JP7556830B2 - Power distribution system and method for detecting faults in power distribution system - Google Patents

Description

The present invention relates to the configuration of a power distribution system and a method for detecting faults in the system, and in particular to technology that can be applied to identifying faulty power supply lines.

In the power distribution network, transmission and distribution lines play an essential role in efficiently transmitting electricity over long distances from centralized power plants. However, these power supply lines often fail due to storms, lightning, snow, freezing rain, insulation breakdown, or short circuits caused by birds or other external objects. After a fault occurs, protective relays detect the fault and trip circuit breakers to open the faulted wiring section, resulting in a power outage. Power outages are a serious problem in modern society.

If a faulty power supply line can be accurately identified, it can be restored quickly. This can reduce outages for users connected to power supply lines that are not directly affected by the fault. In many cases, electrical faults lead to mechanical damage to the facilities and equipment receiving the power, necessitating repairs. Identifying the faulty power supply line allows for early repairs, preventing recurrence of the fault or serious damage.

Therefore, in power distribution networks, fast and reliable methods for detecting faults and restoring power are essential for high quality of service and overall cost reduction.

As one of the main grounding modes, neutral ineffectively grounded or highly grounded is widely used in medium voltage (MV) distribution grids, especially in Europe and China. Such neutral ineffectively grounded distribution grids (NIGDG) have the advantage that the phase-to-phase voltages remain symmetrical after a single-phase earth fault occurs. The distribution system can operate under fault conditions for 1-2 hours, and sometimes even longer. This allows the utility company time to clear the fault and provide uninterrupted power.

However, an inherent drawback of NIGDG is that the magnitude of the fault current does not significantly exceed the normal operating current, making detection and identification of faulted power lines very difficult.

In addition, electrical conductors in the power distribution network often come into contact with poorly grounded objects such as trees, wooden fences, and vehicles. Sometimes, these energized conductors can break and come into contact with high-impedance ground such as asphalt, concrete, grass, or sand. These contacts limit the fault current to only a few milliamps to tens of amps. Traditional protection schemes based on overcurrent relays, reclosers, and fuses have difficulty detecting such low-current high-impedance faults (HIFs).

Undetected HIFs do not damage components of the power grid, but energized conductors above ground can pose a personal injury hazard. Also, if such faults cause arcing, they pose a fire hazard.

According to several reports, HIFs account for around 20-25% of all faults in the power grid. However, in reality, this percentage is higher than reported, making it essential to address the problem of HIF detection.

In recent years, power utilities aiming for a sustainable society are moving to smart grids to provide more renewable energy and electric vehicles (EVs). Power conversion equipment is required for power conversion from renewable energy and EVs. By using intelligent monitoring and control from multi-function power conversion equipment, power utilities can reduce power outages.

As background technology in this technical field, for example, there is technology such as that in Patent Document 1. Patent Document 1 discloses a "fault location method for determining the type of fault (ground fault or short circuit fault) by measuring the phase-to-phase impedance and the phase-to-ground impedance of a power distribution line using a high-frequency power source, a voltage measurement sensor, and a current measurement sensor."

Patent Document 2 also discloses a system that "ensures that distributed resources of a power distribution system remain connected to the power distribution system circuit when a fault occurs in a distributed resource node."

Patent document 3 also discloses a ground fault detection device that includes "a detection signal injection means for injecting two detection signals with a phase difference of 180° between an underground distribution line and the ground, a magnetic field detection means for detecting the magnetic field generated by the two detection signals propagating through the underground distribution line, and a ground fault detection means for detecting a ground fault point in the underground distribution line based on the detected magnetic field detected by the magnetic field detection means."

JP 2002-122628 A U.S. Pat. No. 9,459,308 JP 2007-279031 A

Previous inventions apply single high frequency current injection from power devices at multiple locations on a power line for fault detection and identification. These power devices not only inject high frequency current during a grid fault, but also block high frequency current injected by other adjacent power devices. High voltage (HV) voltages and currents are detected by sensors installed with the power devices on multiple power lines. The faulty power line is identified by evaluating and comparing the high frequency (HF) impedance of the multiple power lines.

However, in the case of high impedance faults (HIF) or in distribution networks with ungrounded or highly grounded neutrals (NIGDG), the difference between the HF impedances of the power supply lines is small, so this method may not be effective.

The technologies described in Patent Documents 1 to 3 above may not be able to properly identify high impedance faults (HIF) or in power distribution networks where the neutral point is ungrounded or highly resistively grounded (NIGDG).

