Overview of electrical protection requirements for integration of a smart DC node with bidirectional electric vehicle charging stations into existing AC and DC railway grids

Electric Power Systems Research 122 (2015) 104–118

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Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr

Overview of electrical protection requirements for integration of a smart DC node with bidirectional electric vehicle charging stations into existing AC and DC railway grids F. Sanchez-Sutil a , J.C. Hernández a,∗ , C. Tobajas b a b

Dept of Electrical Engineering, Jaén University, Campus Lagunillas s/n, Edificio A3, Jaén 23071, Spain ADIF (Spanish Railway Infrastructure Manager), Strategy Management and Development, C/Titan 8, Madrid 28045, Spain

a r t i c l e

i n f o

Article history: Received 21 March 2014 Received in revised form 30 December 2014 Accepted 2 January 2015 Keywords: Protection Electric vehicles Railways Distribution networks Photovoltaic power systems Power system protection

a b s t r a c t Distribution network operators (DNOs) and railway traction system operators (RTSOs) who will connect bidirectional electric vehicle charging stations (EVCSs) (treated as both load and source) to their power networks need a unified set of requirements for safe operation. However, such requirements are currently unavailable. Accordingly, the authors in cooperation with the Spanish national RTSO and the most important Spanish DNO have elaborated a unified regulatory framework of requirements for the interconnection protection systems and the earthing arrangement of a DC node used as a reference to feed bidirectional EVCSs, now under construction. This node connects a 0.4-kV AC secondary distribution network (SDN), a 25-kV AC railway traction system (RTS), a 3-kV DC RTS, a local distributed resource (DR) system, and two bidirectional EVCSs. The DR system includes both a photovoltaic PV system as well as backup storage systems (battery and supercapacitor). Thus, the DC node has all the potential interconnections to serve as a reference for requirements regarding the interconnection of bidirectional EVCSs. This unified regulatory framework is the result of a critical review which enabled us to modify, harmonise, and adapt requirements in a wide range of grid-interconnection standards and codes as well as company operation practices to the specific behaviour of bidirectional EVCSs. This is particularly important for RTSs where the interconnection of this reference DC node significantly changes the protection practices of RTSOs and the RTS earthing arrangement. This unified framework is proposed as a technical specification for the companies involved in the implementation of this type of DC node to feed bidirectional EVCSs. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aim of the study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Configuration of reference DC node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interconnection systems in the reference DC node and their functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical protection requirements for the grid-interconnection of the reference DC node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Detection of faults in the AC/DC RTS or SDN and insulation of the reference DC node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Islanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Coordination of RTSO and DNO reclosing practices and the reference DC node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Reconnection to the RTS (SDN) of the reference DC node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Grid-interconnection of the reference DC node in the 25-kV AC RTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. 25-kV AC RTS protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Electrical protection for the reference DC node grid-interconnection in the 25-kV AC RTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Earthing and safety protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +34 953212463; fax: +34 953212478. E-mail address: [email protected] (J.C. Hernández). http://dx.doi.org/10.1016/j.epsr.2015.01.003 0378-7796/© 2015 Elsevier B.V. All rights reserved.

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7.

Grid-interconnection of the reference DC node in the 3-kV DC RTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. 3-kV DC RTS protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Electrical protection for the reference DC node grid-interconnection in the 3-kV DC RTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Earthing and safety protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Grid-interconnection of the reference DC node in the 0.4-kV AC SDN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Electrical protection for the reference DC node grid-interconnection in the 0.4-kV AC SDN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Earthing and safety protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Grid-interconnection of the bidirectional EVCSs, backup storage system, and PV system with the 750 V-DC bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. Electrical protection for the grid-interconnection of the bidirectional EVCSs, backup storage system, and PV system in the 750 V-DC bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Earthing and safety protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Electric vehicles (EVs) reduce the CO2 emissions generated by the transportation sector [1–3]. However, their large-scale use involves the massive implantation of EVCSs in traditional distribution networks with numerous associated grid impacts [4–6]. Furthermore, it is expected that this implantation will include RTSs [7–10]. The first challenge arises from the fact that transportation movements end at approximately the same time as the peak electricity demand. There should thus be a demand side management focused on encouraging users to recharge EVs at off-peak times, when the power demand is lower [4,6,11–13]. Although EVs now only store power from the grid (grid to vehicle, G2V), the second challenge will surface when EV batteries must also deliver power to the grid (vehicle to grid, V2G) to provide ancillary services to the power system [14–16]. Furthermore, EVCS should be treated as both load and source, with bidirectional power flow capacity (i.e. DR). Therefore, the limited hardware requirements and simplified interconnection issues for unidirectional EVCSs do not apply. The third challenge is related to the heterogeneous sources that generate power for EVCSs. The reduction in CO2 emissions should be based on the inclusion of renewable power in these stations [1–3] (e.g. PV power or ‘renewable’ power, such as regenerative braking power in RTSs [7,17]). Despite the fact that such power is relatively easy to obtain, its integration into the network is problematic because of its intermittent behaviour and also because production is difficult to predict. In this context, the definition of a DC node used as a reference to feed bidirectional EVCSs in the SDN and RTS context requires a combination of new technologies, the optimal exploitation of existing infrastructure, as well as changes in the operation and protection practices of power companies. To make this a reality, the FerroSmartGrid project [18] is now developing new products and services for this type of DC node. The reference DC node defined connects the following elements: (i) a 0.4-kV AC SDN; (ii) a 25-kV AC RTS; (iii) a 3-kV DC RTS; (iv) a local DR system including PV power and backup storage systems (i.e. battery and supercapacitor); and (v) two bidirectional EVCSs. FerroSmartGrid lays the foundations for the safe, efficient, and smart interconnection of complementary AC and DC sources which feed bidirectional EVCSs, thanks to a reference DC node. The research presented in this paper is a part of this project which focuses on the elaboration of a unified set of requirements for interconnection protection systems and the earthing arrangement of this reference DC node. This will guarantee its safe use. Section 2 of this paper discusses the aim of the study. Section 3 presents the configuration of the reference DC node, where the research was performed, and Section 4 describes the functions of an interconnection system. Section 5 presents the electrical protection

