1

Iernut Substation Automation System

Dr. Ing. Florin Balasiu TRANSELECTRICA Romania

Dr. Ing. Gheorghe Moraru SMART Romania

ABSTRACT

Transelectrica as the Romanian Transmission and System Operator (TSO) has developed an intensive process of rehabilitation of the transmission substations including the substation automation systems (SAS). As a result different types of protection and control systems were put into service improving the know-how of the protection engineers. Different substation layouts and different SAS architectures challenged the protection engineer to improve the know-how in this field.

The proposed paper presents the SAS implemented in the 400/220 kV Iernut substation and some practical lessons learnt on numerical protection system engineering.

The 400 kV substation layout is a classical one with double bus-bar and a coupler, while the 220 kV is one-and-a-half circuit breaker (1 CB) layout. The two distinct layouts impose a different approach to the protection and control system arrangement. Advantages of using numerical relays to cope with this challenge are shown in the paper.

The conclusions highlight on the advantages and limitations of the numerical protection systems implemented in Iernut substation.

1. INTRODUCTION

The 400/220/110/6 kV Iernut substation is located into an important node of the Romanian Transmission Network (RTN) to interconnect the 400 kV northern part of the country to the central one and on the 220 kV side to evacuate the energy generated by Iernut power plant (TPP) to the grid and provide the 220 kV back-up way for both northern and eastern part of the country. The double bus-bar 400 kV substation layout consists from two overhead lines (OHLs), a coupler and an autotransformer, namely the 400 kV Sibiu South and Gadalin OHLs and AT-1 400/220 kV, 400 MVA. The protection system arrangement on the OHLs is based on the main one (MP-1) and the main two (MP-2) protections implemented using multi-function numerical relays. The AT-1 protection system provides beside the technological protections

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two multi-function differential protections and on each voltage level one numerical multi-function distance relay. To protect the 400 kV bus-bars a numerical bus-bar protection (BBP) is provided. The 220 kV 1 CB layout with five diameters (refer to figure 7) consists of four 220 kV OHLs, two autotransformers (AT-1 and AT-3) and the four links of the 220 kV generators from TPP (T206, T205, T201+T202). The protection system for each of the 220 kV OHLs is based on a main and a back-up protection. The main protection is basically a numerical distance protection, while the back-up is a multi-function over-current protection relay. The AT-3 220/110 kV, 200 MVA protection system provides beside the technological protections two multi-function differential relays and on each voltage level one distance relay. To protect the two 220 kV bus-bars two numerical BBPs are provided. The 110 kV substation consists of eight 110 kV OHLs, a coupler, two 110 kV generator bays, one 110 kV bay for AT-3 and two 110/6 kV transformer bays. The protection system is based on numerical relays and is not detailed in this paper. The control system is organised to provide full control of the entire substation, including remote control from the National Dispatching Center (NDC) and is based on a redundant SCADA system. To monitor the protection systems an independent LAN is used. Both SCADA and protection LANs are optical fiber and use the IEC 60870-103 protocol. The communication to the NDC is based on the IEC 60870-101 protocol and uses two independent communication channels.

2. 400 kV PROTECTIVE SYSTEMS

2.1 Protection of the 400 kV OHLs

Each of the protection system for the 400 kV OHLs uses two multi-function numerical relays, F21.1 and F21.2 connected as in figure 1. Each multi-function relay provides the following protection and automation functions: 21 distance protection function, five quadrilateral zones against phase and ground faults. It is common practice to use three forward zones, one reverse zone and one non-directional zone.

50+50N phase over-current and ground over-current protections, as back-up protection against phase and ground faults 67N1+67N2 directional earth fault protections, as ground back-up protection or against high resistance ground faults. 50HS switch-onto-fault protection to fast tripping in case of closing the CB onto a fault. 59 phase-to-ground over-voltage protection against dangerous over voltages in the network. Such a condition could come out when the line is operated with one end open. 85 teleprotection system to provide fast clearance of faults located anywhere on the line, based on a Permissive Under-reach Transfer Scheme (PUTT) and on two independent physical communication channels, an optical fiber (FO) and a power link carrier (PLC). Each MP-1 and MP-2 form both line-ends transmit and receive the signals on/from both physical channels. The transmit signal is limited to only zone 1 trip. The received signal is used to fast trip the CB assuming a zone 2 start condition. 68 power swing detection. When a power swing condition is detected then zone 1 of the distance protection function is blocked to prevent an unwanted trip. 79 one-shot, one-pole autoreclosing (AR). The AR function provides a fast re-energize of the line once the fault is extinguished (providing it is a transient fault) only for single-line-to-ground faults (SLG). For line-to-line faults (LL) or line-to-line-to-ground faults (LLG) only a