The object of the present invention is to provide a power distribution system and a fault detection method that can accurately identify a faulty power supply line, regardless of the type of power distribution network or the type of fault that occurs.

In order to solve the above problems, the present invention provides a fault detection method for a power distribution system for identifying a faulty power supply line in a power distribution network, which is characterized by injecting high-frequency currents of multiple frequencies from a power converter or any active device into multiple power supply lines, detecting high-frequency impedances of the multiple power supply lines at the multiple frequencies , identifying a faulty power supply line based on the detected high-frequency impedances , and performing subtraction of the high-frequency impedances at the multiple frequencies to eliminate the effect of fault resistance .

The present invention also includes a power converter or any active device that injects high-frequency currents of multiple frequencies into a plurality of power supply lines, and a sensor that detects current values of the plurality of power supply lines and a bus voltage of a bus to which the plurality of power supply lines are connected, and is characterized in that a plurality of high-frequency currents are injected from the power converter or the any active device into the plurality of power supply lines, high-frequency impedances of the plurality of power supply lines at the plurality of frequencies are detected based on the current values and the bus voltages detected by the sensor, a faulty power supply line is identified based on the detected high-frequency impedance , and a subtraction is performed on the high-frequency impedances at the plurality of frequencies to eliminate an effect of a fault resistance .

The present invention makes it possible to realize a power distribution system and a fault detection method that can accurately identify faulty power supply lines, regardless of the type of power distribution network or the type of fault that occurs.

This will enable the creation of a highly reliable smart grid.

Issues, configurations, and advantages other than those described above will become clear from the description of the embodiments below.

FIG. 1 is a schematic diagram of a comb-type medium voltage power distribution network with power converters installed on multiple buses. FIG. 1 is a schematic diagram of a meshed medium voltage power distribution network with power converters installed on multiple buses. FIG. 1 illustrates a power supply line configuration and components when a high impedance fault (HIF) occurs on one of multiple power supply lines in a medium voltage distribution network. FIG. 1 illustrates a power supply line configuration and components in a non-effectively grounded neutral distribution grid (NIGDG) where a fault occurs on one of the power supply lines. FIG. 2 illustrates a method for identifying a faulty power supply line in accordance with an embodiment of the present invention. FIG. 6 is a diagram showing a detailed configuration and operation of a fault power supply line identification (FFI) block in FIG. 5 . 4 is a flow chart illustrating a method for identifying a faulty power supply line in accordance with an embodiment of the present invention.

Below, an embodiment of the present invention will be described with reference to the drawings. Note that the same components in each drawing will be given the same reference numerals, and detailed descriptions of overlapping parts will be omitted.

In the following, the multiple interconnected distribution networks will be referred to as a high-voltage (HV) distribution network, a medium-voltage (MV) distribution network, and a low-voltage (LV) distribution network, based on the relative relationships between their voltage ranges. However, the present invention is not limited to a specific voltage range and can be applied to distribution networks of any voltage range.

First, the power distribution network to which the present invention is applied will be described with reference to Figs. 1 and 2. Figs. 1 and 2 show medium voltage (MV) distribution networks with two different configurations. Fig. 1 shows a comb-type medium voltage distribution network in which power converters are installed on multiple buses, and Fig. 2 shows a mesh-type medium voltage distribution network in which power converters are installed on multiple buses.

The medium voltage (MV) distribution network 1 in FIG. 1 is configured as a comb-type medium voltage distribution network with one end connected to a high voltage (HV) distribution network by an HV/MV transformer 101 and the other end connected to a low voltage (LV) distribution network by an MV/LV transformer 115.

The medium voltage (MV) distribution network 1 is divided into multiple power supply lines 104, 108, 112 by multiple buses 102, 106, 110, 114.

Now, a fault in any of the power lines can be isolated by the circuit breakers 103, 105, 107, 109, 111, 113, allowing efficient restoration of the remaining healthy power lines. Each of the different buses 102, 106, 110, 114 is equipped with a power converter 116, 117, 118, 119 that performs the additional function of Faulty Feeder Identification (FFI).

The medium voltage (MV) distribution network 1 in FIG. 2 is configured as a mesh-type medium voltage distribution network with one end connected to a high voltage (HV) distribution network by an HV/MV transformer 201 and the other end connected to a low voltage (LV) distribution network by an MV/LV transformer 207.

