QUANTIFYING THE POTENTIAL IMPACTS OF DISTURBANCES ON POWER SYSTEM PROTECTION

F Wang M H J Bollen

Chalmers University of Technology, Sweden

The performance of power system protection plays a vital role in system security and reliability. The presence of disturbances may affect the operation of protective relays, and thus endanger security and reliability of the supply. The effect of disturbances on protection operation increases when faster and more sensitive algorithms are used. In this paper, two methods are presented to quantify the potential impact of measured disturbances on relay operation. Both methods are applied to a number of disturbances.

INTRODUCTION

Protection mal-trips due to voltage and current disturbances form a potential threat to the reliability of the power supply. If such a disturbance is due to a fault, it will lead to the loss of the faulted as well as the non- faulted component. For other disturbances, several relays may react in the same way. The loss of multiple components in a transmission system may trigger a large-scale blackout. The risk of a mal-trip due to a disturbance is minimized during the design process of a relay (mainly through the use of filters) and by a conservative choice of threshold and time delay settings. All this typically leads to an increase of the fault- clearing time and greater risk of fail-to-trip. To make an accurate trade-off between fail-to-trip, fault-clearing time and mal-trip, a detailed knowledge is needed of voltage and current disturbances at the relay terminals.

Normally, protective relays are tested using a set of synthetic disturbances. But the use of measured disturbances will form an important and necessary complement to this. It is practically impossible to test each relay for all possible disturbances. Some first pruning is needed, based on the potential impact of a disturbance on a certain relay type. It is difficult to intuitively tell the potential risk caused by disturbances on relays. However the quantifkation technique adopted in this paper demonstrates the nuance by means of a disturbance factor. While the voltage and current waveforms of different voltage disturbances are quite similar, the disturbance factors may be completely different.

DISTURBANCE QUANTIFICATION IN TWO WAYS

The impact of disturbances on relay operation will be addressed in two ways in this paper. A disturbance may cause a measured parameter to falsely enter the relay

tripping range, potentially leading to a mal-trip. This effect will be referred to as setting-based quantification. Disturbances will affect the relay operation in another way as well. A relay, especially a digital relay, can be seen as a (digital) filter that extracts the desired component from the measured voltages and/or currents. The presence of disturbance will give an error in extracting the desired component. This will be referred to as design-based quantification. It will be assumed that the desired component is the fundamental component; unwanted components being harmonics, interharmonics, dc, transients, etc.

Setting-based Quantification

In practical installation, the main concerns are whether the disturbances may trigger the relay protection by falsely entering the protection tripping region. A disturbance studied in this project means signal with a short-term transient, or a long-term distortion, or both. The practically measured disturbance signals have limited duration, from several cycles to more than ten cycles. The status at both the beginning and the end of the measurement window are considered stable; the variation in the signal in between determines the impact on relay operation. The severity of the disturbance is given relative to a steady state value. Either the pre- disturbance or the post-disturbance value is used, whichever one is more severe. Two examples are given in Figs. 1 and 2 (2).

In Fig. 1, the impact region starts from the post- disturbance value. In Fig.2, the impact region is counted from the predisturbance point because it is closer to the setting region compared with the post-disturbance point.

patd$labncsStahn,

pbdis-- Tme

0 b

SBmnO m - C u r a n R M S ~ t s e a

Figure 1. Impact region for overcurrent relay

To quantify the severity of an impact, a disturbance factor D is introduced. Let s e t 0 = max (set-Os, seLOe), or set-- (set-Os, set-Oe), depending on the type of protection, then

262 Developments in Power System Protection, Conference Publication No.479 0 IEE 2001.

D = I (set-h-set-O)/set-h I

- R

+SMWPoht XEnQngpoint

4npedanceVariatiOnTnce

Figure 2. Impact region for impedance relay

Overvoltage, undervoltage, overload, overcurrent and impedance relays are the main force in feederfline protections. Whenever a disturbance occurs, it is necessary to know its potential impact on various protections. For an overall evaluation of the disturbance, the impact region of the following relays are studied

0 Overvoltage ( v t ) 0 Undervoltage ( V 3. ) 0 Overcurrent ( I T ) 0 Impedance(Z.1)

Table 1 lists the possible disturbances and their impacts on various relay protections.