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final three-pole trip is allowed. In case of both teleprotection channels being out-of-service, the zone 1 is extended but only for SLG faults and only before AR. In this case is the SLG fault is permanent and located towards remote end, the second trip will be a normal zone 2 trip. 25 synchro-check function to assist the manual CB closing command. This ability is used as back-up to the similar function provided by the bay control unit (BCU) and it is only activated when voluntary selected by way of a switch. The synchro-check function allows closing the CB only if the synchronism is fulfilled or if some dead line/bus condition is satisfied. FL the fault locator indication provides useful information on the fault location for the line maintenance crews. ER+DR event and disturbance recording to assist post-factum analysis.

Q0

P1

P2

Q9

Q51

Q8

Q52

Q1 Q2

CT w25P3030 VA

CT w10.5 FS 530 VA

CT w35P3030 VA

FBCU-1AM17031

MET

F87BDRS-CBB

F21.17SA612

F21.27SA612

W1 cl , 30VAw2 cl 0.5/3P,50VAw3 cl 0.5/3P,50VA

0.5

CVT

F21.1212579685050HS5950N, 51N, 67N85FLERDR

F87B87B50BF50EZ

BCU25Interlocking

Q1

Q1VT-BB1

BB1-400 kV

BB2-400 kV

VT-BB2

Q2

Q2 Q62Q61

F21.22179685050HS5950N, 51N, 67N85FLERDR

Figure 1. Typical 400 kV OHL protection system

2.2 Protection of the 400 kV Coupler

The protection system for the 400 kV Coupler uses only one multi-function numerical relay, F21 connected as in figure 2. The multi-function relay provides two operating method, one as normal coupler and the other one to bus-bar check when energising. The following protection and automation functions are used: 21 distance protection function, four quadrilateral non-direction zones against phase and ground faults. When energising a dead bus all zones become instantaneous by means of a selector switch. 68 power swing detection. When a power swing condition is detected all four zones of the distance protection function are blocked to prevent an unwanted trip.

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25 synchro-check function to assist the manual CB closing command. This ability is used as back-up to the similar function provided by the BCU and it is only activated when voluntary selected by way of a switch. The synchro-check function allows closing the CB only if the synchronism is fulfilled or if some dead line/bus condition is satisfied. FL the fault locator indication provides useful information on the fault location for the line maintenance crews. ER+DR event and disturbance recording to assist post-factum analysis.

Q0

P1

P2

Q53

Q11 Q12

CT w25P3030 VA

CT w10.5 FS 530 VA

CT w35P3030 VA

FBCU-1AM17031

F87BDRS-CBB

F217SA612

F21212550HS50N, 51N, 67NFLERDR

F87B87B50BF50EZ

BCU25Interlocking

BB

-1

Q1VT-BB1

BB1-400 kV

BB2-400 kV

VT-BB2

Q2 Q62Q61

Q51

BB

-2

Figure 2. Protection system for the 400 kV Coupler

2.3 Protection of the 400 MVA Autotransformer

The protection system for the 400 MVA autotransformer AT-1 provides technological protections, two independent differential protections with thermal overload protection (F87T.1, F87T.2), distance and current protections (F21.1) on both voltage sides as shown in figure 3. The technological protections placed on the autotransformer are of over-pressure type and Buchholz type and are beyond the scope of this paper. The multi-function relay F21.1, connected to the 400 kV side provides the following protection and automation functions: 21 distance protection function, five quadrilateral zones against phase and ground faults. It is common practice to use three forward zones, one reverse zone and one non-directional zone.