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requirements, specifically adapted and harmonised to the reference DC node, Sections 6–9 discuss the requirements for each interconnection protection system and earthing arrangement of the reference DC node, namely, 25-kV AC RTS, 3-kV DC RTS, 0.4-kV AC SDN, bidirectional EVCSs, a backup storage system, and PV system. This proposal targets protection relays, their settings, and earthing requirements. For interconnections to RTSs, the relevant network protection is first described since careful coordination is required. Finally Section 10 lists the conclusions that can be derived from this research. 2. Aim of the study The interconnection of bidirectional EVCSs (i.e. DRs) to SDNs and AC/DC RTSs can only be implemented when safe network operation is guaranteed by suitable electrical protection requirements. These requirements constitute the electrical interconnection protection that allows the SDNs and AC/DC RTSs to operate safely and is often viewed as the single most important technical requirement [19]. It is vital to distinguish interconnection protection from network protection and EVCS protection. Currently, the interconnection of bidirectional EVCSs must be approved by the DNO or RTSO as the case arises. Because of the recent proliferation of EVCSs, the DNO or RTSO now has greater responsibility, and would doubtlessly benefit from a unified set of requirements for the interconnection protection system and the earthing arrangement if they were available. In addition, when an intermediate DC power bus is required, its security should also be guaranteed. An EVCS is connected to SDNs or AC/DC RTSs by means of an AC/DC or DC/DC converter. Nevertheless, when more than one source feeds the EVCS, as in the case of the interconnection of different DRs [20], an intermediate DC power bus is advisable for the interconnection. Many references focus on the monitoring, operation, and power sharing in a DC power bus [20–24]. The connections of EVCSs usually have unidirectional power flow capability, but the new V2G approach [14–16] requires that this capability be extended to a bidirectional power flow. Bidirectional capability is the usual condition for the connection of several sources in an intermediate DC bus [20]. Despite their importance, there are currently no electrical protection requirements for the grid-interconnection of bidirectional EVCSs (i.e. DRs). Thus, in regards to SDNs, recent regulations [25,26] only include requirements to protect EVCSs from internal faults and abnormal internal operating conditions. This is the EVCS protection. The reason for this is that EVCSs have always been regarded as passive loads. This approach is also followed in the only European standard available (series IEC 61851 [27]), which is the only one that deals with EVCS protection, among other issues. Therefore, these requirements [25–27] are insufficient for the grid-interconnection

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Fig. 1. The reference DC node with interconnections to different networks along with the local DR system and bidirectional EVCSs.

of bidirectional EVCSs. The same approach to SDN regulations is followed in recent regulations for the grid-interconnection of EVCSs to RTSs, e.g. in 25-kV AC RTS [28] or 3-kV DC RTS [29]. In regards to the grid-interconnection of distributed generation (DG) or DRs to RTSs, no codes or standards are available. However, for SDNs, there is a wide range of codes (e.g. [30–35] for DG/DR or [36] for PV-DG) and national standards (e.g. [19,37–46] for DG/DR or [47–51]for PV-DG) that regulate interconnection protection. Nonetheless, the arrangement of interconnection protection systems and the associated settings vary significantly, depending on factors such as DG/DR type and size, interconnection transformer configuration, earthing arrangement, and the network protection scheme. After analysing documents [19,30–51] that could help the DNO/RTSO to define protection requirements for the gridinterconnection of bidirectional EVCSs (regarding SDNs, AC/DC RTSs, DC power bus), marked differences were detected, and even worse, a wide range of inconsistencies were found when requirements were applied to the grid-interconnection of bidirectional EVCSs. The reason for this lies in the significant technological differences between each DG/DR class. This was worse for interconnections with RTSs, as most documents are only applicable to distribution networks. Thus, there were difficulties in the harmonisation of RTS protection practices and earthing arrangements. This lack of clarity led the authors in cooperation with the Spanish national RTSO and the most important Spanish DNO to elaborate a unified set of requirements for each interconnection

protection system and earthing arrangement of a DC node used as a reference to feed bidirectional EVCSs. For this purpose, we performed a critical review. Based on this analysis, we modified, harmonised, and adapted requirements in a wide range of gridinterconnection standards and codes as well as company operation practices to the specific behaviour of bidirectional EVCSs. These requirements also accounted for the impact of different sources [52–54] on SDNs, as well as the impact of EVCSs [4–6] on SDNs and on RTSs [55]. The critical review and the subsequent elaboration of a unified set of requirements were carried out in the framework of the FerroSmartGrid project [18]. 3. Configuration of reference DC node A DC power bus can potentially offer a more efficient and reliable grid-interconnection for networks at different frequencies (e.g. SDN and AC RTS in comparison to DC RTS). In addition, the connection to these networks of a local DR system (e.g. PV power and backup storage systems), and bidirectional EVCSs (i.e. DRs) with this arrangement obtain the same benefits. This is because the DC power bus, as opposed to the AC bus, reduces the power conversion stages for interconnections and connection, which thus increases the overall efficiency of the system [20,23]. Furthermore, the paralleling of multiple sources onto a DC bus is more straightforward than for an AC bus since tight frequency regulation is not required [20,23]. Fig. 1 shows the reference DC node connected to an external 0.4-kV AC SDN, 3-kV DC RTS, and 25-kV AC RTS with bidirectional

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power flow capability. The node is also equipped with local backup storage systems. These are all connected to the DC bus with bidirectional power flow. The PV system connected to the DC bus has a unidirectional power flow. All interconnection powers are rated at 50 kW. The power management strategy is geared to controlling the power from different sources so that the load demand of bidirectional EVCSs can be satisfied with the least possible cost, while giving the highest priority to PV and regenerative braking power.

