CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

BKW_Schenk_A2 Session 3 Paper No 52 - 1 -

ASSESSMENT OF PROTECTION SCHEMES: A KEY FACTOR FOR RELIABILITY

Alain SCHENK, David ORZAN, Fritz GANTENBEIN, Markus VON ARX BKW FMB Energy Ltd. - Switzerland

alain.schenk@bkw-fmb.ch

INTRODUCTION Assessment of protection schemes represents an important aspect of the network reliability, especially in a liberalised market, where the operator tends to reduce the maintenance costs and to postpone the replacement of the old components. This article presents the experiences made by an electrical utility in assessing protection schemes. A systematic approach is used in order to check the efficiency of the whole fault clearance system and improve the reliability of the protection by testing the relays with real fault recorder events or simulated data. The paper is illustrated with real case studies, reflecting the way the protection should be tested. The focus is set on the new constraints resulting from the deregulation (higher cost pressure and unusual network configurations). Keywords: Protection - Relay setting - Reliability - Check - Disturbance recorder - EMTP - Simulation CHECK AND CALCULATIONS WITH PROGRAMS A software is used in order to calculate and check the settings of the distance protection relays in the 380, 220, 132, 50, and 16 kV networks. Figure 1 shows an example of computation of tripping zones.

Figure 1 - Example of tripping zones

Such a modern tool presents the following advantages: • Ability to simulate numerous network configurations -

systematic study of main and back up protection reliability, best compromise for relay settings;

• Precise calculation of special cases - for example, zero sequence coupling of double lines, or transformer with delta tertiary winding;

• User-friendly database - relay settings represented either in list, graphically, or printed in reports.

The program calculates load flows and short circuits as well. Only one database must be looked after making it easier to have up to date short circuit values for simulation. Some examples of application are provided below. They highlight the possibilities of such a program. General considerations The relay settings can not be dynamically adjusted according to the changing conditions of the network. However, they must ensure a reliable protection not only for normal conditions but also for extraordinary and though probable conditions. The distance relay can be influenced by many parameters such as fault resistance, zero sequence coupling from parallel circuits, additional current in feed, or teed transformer. Modern programs can assess the protection reliability for the most probable conditions.

Z1 Z2

I

I'

Breaker orRelay failure

Figure 2 - Additional in feed current effect

Figure 2 defines the additional in feed current effect. On this case, the voltage U is defined as

U = Z1 I + Z2 (I + I'). (1)

If I' increases, U will increase as well. The relay will measure a larger impedance and it will trip later. The measured impedance in the second and third zone depends largely on the throttling factor, i.e. the ratio between I and (I + I') [2, 3]. Figure 3 depicts the effect of current in feed on the tripping time. An increase of the measured impedance and thus a delayed tripping time is showed.

The relay settings must take into account several network configurations in order to ensure selectivity for all current in feeds and security (fault detection, tripping times as short as possible). The practice shows that compromises have often to be found because of the incompatibility of these two requirements.

CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

BKW_Schenk_A2 Session 3 Paper No 52 - 2 -

The significant advantage of the modern programs is their ability to simulate many cases in order (a) to get an overview

of the protection reliability and (b) to optimise it for the most probable operation conditions.

Effect of current in feed

Real tripping time (1.5 s)Tripping time without current in feed (0.5 s)

Effect of current in feed

Real tripping time (1.5 s)Tripping time without current in feed (0.5 s)

Figure 3 - Example of the additional in feed current effect on distance relays

Multiterminal line Some large industrial customers require a multi-terminal line (T line) because it enables an economical access to the high voltage system, typically regional distribution network (150 kV). Unfortunately these networks have features that make it difficult to find the optimal relay settings: • Relatively short line length (15 km) - difficulty to obtain

selectivity with distance relay; • High short circuit currents and quite well interconnected

grid - adverse effect of additional current in feed; • No signalling equipment for teleprotection - realization of

the main protection with the tripping zones only; • Heterogeneous lines (overhead lines and cables) -

different impedances.

Moreover, the industrial plants have often their own generation capabilities or are fed by other networks. Therefore, a compromise has to be found between selectivity and security. Programs can help the protection engineer to find the setting ensuring short tripping times.

