuit breakers at both ends of the positive and negative lines act; the fault pole selection criterion is:

In the formula: Sp is the positive fault identification signal, _SN is the negative fault identification signal; when Sp ₌ 1, it is determined as a positive fault, and when _{SN =} 1, it is determined as a negative fault.

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