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Flexible grids protection schemes in ELIA vision: from traditional to intelligent ones Felicia Lazar, Member IEEE, Grégory Huon, Luc Uyttersprot

Abstract-- In this moment inside the structures of electrical power grids worldwide the building of future super-grids and smart grids is starting to take shape. The utility industry, including ELIA the Belgian TSO, is facing the need to be proactive towards the challenges introduced at transmission and distribution levels by the process of massive implementation of both centralized and distributed generation. The paper’s aim is to present studies in the protection and automation domain developed for new primary topologies to be deployed in the Belgian network and to cope with an increasing level of power flowing towards the transmission grid from decentralized generation installed in the distribution grid. The first study leads to the conclusion that an evolution from traditional towards flexible and adaptive protection and automation schemes is necessary in order to cope with potential system stability problems related to fault clearance time and power swing oscillations. The second study shows that the use of a flexible protection and automation scheme allows avoiding heavy investments in the primary system. The paper presents also some considerations about asset management strategy for a future deployment of these protection solutions in ELIA network. Index Terms— network protection assessment, multi-terminal connection, centralized generation, decentralized generation, flexible generation, active three ends topology, asset management

I. INTRODUCTION

T

HE structure of the electric power network in the European countries is nowadays facing the process of a dramatic change in order to be prepared to cope with the integration of both distributed and centralized generation. This preparation program represents also a concern for ELIA the owner and the operator of the Belgian Electrical Power System at the transmission, sub-transmission and distribution level (until 30kV) operating as an integrated part of the European interconnected systems inside the ENTSO-E organization (European Network of Transmission System Operators for Electricity). Schemes of primary topologies best fit to facilitate the integration of these generation were deeply analyzed in order to respond at two major demands: to F. Lazar, G.Huon and L. Uyttersprot are with ELIA Engineering –TGX, Culliganlaan1G, 1831 Diegem Belgium (e-mails: [email protected]; [email protected]; [email protected])

preserve the power system reliability and to have a reduced cost. From the system reliability point of view the chosen topology should not jeopardize the adequacy, the ability of the system to satisfy the customer requirements (power, energy) considering the system equipment outages possible to occur and in the mean time to preserve the security, the system ability to remain in operation after sudden disturbances as short circuits, loss of equipment etc [7]. The paper presents the analyze of 2 new designs of flexible protection and automation schemes, one applied to a new primary topology consisting of three active ends and a second applied to a new mode of operation for HV/MV transformer to cope with the increasing level of power flowing towards the transmission grid from decentralized generation installed in the distribution grid. The introduction of new protection schemes in an existing network demands a special attention from the asset management point of view, as, for instance, which will be the impact of these new protection schemes on existing and recent protection cubicles of concerned links. This has to be considered, taking into account the fact that the more and more generalized line differential protection, despite of its intrinsic advantages, has also the drawback of making the substation ends dependent. Also, how to best introduce a new centralized function like G-Flex in a substation. Those asset management related issues are debated in the last section of this paper. II. STUDY TO DETERMINE THE PROTECTION SOLUTION FOR MULTIPLE TERMINAL AND THREE ACTIVE ENDS CONNECTIONS The cost of a high voltage switching substation and the difficulties to obtain the legal rights for the way of transmission line were convincing enough that the tapping of high voltage line and cables are the best to serve the purpose of connection centralized/decentralized generation [1]. The problem of multiterminal lines resides in the difficulty of choosing and coordinating an appropriate protection solution for these primary topologies [3] and for this purpose in ELIA it was performed a study. The study goal was to establish the most dependable, secure, fast, sensitive, selective protection solution for two new primary topology to be deployed in ELIA network: multiterminal connected centralized generation and three active (with sources) end connection. The study was based on the comparison of several protection solutions possible to be

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developed in order to cover these primary topologies. The main comparison criteria considered in this study were: selective settings and action logic with respect to fault clearing time stated by ELIA requirements. A. Steps in the study development From the multiple terminal line topologies possible to be developed in ELIA network two were chosen to be studied: - the teed connected centralized generation, presented in Figure 1

Fig.1. Topology of a teed centralized generation connection

- the active three ends connection, presented in Fig. 2. It was identified that these topologies are possible to be deployed in networks with voltage levels in the range of 36 kV to 380kV.