Circuit components such as buses 202, 206, and 212, power supply lines 204, 210, and 215, and power converters 208, 213, and 217 perform functions similar to those of the components described in FIG. 1.

Circuit breakers 203, 205, 209, 211, 214, and 216 are used to isolate and reconfigure each power supply line from the main system in the event of a fault or during maintenance.

Next, referring to Figures 3 and 4, a case where a fault occurs in the power supply line 204 will be described. Figure 3 shows an equivalent circuit when a high impedance fault (HIF) occurs in the power supply line 204, and Figure 4 shows an equivalent circuit when a fault occurs in a power distribution network (NIGDG) in which the neutral point having a ground resistor 401 is highly resistively grounded.

Figure 3 shows the components of the power supply lines 204, 210 connected to the bus 206. A high impedance fault occurs through a fault resistance 304, which is a resistance between a fault point 302 and ground 305. The fault point 302 splits the faulted power supply line 204 into two parts with respective impedances 301 and 303. The total line impedance of the normal power supply line 210 is 306.

In Figure 4, the total fault resistance includes fault resistance 304 and ground resistance 401, regardless of the type of fault.

Next, the configuration of the power distribution system and the fault detection method of the present invention will be described with reference to Figs. 5 to 7. Fig. 5 is a diagram showing a method for identifying a faulty power supply line in the medium voltage power distribution network of Fig. 3. Fig. 6 is a diagram showing the detailed configuration and operation of the faulty power supply line identification (FFI) block 510 of Fig. 5. Fig. 7 is a flowchart showing the method for identifying a faulty power supply line in the present invention.

In the present invention, once a fault is detected, the next step is to identify and isolate the faulty power supply line, allowing the healthy parts of the system to continue operating.

As shown in FIG. 5, the power converter 208 is typically mounted on a common bus 206 to which multiple power supply lines 204, 210 are also connected.

In the present invention, after the fault detection signal 501 is generated, the block 502 injects two different high frequency (HF) currents 504. Also, after the fault detection signal 501 is generated, the converter control block 503 stops the basic current injection from the power converter 208 and injects only the two different high frequency (HF) currents 504. For example, high frequency (HF) currents with a frequency of 50 Hz or higher are used as the two different high frequency (HF) currents 504.

The injected high frequency (HF) current 504 is split between the power lines as high frequency (HF) current 505 on power line 204 and high frequency (HF) current 506 on power line 210. Sensors 507, 508, 509 record the bus voltage and power line current signals from the two power lines 204, 210 and transmit them to a faulty power line identification (FFI) block 510 for FFI operation.

Note that the injection of two separate high frequency (HF) currents 504 can be done simultaneously or sequentially and the resulting output of the FFI operation will be the same.

Figure 6 shows the detailed steps performed by the FFI algorithm of the power converter 208.

The Fault Line Identification (FFI) block 510 requires recording of bus voltage and power line current by sensors 507, 508, and 509 to evaluate high frequency (HF) impedance at different frequencies.

This evaluation requires the extraction of separate high frequency (HF) components from the detected bus voltage and power line current, which are then passed through a second order filter tuned to the selected separate high frequency (HF). These filters are typically band pass filters 601, 602, and 603, which generate two output signals 604, 605, 606, 607, 608, and 609 corresponding to the two separate high frequencies (HF) from the input signals of the bus voltage VA and the power line currents IL1 and IL2, respectively. The output signals 604, 605, 606, 607, 608, and 609 are VAh1, VAh2, IL1h1, IL1h2, IL2h1, and IL2h2, respectively.

The transfer function of this filter is defined by equation (1).

where s is the Laplace operator, WHF is the selected high frequency (HF), and ξ is the damping ratio.

The output signals 604, 605, 606, 607, 608, 609 of the bandpass filters 601, 602, 603 are used by impedance estimation blocks 610, 611, 612, 613 to calculate high frequency (HF) impedances 614, 615, 616, 617. The high frequency (HF) impedances 614, 615, 616, 617 are ZL1h1, ZL1h2, ZL2h1, ZL2h2, respectively.

For the faulty power supply line 204, the high frequency (HF) impedance is calculated using equations (2) and (3).

On the other hand, for a normal power supply line 210, the high frequency (HF) impedance is calculated using equations (4) and (5).

Where, VAh1 and VAh2 are the bus voltages at the selected high frequency (HF) h1 and h2, IL1h1, IL1h2, IL2h1, and IL2h2 are the high frequency (HF) currents flowing through the faulty and normal power supply lines, respectively. Also, ZL1h1, ZL1h2, ZL2h1, and ZL2h2 are the line impedances of the faulty and normal power supply lines at high frequency (HF), respectively.