50+50N phase over-current and ground over-current protections, as back-up protection against phase and ground faults 67N1+67N2+67N3 directional earth fault protections, as ground back-up protection or against high resistance ground faults, looking forward, reverse and non-directional.

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68 power swing detection. When a power swing condition is detected then zone 1 of the distance protection function is blocked to prevent an unwanted trip. 25 synchro-check function to assist the manual 400 kV CB closing command. This ability is used as back-up to the similar function provided by the BCU and it is only activated when voluntary selected by way of a switch. The synchro-check function allows closing the CB only if the synchronism is fulfilled or if some dead line/bus condition is satisfied. FL the fault locator indication provides useful information on the fault location for the line maintenance crews. ER+DR event and disturbance recording to assist post-factum analysis.

The multi-function relay F21.2, connected to the 220 kV side is described latter on.

The multi-function relays F87T.1 and F87T.2 provide the following protection functions: 87T current differential protection function. The operating characteristic of the restrained current differential protection is double slope, with harmonic restrain against transformer current inrush, with internal current magnitude compensation, internal current saturation detection and current zero-sequence rejection. The unrestrained current differential provides a fast fault clearance in case of in-zone large faults. 49 thermal image overload protection as a back-up of the winding temperature supervision device within the autotransformer. It is operating on the partial thermal image model of the autotransformer and provides both an early alarm stage and a second tripping stage. 50/51 over-current protection as back-up protection. ER+DR event and disturbance recording to assist post-factum analysis. To improve analysis the technological protections trips are passed to the relays just for event and disturbance recording.

Q0

P1

P2

Q9

Q51

Q8

Q52

Q1 Q2

CT w25P3030 VA

CT w10.5 FS 530 VA

CT w35P3030 VA

FBCU-1AM17031MET

F87BDRS-CBB

F21.17SA612

W1 cl , 30VAw2 cl 0.5/3P,50VAw3 cl 0.5/3P,50VA

0.5

CVT

F87T87T4950/51ERDR

F87B87B50BF50EZ

BCU25Interlocking

Q1

Q1

VT-BB1

BB1-400 kV

BB2-400 kV

VT-BB2

Q2

Q2 Q62Q61

F87.2DRS-C

F87.1DRS-C

CT-220

CT-220

AT 1-400 MVA

F21.1256850, 50N50N, 51N, 67NERDR

Figure 3. Autotransformer protection on the 400 kV side

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2.4 The 400 kV Bus-Bar Protection

Short circuits on bus bars have severe consequences, due to their central position in the power system. A fault on a bus-bar endangers system operation more than located on any other element. The main purpose of bus-bar protection is to prevent system blackout by isolating the faulty bus bar zone, before other elements drop out. Furthermore, short-circuit currents on a bus bar are in general very high and lead to serious damages if they are not interrupted in due time. Thus high requirement for sensitivity and security challenge the BBP.

The 400 kV BBP is of a distributed arrangement type, using a central unit (CU) and distributed field units (FU) located in each 400 kV bay. The connection between a FU and the CU is based on a double ring optical fiber. Thus some benefits are obtained:

The expensive and time consuming cable works used in traditional centralised busbar protections are avoided and by the ringshaped arrangement a minimum cable length is achieved. With a fibre-optic ring configuration, even if both rings are interrupted at a single point, each unit can continue to communicate, allowing uninterrupted operation. The FUs are in addition bay back-up protection units and operate independent from each other and independent from the CU. Lower cubicle requirement reducing overall system costs.

The BBP is of low-impedance differential type. Each bus zone is provided with its own software measuring system in which the instantaneous current values of all feeders are summed up and in which this current sum is checked if it exceeds the reference value. A plant replica, factory installed into the system, holds an internal equivalent to the actual bus bar arrangement. This replica uses the isolator positions to select the currents that are to be summed by the measuring systems of the zones. In case a measuring system detects a bus bar fault, the trip commands are given via the same plant replica to the CBs of all feeders which are connected to the individual zone. For redundancy, fast acting current relays in the feeders are used, as well as a check zone, which is built up similarly to the discriminating zones, but covering the whole plant. Therefore, the check zone acts independently of any isolator positions. The current transformer saturation detection function prevents unwanted trips under these conditions. The operating characteristic is shown in figure 4, where the operating current is the phasor sum of currents, while the restrain current is the largest feeder current.