4. Interconnection systems in the reference DC node and their functions An interconnection system is the means by which the reference DC node is electrically connected to the 25-kV AC RTS, 3-kV DC RTS, or 0.4-kV AC SDN at the point of common coupling (PCC) (Fig. 1). Interconnection system functions are the following [19]: synchronization, power source transfer, metering and monitoring, control and dispatch/communication, and electrical protection. Nonetheless, various internal interconnection systems are defined in the reference DC node with only electrical protection functions. Technical requirements pertaining to the interconnection systems of the reference DC node can be classified in three groups [19]: (i) general requirements; (ii) power quality requirements; and (iii) electrical protection requirements. Electrical protection requirements in the interconnection systems of the reference DC node were the focus of this research. Also studied were the general requirements for earthing in the DC node.

5. Electrical protection requirements for the grid-interconnection of the reference DC node Although standard IEEE 1547.2 [19] does not cover the reference DC node (AC/DC RTSs are not within its scope), its adaptation allowed us to establish that electrical protection requirements should protect the AC/DC RTS and AC SDN so that its equipment would not be damaged by the reference DC node. They also protect utility workers from exposure to unnecessary hazards. Such requirements concern the response of the reference DC node to abnormal conditions in the AC/DC RTS and AC SDN. Thus, they focus on the following functions: (i) the performance of the reference DC node when faults occur in the AC/DC RTS or AC SDN; (ii) coordination between the reclosing practices in the AC/DC RTS or SDN and the performance of the reference DC node; (iii) reconnection of the reference DC node to the AC/DC RTS or SDN. Given the absence of specific standards or codes and the lack of company operation practices for the grid-interconnection of bidirectional EVCSs, the next sections discuss how these functions can be achieved for each interconnection protection system of the reference DC node. In our study, this was accomplished by the harmonisation of network protection practises at each gridinterconnection. 5.1. Detection of faults in the AC/DC RTS or SDN and insulation of the reference DC node According to the adaptation of standard IEEE 1547.2 [19], the interconnection protection systems of the reference DC node should detect and respond to abnormal conditions (e.g. faults and unintentional islanding) in the AC/DC RTS or SDN. There are two methods of detecting such conditions, both of which stem from the fact that a fault reduces the apparent system impedance. The first method detects the impedance reduction in an overcurrent at the PCC, whereas the second method detects it as a reduction in voltage at the PCC.

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The usual method for detecting faults in the interconnection protection systems, based on overcurrent principles, is not applicable to the reference DC node because interconnection converters cannot produce or sustain significant fault currents during fault conditions in the AC/DC RTS or SDN (a maximum of 1.2–1.5 times the rated current) [51,56]. Consequently, in our proposal, we established that fault detection for the interconnection protection systems of the DC node should be based on passive methods for detecting the loss of mains (LOM). These included voltagebased fault detection (under/overvoltage relay 27/59 in RTSs and SDN and under/overfrequency relay 81U/O in the AC RTS and SDN). This proposal is in accordance with many gridinterconnection standards for DG/DR in distribution networks [19,30,32–34,39,44,46,47,51], which state that when a fault occurs in the grid and the monitored DG/DR parameters exceed acceptable limits, the interconnection protection system must stop energizing the grid (see Table 1 for the most important standards). However, in the context of RTSs, available standards [28,29] do not regulate behaviour in the case of network (AC/DC RTS) faults. Our review only detected interrelating requirements for RTSs in the IEC 60850 standard [57] (supply voltages), which only specifies minimum and maximum voltages/frequencies that should not be exceeded during specific time periods (see Table 1). Also taken into account was the standard for SDNs (EN 50160 [58]). The settings of both standards should be compatible with the protection settings proposed. According to our findings, the internal interconnection systems of the reference DC node (Fig. 1) should also detect faults in the 750-V DC power bus by means of the voltage-based fault detection (under/overvoltage relay 27/59) although this interconnection is not regulated by IEEE 1547.2 [19]. However, the response to DC bus fault conditions was required because of connection converters. As shown in Table 1, voltage and frequency settings and disconnection times (tD ) differ considerably. The reason for this is that each DNO must adjust disconnection to the specific characteristics of the distribution network. It should be underlined that there are currently neither grid-interconnection standards for DG/DR in RTSs nor any settings for protection against abnormal conditions in the RTSs in Table 1. Nonetheless, for all interconnection protection systems (25-kV AC RTS, 3-kV DC RTS, 0.4-kV AC SDN), this research established a 0.3 s disconnection time (Table 1) for effective islanding detection (see Section 5.2) as well as the rapid disconnection of the reference DC node before a quick reclosing attempt (see Section 5.3). Delayed disconnection times may significantly hinder and disrupt islanding detection and reclosing practices. In contrast, fast disconnection times may increase nuisance trips if phase-timeovercurrent relays (51) are used in network protection (RTSs or SDN). Moreover, a single maximum voltage setting, rated at 110%, (85% minimum) was established for the two RTSs, and the SDN (Table 1). This detects light as well as severe faults and is also compatible with settings in supply voltage standards. This voltage setting requirement was also established for internal interconnection protections (750-V DC bus, Table 1) since voltage variation resulting from the DC node control strategy is lower (±5%). The frequency setting specified (AC RTS and SDN) was closer to the stricter threshold of the grid-interconnection standards analysed for effective islanding detection (Table 1). The detection of earth faults depends on the winding configuration in the interconnection transformer. Unearthed primary interconnection transformers (e.g. delta-floating wye in the 3-kV DC RTS or floating wye-delta in the SDN) did not contribute to earthing current.

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Table 1 DG/DR response to abnormal voltage and frequency at the LV and MV level in distribution networks along with the supply voltages for RTSs and the settings for the response of the reference DC node to abnormal conditions in different networks.