If the fault resistance Rf is near zero, the fault point is contained in the power supply line with the lowest impedance, and FFI can be performed using only one high frequency (HF) data.

For higher fault resistances, the difference between the impedance of the faulted power supply line and the impedance of the normal power supply line becomes smaller.

The effect of fault resistance can be eliminated by subtracting the high frequency (HF) impedances ZL1h1 and ZL2h1 at the two separate high frequencies (HF) from ZL1h2 and ZL2h2 in subtraction blocks 618 and 619, respectively.

The outputs 620, 621 of the subtraction blocks 618, 619 are then compared in a comparison block 622 as new high frequency (HF) impedances, and the power supply line with the lowest value is identified as the faulty power supply line. It is also possible to inject three or more high frequency (HF) currents to evaluate and compare the high frequency (HF) impedances to identify the faulty power supply line.

After identifying the faulty power line, the recorded high frequency (HF) data of the faulty power line, which is the output 624 of the comparison block 622, is used to determine the type of fault (fault type 626) in the comparison block 625.

The calculated impedance ZL1h has three components corresponding to the three phases of the faulted line. In comparison block 625, the type of fault (fault type 626) is determined by comparing the impedances of the three phases.

For all line-to-ground (LG) faults, the phase with the lowest impedance contains the fault point, and the remaining two phases have relatively high high frequency (HF) impedances.

If the two phases of the faulty power supply line show similar impedance and the impedance of the third power supply line is relatively high, a double line (LL) fault or a double line-to-ground (LLG) fault is likely.

Also, if the impedance of all three phases is low compared to the typical line impedance under normal grid conditions, a triple line (LLL) fault or triple line-to-ground (LLLG) fault may be suspected.

Figure 7 shows a flowchart of the method for identifying a faulty power supply line in the present invention.

First, in step S701, it is determined whether or not a fault exists. If it is determined that no fault exists (NO), operation continues without taking any action (step S702). If it is determined that a fault exists (YES), the process proceeds to step S703, where the power converter injects two separate high frequency (HF) currents simultaneously (or sequentially).

Next, in step S704, the power converter's sensors detect and record the bus voltage and the currents flowing through the multiple power feed lines.

Then, in step S705, the recorded data (bus voltage VA and power supply line currents IL1, IL2) are passed through a band-pass filter to extract samples at the selected high frequency (HF) (steps S706, S707).

Based on the bandpass filter output signals VAh1, VAh2, IL1h1, and IL1h2 extracted in steps S706 and S707, respectively, high frequency (HF) impedances are calculated at the selected high frequency (HF) in steps S708 and S709, and are subtracted from each other to cancel the effect of the fault resistance.

Now, the new parameter shown in equation (6) is used to distinguish between faulty and normal power supply lines.

However, power supply lines may be connected in various grid configurations with different cable materials, line impedances, and lengths. Therefore, to correct for this, the new high frequency (HF) impedance is normalized in steps S712 and S710 based on the pre-fault impedance values Z'L1pre-fault and Z'L2pre-fault (steps S713 and S711), respectively.

Then, in step S714, the new normalized high frequency (HF) impedance is compared to identify the faulty power supply line. The faulty power supply line is isolated from the power grid (step S716), and the normal power supply line is restored to the power grid and continues operation (step S715).

Finally, in step S717, the high frequency (HF) impedance of the faulty power supply line is compared between phases to identify the type of fault. Knowledge of the fault type is useful for estimating the fault location.

The present invention is not limited to the above-described embodiments, but includes various modified examples. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those having all of the configurations described. It is also possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

1...Medium voltage distribution network 101, 201...HV/MV transformer 115, 207...MV/LV transformer 102, 106, 110, 114, 202, 206, 212...Bus
103, 105, 107, 109, 111, 113, 203, 205, 209, 211, 214, 216...circuit breaker 104, 108, 112, 204, 210, 215...power supply line 116, 117, 118, 119, 208, 213, 217...power converter 301, 303, 306...impedance 302...fault point 304...fault resistance 305...ground 401...ground resistance 501...fault detection signal 502...block for injecting two different high frequency (HF) currents 503...converter control block 504, 505, 506...high frequency (HF) current 507, 508, 509...sensor 510...fault power supply line identification (FFI) block 601, 602, 603...Band pass filter 604, 605, 606, 607, 608, 609...Band pass filter output signal 610, 611, 612, 613...Impedance evaluation block 614, 615, 616, 617...High frequency (HF) impedance 618, 619...Subtraction block 620, 621...Subtraction block output 622...Comparison block 623...Identification and isolation of faulty power supply line 624...Comparison block output 625...Comparison block 626...Fault type