Figure 4. The 400 kV BBP operating characteristic

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To avoid unnecessary BBP trip due to a fault located between the CB and CT, each feeder is provided with the End-Zone Protection (EZP) that trips only the remote end line CB. The same relays provide also the breaker failure protection (BFP) for the 400 kV substation. In case of a CB failure, detected by criteria, a tripping command and current flow above a certain value, than after the set time delay all feeders connected to the faulty bus are tripped. The bus-bar trip is achieved using the same plant replica as for the BBP. For redundancy reasons the re-trip feature is used in relationship with the single-pole trip mode of the related protection functions.

2.5 Field Experience

The 400 kV Iernut substation was put into service in November 2006 after finishing the rehabilitation works. Since then some events validated the accurate operation of the new protection system. Instead of conclusions, two disturbance records are given below. In figure 5 is shown the F21.1 relay operation on a permanent fault on 400 kV Sibiu South line. The distance protection function tripped single-pole in zone 1 and after the AR dead time and re-close command tripped again, this time three-pole.

Figure 5. Operating of the 400 kV OHL protection on a permanent fault

Another example is shown in figure 6, where the current differential relays tripped and cleared a fault inside the autotransformer. To be noticed also the tap-changer tank over-pressure protection trip. After investigations it was found an insulation break-down inside the on-load tap-changer area, thus proofing the right operating of the protection system.

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Figure 6. Operating of the autotransformer protections on an internal fault

3. 220 kV PROTECTIVE SYSTEMS

The 220 kV substation is based on the 1 CB arrangement as shown in figure 7.

Q51 Q51 Q51

Q55 Q55

Q54 Q54 Q54

TC11 TC11 TC11

TC31 TC31

TC21 TC21 TC21

TT 7 TT 7

TT 8 TT 8 TT 8

TT5 TT5

TT6 TT6 TT6

Q53 Q53 Q53

Q56 Q56

Q52 Q52 Q52

Q91 Ungheni-1 Q91

Q92 Q92 Q92

Q81 Q81

Q82 Q82 Q82

Q57 Q57

Q58 Q58 Q58

Q1 Q1 Q1

Q13 Q13 Q13

Q31 Q31

Q23 Q23 Q23

Q32 Q32

Q2 Q2 Q2

Q01

11 21 13

BB1-220 kV

BB2-220 kV

Q01 Q01

Q02 Q02 Q02

Q03 Q03

12 22 32

Q51 Q51

Q55 Q55

Q54 Q54

TC11 TC11

TC31 TC31

TC21 TC21

TT 7 TT 7

TT 8 TT 8

TT5 TT5

TT6 TT6

Q53 Q53

Q56 Q56

Q52 Q52

Q91 Q91

Q92 Q92

Q81 Q81

Q82 Q82

Q57 Q57

Q58 Q58

Q1 Q1

Q13 Q13

Q31 Q31

Q23 Q23

Q32 Q32

Q2 Q2

41 51

Q01 Q01

Q02 Q02

Q03 Q03

42 52

Ungheni-2 Campia Turzii Baia Mare

D1 D2 D3 D4 D5

T206 T205 AT-3 AT-1T201+T202

Figure 7. Single line diagram of the 220 kV substation

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3.1 Protection of the 220 kV OHLs

Each of the protection system for the 220 kV OHLs uses four multi-function numerical relays, one main protection (F21), a back-up protection (F67N) and two breaker management relays (FBM1, FBM2) connected as in figure 8. The following functions allocation for the main protection F21 is used: 21 distance protection function, five quadrilateral zones against phase and ground faults. It is common practice to use three forward zones, one reverse zone and one non-directional zone.