Collecting all the quantified impact information together gives an overall estimation of the disturbance effect on the infringement of relay setting region. Examples are shown below.

Table 1. Disturbances and corresDonding imDact

v t v3. I t Z-L Disturbance type . J J X X Voltage transient

X X J X Current transient J X X X Voltage swell X J J J Voltage sae J J X J Voltage fluctuation X X J J Short-time overload J potential impact X minor or no impact

Design-based Quantification .

An important part of the design of digital relays involves the extraction of the fundamental component by a digital filter. The window size and shape significantly affect the filter output. This is shown in Fig. 3 where the amplitude of the fundamental component is plotted for cosinelsine windows of different size, together with the (time-domain) disturbance factor.

-d 0 w . 40 . 63 . 0 . 1m . 1w . 140 . 163 . IP . 200 I Figure 3. Outputs of filtered signal by different filters

The window determines the iiquency response of the filter. Table 2 shows the capability of different filters in removing various unwanted components.

Table 2 Capability of filter in removine. disturbances

0.5-cycle 1 -cycle 2-cycle 3-cycle Odd harm. J ' J J J Even harm. J J J Dc J J J 1/2 interharm. J

Whenever there is a transient, no filter can work hundred percent correctly. Depending on the window size, a filter will not yield correct output in less than 1 sampling window time, which means there is no way for a filter to extract the exact fundamental component.

From table 2, it is clear that normally a longer sampling window gives a more accurate output. However, more cycles for sampling also means more time for decision- making. So this is a trade-off between time and accuracy. A compromise of these factors makes 1-cycle filter most commonly adopted in practice. By comparing the filter outputs at different window sizes, an approximate evaluation of the possible unwanted components can be made. Let the case of 1-cycle filter be the reference, the relative shift of relay setting characteristics fkom the reference is checked for each other filter, as an example shown in Fig. 4. The 'delay time' in the diagram is the relay decision-making time, i.e. the time for relay to c o d m the over-threshold event after the setting limit is exceeded. The setting value at the knee point is the maximum setting limit under a certain time delay. Such a relay setting characteristic diagram can easily be obtained f?om the output of relay filter (4).

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- 1st I r i

I . . . . . I "0.1 0 2 03 0 4 05 Ob 0.7 Od 03 1

5.- IA)

Figure 4. Shift of relay setting characteristics

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The same approach can be adopted to evaluate the disturbance severity. The same formula as in eqn.1 can be used to calculate the disturbance factor, for a certain filter at a certain time delay.

CASESTUDIES ,

Case 2 Voltage fluctuation (interhamonics)

The disturbance in this case study occurred in an 11 kV industrial system in Europe. The voltage shows obvious fluctuation while the current is relatively stable.

10' 2d773ado (Sampling Rate 12800 Hr)

In the following case studies, some measured disturbances are applied to an ordinary digital relay to demonstrate the disturbance impact severity. All the relays in the study use a sampling frequency of 800 Hz.

Setting-based Disturbance Evaluation

Case 1 Voltage sag

The disturbance in this case study occurred in a 400 V residential system in Europe. This is a 4-cycle voltage sag, as shown in Fig. 5.

st=mlngar990819 (Sampling Rate 6400 Hz) I I I I I 1

E m 0 0 'i, =. -200

,_ . . , . . ,_ , ,. . , . , I I I I I I

0 50 100 150 200 250 300 -4001

lime (N)

s k m I n g ~ l 9 (Sampling Rate 6400 H r ) 100,

I I I I I J 0 50 100 150 200 250 300

-1001

lime (N)

Figure 5. Voltage sag

By applying the approach described in the previous section, the impact of this disturbance on the setting regions of various types of relay is illustrated in Fig. 6.

Fig. 6 tells that there is much impact on overcurrent and impedance relays, less impact on undervoltage relay and almost no impact on overvoltage relay. The impact duration is quite long (more than 20 ms). The heights of the bars show the relative disturbance variation towards the relay setting region. The higher the bar, the greater the potential risk of relay maloperation.