Claims (13)

1. A method for fault detection in an electrical power distribution system for identifying a faulted power supply line in an electrical power distribution network, comprising:
Injecting high frequency currents of multiple frequencies from a power converter or any active device into multiple power supply lines;
detecting a radio frequency impedance of the plurality of power supply lines at the plurality of frequencies ;
A method for detecting a fault in a power distribution system , comprising: identifying a faulty power supply line based on the detected high frequency impedance; and performing a subtraction of the high frequency impedance at the multiple frequencies to eliminate the effect of fault resistance . 2. A method for detecting a fault in a power distribution system according to claim 1, comprising:
injecting high frequency currents having two different frequencies from the power converter or any one of the active devices into the plurality of power supply lines;
Detecting high frequency impedances of the plurality of power supply lines at the two different frequencies;
calculating a difference between the detected high frequency impedances at the two different frequencies for each power supply line;
A fault detection method for a power distribution system, comprising: identifying a faulty power supply line based on the calculated difference. 2. A method for detecting a fault in a power distribution system according to claim 1, comprising:
The method for fault detection in a power distribution system, wherein the power converter or any of the active devices injects high frequency currents at the plurality of frequencies simultaneously or sequentially. 2. A method for detecting a fault in a power distribution system according to claim 1, comprising:
11. A method for fault detection in an electrical power distribution system, wherein the power converter or any of the active devices injects high frequency currents at any number of frequencies greater than or equal to two. 3. A method for detecting a fault in a power distribution system according to claim 2, comprising:
Detecting current values of the plurality of power supply lines and a bus voltage of a bus to which the plurality of power supply lines are connected;
Passing the detected current value and bus voltage through a band pass filter to extract voltage values and current values at the two different frequencies;
A method for detecting a fault in a power distribution system, comprising the steps of: calculating a high-frequency impedance based on the extracted voltage value and current value; 2. A method for detecting a fault in a power distribution system according to claim 1, comprising:
A method for detecting a fault in a power distribution system, comprising: identifying a faulty power supply line from among a plurality of power supply lines constituting a medium-voltage power distribution network. 2. A method for detecting a fault in a power distribution system according to claim 1, comprising:
A method for fault detection in an electrical power distribution system, characterized in that it is applicable to electrical power distribution networks having any type of grounding system. 3. A method for detecting a fault in a power distribution system according to claim 2, comprising:
A method for detecting a fault in a power distribution system, wherein the two different frequencies are 50 Hz or higher. 3. A method for detecting a fault in a power distribution system according to claim 2, comprising:
11. A method for fault detection in an electrical power distribution system comprising: normalizing said high frequency impedance using a pre-fault impedance. 2. A method for detecting a fault in a power distribution system according to claim 1, comprising:
A method for detecting a fault in a power distribution system, comprising: comparing high frequency impedances of three phases of three-phase currents flowing through said plurality of power supply lines. A power converter or any active device for injecting high frequency currents at multiple frequencies into multiple power supply lines;
a sensor for detecting a current value of the plurality of power supply lines and a bus voltage of a bus to which the plurality of power supply lines are connected;
injecting a plurality of high frequency currents from the power converter or any of the active devices into the plurality of power supply lines;
detecting high frequency impedances of the plurality of power supply lines at the plurality of frequencies based on the current values and the bus voltages detected by the sensor;
and identifying a faulted power supply line based on the detected high frequency impedance and performing a high frequency impedance subtraction at the multiple frequencies to eliminate the effect of fault resistance . 12. The power distribution system of claim 11 ,
injecting high frequency currents having two different frequencies from the power converter or any one of the active devices into the plurality of power supply lines;
Detecting high frequency impedances of the plurality of power supply lines at the two different frequencies;
calculating a difference between the detected high frequency impedances at the two different frequencies for each power supply line;
The power distribution system further comprises: identifying a faulty power supply line based on the calculated difference. 12. A method for detecting faults in a power distribution system as claimed in claim 11 , comprising:
The power converter or any of the active devices injects high frequency currents at the multiple frequencies simultaneously or sequentially.

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