# Protection design for pole-to-pole faults in DC networks

Fabian Benedikt Witt, Melanie Hoffmann, Michael Kurrat

TU Braunschweig - elenia Institute for High Voltage Technology and Power Systems

Schleinitzstraße 23, 38106 Braunschweig, Germany

Tel.: +49 531 3917703

E-Mail: fabian-benedikt.witt@tu-braunschweig.de

URL: https://www.tu-braunschweig.de/elenia

## Keywords

«Fault handling strategy», «Short circuit», «AC-DC microgrid», «Active protection», «Protection device».

## Abstract

Fault protection is one of the primary challenges impeding the widespread adoption of DC and hybrid AC/DC grids. A structured planning process for protection systems is necessary with regard to standardization. In this paper, a novel three-step protection system design process is proposed building upon the CENELEC (CLC/TS 50654-1) approach. The proposed planning process is separated into concept, system and component level. On concept level, the operational modes "continued operation", "temporary stop" and "permanent stop" according to CENELEC are used to define desired system behaviour in the event of a fault. On system level, protective action sequences are defined to realize the system behaviour defined on concept level. Finally on component level, protective devices are dimensioned and tripping thresholds are determined.

## Introduction

Direct current (DC) grids are gaining increasing importance in the context of the energy transition. The use of DC enhances efficiency in connecting DC components such as photovoltaic systems, battery storage systems, and computer systems [1,2] and in transmitting energy over long distances via high-voltage direct current (HVDC) systems [3-5].

However, the use of DC introduces new protection challenges. Fault currents rise more rapidly due to the absence of transformers, which results in lower inductance values. Additionally, unlike alternating current (AC), DC fault currents do not naturally extinguish due to lack of zero crossing. Furthermore, conversion processes in

DC networks are exclusively managed by power electronics based on semiconductor components, which generally have a lower thermal load capacity compared to the transformers typically used in AC systems [6].

The fault clearing process in DC systems can be divided into two sub-processes: initial fault current interruption and galvanic fault isolation [7]. Fault current interruption can for example be realized by suppressing the fault supply through power electronics or increasing the arc voltage above the dc-supply voltage [8].

Fault isolation is the creation of a galvanic isolation path, which by definition can only take place after fault interruption has been completed and current values close to zero (residual current) have been achieved. [7]

In this paper, fault current interruption and fault isolation are regarded as two strictly separate processes, even if they are realized by the same protective device (PD) and the same switching action. Fault current interruption and fault isolation do not have to take place at the same point in the network, which results in multiple different fault-clearing strategies that have not yet been integrated into a standardized protection design methodology. A distinction is made between breaker-based and breaker-less fault clearing strategies as depicted in Figure 1.

<figure><img src="image_1.png" /><figcaption>Fig. 1: Single-pole representation of breaker-based and breaker-less protection</figcaption></figure>
Breaker-based methods use circuit breakers (or fuses) to interrupt the fault current. If the circuit breaker does not provide intrinsic galvanic isolation, the switch manufacturer installs a serial disconnector which opens immediately after the fault interruption without further external control. If the fault-current interruption is fast enough, the supply of critical loads can continue without any safety-relevant stop. [9,10]

Breaker-less protection uses the ability of power electronics already in the network to limit and switch off the fault current. Power electronics therefore assume the role of a fault-current-interrupting protective device and also enable the system to be restarted quickly after the initial fault interruption. After fault current interruption by remote power electronics, disconnectors close to the fault are opened to isolate the fault. After fault isolation, the healthy part of the system is quickly reenergized. [10-12]

In this paper, a protection system design process for line-to-line faults in general DC systems is proposed. The aim is to allow fault interruption and fault isolation at different locations in the network in the design process and thus to combine breaker-based and breaker-less protection methods in one overall process. The proposed process is divided into three steps as shown in Figure 2:

<figure><img src="image_2.png" /><figcaption>Fig. 2: Proposed protection design process</figcaption></figure>

At concept level, boundary conditions on power flow during and after the fault clearing process are analysed. The result of this step is the description of the desired fault clearing process for various fault locations using power flow without considering the location of the fault clearing action or protective devices.