Section C

Fig.2. Topology of a active three end connection

B. The proposed protection solutions Several protection schemes could be applied for the primary topology of teed centralized generation presented in Figure 1. Variant1: Main 1: Distance Protection with POTT logic carried for all three line end Main 2: Line Differential Protection for three ends Variant 2: Main 1: Distance Protection with POTT logic carried for two line ends with transfer trip commands sent from both these ends towards the third end. [5] Main 2: Line Differential Protection for three ends Variant 3: Main 1: three ends line differential protection

Main 2: three ends line differential protection Back-up protection: a distance protection, with zones set for remote faults. For the active three ends topology presented in Figure 2 it was determined as possible protection schemes the variant 1 and variant 3 presented above. Remarks related to variant 1: v The use of distance protection as a protection scheme for a three ends (terminal) lines is more complex than the application for two terminal lines because of the variety of tap location, line or cable impedances, source impedances, system loading requirements. An in-feed between the relay and the fault location will cause an under-reach effect in impedance measurement; the measured impedance seems to be greater and will cause the trip in a higher zone with a higher time delay, due to the current intermediate in-feed that will produce an additional voltage drop in the short-circuit loop. v A dynamic behavior of the POTT with 3 ends scheme could be expected. That implies that a non-simultaneity in reaching the signals from both opposite ends could jeopardize the trip of the third end in POTT logic. v Terminal C may be part-time with week back feed when generation is switched-off. For these cases the scheme should have active a weak-infeed supplementary function. For variant 2 an additional concern is that in case the overreaching zone from both line ends would not be able to detect a fault inside the protected topology the inter-trip signal will not be transmitted towards the third end. That will lead to additional delay in fault clearance (cascade trip expected). For variant 3, three terminals line differential protection for both main 1 and main 2 should be provided by different manufacturers. Using this protection the necessity of weak end in-feed trip with a voltage criterion validation would not be necessary. The distance protection representing a back-up protection will be active to operate only with time delayed distance zones. C. The network variables to influence the protection solutions For the primary topology of teed connected centralized generation 7 main categories of variables were identified: a) the voltage level in which such connections are foreseen to be implemented, b) the step up transformer (variable data): ® possible ratings, min/max ® voltage level HV and LV ® short circuit reactance min/max (reactance related on load tap position, if on load tap exists) ® the earthing connection mode on HV side, if any, c) the data related to the in-feeding network for A and B ends, source impedance maximum (complete configuration) and minimum d) the maximum and the minimum current in-feed related to the concentrated generation possible to be connected The electrical parameters of the generation, including subtransient reactance of the generator should be also treated as variables

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e) the data( electrical parameters) of the network element (OHL, cable, mixed cable and OHL) f) The variable location of the centralized generation branch connection point alongside the initial OHL, cable. g) The variable length of the derivation: short, medium, long in comparison with the main two end connection. For the active three ends topology another influencing factor would be the existence of parallel paths (AB, AC and/or BC)[2] Figure 3 presents the proposed case study with the variable electrical parameters. For the studied case the step-up transformer is grounded wye (particular case through a reactance) there will be a significant source of zero-sequence current. In this case the in-feed current (the current is also incoming through the ground reactance) does not affect the phase-distance elements; however the ground distance elements will be affected.

Z R = Z AL - Ph + mZ LB - Ph ( 1 + IT-ph /IL-ph) (5) Measurement error for this case: mZ LB - Ph · IT-Ph / I L-Ph (6)

Where: Zn-ph, Zn-E represents the phase impedance, respective earth fault impedance of the element n (n: portion of OHL, transformer..) as presented in Figure 3. In-Ph In-E represents the faulty phase current, respective the earth-fault current flowing through element n (n: portion of OHL, transformer…) IL1,IL0 represents the positive-sequence and zero sequence impedance of the line Additional difficulties are introduced by the underground cable: - non linear relationship between the measured reactance and the fault location - non-linear relationship between the measured resistance and the fault location - the difficulty of choosing the correct zero sequence compensation factor in mixed line-cables circuits should also be considered, having in mind the significant different Z1/Z0 relationship of cables compared with OHL. The magnitude of the distance protection measurement error for each of the three ends is depending of: o the current introduced by the in-feed: the greater the current the higher the measurement error, o the structure of the existing network connection where the branch is foreseen to be added, othe effect a pre-fault load will introduce in the distance relay displacement of the detected fault impedance. An additional important variable is represented by the fault: the fault location, the fault type and the presence or absence of fault arc resistance. In case of a fault with arc resistance an additional voltage drop across this resistance will appear. The Fig. 3 Case Study: the distance protection (overreaching zone) present study for the POTT logic was performed considering behavior that an independent distance zone is possible to be used as For the terminal A distance protection the measurement of extended zone (different as Z2). Otherwise the configuration of the network outgoing from all opposite ends and the phase to earth impedance is done with the equation (1): corresponding distance protection settings should be also considered when choosing the extended zone settings. (1)

