CN109066584B - Capacitor harmonic protection method with high protection action precision - Google Patents

Capacitor harmonic protection method with high protection action precision Download PDF

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CN109066584B
CN109066584B CN201810919122.9A CN201810919122A CN109066584B CN 109066584 B CN109066584 B CN 109066584B CN 201810919122 A CN201810919122 A CN 201810919122A CN 109066584 B CN109066584 B CN 109066584B
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current
voltage
protection
harmonic
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CN109066584A (en
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俞永军
张亮
陈晓宇
曾幼松
王昕�
蔡重凯
李俊
许海峰
王新华
秋勇
许涛
姚建生
匡申
耿介雯
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CEIEC ELECTRIC TECHNOLOGY Inc
State Grid Zhejiang Electric Power Co Ltd
Shaoxing Power Supply Co of State Grid Zhejiang Electric Power Co Ltd
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CEIEC ELECTRIC TECHNOLOGY Inc
State Grid Zhejiang Electric Power Co Ltd
Shaoxing Power Supply Co of State Grid Zhejiang Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
    • H02H3/50—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to the appearance of abnormal wave forms, e.g. ac in dc installations
    • H02H3/52—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to the appearance of abnormal wave forms, e.g. ac in dc installations responsive to the appearance of harmonics

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Supply And Distribution Of Alternating Current (AREA)
  • Emergency Protection Circuit Devices (AREA)

Abstract

The invention discloses a capacitor harmonic protection method with high protection action precision, which is characterized by detecting a break variable of a three-phase alternating voltage full-wave effective value and a three-phase current full-wave effective value of a power grid within a certain time, comparing an absolute value or a change rate of the break variable with a fixed value of harmonic protection, and locking corresponding N-time voltage or current harmonic protection when the variation of the effective value of voltage or current within a 5ms calculation interval is larger than N-time voltage or current harmonic protection fixed value of 0.01 multiplied by N; and locking the voltage or current distortion rate protection when the change rate of the effective value of the voltage or the current in the 5ms calculation interval is more than 0.5 time of the voltage or current distortion rate protection fixed value. Therefore, harmonic protection misoperation of the capacitor can be effectively prevented when a transient event occurs, and safety and reliability of the power system are improved.