Remarks: a Disconnection time. b Maximum disconnection time (DG ≤ 30 kW), default disconnection time (DG > 30 kW). c According to ER G47, d DG > 30 kW. e MV. g Level required by Spanish royal decree 1663/2000 [72]. h LV. m DG < 150 kVA. k The typical lowest value. p Apply to AC RTSs and SDNs.

5.2. Islanding Based on our results, the unintentional islanding operation of the reference DC node in the SDN or AC/DC RTS was not permitted for reasons of safety and reliability. This is in consonance with many grid-interconnection standards regarding DG/DR in SDNs [59]. In the context of RTSs, interrelated standards also prohibit the unintentional islanding operation, e.g. standard for connecting unidirectional EVCSs in AC RTSs [28] (DC RTSs [29]) and for regenerative braking [60]. There are passive and active LOM detection methods for islanding detection. The passive ones can fail in the surroundings of the load-generation equilibrium of the islanded part of an RTS after a fault. The probability of this event is insignificant given the low-null penetration level of bidirectional EVCSs in such systems. However, the lower-rated power of the SDNs increases this probability. Active methods may thus be recommended in such networks [32,40,43,44,48], which require a disconnection period of 1.66–5 s [19,37,40,47].

5.3. Coordination of RTSO and DNO reclosing practices and the reference DC node Various utilities improve the reliability of the electricity supply in distribution networks by using reclosing practices in their network protection schemes, mainly in regards to fuse-saving schemes. These schemes result in the quick tripping of the reclosing relay for initial faults. For subsequent tripping for the same fault, the reclosing relay may be much slower and time-delayed. Reclosing practices vary widely [61]. For instance, some utilities use oneshot reclosing (0.4–15 s or more), whereas others use up to three subsequent time-delayed attempts of varying intervals (periods of roughly 1–3 min). Fault-current contribution from the reference DC node can cause the fuse to operate much more quickly than it otherwise would, and thus disrupt the fuse-saving scheme. Therefore, as part of our proposal, we established a 0.3 s setting for the mandatory disconnection of the reference DC node from the SDN by means of LOM protection. This is compatible with the faster time for the first reclosing attempt usually applied by utilities (0.4 s).

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Fig. 2. Conventional 25-kV single-phase AC RTS arrangement in Spain.

Reclosing practices also improve reliability in the AC RTS and DC RTS. Thus, the Spanish national RTSO (ADIF, Administrador de Infraestructuras Ferroviarias [Railway Infrastructure Manager]) generally uses a reclosing relay (79) in the network protection of 25-kV AC RTSs (Fig. 2) and an 82 or 83 relay in the network protection of 3-kV DC RTSs (Fig. 4) for reclosing the overhead contact line breaker. ADIF reclosing practices have a first one-shot reclosing (1–5 s or longer, regulated by [62]). In addition, in the 25-kV AC RTS, there are up to three subsequent time-delayed attempts (180 s between attempts according to [63]). However, in the 3-kV DC RTS, the attempts are of varying length (15 s, 60 s, and 180 s according to [64]). The first attempt to automatically reclose the overhead contact line to which the reference DC node is connected in the AC/DC RTS can fail because the fault may be fed by the reference DC node. In addition, a change in frequency may occur in the insulated part of the AC RTS with unacceptable stress on both the reference DC node and the AC RTS equipment. For this reason, in our proposal, we established a 0.3 s setting for the mandatory disconnection of the reference DC node from the AC/DC RTS by means of LOM protection. This has the advantage of being compatible with the faster time for the first reclosing attempt usually applied by the ADIF or other standards [62] (1 s). Our proposal is in accordance with the mandatory disconnection in reclosing practices in AC/DC RTSs for unidirectional EVCSs [28,29] and regenerative braking [60,62]. The results of our research also propose a communication channel between the overhead contact line protection and the reference DC node which permits a transfer tripping (TrTr) of the interconnection system. This enables fast reclosing. Synchronism-check relay (25) can also supervise reclosing. 5.4. Reconnection to the RTS (SDN) of the reference DC node It is necessary to protect RTS/SDN equipment during restoration activities after a fault and also protect utility workers from risks during line maintenance. Consequently, in our proposal, we established that the interconnection systems of the reference DC node should prevent the energization of the RTS or SDN until its frequency (AC RTS and SDN) and/or voltage could be maintained

in rated ranges during a delay time. This is also stated in most grid-interconnection standards for DG/DR in distribution networks [19,30,32,39,44,47,50] (see Table 2 for the most important standards). Although there are no specific grid-interconnection standards for DG/DR in RTSs, a similar statement can also be found in standard IEC 62313 [62] which coordinates rolling stock and substations (Table 2). Based on our findings, we proposed that the voltage and frequency settings for the reference DC node reconnection in all interconnection protections (Table 2) match those for fault detection (Table 1, Section 5.1). However, as the reference DC node reconnection should be harmonized with the DNO’s and RTSO’s reclosing strategy, specific delay times were given. A 5-min delay setting was established in the SDNs (Table 2). In RTSs, however, the reclosing coordination [63,64] required a 9-min delay setting (Table 2). 6. Grid-interconnection of the reference DC node in the 25-kV AC RTS 6.1. 25-kV AC RTS protection This section describes the network protection system of a 25kV single-phase AC RTS in Spain, though this same system is also used throughout the world [65–67]. This protection system and its design is a key issue because it was necessary to harmonise it with our proposal. Fig. 2 shows the feeding diagram for a typical two-track railway using a directional feed, with coupling via a switching post (SP). Feeder substations usually have two transformers in the 20–60 MVA size range. The infeed to the tracks in the ‘northbound’ direction is via the grid transformer T2 at the feeder substation (FS). The power is then distributed by means of catenaries 1 and 2 above the northbound and southbound tracks. At intervals, it is usual to parallel the two catenaries at SP. The infeed from the T2 generally feeds only as far as the normally open bus section circuit breaker (CB1) at the coupling post (CP). Beyond the CP, there is a mirror image of the electrical configuration from the T2 to the D2.