ZR =

U APh - E I Ph - L + k e × I E - L

D. Analyze of the distance protection behavior with POTTcase study Considering Ke, the residual compensation factor, setting For the primary topology presented in Figure 3 it was adapted to the OHL characteristics (2): studied the possibility of choosing plausible settings for the Z L 0 - Z L1 distance protection overreaching zone used by POTT logic for Ke = (2) all three ends[4]. 3Z L1 The meaning of “plausible settings” is that the values the relation (1) can be presented as: chosen for the overreaching zone, phase-ground and phasephase impedance: reactance and resistance, residual (zero ×I T - Ph×+ IT - E×ke Z R = Z AL - ph + mZ LB - ph + m Z LB - ph (3) sequence) compensation factor should correctly detect all I L- Ph×+ I L- E×ke possible fault types with possible location on any place of this primary topology. This requirement should be fulfilled for the Measurement error(under-reach) for the case of single cases of system and topology variables considered as most representative and presented in Table 1. × I T - Ph×+ I T - E×ke phase fault: m Z LB - ph (4)

I L - Ph×+ I L - E×ke

For the case of a three phase fault (1) can be presented as:

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Network characteristics: 380kV, AB, LC portions: OHL Table 1. System and Topology variables for the case studied

calculation are presented in table 3. Table 3 Data of the studied active three end topology

Elemen l [km] r x t [Ohm/km] [Ohm/km] BL 1, 10 0.031 0.327 AL 29 0.031 0.327 Element r x ro/r CL 35, 50 0.031 0.327 Name [Ohm/km] [Ohm/km] BCBL, 150,033 0.031 0,33 0.327 6.36 AL, AC CL 30 0.031 0.327 AB 20 0.031 0.327

ro/r

xo/x

7.58 7.58 7.58 7.58 7.58 7.58

1.06 1.06 xo/x 1.06 2.761.06 1.06 1.06

For the study simplification it was considered that the OHL, AB and LC, have the same specific electrical parameters homogenous on the entire length, as presented in Table 2. Element Name T 490MVA, 20/380kV XNG Gen 480MVA

X [Ohm]*) 47.15

Xo [Ohm]*) 37.72

76. 71

30 -

.

*) reference to 380kV

Table2. Data of the network elements Using the PSS SINCAL software tool there were performed three-phase and single-phase short circuit calculation for different scenarios, mainly searching to detect those cases where the distance protection with POTT logic would not be able to detect and clear all the faults possible to occur on it. For this connection another behavior is particular: the impedance measured by the protection device may also decrease due to “negative in-feed” or “out-feed” when the current flows out of the feeder during an internal fault, as presented in Figure 4. For this case a reduction of the fault impedance occurred due to a parallel path in the short circuit loop.

Fig.4. Out–feed in case of an internal fault. Distance protection in B is “seeing” the fault in reverse direction

The system and topology data considered for the

Network characteristics: 380kV, AB, LC portions: cable As the calculation performed with the same software tool showed in the configuration presented in Figure 4 there is important parallel impedance due to the meshed grid. This fact leads to non operation of the POTT scheme. The distance protections in A and C will detect the fault in forward direction; side B distance protection however will “see” the fault in the reverse direction. In stead of a trip with the POTT system there will be a trip in first distance zone for the sides A and C. After the circuit breakers A and C trip the current flow in BB will turn in the opposite direction and there will be a cascade trip in B (distance protection zone 1) with an elimination of the fault. Due to this cascade trip the total elimination time of the internal fault will be higher than 100ms (requirement for 380kV overhead lines). E. Comparison of results, selection of the solution The calculation results prove the existence of numerous cases for which a safe setting value for the distance protection overreaching zone is very difficult to be chosen. Considering as error calculation: Z protection-Z fault real ε= × 100% , (7) where Z fault real Z protection: the fault impedance measured (“seen”) by the distance protection Z fault real: impedance up to the fault location, the results obtained showed an error variable from 50% to 250% for three phase faults and from 60% to 80% in case of single phase faults for the topology presented in Figure 1. The calculation results proved the known fact that in case of multi terminal lines the simple linear connection between fault impedance and distance to fault can not be assumed due to intermediate in feed (or out-feed). The results showed a variable zone reach that caused consequently problems to achieve sensitive zone settings for a dependable and secure fault clearance. Another aspect in setting the overreaching distance zone for both POTT configurations presented above is that a very large impedance value chosen for this zone would increase the possibility of incorrect operation in case of Variables Source impedance A (GVA)