At system level, desired protective action sequences at certain locations in the network are defined without considering realizing protective devices. Fault-localizing mechanisms are chosen to enable the realization of these desired protective reactions during operation. The timing of all protective actions is also considered in order to guarantee proper interplay.

At component level, protective devices are placed in the network to realize the protective action sequences defined on system level. Finally, protective devices are dimensioned and trigger thresholds are selected in order to enable the required reaction of all components.

The protection design process presented in Figure 2 is described in more detail in the following chapters.

As the physical principles of DC protection are fundamentally independent of the voltage level [13], the applicability of the protection design process proposed in paper is not limited to one voltage class (low voltage (LV), medium voltage (MV) and high voltage (HV)). However, the challenges in the design of protective devices are generally much lower in low voltage. Also, the requirements on grid protection generally vary with voltage level and use case. [13]

## Concept level - Definition of the required power flow in case of fault

In this chapter, a methodology for characterizing protection schemes is presented. For HVDC systems, CLC/TS 50654-1: 2020 [13] may be used, the methodology of which is applied to general DC systems limited to pole-to-pole faults in the following. To characterize the desired system behavior in the event of a fault, power flow is defined at PoCs (Point-of-Connection) for different fault locations, differentiated by protection zones. In this paper, the DC-network in Figure 3 is considered as an example.

The grid in Figure 3 consists of two loads in zones 3 and 5, a DC source in zone 2 and an AC grid, both connected via power electronics to the DC bus in zone 1. The load-interface is a local energy storage device which can continue to supply the critical load in zone 5 for a short time in case of temporary outage of the DC bus [14]. The load interface may not discharge into the fault in case of a fault in zones 1-4. The grid in Figure 3 can for example represent a MVDC distribution grid.
<figure><img src="image_3.png" /><figcaption>Fig. 3: Single-pole representation of the network considered as an example. Green lines are PoCs, red areas are protection zones.</figcaption></figure>

The choice of protection zones and the placement of PoCs is a critical step in the protection design process. Guidelines for the selection of protection zones and PoCs will be provided in a future publication. In the following, the characteristics of PoCs and protection zones will be briefly described.

A PoC represents an interface in the network. PoCs divide the network into several smaller areas, referred to as protection zones. Protection zones describe fault locations to be considered in the protection design process. The protection zones according to CENELEC [13] used here, which indicate possible fault locations, are not identical to the classical protection zones used for HVDC by Cigre [15]. Protection zones according to Cigre [15] require fault clearing within the affected protection zone or by directly adjacent PDs. Unfortunately, both these fundamentally different concepts share the same name in literature.

The entire network does not have to be covered by protection zones. However, areas not covered by protection zones are not taken into account in the protection system design process. An example of this is shown in Figure 3: Pole-to-pole faults in the AC-Grid, within the AC/DC converter and the DC/DC converter are not covered by a protection zone and must be cleared separately.

PoCs and protection zones may be used to define requirements on grid protection as part of the stakeholder analysis. All faults within a protection zone are considered identical from a system perspective. Each component within a faulty protection zone sees the fault for the entire

fault duration. By definition, no protective action can take place within a protection zone. All protective functions must be assigned to a PoC. However, the need for a protective device is not a prerequisite for defining a PoC. Since power electronics often perform protective functions, PoCs must be defined directly adjacent to power electronics which represent these protective functions. The network in Figure 3 is considered as an example. PoC-AC1 contains the ability of the AC/DC power electronics to limit and switch off AC-side fault currents. PoC-DC1 contains the ability of the AC/DC power electronics to limit and switch off DC-side fault currents. The AC connection is therefore, for example, a full-bridge Modular Multi-level Converter (MMC) or a Voltage Source Converter (VSC) with DC/DC stage [15]. PoC-DC2 includes the ability of the DC/DC converter to limit and switch off fault currents in the direction of zone 1. It should be noted that the DC/DC converter is not able to switch off faults in zone 2.