85% < V < 110% 99% < f < 101% Delay time > 9 min RTSs V, f in interval of IEC 60850 [57] (Table 1) Delay time > 5 min

85% < V < 110% 99% < f < 101% Delay time > 5 min SDNs 88% < V < 110% 98.8% < f < 100.8% Delay time >5 min 85% < V Delay time >3 min

MV

LV

The usual protective devices of the network protection system are shown in Fig. 2. Thus, the overhead contact line is protected by means of a multi-step distance protection system (21 relay) with two zones [63,65,66,68–70]. This system is based on the two following premises: (i) the protection needs to be discriminative to ensure that only the two circuit breakers associated with the faulted line section are tripped; (ii) the prospective fault current levels at the SP and CP are progressively smaller and may be lower than rated current. However, the AC RTSs protection also use overcurrent protection as time-delayed back-up protection for the main distance protection [63,65,66]. Two philosophies are at work here. The first is to implement definite-time overcurrent protection (50/51 relay) with settings chosen to ensure that the distance relay should operate first. The second is to use back-up overcurrent protection (51 relay) in those cases that distance protection stages fail. Catenary thermal overload protection (49 relay) is also required [63,65,66] so that railway catenaries remain in the correct position relative to the track, thus ensuring good current collection by train pantographs. Auto-reclosers (79 relay) are also required [62,63]. Optional protection relays in some AC RTS protections are the following: over/undervoltage protection (27/59 relay) [63], high speed circuit-breaker failure protection (50BF relay), and trip circuit supervision (74TC relay). Potential network protection relays that must be coordinated with the interconnection protection system are auto-reclosers (79) and 27/59 relay (if present). 6.2. Electrical protection for the reference DC node grid-interconnection in the 25-kV AC RTS

88. 9% < V < 106% 98.8% < f < 100.8% Adjustable delay time <5 min

85% < V Delay time >3 min

85% < V < 115% Delay time >3 min

85% < V < 110% 98% < f < 102% Delay time = 0 s

25-kV AC RTS 3-kV DC RTS

IEEE 929 [47] IBERDROLA code [32] IEEE 1547-2 [19]

ENDESA code [30]

SPANISH order 5/9/1985 [39]

Royal decree 1663/2000 [50]

IEC 62313 [62]

Proposed settings for reference dc node reconnection to SDNs and RTSs

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Table 2 Voltage and frequency window and delay time for DG/DR reconnection in distribution networks along with relevant settings in the RTS context and proposed settings for reference DC node reconnection to different networks.

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In consonance with the adaptation of standard IEEE 1547.2 [19], we established a single 25-kV interconnection protection system for the reference DC node at the PCC (see Figs. 1 and 3). This interconnection protection system must thus be equipped with under/overvoltage detection (27/59 relay) and under/overfrequency detection (81U/O relay) (Section 5.1). The recommended settings are shown in Table 1. Additionally, as part of our proposal, we specified that an overcurrent protection by means of a circuit breaker (52), triggered by the 50/51 relay, must guarantee the disconnection of the reference DC node from the 25-kV AC RTS in the event of 25-kV AC RTS faults. Sometimes, this protection can be performed by means of distance protection (21 relay) as specified in [63,65,70]. A timedelayed, instantaneous earth overcurrent relay (50/51N) also should trip this circuit breaker to control earth faults in the LV transformer side. To ensure safety in the 25-kV AC RTS operation, we established that the reference DC node must be obligatorily included in the remote control scheme of the RTSO [63]. In this sense, a TrTr to the interconnection circuit breaker (52) is currently required in similar standards when output power exceeds a certain threshold value (e.g. 100 kVA [51], 1 MVA [30,31,39]). Moreover, there should be a bidirectional TrTr between the 750-V and 25-kV interconnection protection systems that guarantees their simultaneous trip. This ensures the safety of the 750-V DC bus. To coordinate the reference DC node reconnection with the reclosing strategy of the RTSO, we also included an auxiliary undervoltage relay (27×) in our proposal, as specified in similar standards [19,30–32,39,47,62]. However, specific anti-islanding protection was not a requirement in our proposal, as advised in similar standards [32,46,47]. Finally, our proposal established that these protective functions should be built into the control software/hardware of the reversible 750-V DC/0.4-kV AC converter (AC side). Requirements outside the scope of the interconnection protection system were the following (Fig. 3): (i) the reference DC node

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111

25-kV AC RAILWAY TRACTION SYSTEM (RTS) 52 PCC PCC

AC, 25 kV, 50 Hz 52b TrTr

Running rails (return circuit)

OVERALL EARTHING SYSTEM of the 25-kV AC RTS

AC traction substation

M

Catenary switch

Visible lockable switch

89

Fuse

SMART DC NODE

25 kV AC Interconnection transformer

55 kVA

0.4 kV AC

52

TN (Preferred)

3

1

50 51

50/ 51N

25-kV Interconnection Protection System (within dashed lines)

1

52TOC/b

52

1 (2) 1

50 51

TrTr

1

1

27 59

AC circuit breaker

50/ 51N

1 (3)

21

81 U/O

1 Trip

TrTr

1

AI 1

1

27x

79

Delayed closing (>5 min)

25-kV AC RTS faults

Auto-recloser

System earthing of 0.4-kV single phase AC systems

TT

Communication Channel

Earthing switch 57

(1)

25

Communication Channel Transfer tripping (TrTr)

(1)

Visible lockable switch 89

(2) (3)

Only for self-commutated inverters that operate as voltage source 25-kV AC RTS fault protection: option 1 25-kV AC RTS fault protection: option 2

2

Transfer tripping (TrTr)

INSULATION MONITORING DEVICE

INNER ELECTROMECHANICAL RELAY/CONTACT

Communication Channel

50 kVA

EARTHING SYSTEM of the 750-V DC bus

750-V Internal Interconnection Protection System (within dashed lines)

Visible lockable switch

89

1

IMD

72TOC/b

-FLOATING CONFIGURATION -

TrTr

72

1

27 59

DC circuit breaker 2

Trip

76

TRTR

750-V DC bus faults

TrTrs for 750 V DC bus faults

Negative conductor

750-V DC BUS

Fig. 3. Basic functions of the interconnection protection system for the reference DC node in the 25-kV AC RTS.