30, 10, 5

Source impedance B(GVA) Branch connection point AL/AB (% )

30, 10, 5 6.25, 50, 90

Branch length CL/AB (%)

3, 62.5, 100, 125,150

Fault location from ZAC,ZAB,ZBC (%)

85-99.8

reverse faults in the close neighborhood of the connection. It is also necessary to be mentioned that for certain topologies of teed connection as equal braches accompanied by certain network conditions (as relatively balanced source

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impedances) a correct overreaching distance zone for all three ends is possible to be calculated, but, for the large majority of the cases the distance protection with communication scheme as POTT is not complying with the fault clearance requirements. For both studied configuration the most suitable: reliable, fast, sensitive and selective protection solution is: main 1(and main 2 in case of transmission network): three terminals line differential protection back-up: distance protection (remote back-up) only delayed stages. Other important advantages of using the differential protection solution[8] for such topologies are: o suitable to be extended with additional terminal devices in case a new branch proves necessary to be introduced in the already existing three ends primary topology. In this way the adaptability of this protection solution would be very useful for ensuring a reliable protection solution for the flexible smart grids of the future. For cases of more then three ends primary topologies the distance protection with communication scheme would be even more difficult to set due to additional variable in-feed. o a fault clearance time substantially reduced in comparison with distance protection with communication schemes (as presented afore). This fact will significantly contribute at the enhancement of the transient stability of the network area where the fault occurred [7]. o immune to power swing phenomena. For that reason no additional blocking logic is necessary as in the case of distance protection [6]. This could represent a useful feature in case of teed connected centralized generation that could face active power oscillation in relation with the interconnection network. III. FLEXIBLE ACCESS CONTRACT FOR DECENTRALIZED GENERATION (GFLEX) Until now, the maximum allowable decentralized generation for a HV/MV substation has been based on the N1 criterion. Due to the rapidly growing of the decentralized generation, this capacity is already reached in several substations. Based on the fact that the occurrence of a N-1 situation is quiet low, one possibility to quickly increase this capacity consists in abandoning the N-1 criterion for the new decentralized generators which exceed the maximum allowable capacity based on the N-1 criterion. The idea is to use the total installed transformation capacity in the HV/MV substation instead of only 50 %. This could almost – in theory - double the capacity for decentralized generation without heavy investment. In case of an N-1 situation, the decentralized generators with a flexible contract will be automatically tripped to avoid severe overloads of the grid whereas the decentralized generators with a traditional

contract will stay in service. The withdrawing of the N-1 criterion for decentralized generation requires adaptations of the protection scheme used on HV/MV transformers and new automatic functions. Nevertheless, in spite of these adaptations, the protection scheme will also limit the allowable capacity for decentralized generation. These points are described further in the paper A. Operation of the HV/MV transformers in Elia with decentralized generation Figure 5 presents the 2 different types of operation of the transformers within Elia TSO, namely transformers working in parallel and transformers working in solo. In both cases, the load can be fed by one transformer. With transformers working in parallel, in the case of a faulty transformer, the load will be fed by the remaining transformer without any interruption. Solo operation

Parallel operation

HV

HV

MV

MV

Load

Load Gtrad GFlex

Gtrad GFlex

Fig. 5 Operation of the HV/MV transformers in Elia

With transformers working in solo, one transformer is in service and the second is in standby. In the case that the transformer feeding the load is unavailable, the transformer autoclose function will switch on the standby transformer. In both cases, the N-1 criterion is fulfilled for the loads. Regarding decentralized generators, we have to distinguish between the first installed generators which are using the allowable capacity according to the N-1 criterion and the later installed generators exceeding the allowable capacity. The first ones have a traditional contract with the guarantee to export power to the transmission grid via the remaining transformer in case of N-1 situation. In the case of parallel operation of the transformers, the power generated will be exported to the transmission grid without any interruption. With solo operation, the decentralized generators connected to the faulty transformer will be tripped in a first stage by their anti-islanding protection. In a second stage, they will reconnect to the grid. The later installed generators have a flexible access contract which means that they will be automatically cleared in a N-1 situation and put out of service until the healthy situation is back. The level of decentralized generation with a traditional