According to CLC/TS 50654-1: 2020 [13], there are five possible operating modes describing power flow through a PoC in case of a fault, shown in Table I. Reactive power transfer is not considered at PoCs in the DC-grid. To characterize the desired system behavior in the event of a fault, these operating modes are defined at PoCs for different fault locations, differentiated by protection zones. The desired system behavior can also be defined separately for primary and backup protection and for different fault types.
<table><caption>Table I: Possible operating modes of a PoC in case of fault based on [13]</caption><thead><tr><td>Required mode of operation</td><td>Description</td></tr></thead><tbody><tr><td>Continuous operation (CO)</td><td>In the event of a fault, both active and reactive power must remain interchangeable without any safety-relevant interruption.</td></tr><tr><td>Temporary stop P (TS-P)<br/>Temporary stop P and Q (TS-PQ)</td><td>Temporary interruption of active power transmission/ active and reactive power transmission is required in the event of a fault. Can be used for fault clearing in the event of an isolation fault on overhead lines. The temporary stop can also contribute to reaching current zero in order to enable fault isolation.</td></tr><tr><td>Permanent stop P (PS-P)<br/>Permanent stop P and Q (PS-PQ)</td><td>Long-term interruption of active power transmission/ active and reactive power transmission is allowed/ required in the event of a fault.</td></tr></tbody></table>

When designing the grid protection, the grid topology, protective capabilities of components already present in the grid, possible locations for local energy storage, critical loads and others must be taken into account. The requirements of all stakeholders must be considered. Needs and requirements are defined from the view of the stakeholder. These needs then must be translated into basic functions and a quantifiable set of performance requirements using key performance indicators (KPIs) for evaluation and use in the system design process [16]. In general, not every requirement can always be met. Conflicting requirements must be balanced within the scope of requirements management. In general, DC protection is a trade-off between availability and cost. [15]

On concept level, the protection strategy is designed with regard to selectivity. It is determined which network sections are to be (temporarily) switched off and which are to be galvanically isolated.

As part of the protection design at concept level, the DC grid under consideration may be divided into several smaller sub-grids. These classical protection zones according to Cigre [15] require fault clearing within the affected protection zone or by directly adjacent PDs. In other words: All PoCs within healthy classical protection zones must remain in continued operation (CO), disallowing the temporary (TS) or permanent stop (PS) as in Table I.

Using these classical protection zones, the hierarchy of the DC grid may be taken into account. In general, faults in subordinate network sections should not lead to outages in higher-level network sections.

The process of finding the protection properties listed below as examples will be described in a future publication. These properties are the result of a fictitious requirements analysis and allow a wide range of protection concepts to be considered in the context of this paper.

* For faults in zones 2 and 3, the DC-bus (zone 1) must be temporarily shut down in order to support the fault isolation process at PoC-DC2 or PoC-DC3. This is accompanied by the temporary interruption of active power transmission at PoC-AC1, PoC-DC1, PoC-DC2, PoC-DC3 and PoC-DC4.

* Temporary shutdown of the DC-bus (zone 1) is not permitted for faults in zones 4 and 5.

* The load in zone 5 should continue to be supplied without interruption in the event of faults in zones 2 and 3. This is made possible by the load interface at PoC-DC5. This is an energy storage device that allows the load to continuously be supplied for a short time even if the supply voltage (zone 1) temporarily drops. It should be noted that the capacity of the load interface is limited. It is therefore not possible to supply the load in zone 5 in the event of a permanent stop in zone 1. [14]

* In the event of faults in zones 1 to 5, the AC-connection at PoC-AC1 must be able to provide ancillary services in the form of reactive power without interruption.

The desired system behaviour in the event of a fault, defined holistically in Table II, contains all the requirements defined above.
<table><caption>**Table II: Required properties of the primary protection by fault location**</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Primary protection</th><th colspan="6">Point-of-Connection (PoC)</th></tr><tr><th>AC1</th><th>DC1</th><th>DC2</th><th>DC3</th><th>DC4</th><th>DC5</th></tr></thead><tbody><tr><th rowspan="4">Faulty zone</th><th>Zone 1</th><td>PS-P</td><td>PS-P</td><td>PS-P</td><td>PS-P</td><td>PS-P</td><td>PS-P</td></tr><tr><th>Zone 2</th><td>TS-P</td><td>TS-P</td><td>PS-P</td><td>TS-P</td><td>TS-P</td><td>CO</td></tr><tr><th>Zone 3</th><td>TS-P</td><td>TS-P</td><td>TS-P</td><td>PS-P</td><td>TS-P</td><td>CO</td></tr><tr><th>Zone 4,5</th><td>CO</td><td>CO</td><td>CO</td><td>CO</td><td>PS-P</td><td>PS-P</td></tr></tbody></table>

If necessary, the desired behaviour of the backup protection can be defined separately in case of failure of the primary protection fault-current interruption and fault isolation equivalent to Table II.

