^{1}

^{*}

^{2}

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This paper introduces a quick classification method of the power quality disturbances. Based on analyzing the characteristics of different electrical disturbance signals in time domain, four distinctive features are extracted from electrical signals for classifying different power quality disturbances and then an automatic classifier is proposed. Using the proposed classification method,a PQ monitor of the classifying power quality disturbances is developed based on the TMS320F2812DSP micro-processor. Semi-physical simulation, lab experiment and field measurement results have verified that this proposed method can classify single or complex disturbance signals effectively.

To detect and improve power quality, we first need to monitor and analyze the power quality disturbances. There have been many methods presented, such as Fourier transform [

The general single-phase voltage signal can be expressed as the superposition of the fundamental wave voltage and the disturbance signals:

where _{th} harmonic RMS voltage; _{th} harmonic initial phase angle;

In Equation (1), when

Equation (1) represents the ideal voltage. So the voltage disturbance can be divided into two categories. One is the disturbances with the change of the

For voltage sag, swell and interruption has the similar characteristics, the author only takes voltage sag as the analyzing object. The other two can also be identified by the method presented in this paper.

If

. Typical characteristics of power system disturbances

Disturbances | Typical spectral content | Typical duration | Typical voltage magnitude |
---|---|---|---|

Voltage sag | 0.5 cycles - 1 min | 0.1 - 0.9 pu | |

Fluctuation and flicker | <25 Hz | Intermittent | 0.1% - 7% |

Harmonics | 0 - 100^{th} Hz | Steady state | 0% - 20% |

Oscillatory transients | <5 kHz | 0.3 - 50 ms | 0 - 4 pu |

Interharmonics | 0 - 6 kHz | Steady state | 0% - 2% |

. The categories of the voltage disturbance

Voltage amplitude disturbances | Stationary | Fluctuation and flicker, under voltage ,over voltage |
---|---|---|

Transient | Voltage sag, swell, transient interruption | |

Additive disturbances | Stationary | Harmonics, interharmonics |

Transient | Oscillatory transients, impulse voltage |

where,

Equation (2) consists of three parts. First is DC component

The square of Equation (1) is:

The expanded formula of (3) is long, but also is consists of three parts: DC component

The curve of a full cycle integral of (3) is shown in

By comparing _{d}/U_{N} in _{1}/U_{N} in

ference between them is the amplitude of the additive disturbance

terharmonics are the additive disturbances. So the U_{d}/U_{N} in _{1}/U_{N} in

The curve of full cycle integral of (2). (a) Voltage sag; (b) fluctuation and flicker; (c) harmonics; (d) Oscillatory transients; (e) Interharmonics

The curve of full cycle integral of (3). (a) Voltage sag; (b) Fluctuation and flicker; (c) Harmonics; (d) Oscillatory transients; (e) Interharmonics

Though above analysis, some individual features of the power quality disturbance singles can be shown in time- domain:

1) Equation (2) has the effect of selecting system fundamental frequency. The DC component, which can be gotten by low pass filter from Equation (2), is the RMS voltage with system fundamental frequency, which is not affected by additive disturbance. And its change is equivalent to the amplitude disturbance of the RMS voltage (like voltage sag, swell, transient interrupt, under voltage, over voltage, continuous interrupt, fluctuation, flicker and so on).

2) The DC component of Equation (3) is the geometric sum of the RMS voltage with system fundamental frequency and all other additive disturbance RMS voltages. So the geometric difference of the Equation (3) and Equation (2)’s DC component is exactly equivalent to the amplitude of the additive disturbance (like harmonics, oscillatory transients, impulse voltage and interharmonics).

3)

So, the next 4 features (F_{1} - F_{4}) can be used to classify power quality single disturbances and the mixed disturbances can be considered as the “superposition” of the single disturbances. The calculating flow chart of the 4 features is shown in

1) _{1} is the system fundamental wave voltage amplitude variation value which is not affected by additive disturbance. It reflects the extent of the fundamental wave amplitude change. So the information of system fundamental RMS voltage change can be known from the extent of the F_{1} change. Then it can be used to identify whether there are the voltage sag, swell, instantaneous interruption, under voltage, over voltage, continuous interruption, fluctuation and flicker in electrical singles (shown in

2)

(for F_{2} is used to detect stationary additive disturbances, the analysis period of time can be enlarged. This paper takes

3) If

classify the additive disturbances harmonics and interharmonics. If interharmonics exist in Equation (2), the curve of