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contract and with a GFlex contract is evaluated as follows : § Maximum allowable power in the transfo towards transmission grid = 0,9 * Pnominal transfo; this is a thermal limitation of the tap changer. § Maximum decentralized generation with a traditional contract : 0,9 * Pn transfo + minimum load (see fig. 6);

1. Directional phase overcurrent protection

Max. allowable decentralized generation with traditional contract 50,00

Power [MW]

40,00 30,00

Load profile

20,00

Min. load

10,00 Gtrad,t = Min. load + 0.9 x Pnominal transfo

0,00 -10,00

1

96 Max. allowable power to the transmission grid (0.9 x Pn transfo)

-20,00 -30,00

Fig. 7 Fault on the HV side fed by the other transformer

-40,00 -50,00 1/4h (one day)

Fig. 6 Maximum allowable decentralized generation

§

Maximum decentralized generation with a GFlex contract : 0,9 * Pnominal transfo § Total maximum allowable decentralized generation : 1,8 * Pnominal transfo + minimum load § Depending on local circumstances, this maximum allowable decentralized generation with a GFlex contract can be lower (see below) B. Impact on the protection The settings used by the protections of the transformer will limit the maximum allowable power flowing in the transformer towards the transmission grid. There are several protection schemes used on distribution transformers within Elia depending on the short-circuit voltage of the transformer, the coupling of the transformer and the operation of the transformers – parallel or solo as described in table 4 : Yd11 transformer, Usc 20%, parallel operation Internal protections (Buchholz) Transformer differential protection Time-delayed overcurrent protection (phases and neutral) at the primary side – back-up protection Time-delayed combined overcurrent and undervoltage protection at the secondary side – back-up protection for faults in the MV network Time-delayed overcurrent protection at the secondary side – back-up protection for faults in the MV network Time-delayed zero-sequence overvoltage at the primary side – backup protection for single-phase faults at the primary side feed by the decentralized generators Time-delayed directional phase overcurrent protection at the secondary side – back-up protection for multiplephase faults at the primary side feed by the decentralized generators

Yd11 transformer, Usc 10%, solo operation

X

X

X

X

X

X

X

-

-

X

X

X

X

-

Table 4 Protection schemes for HV/MV transformers

This protection is used on transformers with parallel operation. This is a back-up protection for faults at the HV side of the transformer fed by the other transformer (see fig. 7). It is based on a directional overcurrent relay installed at the MV side of the transformer. The setting of this relay is 0.9 nominal current of the transformer with a time delay of 0.8 s. There is clearly a conflict between these settings and the allowable power flowing towards the transmission grid which is also 0.9 nominal power of the transformer. 2 situations require attention: § No fault, power flowing to the transmission grid = 0,9 * Pnominal in each transformer In that case, the directional overcurrent protection will be activated and both transformers will trip ! This situation is unacceptable because the loads will not be supplied anymore. To solve this problem, we can increase the setting of the current which will reduce the sensitivity of this protection. In cases where this reduction is not acceptable, we have to reduce the maximum allowable decentralized generation for example down to 0,6 * Pn of each transformer. § Faulty transformer, power flowing to the transmission grid ≥ 0,45 * Pn in each transformer before the occurrence of the fault In this case, after the tripping of the faulty transformer, all power flowing towards the transmission grid will go via the healthy transformer. This situation will activate the directional overcurrent protection and a tripping of the transformer will occur after 0,8’’. To avoid this tripping, we have to trip the decentralized generators with a GFlex contract before the time delay of this protection. This tripping will be executed by an automatic function based on the detection of an overcurrent flowing towards the transmission grid in the transformer. 2. Time-delayed overcurrent protection at the secondary side – back-up protection for faults in the MV network This is a back-up protection for faults in the MV network used by transformer with Usc = 10 %. The settings are 1,8 of the nominal current of the transformer with a time