Description

Capacitor harmonic protection method with high protection action precision
Technical Field
The invention relates to the field of transformer substation intellectualization, in particular to a capacitor harmonic protection technology.
Background
With the development of power systems and the extensive use of power electronics technology, the load structure of power grids has changed significantly. The non-linear electric equipment such as variable frequency motors, high-power electronic devices, thyristor voltage regulating devices and the like are greatly increased, the power quality environment of a power grid is increasingly deteriorated, and the operation safety of the variable frequency motors and the power grid equipment connected with the variable frequency motors and the power grid equipment is seriously influenced due to the pollution of the power grid, namely harmonic pollution caused by the increasingly deteriorated power quality environment.
The intelligent degree of a transformer substation is gradually improved along with the development of a power grid, the switching operation requirement of a capacitor bank is also gradually improved, but due to harmonic pollution of the power grid, the traditional parallel capacitor bank has an increasingly harsh operating environment, faults occur frequently, and the operating safety of the capacitor and other equipment of the power grid connected with the capacitor are seriously influenced.
Harmonic current can cause the increase of the loss of an additional insulating medium of the power capacitor, the temperature rise, the insulation aging of the capacitor is accelerated, and even the capacitor is damaged due to overheating; partial discharge caused by harmonic voltages can seriously affect the service life of the capacitor and can also cause the capacitor to bulge, break down or explode.
If the natural frequency is close to or matched with the frequency of a certain harmonic in the higher harmonic, the current and voltage of the harmonic in the parallel loop of an inductor (system inductive reactance) and a capacitor (compensation device) are amplified and even resonated, so that the harm of the harmonic in the power system is aggravated.
A large amount of capacitor bank operation data show that harmonic pollution of a power grid is increasingly serious along with the access of a large amount of nonlinear loads such as frequency conversion equipment of the power grid, more and more accidents are caused by harmonic damage of the capacitor bank in the power grid, and a plurality of faults of capacitor bank overload and burnout occur in the power grid every year.
The existence of a large amount of harmonic waves and the use of frequency conversion equipment cause voltage interruption and other phenomena of voltage waveform, the zero crossing point of the voltage and current waveform is influenced, and the correctness and the precision of harmonic protection action are influenced.
In applications, harmonic protection implementations often mistake transients for harmonic distortion. The measured waveform of the transient process has the characteristics of obvious high-frequency components, but the transient and the harmonic are two completely different phenomena. The high-frequency component brought by the distorted waveform of the power system after being suddenly disturbed is irrelevant to the fundamental frequency of the system.
Although the change of the instantaneous value such as a notch or a commutation gap occurring in the commutation of the commutation device is also periodic, it does not belong to the harmonic category.
The existing capacitor has larger protection error, and has no corresponding misoperation prevention strategy of transient events, so that when transient disturbance occurs in a system, misoperation of a protection device easily occurs.
Disclosure of Invention
The invention aims to provide a capacitor harmonic protection method with high protection action precision, and solve the problem that harmonic protection is possibly mistaken for a transient event of a power system.
In order to solve the technical problems, the invention adopts the following technical scheme: a method for protecting harmonic wave of capacitor includes such steps as,
sampling the voltage and current of the protected capacitor bank;
low-pass digital filtering the sampled data;
calculating the effective value of three-phase voltage or current in real time, and calculating the absolute value of the change of the effective value within 5ms and the ratio of the change of the effective value;
locking harmonic protection when the absolute value of the change of the effective value of the three-phase voltage or current and the ratio of the change of the effective value are greater than a set value within 5 ms;
wherein | Δ U | is defined as an absolute value of a variation of the voltage effective value within 5ms, | Δ I | is defined as an absolute value of a variation of the current effective value within 5ms,
Figure RE-GDA0001839705910000021
defined as the absolute value of the rate of change of the effective value of the voltage within 5ms,
Figure RE-GDA0001839705910000031
is defined as the absolute value of the rate of change of the effective value of the current in 5ms, U (n)setDefined as the constant value of N-th harmonic protection, I (N)setDefined as the current harmonic protection constant value of Nth, N is the harmonic protection number, UTHDsetDefined as the voltage distortion rate protection constant, ITHDsetDefined as the current distortion rate protection constant,
the logic for harmonic protection is as follows:
when | Δ U | > 0.01 XNXU (N)setLocking N times of voltage harmonic protection and voltage effective value protection;
when | DeltaI | > 0.01 XNxI (N)setLocking N times of current harmonic protection and current effective value protection;
when in use
Figure RE-GDA0001839705910000032
Latching voltage distortion rate protection;
when in use
Figure RE-GDA0001839705910000033
Latching current distortion rate protection.
Preferably, the method further comprises dynamically evaluating and selecting the real-time sampling frequency, and the dynamically evaluating and selecting the real-time sampling frequency comprises the following steps:
step 1), a hardware frequency measurement circuit performs analog low-pass filtering on line voltage to filter out frequent zero crossing points and zero crossing point offset caused by harmonic distortion to influence hardware frequency measurement;
step 2), the hardware frequency measurement circuit triggers the external interrupt of the protection device to record the zero crossing point time when the hardware frequency measurement circuit crosses the zero point, and the protection device calculates the real-time hardware frequency measurement frequency fh by recording the time difference of the two zero crossing points;
step 3), using the formula of the variance
Figure RE-GDA0001839705910000034
Calculate the last 5 times fhVariance S ofh 2；
Step 4), sampling the voltage and current of the protected capacitor;
step 5), performing dead pixel removal and low-pass digital filtering on the sampled data, removing dead pixels of AD conversion and filtering out the distortion influence of higher harmonics on the waveform;
step 6), searching for a sampling point n crossing a zero crossing point in the sampling value1＞0，n2≥0，n3＜0，n4< 0 or n1＜0，n2≤0，n3＞0，n4> 0, where n1，n2，n3，n4For continuous sampling points, and respectively recording the time t of the four sampling points1，t2，t3，t4；
Step 7), using the formula
Figure RE-GDA0001839705910000041
Calculating the zero crossing point t0；
Step 8), calculating the frequency f of the software frequency measurement by using the time difference of two continuous zero-crossing points calculated last timesAB，fsBC，fsCA；
Step 9), again using the variance formula
Figure RE-GDA0001839705910000042
Calculate the last 5 times fsAB，fsBC，fsCAVariance S ofsAB 2，SsBC 2，SsCA 2；
Step 10), selecting Sh 2，SsAB 2，SsBC 2，SsCA 2Minimum value of
Shmin 2＝min{Sh 2，SsAB 2，SsBC 2，SsCA 2F, corresponding frequency;
and 11) adjusting the system sampling interval by using the obtained f.
Preferably, the specific steps of calculating the effective value are as follows:
step 1), applying a formulaAnd
Figure RE-GDA0001839705910000052
calculating and memorizing effective values of the three-phase voltage and current after low-pass filtering;
step 2) reuse
Figure RE-GDA0001839705910000053
And
Figure RE-GDA0001839705910000054
calculating effective values of the current voltage and current;
step 3) calculating the effective value U of the current voltagermsAnd the effective value of the current IrmsAnd 5ms front voltage effective value Urmspre5Or effective value of current Irmspre5The absolute values of the differences deltau and deltai,
|ΔU|＝|Urms-Urmspre5|
|ΔI|＝|Irms-Irmspre5|
step 4), calculating the change ratio of the effective values of the current voltage and current to the effective values of the voltage or current before 5msAnd
Figure RE-GDA0001839705910000056
the absolute value of (a) is,
Figure RE-GDA0001839705910000057
Figure RE-GDA0001839705910000058
step 5, utilizing discrete Fourier algorithm
Figure RE-GDA0001839705910000059
Calculating the voltage and current amplitude of the N-th harmonic wave;
step 6), according to the formula
Figure RE-GDA00018397059100000510
Wherein U is1Calculating the total distortion rate of the voltage for the fundamental wave value of the voltage;
step 7), according to the formula
Figure RE-GDA0001839705910000061
Wherein I1For the current fundamental value, the total current distortion rate is calculated.
The method comprises the steps of detecting the break variable of a three-phase alternating voltage full-wave effective value and a three-phase current full-wave effective value of a power grid within a certain time, comparing the absolute value or the change rate of the break variable with a fixed value of harmonic protection, and locking corresponding N-time voltage or current harmonic protection when the variation of the effective value of voltage or current within a 5ms calculation interval is larger than an N-time voltage or current harmonic protection fixed value which is 0.01 multiplied by N times; and locking the voltage or current distortion rate protection when the change rate of the effective value of the voltage or the current in the 5ms calculation interval is more than 0.5 time of the voltage or current distortion rate protection fixed value. Therefore, harmonic protection misoperation of the capacitor can be effectively prevented when a transient event occurs, and safety and reliability of the power system are improved.
Detailed Description
The method of harmonic protection of the present invention is specifically described below:
a capacitor harmonic protection method with high protection action precision comprises
Sampling the voltage and current of the protected capacitor bank;
low-pass digital filtering the sampled data;
calculating the effective value of three-phase voltage or current in real time, and calculating the absolute value of the change of the effective value within 5ms and the ratio of the change of the effective value;
and locking harmonic protection when the absolute value of the change of the effective value of the three-phase voltage or current and the change ratio of the effective value are greater than the set value within 5 ms.
And the final sampling frequency is screened from the hardware frequency measurement and the three-phase voltage software frequency measurement by integrating the frequency break variable and the sliding window minimum variance algorithm.
When a frequency measurement signal loop is seriously distorted, zero crossing point deviation or continuous zero crossing points occur, so that serious interference is caused to frequency measurement, and even real-time frequency cannot be acquired. The general power quality analysis device adopts a long-period average value to acquire frequency to overcome the influence, the data window acquired by the frequency is long, and the requirement of real-time and high speed of the protection device cannot be met due to the long-period hysteresis. If the sampling frequency is inconsistent with the actual frequency, the error of harmonic analysis is increased, and protection misoperation can be caused after the deviation is carried out to a certain degree. In order to solve the problems, the device adopts special hardware low-pass filtering on a frequency measurement loop, and adopts a high-performance digital filter to extract fundamental wave components of the device in data processing, so that the interference of harmonic waves on zero crossing points is removed, the high-precision acquisition of real-time frequency and frequency tracking sampling are realized, and the precision of harmonic wave analysis is ensured.