The flow chart of calculating classification features

. Amplitude change disturbance classifications with F_{1}

c | F_{1} = 1.1 - 1.2 pu, duration > 1 min |
---|---|

Under voltage | F_{1} = 0.8 - 0.9 pu, duration > 1 min |

Voltage interruption | F_{1} < 0.1 pu, duration: 10 ms - 3 s instantaneous interruption; 3 - 60 s temporary interruption; >60 s power off |

Voltage sag | F_{1} = 0.1 - 0.9 pu, duration 10 ms - 1 min |

Voltage swell | F_{1} = 1.1 - 1.8 pu, duration 10 ms - 1 min |

Fluctuation and flicker | F_{1} is voltage fluctuation in 0.9 - 1.1 pu randomly |

and

4)

that the amplitude of instantaneous disturbance changes fast determines _{4} can determine the instantaneous disturbances exist or not. The MATLAB simulation shows that the threshold can be 15. If

. Figure 4 is the flow chart of the automate classification of the disturbances

Comparing with the other power quality disturbance classification methods using some kinds of transforms, the method presented by this paper has advantages as follows:

1) The single disturbance can be identified by one feature or the combination of some features. That means if one feature or some features satisfied some conditions, a disturbance or mixed disturbances can be sure. Then the disturbance classification will not be probable, but be definitive and the correct rate of the disturbance classification would be very high. And it makes the classification simpler. For example, as shown in

The flow chart of the automate classification. A: Har- monics + Interharmonics; B: Harmonics; C: Interharmonics; D: No stationary additive disturbance; E:?; F: Identify the amplitude disturbances by; G: No amplitude dis- turbance; H:?; I: Osillatory transients

. The simulation value of the features

The disturbances | The disturbance parameters | F_{2} | F_{3} | F_{4} |
---|---|---|---|---|

Fluctuation and flicker | Amplitude: 0.95 - 1.05 | 0.68 | 0. 91 | 1.96 |

Voltage sag | Sag amplitude: 0.5 pu | 0.49 | 0. 40 | 38.66 |

Harmonics | ^{*}5^{th} 4%; 7^{th} 3% | 5.0 | 0 | 0 |

Oscillatory transients | M_{s} = 0.8 pu, f_{s} = 1025 Hz; U_{s} = 0.1 | 0.49 | 0.22 | 59.95 |

Interharmonics | f = 125 Hz, amplitude 2% | 1.99 | 1.27 | 1.78 |

Voltage sag and harmonics | 0.5 pu, 5^{th} 4%; 7^{th} 3% | 5.41 | 0. 40 | 39.45 |

Fluctuation and harmonics | Amplitude: 0.95 - 1.05; 5^{th} 4%; 7^{th} 3% | 5.05 | 0. 91 | 1.31 |

Harmonics and oscillatory transients | 5^{th} 4%; 7^{th} 3%, γ_{s} = 0.1, U_{s} = 0.8 pu, f_{s} = 1025 Hz | 5.34 | 0. 22 | 59.92 |

Harmonics and interharmonics | 5^{th} 4%; 7^{th} 3%; f_{s} = 125 Hz, U_{s} = 2% | 5.37 | 1.27 | 1.73 |

2) The features extracted from one disturbance won’t change a lot for the existence of the other disturbance. This is shown clearly in

summation of their amplitude (

ample,

3) The classifying features have clear physical meanings. So it profits the evaluation of the power quality disturbance. The physical meaning of

4) The calculating time is much less, and profits to be used in real-time power quality disturbance classification.

5) The features extracted are low-pass filtered or the full cycle integral values, so it has good ability of noise proof.

Using the proposed power quality disturbance classification method, a PQ monitor is developed based on the TMS320F2812 DSP micro-processor. Semi-physical simulation, lab experiment and field measurement results have verified the proposed method.

The authors use D space semi-physical experiment platform as the disturbance signal generator. The PQ monitor samples the signals generated by D space, identifies disturbances and evaluates their parameters.