50+50N phase over-current and ground over-current protections, as back-up protection against phase and ground faults 67N1+67N2 directional earth fault protections, as ground back-up protection or against high resistance ground faults. 50HS switch-onto-fault protection to fast tripping in case of closing the CB onto a fault. 59 phase-to-ground over-voltage protection against dangerous over voltages in the network. Such a condition could come out when the line is operated with one end open. 85 teleprotection system to provide fast clearance of faults located anywhere on the line, based on a Permissive Under-reach Transfer Scheme (PUTT) and on either FO channel or PLC channel. The logic is similar to the 400 kV one. 68 power swing detection. When a power swing condition is detected then zone 1 of the distance protection function is blocked to prevent an unwanted trip. FL the fault locator indication provides useful information on the fault location for the line maintenance crews. ER+DR event and disturbance recording to assist post-factum analysis.

The back-up protection F67N has the following functions allocation: 50+50N phase over-current and ground over-current protections, as back-up protection against phase and ground faults 67N1+67N2 directional earth fault protections, as ground back-up protection or against high resistance ground faults. ER+DR event and disturbance recording to assist post-factum analysis.

The breaker management relay on Q01 provides the following functions: 79 one-shot, one-pole autoreclosing (AR). The AR function provides a fast re-energize of the line once the fault is extinguished (providing it is a transient fault) only for single-line-to-ground faults (SLG). For line-to-line faults (LL) or line-to-line-to-ground faults (LLG) only a final three-pole trip is allowed. In case of both teleprotection channels being out-of-service, the zone 1 is extended but only for SLG faults and only before AR. In this case is the SLG fault is permanent and located towards remote end, the second trip will be a normal zone 2 trip. 25 synchro-check function to assist the manual CB closing command. This ability is used as back-up to the similar function provided by the BCU-1 and it is only activated when voluntary selected by way of a switch. The synchro-check function allows closing the CB only if the synchronism is fulfilled or if some dead line/bus condition is satisfied. ER+DR event and disturbance recording to assist post-factum analysis. The breaker management relay on Q03 provides in addition the following functions: 50BF breaker Q03 failure protection. In case of Q03 trip failure, detected by both criteria, than after the set time delay Q01, Q02 CBs as well as remote line end CB are tripped.

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50EZ end-zone protection. This protection function detects faults between the CB and CT and trips Q02 and the 400 kV CB of AT-1.

Q51

Q55

Q54

TC11 0.5FS5

0.5FS5w2

5P30

0.5FS5

0.2SFS5w1

5P30w45P30w5

5P30

5P30

5P30w6

F217SA612

F87T.2

F87T.1

FBM 17VK611

FBCU-1AM17031

F21.27SA611

MET 1F212125685050HS5950N, 51N, 67N85FLERDR

F67N5050N, 51N, 67NERDR

FBM250EndF8579ERDR

F87BDRS

F67N7SJ621

F87BDRS-CBB

MET 2

FBM 27VK611

5P30

0.5FS5w3

5P30w7

TC31

TC21

TT 7

TT 8

TT1

TT2

TT5

TT6

W1 cl , 30VAw2 cl 0.5/3P,50VAw3 cl 0.5/3P,50VA

0.5

W1 cl , 30VAw2 cl 0.5/3P,30VAw3 cl 0.5/3P,30VA

0.5

CT-400 kVw3

CT-400 kVw2

Q53

Q56

Q52

Q91

Q92

Q81

Q82

Q57

Q58

Q1

Q13

Q31

Q23

Q32

Q2

Q03

Q02

Q01

3

4

BB1-220 kV

BB2-220 kV

Baia Mare

AT 1 - 400

LT

FBCU-2AM17031

FBM12579ERDR

W1

cl

, 30

VAw

2 cl 0

.5/

3P,5

0VA

w3

cl 0

.5/

3P,5

0VA

0.5

W1

cl

, 30

VAw

2 c

l 0.5/

3P,5

0VA

w3

cl 0

.5/

3P,5

0VA

0.5

F21.17SA611

F87B-400DRS

Figure 8. Typical 220 kV OHL protection system

For the AR function different philosophy can be used. One is to use single-pole AR on both Q01 and Q03 based on time discrimination. In this case, for SLG faults Q01 and Q03 are tripped single-pole and after the Q01 dead time the AR in FBM1 re-closes the Q01 CB. If successful, the Q03 CB is also re-closed by the AR function in FBM2. If Q01 is closed onto-fault the second trip is three-pole on both Q01 and Q03 and the AR cycle on Q03 is blocked. Another AR philosophy is to use single-pole AR on Q01 and three-pole AR on Q03. In this case Q03 is always three-pole tripped and time delayed reclosed assuming a successful AR cycle on Q01 and assuming that the synchro-check conditions on Q03 are fulfilled. The advantage is a constant AR dead time and less signals exchange among the different multi-