Dynamically evaluating and selecting real-time frequencies comprises the following steps:
the hardware frequency measurement circuit carries out analog low-pass filtering on the line voltage, and filters out frequent zero crossing points and zero crossing point offset caused by harmonic distortion to influence the hardware frequency measurement.
The hardware frequency measurement circuit triggers the external interruption of the protection device to record the zero crossing point time when the zero crossing point is reached, and the protection device calculates the real-time hardware frequency measurement frequency f by recording the time difference of the two zero crossing pointsh。
Formula using variance
Figure RE-GDA0001839705910000071
Calculate the last 5 times fhVariance S ofh 2；
The current and voltage of the protected capacitor are sampled.
And (3) carrying out dead pixel removal and FIR low-pass digital filtering on the sampled data, removing dead pixels of AD conversion and filtering out the distortion influence of higher harmonics on the waveform.
Finding a sampling point n crossing a zero crossing point in a sampled value1＞0，n2≥0，n3＜0，n4< 0 or n1＜0，n2≤0，n3＞0，n4> 0, where n1，n2，n3，n4For continuous sampling points, and respectively recording the time t of the four sampling points1，t2，t3，t4。
Using formulas
Calculating the zero crossing point t0。
Calculating the frequency f of the software frequency measurement by using the time difference of two successive zero crossings calculated in the previous stepsAB，fsBC，fsCA。
Again using the variance formula
Calculate the last 5 times fsAB，fsBC，fsCAVariance S ofsAB 2，SsBC 2，SsCA 2；
Selecting Sh 2，SsAB 2，SsBC 2，SsCA 2Minimum value of
Shmin 2＝min{Sh 2，SsAB 2，SsBC 2，SsCA 2}
The corresponding frequency f.
And adjusting the system sampling interval by using the obtained f.
The method for calculating the change quantity and the change rate of the three-phase voltage and current effective values comprises the following steps:
three-phase voltage and current are collected, FIR low-pass digital filtering is carried out on the collected data, and high-frequency interference or aliasing of noise signals is prevented.
Using a formula
Figure RE-GDA0001839705910000083
And
Figure RE-GDA0001839705910000084
and calculating and memorizing effective values of the three-phase voltage and the three-phase current after low-pass filtering.
Is used again
Figure RE-GDA0001839705910000091
And
Figure RE-GDA0001839705910000092
and calculating effective values of the current voltage and the current.
Calculating the effective value U of the current voltagermsAnd the effective value of the current IrmsAnd 5ms front voltage effective value Urmspre5Or effective value of current Irmspre5The absolute values of the differences Δ U and Δ I.
|ΔU|＝|Urms-Urmspre5|
|ΔI|＝|Irms-Irmspre5|
Calculating the present voltage and currentRatio of effective value to effective value of voltage or current before 5ms
Figure RE-GDA0001839705910000093
And
Figure RE-GDA0001839705910000094
absolute value of (a).
Figure RE-GDA0001839705910000095
Figure RE-GDA0001839705910000096
Using discrete Fourier algorithm (DFT)
Figure RE-GDA0001839705910000097
And calculating the voltage and current amplitude of the Nth harmonic wave.
According to the formula
Figure RE-GDA0001839705910000098
Wherein U is1For the voltage fundamental value, the total distortion rate of the voltage is calculated.
According to the formula, the method comprises the following steps of,wherein I1For the current fundamental value, the total current distortion rate is calculated.
The logic of the method for preventing the malfunction of the microcomputer harmonic protection is as follows:
when | Δ U | > 0.01 XNXU (N)setAnd locking N times of voltage harmonic protection and voltage effective value protection.
When | DeltaI | > 0.01 XNxI (N)setAnd locking N times of current harmonic protection and current effective value protection.
When in use
Figure RE-GDA0001839705910000101
Rate of latch-up voltage distortionAnd (4) protecting.
When in use
Figure RE-GDA0001839705910000102
Latching current distortion rate protection.
On the basis, the harmonic protection device has harmonic protection functions of single harmonic voltage, current, effective voltage value, effective current value, voltage distortion rate, current distortion rate and the like, has high protection action precision and correctness, can prevent misoperation of capacitor harmonic protection, and provides better harmonic protection function for power capacitor equipment. In conclusion, the invention has the following beneficial effects:
1. and the line voltage channel of the protected capacitor bank is subjected to hardware frequency measurement, so that the influence of common-mode interference and 3N harmonic waves in phase voltage on frequency measurement precision is effectively reduced.
2. The voltage sampling data of the protected capacitor bank is subjected to digital low-pass filtering and then subjected to software frequency measurement calculation, so that the influence on the sampling frequency precision of a system due to the interference and failure of a hardware frequency measurement loop caused by transient events such as voltage interruption is prevented.
3. According to the actual frequency, the system sampling interval is dynamically adjusted, and the calculation error and the frequency spectrum leakage of asynchronous sampling are prevented.
While the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that the invention is not limited thereto, and may be embodied in other forms without departing from the spirit or essential characteristics thereof. Any modification which does not depart from the functional and structural principles of the present invention is intended to be included within the scope of the claims.