II'

Figure 4 - Multiterminal line: When connected, the additional generation unit (in dashed line) will increase the short circuit current Remote back up protection If one element of the fault clearance system fails, the back up protection must eliminate the fault [4]. Remote back up protection offers significant advantages because of its

reliability and low cost: • It is completely independent from the main protection, as

it is located in another substation; • It does not require additional devices like circuit breaker

failure protection relays or double protection relays arrangement.

The limitation of the remote back up protection comes from the fact, that the conditions allowing its applicability are not always fulfilled, especially with the large short circuit currents in well interconnected systems. When considering the design of the back up protection, the engineer begins with studying the feasibility of remote back up protection, because it avoids the costs of the local back up protection. This task can easily be performed through programs that enable to check whether the distance protection can operate as back up protection for the next substation or not. This situation is typical for a regional distribution network (150 kV), where the short circuit currents can reach a significant values and the costs are to be held in reasonable limits.

BRU

HAB

UTZ

WAN

BRI

TR1

TR2

Breaker or relay failure

HEI

BIK

BRU

HAB

UTZ

WAN

BRI

TR1

TR2

Breaker or relay failure

HEI

BIK

BRU

HAB

UTZ

WAN

BRI

TR1

TR2

Breaker or relay failure

BRU

HAB

UTZ

WAN

BRI

TR1

TR2

Breaker or relay failure

HEI

BIK

Figure 5 - Remote backup protection: Case study

CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

BKW_Schenk_A2 Session 3 Paper No 52 - 3 -

Figure 5 shows the definition of a case study for a regional distribution network with seven radial lines. Table 1 summarizes the values of the measured impedances, when a short circuit occurs on the line BIK-HEI and simultaneously one element of the clearance system in BIK fails. It shows which relays trip and how long their tripping times are. In this example, the back up protection is insured because all neighbour relays operate and clear the fault within 2 to 2.2 second (end time).

Seeing that some circuit breakers are quite old and sometimes near their interruption capabilities, a systematic study was undertaken in order to check the reliability of the back up protection. The calculations showed the necessity of some simple improvements, most of them being possible by using the installed relays with an additional logic.

Table 1 - Values for a fault on the line BIK-HEI with a failure of the clearance system in BIK Distance relay Zone I [kA] Z [Ω] Φ [°] t [s] DS1_HEI-BIK 1 3.812 2.52 83.0 0.1DS1_BIK-HEI 1 19.137 1.67 82.8 --DS1_SUM-BIK 4 1.316 41.99 67.9 2.0DS1_BRI-BIK 4 1.520 23.15 85.8 2.0DS1_BUR-BIK 4 1.532 27.67 83.6 2.0DS1_HAB-BIK 4 0.928 38.12 80.4 2.0DS1_UTZ-BIK 4 1.175 33.88 72.6 2.0DS1_WAN-BIK 4 3.826 27.32 86.9 2.0DS12BIK1 -1 6.154 5.19 265.0 2.2DS12BIK2 -1 6.154 5.19 265.0 2.2

Back up protection with sequential tripping In case of a large short circuit current, the distance protection is likely not to detect the fault further than the remote substation. However, remote back up protection can be obtained through sequential tripping. As this form of back up protection can be tricky, it is advisable to check the sequences very carefully with software simulations. SIMULATION AND ON SITE TESTS The protection relays are tested regularly on site with standard procedures in order to detect a malfunction. Of course, not only the relay but also the whole fault clearance system is checked (current transformer, potential transformer, remote protection equipment, DC system, wires, protection relay, and circuit breaker).This section presents some experiences made when testing protection relays with modern testing devices for special cases. These testing devices offer a wide range of possibilities, one of which is the replay of transient signals. The current and voltage waveforms come from simulation programs (for example EMTP) or from fault recorders. The replay function of the testing device enables to assess the relay reactions for a special situation. It makes it possible to investigate if the relay scatters. Some cases, where the so-called dynamic relay testing was used for the assessment of the protection schemes, are presented here.

Saturation of a current transformer In a 220 kV substation, the short circuit current has doubled within ten years. Simulations show that the current transformer saturates in case of a maximum direct component of the short circuit (see Figures 6 and 7). This could result in a non-operation or a delayed operation of the relay, what means more damage and non-selectivity.

The protection engineer must now answer some classical questions like: • What is the tripping delay to expect? • Must the current transformers be replaced? • What about installing intermediate current transformers

for the transmission, and/or increase the section of the cable conductor?