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function relays. A major limitation takes place if Q01 is out-of-service. In this case either the three-pole AR cycle is allowed or somehow the single-pole AR on Q03 has to be selected. For Iernut the latter AR philosophy is used and a typical time sequence for a transient SLG fault is shown by the disturbance record in Figure 9. The A-G fault is fast cleared by the distance protection function in zon1 extended by tripping pole A of Q01 and three-pole trip on Q03. The AR functions included in the FBM1 relay and FBM2 relays start their AR single-pole respectively three-pole dead time. About 1000 ms latter FBM1 issues the reclosing command and the Q01 CB recloses successfully, while Q03 CB is still three-pole opened. At about 3000 ms FBM2 issues the reclosing command on Q03 as a result of the successful reclosing of Q01. The reclosing command on Q03 is always checking the fulfillment on the synchronisation conditions. The total system dead time on phase A is about 1000 ms although the dead time for Q03 is about 3000 ms.

Figure 9. Operating of the 220 kV OHL protection on a A-G transient fault

The multi-function protection relay F21 on the OHL and the two AR functions included in each breaker management relay have to exchange some signals either based on wired logic or using the communication features of the relays. These signals carry out the following meanings (as shown in Figure 10):

Trip pole A, B, C (signal no. 1) to notify FBM1 and FBM2 in case of any pole trip. General Pick-up (signal no. 2) to notify FBM1 and FBM2 about protection pick-up at least on one phase. This signal is requested by the AR function to allow AR only for instantaneous trips. AR 3Pole Forced (signal no. 3) the AR function is not prepared for single-pole tripping thus any trip will result in a final three-pole trip.

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AR Z1 Extension (signal no. 4) signal intended to notify the F21 protection relay that the extension of distance zone 1 should be allowed.

In this case if at least one such signal is active, either from FBM1 or from FBM2 then zone 1 extension is allowed but only for single-pole faults and only before the AR cycle. Other conditions to be fulfilled are the CB Ready condition, the switch AR On condition and the CB Closed position related to the corresponding CB. If one CB is out-of-service, i.e. it was open at the AR start instant then this CB is not reclosed. If one of the AR switches is off (for Q01 or Q03) then the corresponding CB is final tripped three-pole ant no reclosing will take place on this CB. If one of the autotransformers protections or by the BBP or by the BFP trips Q03 then the AR function on Q03 is blocked (signal no. 5).

Trip Q01

Trip Q03

Trip Q02

Close Q01

Close Q03Q03

Q02

Q01

BB1-220 kV

BB2-220 kV

OHL

TR

21, 50, 85

21, 50, 67N

87T, 49

F21

F21

F87T

CBM.1

CBM.2

1

1

5

2

2

3

3

4

4

7925

7925

Figure 10. Example of different signals connection

3.2 Protection of autotransformers on the 220 kV side

As above mentioned, the protection system for the 400 MVA autotransformer AT-1 provides technological protections, two independent differential protections with thermal overload protection, a distance protection on the 400 kV side and one distance protection on the 220 kV side. The multi-function relay F21.2, connected to the 220 kV side is connected to the sum of currents delivered by CT31 and CT21 (please refer to figure 8) and provides the following protection and automation functions:

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21 distance protection function, five quadrilateral zones against phase and ground faults. It is common practice to use three forward zones, one reverse zone and one non-directional zone.

50+50N phase over-current and ground over-current protections, as back-up protection against phase and ground faults 67N1+67N2+67N3 directional earth fault protections, as ground back-up protection or against high resistance ground faults, looking forward, reverse and non-directional. 68 power swing detection. When a power swing condition is detected then zone 1 of the distance protection function is blocked to prevent an unwanted trip. 25 synchro-check function to assist the manual 220 kV Q02 CB closing command. This ability is used as back-up to the similar function provided by the BCU-2 and it is only activated when voluntary selected by way of a switch. The synchro-check function allows closing the CB only if the synchronism is fulfilled or if some dead line/bus condition is satisfied. ER+DR event and disturbance recording to assist post-factum analysis.