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delay ≥ 1s. During a healthy situation, there is no risk with this protection because the current flowing towards the transmission grid in each transformer is always lower than 1,8 of the nominal current of the transformer. In case of an internal fault in a transformer, after the tripping of the faulty transformer, all power flowing towards the transmission grid will go via the healthy transformer and will activate this protection and a tripping of the transformer will occur after the time delay of this protection. This is another reason to trip the decentralized generators with a GFlex contract before the time delay of this protection expired. 3. Time-delayed combined overcurrent and undervoltage protection at the secondary side – back-up protection for faults in the MV network This is a back-up protection for faults in the MV network used by transformer with Usc = 20 %. It is described in fig. 8. The settings are 0,9 of the nominal current of the transformer, 0,8 of the nominal voltage with a time delay equal or greater than 1’’. During a healthy situation, with the maximum allowable infeed in the transmission grid, the overcurrent relay will be permanently activated which is not allowable. To solve this problem, the setting of this relay will be changed to a higher value without compromising the sensitivity of this protection

lower than local functions. As a general rule, local functions will be implemented when speed is of high importance. § Several approaches are available for the detection of the N-1 situation : o Topologic recognition in the substation(s) : simple when the transformers are between bus bars, complicated for transformers in antenna because several signals have to be read at remote locations; security problem; o Overload detection on the transformer; this overload needs to be directional because overload capacity of the transformer from HV to MV is much higher than in the opposite direction : 130 % from HV to MV but only 90 % from MV to HV. o Topologic recognition at the remote control center : simple, cheap but slow; Based on the requirements – high speed, universal solution – the new function will be locally implemented and will use a directional overcurrent protection. The figure 9 below shows an example of this function applied to the case of parallel operation of the transformers. As the healthy transformer can be highly overloaded in case of an internal fault in the other transformer (up to 180 % of the nominal power of the transformer), a back-up tripping of the transformer after 2’’ by this function has been added.

Fig 8 Back-up protection for faults in the MV network

In case of a faulty transformer, after the tripping of the faulty transformer, all power flowing towards the transmission grid will go via the healthy transformer. This situation will activate this protection and a tripping of the transformer will occur with the time delay of this protection. This is another reason to trip the decentralized generators with a GFlex contract before the time delay of this protection expired. C. New automatic function The integration of more decentralized generation requires a new high speed automatic function to disconnect the decentralized generators with a flexible contract in case of an N-1 situation. Several possibilities are available and have been investigated: § Local or remote automatic functions? Remote automatic functions are usually cheaper to install then local functions because in the first case it is only software whereas in the second case, wiring is required. On the other side, speed of remote automatic functions will be

Fig 9 Automatic function to disconnect the decentralized generators with a flexible contract in case of an N-1 situation

Security is an important issue for this function because the detection of the N-1 situation takes place at the transformer level but the actions – tripping of the decentralized generation with a flexible contract - are remote. To minimize this risk, emphasis has been put on the ergonomics of this new function : the function will be installed outside the protection cubicle of the transformer, all functions will be grouped together in a new cubicle and a voltage criterion has been added to avoid to trip the generators in case of tests at the transformer level.

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IV. ASSET MANAGEMENT CONSIDERATIONS The introduction of line differential protection on an existing link requires: · the availability of a high quality private communication path between the different ends of the concerned links. Accordingly, a well adapted policy of communication network development has to been drew up, as clear rules for redundancy for these communications paths, depending on the requirement of the grid code at the considered voltage · the compatibility between the different current transformers at the circuit ends, in order to be able to avoid saturation problems and therefore, unwanted tripping by the line differential protection for an external fault. Generally within Elia, primary systems adaptation are not performed to implement standardized secondary systems solutions, except for protection requirements for the concerned links or in case the concerned substation are integrated in a replacement program for obsolescence reason · a probable adaptation of the protection cubicles at circuit ends where it was not initially planned to work, in case these protection cubicles are not equipped with line differential protection or with a generation that is no more compatible with the new installed ones. The retrocompatibility of line differential protection between the Elia successive frame agreements (duration to 8 years) is a point of discussion between Elia and its suppliers. This retrocompatibility can be of two types: the full retrocompatibility between a generation n and a generation n+1, or at least the mechanical retrocompatibility (the capability to be able to put a bigger line differential protection from another supplier in place of a smaller eg distance or line differential protection) At long term, a telecommunication network with high performance - in terms of latency and asymmetry - could allow to avoid this compatibility problem (eg IEC61850 one box solution – like eg a transformer differential protection - with sampled values exchanges between ends) Elia has developed a substation approach for replacement programs and wants therefore to limit as much as possible the adaptation in protection cubicles at other ends to a light scope, eg to avoid the replacement protection cubicle when it is not strictly necessary. Indeed, the replacement of a protection cubicle in an existing substation could lead to other unwanted replacements because of interfaces with the common parties for instance, and therefore, disturbs the priorities established - by the mean of a risk matrix - in the replacement program. The replacement of recent protection cubicle is also not desirable because of financial impact (amortization period of secondary assets is quite high within Elia network). All the considerations above show that a good balance between the protection requirements of special topologies in the network and asset management constraints is required. This has to be translated in a good asset management policy.