0.00 0.04 0.08 0.12 0.16 0.20[s]-40

-20

0

20

40

60

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120

[kA] Primary

Secondary

100% asymmetry

0.00 0.04 0.08 0.12 0.16 0.20[s]-40

-20

0

20

40

60

80

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120

[kA] Primary

Secondary

100% asymmetry

Figure 6 - Saturation with 100% DC asymmetry

0.00 0.04 0.08 0.12 0.16 0.20[s]-40

-20

0

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40

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120[kA]

Primary

Secondary

50% asymmetry

0.00 0.04 0.08 0.12 0.16 0.20[s]-40

-20

0

20

40

60

80

100

120[kA]

Primary

Secondary

50% asymmetry

Figure 7 - Saturation with 50% DC asymmetry

Tests were performed by varying the saturation level and the fault location [5, 6, 7]. The relay always tripped and, except for some seldom cases, the selectivity could be respected. As a consequence, the risk to operate these current transformers was considered as acceptable. Fault between 16 Hz 2/3 and 50 Hz systems In Switzerland, the power frequency of the railway is 16 Hz 2/3. For economical and ecological reasons, many 50 Hz and 16 Hz 2/3 frequency lines are designed on the same towers. When trying to interrupt a current due to a short circuit between the 220 kV 50 Hz and the 132 kV 16 Hz 2/3 systems, a 220 kV 50 Hz circuit breaker exploded because of

CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

BKW_Schenk_A2 Session 3 Paper No 52 - 4 -

thermal overload. The operation of several relays under such conditions was studied, as the settings were given for the power frequency of each system.

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-30

-20

-10

0

10

20

30

[kA]

Earth contact

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-30

-20

-10

0

10

20

30

[kA]

Earth contact

Figure 8 - Short circuit with earth contact

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-30

-20

-10

0

10

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30

[kA]

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-30

-20

-10

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[kA]

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-30

-20

-10

0

10

20

30

[kA]

Figure 9 - Short circuit without earth contact

Figures 8 and 9 present typical waveforms obtained from EMTP calculations. Numerous short circuits were simulated and fed to the relays. These direct tests proved that the tripping was always obtained. However the relays often could not identify exactly the nature and the location of the fault, especially when the short circuit was not in contact with earth, as it can be inferred when comparing figure 8 and 9. For these reasons, in order to avoid the no selectivity, it was decided that such a transmission line should be equipped with teleprotection,. Note that these measures mainly apply to the transmission lines, whose 16 Hz 2/3 and 50 Hz circuits are operated with a similar voltage. If the voltages were quite different, a contact between the two systems would cause a break down of an insulator, and thus a short circuit with earth contact (Figure 8), which can be properly detected by the distance relay. Teed transformer In order to reduce the costs (one circuit breaker instead of a real high voltage substation with three bays), a teed transformation has to be designed to supply a distribution network (see Figure 10). The following requirements are defined:

• Single pole switching for the 220-kV transmission line; • Teleprotection to avoid under or over reach of the distance

protection due to the non-uniform zero sequence impedance along the line (because of the YNd connected transformer acting as a short circuit for the zero sequence current);

• Control equipment to disconnect the three phases of the transformer on the high voltage side. This scheme is necessary because of the delta winding that would feed back a voltage preventing the extinction of the earth fault, if circuit breakers were only installed on the low voltage side.

YNd

Figure 10 - Teed transformer

The purpose of this study was to get some information about the phenomena that are likely to occur, to check the feasibility with respect to the protection, and to specify a general concept. Figures 11 and 12 show two results of the simulations. A substantial advantage of such simulations is that they give precise results before designing the substation. Moreover, as this teed transformer scheme is quite seldom in Switzerland, the tests enable us to get more confidence in its suitability for several scenarios.