. The experiments results

The disturbance type | The experiments times | The correct ratio of identifying the disturbance type | The average error of the parameters evaluation |
---|---|---|---|

The voltage sag | 100 | 100% | 2.12% |

The fluctuation and flicker | 100 | 99% | 3.01% |

The harmonics | 100 | 98% | 2.00% |

The oscillatory transients | 100 | 98% | 2.45% |

The interharmonics | 100 | 97% | 6.33% |

The harmonics + The voltage sag | 100 | 100% | 3.58% |

The harmonics + The fluctuation and flicker | 100 | 96% | 2.99% |

The harmonics + The oscillatory transients | 100 | 100% | 3.07% |

The fluctuation and flicker + The voltage sag | 100 | 100% | 4.28% |

The fluctuation and flicker + The oscillatory transients | 100 | 100% | 4.14% |

The interharmonics + The voltage sag | 100 | 100% | 2.78% |

The interharmonics + The oscillatory transients | 100 | 99% | 2.99% |

The interharmonics + The fluctuation and flicker | 100 | 96% | 3.46% |

The interharmonics + The harmonics | 100 | 97% | 2.53% |

The interharmonics + The harmonics + The voltage sag | 100 | 99% | 4.01% |

The interharmonics + The harmonics + The voltage sag + The fluctuation and flicker | 100 | 98% | 3.98% |

The authors give three types of disturbances: the voltage sag, the voltage sag plus harmonics, the fluctuation plus harmonics plus interharmonics plus voltage sag to present 5 single disturbances and 11 mixed disturbances.

1) The voltage sag

In the following tables the same symbols have the same meanings.

2) The voltage sag plus harmonics

The authors do not show the identifying results in _{2} and F_{3} are relatively independent. The identifying results and the parameters evaluation are not affected by the superposition of the disturbances.

3) The fluctuation plus harmonics plus interharmonics plus voltage sag

_{2} a little, but doesn’t affect the feature F_{3}. The big amount of harmonics may blanket the existence of interharmonics, because the identifying term of the harmonics plus interharmonics is F_{3}/F_{2} > 0.5.

A lab experiment circuit is shown as _{1}. The voltage signal u_{1} is sampled by the PQ monitor and the oscilloscope.

U = 380 V, R_{1} = 1 kΩ, R_{2} = 2 kΩ, R_{3} = 3.9 Ω. S is an AC contact. FU is a fuse. When S is turned on and the current of the FU branch is large enough, the FU will blowing out and the branch will be cut off. Then there will be a voltage sag in u_{1 }as shown in

The experiment results shown in

. The experiments results of voltage sag

t_{m} (ms) | t_{s} (ms) | e_{t} | a_{m} (pu) | a_{s} (pu) | e_{a} |
---|---|---|---|---|---|

35 | 40 | 5 | 0.112 | 0.1 | 0.012 |

53 | 60 | 7 | 0.211 | 0.2 | 0.011 |

77 | 80 | 3 | 0.305 | 0.3 | 0.005 |

98 | 100 | 2 | 0.360 | 0.35 | 0.01 |

116 | 120 | 4 | 0.389 | 0.40 | 0.011 |

138 | 140 | 2 | 0.447 | 0.45 | 0.003 |

t_{m}: The voltage sag lasting time measured by the PQ monitor; t_{s}: The setting time of the voltage sag lasting time; e_{t}: The error between time 1 and time 2; a_{m}: The voltage sag amplitude measured by the PQ monitor; a_{s}: The setting amplitude of voltage sag amplitude; e_{a}: The error between amplitude 1 and amplitude 2.

. The experiments results of harmonics plus voltage sag

THD | e_{THD} | t_{m} (ms) | e_{t} | a_{m} (pu) | e_{a} | F_{2} | F_{3} |
---|---|---|---|---|---|---|---|

2.08% | 0.0018 | 38 | 3.52 | 0.102 | 0.011 | 2.08 | 0.04 |

3.12% | 0.0029 | 54 | 4.76 | 0.212 | 0.011 | 3.12 | 0.01 |

4.09% | 0.0014 | 78 | 2.60 | 0.307 | 0.005 | 4.09 | 0.02 |

5.06% | 0.0010 | 98 | 1.93 | 0.361 | 0.010 | 5.06 | 0.13 |

5.97% | 0.0019 | 115 | 4.77 | 0.387 | 0.010 | 5.97 | 0.05 |

e_{THD}: The error between the measured THD by PQ monitor and the setting THD.