The definition of the required power flow at a PoC in accordance with Table II does not yet determine the fault clearing process, the location of the fault clearing action and the protective responsibilities of PoCs. This is considered at system level in the next chapter.

## System level – Definition of fault clearing sequence, Fault localization mechanism

The power flow specified at concept level is realized by PDs, each of which is assigned to a PoC. There can be multiple PDs assigned to one

PoC. The desired reactions of all PDs of a PoC can be characterized by their protective function and summarized per PoC. In this sense, PoCs will be regarded as general protective devices in the following. In this section at system level, the required protective reactions of all PoCs are characterized.

The desired reaction of a PoC for fault clearing is coded by the colour selection in Figure 4, which also provides a description of all six possible operating modes. It should be noted that the operating modes in Figure 4 are not characterized by the power flow through the PoC under consideration. This distinguishes the operating modes in Figure 4 from the CO, TS and PS operating modes in Table I. To illustrate this distinction, a fault in zone 1 in Figure 3 is assumed as an example. In case of a fault in zone 1, active power flow through PoC-AC1 and PoC-DC1 must be permanently stopped. As shown in Table II, the PS state is therefore required for PoC-AC1 and PoC-DC1. These required PS states can be realized by the protective action of only one PoC. For example, fault interruption and isolation can be demanded at PoC-DC1, while PoC-AC1 does not realize a protective action and remains in fault-ride-through. The protection actions in Figure 4 can therefore be used to define the protection responsibility of an individual PoC.

<figure><img src="image_4.png" /><figcaption>Fig. 4: Possible protective reactions of a PoC</figcaption></figure>

Exactly one operating mode from Figure 4 must be selected per PoC for each fault location to realize the required power flow in Table II.

"Fault-ride-through" is not to be equated with inactivity. For example, current-limiting power electronics must guarantee self-protection through current limitation during fault-ride-through operation. During protection system design, the fault-ride-through mode has to be

specified further with regard to current limitation and fault current contribution.

"Fault interruption and optional restart" is only permitted in the case of two serial PoCs, whereby one PoC is to realize the interruption and the other the galvanic isolation of the fault. After fault interruption and isolation, the interrupting PoC may in principle be restarted, as the galvanic isolation path prevents the fault from being fed.
However, since in most cases both PoCs can be combined into a single PoC with the operating mode “Fault interruption and isolation”, “Fault interruption and optional restart” is not considered in the example chosen in this paper. “PoC not involved” must be selected if the PoC’s reaction to the fault occurrence does not have any influence on the fault situation. This is the case, for example, for faults at a higher hierarchical network level. Even if the reaction of the PoC under consideration is not critical for fault clearing, permanently tripping PDs at this PoC should not operate, e. g. fuses, and must not be damaged in order to enable the network section to be restarted after the fault has been cleared.

The final protective reaction of each PoCs defined in Table III are a valid solution for realizing the power flow specified at concept level. The desired primary protective reaction of PoC-DC2, as defined in Table III, (highlighted in yellow) is shown as an example in Figure 5.

<table><caption>**Table III: Required final reaction of each PoC by fault location**</caption><thead><tr><th rowspan="2">Primary protection</th><th colspan="6">Point-of-Connection (PoC)</th></tr><tr><th>AC1</th><th>DC1</th><th>DC2</th><th>DC3</th><th>DC4</th><th>DC5</th></tr></thead><tbody><tr><th rowspan="5">Faulty zone</th><th>Zone 1</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Zone 2</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Zone 3</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Zone 4</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Zone 5</th><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Figure 5 shows that the required protective reaction of any one PoC depends on the fault location. For example, for faults in Zone 1, “Fault interruption and isolation” is required from PoC-DC2. For faults in Zone 2 “Fault isolation” is required from PoC-DC2. Table III provides a figure equivalent to Figure 5 for each of the six PoCs individually.