Transfer tripping (TrTr)

Exposed conductive parts (25-kV interconnection electrical equipment )

Earth conductor

89 Line switch

Electrical equipment enclosures (transformer and AC switchgear)

Return conductor (return circuit) Conducting components in the overhead contact line zone of the 25-kV AC RTS (signal, fence, platform, pipelines, cable screens, bridges and viaducts, etc.)

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Fig. 4. Conventional 3-kV DC RTS arrangement in Spain.

connection to the 25-kV overhead contact line must be performed with a visible lockable switch (89) that can be accessed at any time by RTSO staff [19,30,31,47,51]; (ii) the interconnection transformer should be protected against overcurrent both on the HV and LV side; and (iii) the reversible DC/AC converter [71] should be insulated by visible lockable switches (89). Node protection consists of a fuse and voltage surge arrestor [28]. Regarding the internal interconnection protection system, our research proposal specified a single 750-V internal interconnection protection system for the 750-V DC bus (Figs. 1 and 3). This system must be equipped with under/overvoltage detection (27/59 relay) (Section 5.1). The recommended settings are shown in Table 1. In addition, overcurrent protection provided by a DC circuit breaker (72), triggered by the 76 relay, would guarantee the disconnection of the 750-V DC bus in the event of 750-V DC bus faults. Finally, in our proposal, we established that such protective functions must be built into the control software/hardware of the reversible 750-V DC/0.4-kV AC converter (DC side). 6.3. Earthing and safety protection The return circuit, the electrical equipment enclosures in AC traction substations, and the conducting components in the overhead contact line zone should be bonded to the overall earthing system of the 25-kV AC RTS [72] to avoid hazardous touch voltages during normal or fault conditions (Fig. 3). Previous requirements in the reference DC node should be harmonized with those related to the earth connection of the return circuit in the 3-kV DC side [73] and the earth arrangement in the LV side (750-V DC and 0.4-kV AC) [74]. This harmonisation prevents the harmful effects of DC stray currents and avoids hazardous touch voltages. According to standard IEC 62128-3 [75], the return circuit in the 25-kV AC side should be independent from the return circuit in the 3-kV AC side. In addition, an independent earth electrode should be designed and included in

the AC LV side if required [76]. Protection against electric shock in DC LV systems should be specifically regulated [77], such as the definition of an independent earth electrode. In any case, it is preferable to use a single LV earth electrode for both the DC and AC sides [77]. For purposes of harmonisation, as part of our proposal, we decided that enclosures of 25-kV interconnection electrical equipment should be bonded to the overall general earthing system (Fig. 3). However, at the LV level (0.4-kV AC, 750-V DC), a single independent earthing system must be designed for the 750-V DC bus, in other words, for the earthing of the system and equipment [77] (Fig. 3). In this case, a TN system guarantees protection against electric shock at the 0.4-kV AC level. Alternatively, it is possible to design an independent earth electrode for the earthing of the 0.4-kV AC system (TT system). In any case, we also established that enclosures of the electrical equipment of 25-kV interconnection protection system (LV equipment) and the 750-V DC equipment should be bonded to the earthing system of the 750-V DC bus. At the 750 V-DC level, a floating configuration is safer for protection against electric shock [77]. Thus, our proposal includes an insulation monitoring device (IMD) added for protection against electric shock [77]. In this way, an insulation fault or direct contact should simultaneously trip all the 750-V internal interconnection protection systems of the 750-V DC bus by TrTrs. 7. Grid-interconnection of the reference DC node in the 3-kV DC RTS 7.1. 3-kV DC RTS protection Fig. 4 shows the typical arrangement of a 3-kV DC RTS in Spain, which is similar to that used throughout the world [65–67]. An AC supply is transformed and rectified by means of uncontrolled/controlled rectifiers to provide the DC traction

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Fig. 5. Basic functions of the interconnection protection system for the reference DC node in the 3-kV DC RTS.

voltage for connection to the positive and negative busbars of the substation. The DC traction substation usually has two transformer–rectifier units in the 3–6 MVA size range. However, substation configurations vary from a single transformer–rectifier unit to multiple units. Currently, Spain has DC traction substations with DC to AC converters where power regenerated by

traction units in the regenerative braking is returned to the AC supply. The supply system of the DC traction power system is divided into electrical segments to better facilitate electrical protection and safety. Section breaks in the contact rail system are provided by gaps in the contact rail. Switching and tie posts

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System earthing of 0.4-kV three phase AC supply systems

LV Secondary substation

SECONDARY DISTRIBUTION NETWORK (0.4 kV) PCC

SMART DC NODE 0.4-kV Interconnection Protection System (within dashed lines)

52

89

Visible lockable switch

3

1

50 51

50/ 51N

(2)

SDN faults (3)

AC circuit breaker

1

3

3

27 59

52TOC/b 52 TrTr

1

(1) Only for self-commutated inverters that operate as voltage source (2) Earth fault protection: option 1 (3) Earth fault protection: option 2

1

81 U/O

AI 1

25

1

Trip

27x

TrTr

(1)

Delayed closing (<1 min)

Transfer tripping (TrTr)

0.4 kV AC

3

LV exposed-conductive parts

Interconnection isolation transformer 55 kVA

0.4 kV AC Communication Channel

Communication Channel Transfer tripping (TrTr)

Visible lockable switch 89

EARTHING SYSTEM of the 750-V DC bus

50 kVA

750-V Internal Interconnection Protection System (within dashed lines)

Visible lockable switch 89 1

DC circuit breaker 72TOC/b

1

27 59 72

TrTr

2

Trip

76

TrTr

750-V DC bus faults

750-V DC BUS From IMD

Fig. 6. Basic functions of the interconnection protection system for the reference DC node in the 0.4-kV AC SDN.

are sometimes used to prevent voltage drops on double tracks where substations are far from each other. In this case, the up and down tracks are connected by a high-speed circuit breaker.