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-200

-150

-100

-50

0

50

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150

200[kV]

Earth Fault

Opening line circuit breaker

Opening teed transformer circuit breaker

Fault extinction

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-200

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200[kV]

Earth Fault

Opening line circuit breaker

Opening teed transformer circuit breaker

Fault extinction

Figure 11 - Phase-to-earth voltage

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-300

-200

-100

0

100

200

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400[A]

Earth Fault

Opening line circuit breaker

Opening teed transformer circuit breaker

Fault extinction

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-300

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400[A]

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]-300

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400[A]

Earth Fault

Opening line circuit breaker

Opening teed transformer circuit breaker

Fault extinction

Figure 12 - Zero sequence current

CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

BKW_Schenk_A2 Session 3 Paper No 52 - 5 -

Capacitive voltage transformer The capacitive voltage transformer presents some advantages, among them a high reliability (few parts) and lower costs. One of its disadvantages is the transient behaviour, that could disturb some older protection relays. Because of this, inductive voltage transformer has been mainly used yet in Switzerland. However, with the advent of the numerical relays, the problem of the transient behaviour should be largely alleviated. In order to check directly this statement, a dynamic relay testing was performed with simulated capacitive voltage transformer outputs. Figure 13 compares the primary and the secondary voltage in case of a terminal short circuit. As well known, the transient response of the secondary voltage can induce a delayed tripping. The tests mentioned above showed that this problem does actually not occur with modern protection relays.

0.00 0.02 0.04 0.06 0.08 0.10 0.12-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0[kV]

[s]

Secondary voltage

0.00 0.02 0.04 0.06 0.08 0.10 0.12-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0[kV]

[s]

Secondary voltage

Figure 13 - Primary and secondary voltage in case of a terminal short circuit

A significant advantage of such an investigation was the possibility to simulate and study easily many actual network conditions. Disturbance recorder Another possibility of the modern relay testing devices is the replay function of the currents and voltages waveforms coming from a real disturbance instead of simulated results. The following figure depicts the voltage measured during an earth fault in an isolated neutral network, that had not been properly detected and lead to misoperation of many relays. Replaying the fault confirmed that relays were not able to detect it. After discussing this case with the relay manufacturer, the corresponding constructive improvements were carried out on all concerned devices in order to obtain more stability under transient fault conditions.

Current

Voltage (faulted phase)

Voltage (sound phase)

Current

Voltage (faulted phase)

Voltage (sound phase)

Figure 14 - Intermittent earth fault in an isolated neutral network

CONCLUSION This paper reports some possibilities to assess the reliability of protection relays. Programs enable to check the selectivity and the back up protection function. They help the engineer to find the best compromise for the settings and give an overview of the reliability of the whole protection system. Relay testing devices can be used for the studies of unusual configurations or complicated phenomena. Current and voltage waveforms come from simulations or from disturbance recorders. These facilities present significant advantages in the design stage of engineering projects, as they permit to simulate precisely numerous situations by varying easily many parameters. REFERENCES [1] Arbeitgruppe Netzschutz der Kommission des

schweizerischen Verbundbetriebes (SBDK), Auswirkung der Marktliberalisierung auf den Netzschutz des Höchstspannungsnetzes der Schweiz, Jul. 2000

[2] H. Ungrad, W. Winkler, A. Wiszniewski, Schutztechnik in Elektroenergiesystemen, Springler Verlag, ISBN 3-540-57832-3

[3] J.P. Barret, Quelques aspects du comportement des protections de distance contre les courts-circuits et les défauts d'isolement destinés aux réseaux THT, RGE, No 2, Fév. 1974

[4] Arbeitgruppe Netzschutz der Kommission des schweizerischen Verbundbetriebes (SBDK), Empfehlung für den Reserveschutz im Schweizerischen 380/220-kV-Verbundnetz (Reserveschutz-Konzept), Sept. 1996

[5] R. Zahorka, Das Verhalten von Stromwandlern bei Einschwingvorgängen mit Gleichstromgliedern unter Berücksichtigung der Sättigung, AEG Mitteilungen, Jan. 1967

[6] J.R. Marti, L.R. Linares, H.W. Dommel, Current transformer and coupling-capacitor voltage transformer in real-time simulations, IEEE Trans. On Power Delivery, Jan. 1997, Vol. 12, No. 1

[7] Working Group C-5 of the Systems Protection of the IEEE Power System Relaying Committee, Mathematical models for current, voltage and coupling capacitor voltage transformer, IEEE Transaction On Power Delivery, Jan. 2000, Vol. 15, No. 1

[8] H. Ungrad, V. Narayan, Behaviour of distance relays under earth fault conditions on double circuit lines, Brown Boveri Review, 1969

[9] M. Agrasa, F. Uriondo, J.R. Hernández, Evaluation of uncertainties in double line distance relaying, IEEE Trans. On Power Delivery, 1998, Vol. 13, No. 4