<figure><img src="image_5.png" /><figcaption>Fig. 5: Required final protective reaction of PoC-DC2 by fault location</figcaption></figure>

The question arises as to how two fault locations, which require a different reaction from a PoC, can be differentiated during operation. PoC-DC2 should react differently to faults in zone 1, zone 2, zone 3 and zones 4,5 in accordance with Table III and Figure 5. A fault locating mechanism is therefore required to realise the desired reactions of the PoC according to fault location. It should be noted at this point that it is generally not necessary to determine the exact fault location, but only to differentiate between fault locations that require a different response from the PoC. Figure 5 shows that PoC-DC2 should remain in fault-ride-through for faults in both zone 4 and zone 5. It is therefore not necessary to differentiate between faults in zone 4 and zone 5

in order to realise the desired functionality of PoC-DC2.

A distinction is made between communication-based protection systems and protection systems without communication [17]. The coordination of several protection devices of the same PoC is often assumed and not described as communication-based.

Protection without communication is based on local measurements and settings/dimensioning [17]. Additional components that change the fault characteristics, e. g. additional current limiters or inductors, facilitate fault localisation and thus the realisation of the required reaction of the protective device. However, such components
generally also make fault detection more difficult and may reduce the tripping capacity of protective devices [18]. Model-based approaches compare the current system behaviour with stored data sets or system simulations in real time for fault detection and localisation [19].

Communication-based protection is characterised by simplified fault localisation and high selectivity. However, boundary conditions for the communication speed must be considered. [20]

A possible communication-based protection strategy for realizing the required protective reaction of PoC-DC2 is shown in Figure 6. The fault location is determined based on the fault current direction at PoC-DC2, PoC-DC3 and PoC-DC4, which is communicated to PoC-DC2. Fault localization is possible immediately after fault detection, which allows PoC-DC2 to directly enter the operating mode specified in Table III depending on the fault location.

<figure><img src="image_6.png" /><figcaption>Fig. 6: Flow-diagram of required protective reaction of PoC-DC2 and actualizing fault localization mechanism based on communication between PoCs</figcaption></figure>

In general, however, the required operational mode per PoC according to Table III does not have to be the initial reaction of the PoCs after fault occurrence. Requirements for fault localization can be relaxed if other intermediate operating modes from Figure 4 are permitted: “Fault-ride-through” or “Fault interruption and restart”.

Intermediate “Fault-ride-through” can increase the ability of protective devices close to the fault to trip. The interruption of the fault current (or the absence of the interruption) provides the PoC under consideration with additional information for fault localization. During fault-ride-through operation, the PoC in question must guarantee self-protection. This can be realized, for example, by monitoring the load integral. According to Figure 5, PoC-DC2 should remain in fault-ride-through mode for faults in zones 4 and 5. Fault interruption and fault isolation are to be implemented at PoC-DC4 in accordance with Table III. It is not necessary to differentiate between faults in zones 4, 5 and zones 1, 3 directly after the fault occurs if initial fault-ride-through for PoC-DC2 is also permitted for faults in zones 1 and 3. For faults in zones 4 and 5, PoC-DC4 will interrupt the fault during the initial fault-ride-through, but not for faults in zones 1 and 3.

Intermediate “Fault interruption and restart” provides galvanic disconnectors close to the fault with the opportunity to isolate the fault. The fault isolation (or lack of isolation) can be detected by the PoC after restart, which provides the PoC in question with additional information for fault localization. According to Figure 5, PoC-DC2 should switch off temporarily for faults in zone 3 (Fault interruption and restart). Fault isolation is to be implemented at PoC-DC3 in accordance with Table III. It is not necessary to differentiate between faults in zone 1 and zone 3 directly after the fault occurs if temporary shutdown of PoC-DC2 is also permitted for faults in zone 1. For faults in zone 3, PoC-DC3 will galvanically isolate the fault during the resulting current zero, but not for faults in zone 1.