Claims (1)

1. A capacitor harmonic protection method with high protection action precision is characterized by comprising the following steps,
U(n)setdefined as the constant value of N-th harmonic protection, I (N)setDefined as the current harmonic protection constant value of Nth, N is the harmonic protection number, UTHDsetDefined as the voltage distortion rate protection constant, ITHDsetDefined as a current distortion rate protection constant;
sampling the voltage and current of the protected capacitor bank;
low-pass digital filtering the sampled data;
calculating the N-th harmonic voltage amplitude U (N), the harmonic current amplitude I (N), the total voltage distortion rate UTHD and the total current distortion rate ITHD in real time;
the harmonic protection logic is:
when U (n)>U(n)setThe N times of voltage harmonic protection is delayed;
when I (n)>I(n)setThe N times of current harmonic protection is performed with delayed action;
when UTHD>UTHDsetThe total distortion rate protection is delayed;
when ITHD>ITHDsetThe total current distortion rate protection is delayed;
the method for improving the correctness of the harmonic protection action by dynamically evaluating and selecting the real-time sampling frequency comprises the following steps:
step 1), a hardware frequency measurement circuit performs analog low-pass filtering on line voltage to filter out frequent zero crossing points and zero crossing point offset caused by harmonic distortion to influence hardware frequency measurement;
step 2), the hardware frequency measurement circuit triggers the external interrupt of the protection device to record the zero crossing point time when the hardware frequency measurement circuit crosses the zero point, and the protection device calculates the real-time hardware frequency measurement frequency f by recording the time difference of two zero crossing points in the same directionh；
Step 3), using the formula of the variance
Figure FDA0002129558240000011
Calculate the last 5 times fhVariance S ofh 2；
Step 4), sampling the voltage and current of the protected capacitor;
step 5), performing dead pixel removal and low-pass digital filtering on the sampled data, removing dead pixels of AD conversion and filtering out the distortion influence of higher harmonics on the waveform;
step 6), searching for a sampling point n crossing a zero crossing point in the sampling value1＞0，n2≥0，n3＜0，n4< 0 or n1＜0，n2≤0，n3＞0，n4> 0, where n1，n2，n3，n4For continuous sampling points, and respectively recording the time t of the four sampling points1，t2，t3，t4；
Step 7), using the formula
Figure FDA0002129558240000021
Calculating the zero crossing point t0；
Step 8), calculating the frequency f of the software frequency measurement by using the time difference of two continuous zero-crossing points calculated last timesAB，fsBC，fsCA；
Step 9), again using the variance formula
Calculate the last 5 times fsAB，fsBC，fsCAVariance S ofsAB 2，SsBC 2，SsCA 2；
Step 10), selecting Sh 2，SsAB 2，SsBC 2，SsCA 2Minimum value of
Shmin 2＝min{Sh 2，SsAB 2，SsBC 2，SsCA 2F, corresponding frequency;
step 11), adjusting the system sampling interval by using the obtained f;
the method for preventing the harmonic protection unwanted operation of the capacitor comprises the following specific steps:
step 1), applying a formulaAnd
Figure FDA0002129558240000032
calculating and memorizing effective values of the three-phase voltage and current after low-pass filtering;
step 2) reuse
Figure FDA0002129558240000033
And
Figure FDA0002129558240000034
calculating effective values of the current voltage and current;
step 3) calculating the effective value U of the current voltagermsAnd the effective value of the current IrmsAnd 5ms front voltage effective value Urmspre5Or effective value of current Irmspre5The absolute values of the differences deltau and deltai,
|ΔU|＝|Urms-Urmspre5|
|ΔI|＝|Irms-Irmspre5|；
step 4), calculating the change ratio of the effective values of the current voltage and current to the effective values of the voltage or current before 5msAnd
Figure FDA0002129558240000036
the absolute value of (a) is,
Figure FDA0002129558240000037
Figure FDA0002129558240000038
step 5), utilizing a discrete Fourier algorithm
Figure FDA0002129558240000041
Calculating the voltage and current amplitude of the N-th harmonic wave;
step 6), according to the formula
Figure FDA0002129558240000042
Wherein U is1Calculating the total distortion rate of the voltage for the fundamental wave value of the voltage;
step 7), according to the formula
Figure FDA0002129558240000043
Wherein I1Calculating the total current distortion rate for the current fundamental value;
step 8) when | delta U | > 0.01 XNXU (N)setLocking N times of voltage harmonic protection and voltage effective value protection;
step 9) when | Delta I | is more than 0.01 XNxI (N)setLocking N times of current harmonic protection and current effective value protection;
step 10) when
Figure FDA0002129558240000044
Latching voltage distortion rate protection;
step 11) whenLatching current distortion rate protection.
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