The usual protective devices of the network protection system of a 3-kV DC RTS are shown in Fig. 4. Thus, the overhead (positive) contact line is protected by a feeder circuit breaker (72), which can be single-pole, high speed, or semi-high speed. This

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Fig. 7. Basic functions of the internal interconnection protection system for the bidirectional EVCSs, backup storage system and PV system.

breaker is equipped with instantaneous overcurrent relays (76I relay), short-time and long-time overcurrent relays (76D relay), and current rate-of-rise detection relays [64–66]. The rate-of-rise function should include I (change in current, 80 relay) and t (time delay) settings. Some RTSOs employ TrTr as part of the protective scheme for feeder breakers that power the same contact rail section (85 relay). DC feeder circuit breakers powering contact rail sections are normally equipped with automatic reclosing capability [62,64]. Control of the automatic reclosing is provided by combined voltage measuring (82 relay) and load measuring (83 relay). Earth faults should be detected at the negative panel by an

earth detection relay (64F relay) [72]. In some cases, undervoltage protection (27 relay) and high-speed circuit-breaker failure protection (50BF relay) are also added. Potential network protection relays that must be coordinated with the interconnection protection system are reclosing relays (82/83), and 27 relay (if present). 7.2. Electrical protection for the reference DC node grid-interconnection in the 3-kV DC RTS By adapting standard IEEE 1547.2 [19], in our proposal, we established a single 3-kV interconnection protection system for the

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reference DC node at the PCC (Figs. 1 and 5). This interconnection protection system must be equipped with under/overvoltage detection (27/59 relay) (Section 5.1). The recommended settings are shown in Table 1. In addition, as part of our proposal, an overcurrent protection by means of a DC circuit breaker (72), triggered by the 76I/76D relay, must guarantee the disconnection of the reference DC node from the 3-kV AC RTS in the event of 3-kV DC RTS faults. To ensure safety in the 3-kV DC RTS operation, our proposal obligatorily included the reference DC node in the remote control scheme of RTSO [64]. Moreover, it should have a bidirectional TrTr between the 750-V and 3-kV interconnection protection systems that guarantees their simultaneous trip. This guarantees the safety of the 750-V DC bus. To coordinate the reference DC node reconnection with the reclosing strategy of the RTSO, we proposed an auxiliary undervoltage relay (27×), as advised in similar standards [29,62]. Specific anti-islanding protection may be applied [29]. Finally, we established that these protective functions should be built into the control software/hardware of the reversible 3-kV DC/0.4-kV AC converter (DC side). Another specific requirement in our proposal, though outside the scope of the interconnection protection system, was that the reference DC node connection to the 3-kV overhead contact line should be performed with a visible lockable switch (89) that can be accessed at any time by RTSO staff (Fig. 5). Node protection consists of a fuse as well as outdoor-suited arrestors [29]. 7.3. Earthing and safety protection IEC 62128-2 [73] states that parts of the return circuit must not have a direct electrical connection to parts of the buildings that are not insulated against earth to prevent the effects of DC stray currents. If connections are to be made to the return circuit, voltage limiting devices must be used (Fig. 5). The conducting components in the overhead contact line zone should be bonded to the structure earth of the 3-kV DC RTS. Electrical equipment enclosures in DC traction substations are insulated from the structure earth (Fig. 5). Earth fault detection in such equipment enclosures is provided by a low resistive connection at one point. Current monitoring (earth relay) between the return circuit and the DC electrical equipment enclosures and a voltage monitor (voltage-limiting device) in the earth connection trip the protection if insulation faults or unacceptable touch voltages occur [72]. With a view to harmonising these premises, we decided that enclosures of the 3-kV DC/0.4-kV AC converter, interconnection transformer and electrical equipment of 3-kV interconnection protection system should be bonded to the 3-kV interconnection earthing grid. To detect inadmissible touch voltages between this earthing grid and the structure earth, a voltage-limiting device should protect against electric shock by tripping the 3-kV interconnection protection system. Earth fault detection was provided by an earth relay. In addition, enclosures of the 750-V DC/0.4-kV AC converter should be bonded to the earthing system of the 750-V DC bus. 8. Grid-interconnection of the reference DC node in the 0.4-kV AC SDN 8.1. Electrical protection for the reference DC node grid-interconnection in the 0.4-kV AC SDN In accordance with standard IEEE 1547.2 [19], a single 0.4-kV interconnection protection system was defined at the PCC for the reference DC node (Figs. 1 and 6). This interconnection protection system must be equipped with under/overvoltage detection (27/59

relay) and under/overfrequency detection (81U/O relay) (Section 5.1). The recommended settings are shown in Table 1. In addition, most references [25,30,32,33,39,41,42] recommend overcurrent protection by means of a circuit breaker (52), which is triggered by the 50/51 relay. Earth faults are controlled by an earth overcurrent relay (50/51N) or by an independent high-sensitivity residual-current device [25,30,32,36,50]. For safety reasons, the 0.4-kV interconnection protection system must be capable of detecting the loss of the grid supply by means of the anti-islanding protection (AI relay) [19,32,33,39,43,44,47–49]. The reconnection of the reference DC node must be enabled by means of the auxiliary undervoltage relay (27×) after an SDN fault [19,25,30,32,33,39,41,42]. The protective functions may be built into the control software/hardware of the 750-V DC/0.4-kV AC converter [30,32,36,39,41,42,48,50] (AC side). DC node connection to the 0.4-kV SDN should be performed with a visible lockable switch (89) [19,30,32,33,36,42,47–50]. Finally, our proposal established that there should be a bidirectional TrTr between the 750-V and 0.4-kV interconnection protection systems to ensure the safety of the 750-V DC bus. The protection of the reference DC node consists of a voltage surge arrestor and circuit breaker, included in this case in the interconnection protection system.