Also, the introduction of a new centralized function like GFlex in a substation needs a good strategy regarding its execution mode. Elia has generally frame agreement contracts for primary and secondary equipments installed in her network. In particular, Elia has frame agreements with two different suppliers for the protection cubicles at bay level and one for the substation control level (DCS). The introduction of new functions is best realized by the means of existing and revised (when contractually possible) frame agreements. This in order to facilitate the frame agreement management and avoid to increase the number of contracts. A well adapted purchasing policy has to be put in place for the supply of all secondary equipments, taking into account the evolution of the technologies and its impact to the global architecture of secondary systems. Finally, a roadmap has to be drawn up in order to give visibility to the stakeholders to all these changes that are coming in the network. Indeed, engineering and maintenance departments are directly impacted by these changes and need therefore to have a good view on what they have to engineer or to test. Well adapted standards management policy, giving some stability perspectives, can help to aim this objective. V. CONCLUSIONS The Belgian electrical power network as the power systems worldwide is facing a permanent extension. The introduction of competitive energy market and the demand of a decarbonized power generation are enforcing this development. As presented in the present paper the in-feeds of distributed and centralized renewable generation are introducing new challenges in the domain of protection and automation schemes requiring meanwhile an increased transmission capacity and stability of the system. Approaches developed by ELIA to find reliable solution for these problems are presented in this paper, taking into account asset management considerations. VI. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

P.M. Anderson, Power System Protection, IEEE Press 1999 Gerhard Ziegler Numerical Distance Protection- Principles and Application CIGRE brochure 064 Application guide on Protection of Complex Transmission Network configurations CIGRE brochure 359 Modern Distance Protection Functions and Applications Manuel Delgado : Introduction au domaine des Protections et des System des Protections des Réseaux Electrique Belges CIGRE. CIGRE Brochure 465: Modern Techniques for Protecting and Monitoring of Transmission Lines D. Tziouvaras, “Relay Performance During Major System Disturbances”, SEL, Inc 2006. Gerhard Ziegler Numerical Differential Protection- SIEMENS 2005

VII. BIOGRAPHIES Felicia Mihaela Lazăr is from the beginning of 2011 Senior Expert Secondary Systems in ELIA, the Belgian TSO, in the department Technical Governance and Expertise, TGX.

9 Until that date she was Manager for System Safety Romanian Power Grid Company –Transelectrica Bucharest – Romania. She has an engineer diploma from 1981 when graduating from University “Politehnica” Bucharest, Electric Power Engineering Department. She has activated in the Protection and Automation Department of National Dispatch Center since 1984. She is author and co-author of more then 15 papers related to Romanian EPS stability measures and defense plan. Grégory Huon was born in 1977. He received degrees of Master of Engineering (Electrical engineering, Institut Supérieur Industriel de Mons, Belgium – 2000) and Master of Science (IT and management, Faculté Polytechnique de Mons, Belgium – 2003). He began his career in the Transmission Network field in 2000 and is specialised in infrastructure project management, asset management and secondary systems expertise. Gregory is at present head of Secondary Systems department (technical governance & expertise level) within Elia, TSO in Belgium, and is active member in different technical committees, especially the Belgian mirror committee B5 (Protection and Automation) of Cigré. Grégory published recently different papers regarding the Elia’s strategy over IEC61850 standard implementation. Luc Uyttersprot was born in 1959 in Brussels, Belgium. He got his Master in Electromechanical Engineering from the University of Brussels (ULB) in 1983. From 1986 he has been working as an expert for the Belgian transmission network. Today he is senior expert for protections and substation automation within Elia, the Belgian TSO. He is actually the Belgian member of the Study Comity B5 of Cigre and also convenor of the Cigre B5-23 Working Group “Short circuit protection of circuits with mixed conductor technologies in transmission networks”. .