Figure 6. Impact on relay protection zone (case 1)

2dmado (Sampling Rate 6400 Hz) 1o00,

I I I I I 0 60 100 150 200 250

T h e (mr) Figure 7. Voltage fluctuation (interharmonics)

The impact diagram of this case is shown in Fig.8. From the diagram, it is clear that the impact is on all the relays except the overcurrent relay. The duration of the impact is short (less than half cycle).

Figure 8. Impact on relay protection zone (case 2)

Compared with the diagram in Fig.6, this diagram shows less and shorter disturbance impact. Generally speaking, the disturbance in this case is less hazardous to the relay setting regions than in case 1.

Design-based Disturbance Evaluation

Case 3 Harmonics and interharmonics in current

Fig. 9 shows a disturbance in the current taken by a computer in our laboratory. From the waveform it can observed that there exist interharmonic component.

- 2 s E o a -2

I I I I I I 0 50 100 150 MO 250 300

-" (W Figure 9. Harmonics and interharmonics in current

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Figure 10. Impact on relay performance (case 3)

The disturbance impact evaluation diagram is shown in Fig. 10. The bars in the row of l-cycle filter are always of a value of 1.0, as described in the previous section. The heights of the bars in other rows are the ratio of s e 0 over set-0, as illustrated in Fig. 4. For any given output waveform, the bar height at Oms means the ratio of maximum point of this waveform over that of the 1- cycle output. From Fig. 10, it h clear that the existence of interharmonics has made the setting limit of output waveforms shifted, either increased or decreased, depending on the window size of relay filter. The difference is greater when the time delay is shorter. At longer time delay, all the output setting limits except RMS output are closer to each other, which implies that there might be some fluctuation on the output waveforms.

To interpret the diagram in Fig. 10 in a more intuitive way, the smoothness of the plane formed by the bars can be a criterion. The smoother the plane, the less the disturbance impact on relay performance.

Case 4 Transient and harmonics in current

Another example is shown in Fig. 11. This is a disturbance signal measured on industrial site, at 400 V system. A transient occurs at a certain moment and later diminishes. Besides, there is also harmonics in the current waveform.

The disturbance impact in this case is illustrated in Fig. 12. Compared with Fig.10, this diagram shows a smoother surface among the bars of different filters. When the time delay is 20 ms for the filters, the setting limits of the outputs are almost the same for different filters, which implies that the transient impact can be neglected by the filters if the decision-making time is as long as 20 ms.

1 I I I I 0 60 1 0 0 1 M ) 2 0 0 2 5 0

-rhw vs Figure 11. Transient in current

Figure 12 Impact on relay performance (case 4)

Compared with the diagram in Fig.10, it can be concluded that the impact on relay performance is less thanincase3.

CONCLUSIONS

The quantification of the disturbance impact on protective relays can be made in two different ways. The setting-based quantification demonstrates the severity of potential risk that the relay protection zones could be falsely reached due to the disturbance. The design-based quantification illustrates the severity of impact on relay output performance due to the unwanted components in the disturbance signal.

The quantification diagrams also provide some intuitive information on the details of the disturbance. By developing criteria on the impact diagrams, the disturbances can be classified according to their impact severity and duration, based on which databases can be set up for relay testing.

REFERENCES

Saha M. M., Rosolowslci E. et al., 1998, Simulation of a series compesated line for evaluation of relaying algorithms, Eurouean EMTP-ATP Users Grouu News, Februarv-May vol. - - 4, 1-2,93-105

Wang F., Bollen M. H. J., 2000, Qisturbance database setup for protective relay testing, Proceedings of 9* ~ntternational Conference on Harmonics and Oualitv of Power, U, 1059-1064.

David S. Baker et al., 1997, Application considerations of static overcurrent relays: a working group report, IEEE Trans. Industry apdication, vol. 33. no. 6,1493-1500

Wang F., Bollen M. H. J., 2000, Evaluating the effect of measured power disturbances on protective relay operation, Proceedinp of Intemational Conference on Electric Utility Deregulation and Restructurin~. and Power Technolorries 2000,232-237

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