. The experiments results of voltage fluctuation plus harmonics plus interharmonics plus voltage sag

The disturbance parameters | F_{2} | F_{3} | Results | Remarks |
---|---|---|---|---|

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.4 pu | 4.72 | 3.75 | Voltage Fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.03, frequency 8 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.4 pu | 4.80 | 3.80 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.4 pu | 4.91 | 3.82 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.05, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.4 pu | 4.98 | 3.80 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 1%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.4 pu | 3.12 | 1.07 | Voltage fluctuation plus harmonics plus voltage sag | When the interharmonics content is lower than 1%, and the harmonics content is large, the error is a little large |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 1245 Hz; harmonics THD 2%; voltage sag amplitude 0.4 pu | 4.21 | 3.68 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 3%; voltage sag amplitude 0.4 pu | 5.25 | 3.71 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 5%; voltage sag amplitude 0.5 pu | 7.27 | 3.75 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.7 pu | 4.05 | 3.69 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag | |

Fluctuation modulating wave amplitude 0.02, frequency 6 Hz; interharmonics content 2%,frequency 125 Hz; harmonics THD 2%; voltage sag amplitude 0.8 pu | 4.11 | 3.70 | Voltage fluctuation plus harmonics plus interharmonics plus voltage sag |

. The voltage sag experiments results comparison table

Identifying results | t_{m} (ms) | t_{om} (ms) | e_{t} | a_{m} (pu) | a_{om} (pu) | e_{a} |
---|---|---|---|---|---|---|

Voltage sag | 100 | 102 | 2 | 0.842 | 0.854 | 0.012 |

Voltage sag | 102 | 105 | 3 | 0.838 | 0.833 | 0.005 |

Voltage sag | 101 | 103 | 2 | 0.857 | 0.872 | 0.015 |

Voltage sag | 108 | 110 | 2 | 0.813 | 0.801 | 0.012 |

Voltage sag | 113 | 114 | 1 | 0.839 | 0.825 | 0.014 |

a: Error means the difference between values evaluated by PQ monitor and ones measured by the oscilloscope. For the oscilloscope measured value has error itself, but here, no error is considered. The meaning of error in the other tables is the same.

A PQ monitor is equipped to a steel pipe factory substation to monitor the harmonics disturbance and the oscillatory transient disturbances. The results are shown in

The field monitoring results in

To catch the oscillatory transients, a PQ monitor is equipped to Shi-Qiao substation. The oscillatory transient signals are generated by switching three phase capacitors. The three phase voltage waveform is shown as

The voltage sag experiment. (a) The voltage sag experiment circuit; (b) The waveform of u_{1}

The three phase voltage waveform of capacitors switching in Shi-Qiao substation

. The harmonics experiments results comparison table

Identifying results | THD_{m} | THD_{om} | Error |
---|---|---|---|

Harmonics | 4.2% | 4.4% | 0.0020 |

Harmonics | 5.0% | 5.2% | 0.0020 |

Harmonics | 4.8% | 5.0% | 0.0020 |

Harmonics | 4.5% | 4.8% | 0.0030 |

Harmonics | 5.7% | 6% | 0.0030 |

THD_{m}: THD measured by the PQ monitor; THD_{om}: THD measured by the oscilloscope.

. The experiments results of oscillatory transient in Shi-Qiao substation

Identifying results | t_{m} (ms) | t_{om} (ms) | Error | a_{m} (pu) | a_{om} (pu) | Error |
---|---|---|---|---|---|---|

Oscillatory transients | 12.43 | 12.56 | 0.13 | 0.70 | 0.78 | 0.08 |

Oscillatory transients | 13.56 | 13.63 | 0.070 | 0.75 | 0.82 | 0.07 |

Oscillatory transients | 14.28 | 14.39 | 0.11 | 0.78 | 0.79 | 0.01 |

Oscillatory transients | 12.85 | 12.98 | 0.13 | 0.74 | 0.85 | 0.11 |

Oscillatory transients | 14.50 | 14.01 | 0.49 | 0.88 | 0.93 | 0.05 |

t_{om}: The oscillatory transients lasting time measured by the oscilloscope; a_{om}: The oscillatory transients amplitude measured by the oscilloscope.

The author takes one phase wave to analyze (the purple one). The field experiment results are shown in

Comparing with analyzing power quality disturbance signals in frequency domain, the method presented by this paper has some advantages. First, if one feature or some features satisfied some conditions, a disturbance or mixed disturbances can be sure. That is to say, the disturbance classification would not be probable, but be definitive. Second, the features extracted won’t change a lot for the existence of the other disturbances. This characteristic is the key of classifying the mixed disturbances. Third, the features have clear physical meanings. So it profits the evaluation of the disturbance parameters.