8.2. Earthing and safety protection As part of our proposal, we established that there should be a single earthing system in the 750-V DC bus [77] which connected the exposed conductive parts of the electrical equipment in the 0.4-kV interconnection protection system and the 750-V DC equipment. In the 750-V DC side, protection against electric shock was guaranteed by TrTrs from the IMD on all the 750-V internal interconnection protection systems.

9. Grid-interconnection of the bidirectional EVCSs, backup storage system, and PV system with the 750 V-DC bus 9.1. Electrical protection for the grid-interconnection of the bidirectional EVCSs, backup storage system, and PV system in the 750 V-DC bus Based on the adaptation of standard IEEE 1547.2 [19], we established a single internal interconnection protection system for the EVCSs, the backup storage system (battery and supercapacitor bank), and the PV system with the 750 V-DC bus (Figs. 1 and 7). This protection system must be equipped with under/overvoltage detection (27/59 relay) (Section 5.1). The recommended settings are shown in Table 1. Furthermore, one of the requirements in our proposal is overcurrent protection by means of a DC circuit breaker (72), triggered by the 76 relay, which must guarantee the disconnection of the 750-V DC bus in the event of 750-V DC bus faults. Such protective functions should be built into the control software/hardware of each DC/AC converter or DC/DC converter (750-V DC side).

9.2. Earthing and safety protection The earthing and safety protection in the 750-V level follows the requirements in Section 8.2. In addition, a TN system was preferred to guarantee safety protection at the 0.4-kV AC level. However, when EVCSs (PV system) are located in a building far from the 750-V DC bus, their enclosures must be bonded to local equipment earthing.

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10. Conclusion This paper has presented a unified regulatory framework of electrical protection requirements and of earthing arrangements, applicable to the grid-interconnection systems of a DC node used as reference to feed bidirectional EVCSs. Such requirements were derived from a critical review which modified, harmonised, and adapted requirements in many grid-interconnection standards and codes as well as company operation practices to the specific behaviour of bidirectional EVCSs. The creation of a global regulatory framework gives EVCS engineers valuable information for any type of interconnection of bidirectional EVCSs to all the networks proposed in this research. Interestingly, in Spain as well as in other countries, electrical protection requirements for the grid-interconnection of bidirectional EVCSs (i.e. DRs) in SDNs can be easily defined based on available standards of DG/DRs. However, quite surprisingly, there are no bidirectional EVCSs standards or projects for RTSs. Since EVCSs are in the initial stages of regulation, some interconnection protection requirements have been provided for unidirectional EVCSs in SDNs and RTSs. However, these requirements are not useful for bidirectional EVCSs. Finally, in reference to grid-interconnection between SDNs and RTSs, it is urgent to advance farther. Until now, RTSs have been regarded as insulated systems of passive loads, or at most, systems with regenerative braking powers. The interconnection of the reference DC node proposed significantly changes RTSO protection practices. This means that there is a clear need for a unified set of electrical protection requirements for both interconnection protection and the network protection system. Acknowledgements This work was carried out as part of the FerroSmartGrid Research Project: Railway Smart Grid, (ITC 20111044), funded by the Ministry of Science and Innovation in Spain. The project includes nine companies (Telvent Energía, Adif, Inabensa, Indra, Windinertia, Andel, Adevice, Telvent Transporte and Acisa) and four Universities (Seville, Malaga, Jaén, and Cordoba). The performance budget for this project is 9.6 million euros. The authors would like to thank the Company Distribution Endesa for its continued support in the discussion of the configuration of the interconnection protection system of bidirectional EVCSs in distribution systems. References [1] O. Van Vliet, A.S. Brouwer, T. Kuramochi, M. Van den Broek, A. Faaij, Energy use, cost and CO2 emissions of electric cars, J. Power Sources 196 (2011) 2298–2310. [2] P. Tulpule, V. Marano, S. Yurkovich, G. Rizzoni, Economic and environmental impacts of a PV powered workplace parking garage charging station, Appl. Energy 108 (2013) 323–332. [3] B. Tarroja, J.D. Eichman, L. Zhang, T.M. Brown, S. Samuelsen, The effectiveness of plug-in hybrid electric vehicles and renewable power in support of holistic environmental goals: Part 1—Evaluation of aggregate energy and greenhouse gas performance, J. Power Sources 257 (2014) 461–470. [4] M. Moradijoz, M. Parsa, M.R. Haghifam, E. Alishahi, A multiobjective optimization problem for allocating parking lots in a distribution network, Int. J. Electr. Power Energy Syst. 46 (2013) 115–122. [5] C.H. Dharmakeerthi, N. Mithulananthan, T.K. Saha, Impact of electric vehicle fast charging on power system voltage stability, Int. J. Electr. Power Energy Syst. 57 (2014) 241–249. [6] A.M.A. Haidar, K.M. Muttaqi, D. Sutanto, Technical challenges for electric power industries due to grid-integrated electric vehicles in LV distributions: a review, Energy Convers. Manage. 86 (2014) 689–700. [7] M.C. Falvo, R. Lamedica, R. Bartoni, G. Maranzano, Energy management in metro-transit systems: an innovative proposal toward an integrated and sustainable urban mobility system including plug-in electric vehicles, Electr. Power Syst. Res. 81 (12) (2011) 2127–2138. [8] A. Motraghi, Rail research projects: case studies, Res. Transp. Econ. 41 (1) (2013) 76–83.

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