The protection system for the 200 MVA 220/110 kV autotransformer AT-3 provides technological protections, two independent differential protections with thermal overload protection, a distance protection on the 220 kV side and one distance protection on the 110 kV side. The multi-function relay F21.1, connected to the 220 kV side is connected to the sum of currents delivered by CT31 and CT21 (similar to the scheme in figure 8) and provides the following protection and automation functions: 21 distance protection function, five quadrilateral zones against phase and ground faults. It is common practice to use three forward zones, one reverse zone and one non-directional zone.

50+50N phase over-current and ground over-current protections, as back-up protection against phase and ground faults 67N1+67N2+67N3 directional earth fault protections, as ground back-up protection or against high resistance ground faults, looking forward, reverse and non-directional. 68 power swing detection. When a power swing condition is detected then zone 1 of the distance protection function is blocked to prevent an unwanted trip. 25 synchro-check function to assist the manual 220 kV Q02 CB closing command. This ability is used as back-up to the similar function provided by the BCU-2 and it is only activated when voluntary selected by way of a switch. The synchro-check function allows closing the CB only if the synchronism is fulfilled or if some dead line/bus condition is satisfied. ER+DR event and disturbance recording to assist post-factum analysis. The multi-function relay F21.2, connected to the 110 kV side of AT-3 provides the same protection functions as F21.1. The two multi-function differential protections with thermal overload protection are similar to those shown for AT-1.

3.3 Protection of the 220 kV generators All generators protections are located in the TPP, operate their own CBs and their protective system presentation are beyond the scope of this paper. To protect the overhead link to the step-up transformer of a generator one differential protection is provided. As back-up protection, one distance protection on each generator is provided. Both the BFP from the substation and that from TPP interact on the corresponding CBs. Thus, in case of the

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generator CB failure in the TPP, both adjacent 220 kV CBs in the substation are tripped, while in case of a Q02 CB failure, the 220 kV generator CB in the TPP is tripped.

3.4 Protection of the 220 kV bus-bars Each 220 kV bus-bar is protected by one BBP. Each 220 kV BBP is of a distributed arrangement type, using a CU and distributed FUs located in each 220 kV diameter. The connection between a FU and the CU is based on a double ring optical fiber and operation is achieved in a similar manner to the 400 kV BBP already described above.

4. Conclusions

As a consequence of large scale rehabilitation works in the RTN, different types of protection and control systems were put into service challenging the protection engineer. Different substation layouts and different SAS architectures require the protection engineer to improve the know-how in different application fields as protection, control, communication, information technology and maintenance.

Numerical multi-function relays present certain advantages such as: Factory pre-tested functions. This is an important support for protection engineers as

allows focusing on the overall protection system rather than to certain protection functions. Communication features that make possible the use of user-friendly software tools to

parameterize and configure the relay to the actual application and to extract event and disturbance recordings to help post-factum analysis.

Using of different protection functions to manage different system requirements. Thus, fewer devices are necessary and less wiring are to be done, improving dependability and security of the overall protective system.

Self-supervision that improves the reliability of the protective system due to an early alarm in case of on internal failure.

Multiple setting groups that allow handling different system conditions for instance bus-bar energising through the coupler.

Using of numerical testing devices that allow for a better, faster and more accurate testing of the protection system. The play-back of disturbance recordings is a powerful investigation tool in case of post-factum analysis.

The drawback of numerical multi-function relays result in: More skilled protection engineers are needed with strong knowledge in protection,

communication and information fields and this is hard today. Need of intensive factory acceptance tests that are time and money consuming. There

is always a tendency to reduce the time and financial support allocated to such tests and the result could be unpredictable for future protection system operation.

Large amount of diverse relay settings that are to be properly understood, implemented and tested. Many of numerical protection miss operations are due to miss understanding of certain settings or internal logic or due to poor protection system testing. However, testing of all protection functions is time consuming and this is to be kept in mind when scheduling the commissioning plan.