Both of these intermediate operating states are used in the alternative protection strategy for realizing the required protective reaction of PoC-DC2 shown in Figure 7, which enables fault localization and selectivity even without communication between the PoCs.
<figure><img src="image_7.png" /><figcaption>Fig. 7: Flow-diagram of required protective reaction of PoC-DC2 and actualizing fault localization mechanism without communication between PoCs</figcaption></figure>

The protection strategy for realizing the required behaviour of PoC-DC2 shown in Figure 6 requires the differentiation of four different protection zones after initial fault occurrence. The protection strategy shown in Figure 7, on the other hand, only requires the differentiation of two protection areas after the initial occurrence of a fault. The fault is then further localized during the fault clearing process by the reaction of other PoCs (or the absence of their protective reactions).

The protective action sequences in Figures 6 and 7 can also be understood as state machines and implemented as such per PoC. It is critical that both fault clearing processes shown in Figures 6 and 7 realize the final protective reaction PoC-DC2 required according to Table III.

In order to enable proper interplay of all protective actions, the timing of the determined protection sequences must also be considered. For example, the fault interruption of protective devices at PoC-DC2 must be coordinated with PoC-DC1 in order to enable galvanic fault isolation at PoC-DC3 for faults in zone 3 in accordance with Table III.

The result of considerations on system level is the definition of the required protection action sequence for each PoC, shown in Figure 6 and 7 as an example for PoC-DC2. Associated fault localization mechanisms are defined and the timing of protective actions of all PoCs are taken into account.

## Component level – Selection of protective devices, Dimensioning

At component level, protective devices are placed at PoCs to realize their required protective action sequences defined on system level. When selecting PDs, their respective capabilities must

be taken into account, shown as an example in Table IV. Table IV does not claim completeness, as the capabilities of protective devices are always component-specific. As shown as an example in Table IV, the current limiting control of power electronics is not able to provide galvanic isolation by itself. An additional protective device to provide galvanic isolation is needed, should the protective action sequence defined on system level call for that.

<table><caption>Table IV: Exemplary representation of the capabilities of PDs to realize the PoC-operating states in Figure 4</caption><thead><tr><th>Protective device</th><th>Fault-ride-through</th><th>Fault interruption and restart</th><th>Fault interruption and optional restart</th><th>Fault interruption and isolation</th><th>Fault isolation</th></tr></thead><tbody><tr><td>Current limiting control</td><td>✓</td><td>✓</td><td>✓</td><td></td><td></td></tr><tr><td>Fast reclosing circuit-breaker without galvanic isolation</td><td>✓</td><td>✓</td><td>✓</td><td></td><td></td></tr><tr><td>Slow reclosing circuit-breaker with galvanic isolation</td><td>✓</td><td></td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Disconnector</td><td>✓</td><td></td><td></td><td></td><td>✓</td></tr></tbody></table>

At this point, it should be noted that the challenges in the design of protective devices are dependent on voltage level and generally much lower in low-voltage [13]. The dimensioning of protective devices and trigger thresholds requires methods for estimating the fault current. This will be examined in more detail in a future publication.

## Conclusion

The use of a DC allows efficiency increases in energy transmission over long distances (HVDC) [3-5] and in the connection of direct current components by eliminating DC/AC and AC/DC conversion losses [1,2]. One of the obstacles to the widespread use of DC is the lack of
standardization regarding the grid protection
design process. Unlike breaker-based protection
methods [9,10], breaker-less protection [10-12]
allows fault interruption and galvanic fault
isolation at different points in the network. Power
electronics may assume the role of fault-
interrupting protective devices, thus not only
taking responsibility for self-protection, but also
for grid protection. Coordination with other fault
interrupting protective devices and galvanic
disconnectors is critical and requires a holistic
protection design methodology combining
breaker-based and breaker-less protection
methods.

This paper proposes a three-step protection
design process, separated into concept, system
and component level, for line-to-line faults in
general DC systems.

On concept level, boundary conditions on power
flow during and after the fault clearing process
resulting from requirements analysis are
determined. Required power flow is defined using
operational modes according to CLC/TS 50654-
1: 2020 [13], the methodology of which is
transferred from HVDC to general DC networks.
On system level, final protective actions for fault
clearing at PoCs are defined and shown in tabular
form. Fault-localizing mechanisms are chosen to
enable the realization of these desired protective
reactions during operation. Requirements on fault
localization may be relaxed if intermediate
operating modes for PoCs are permitted, resulting
in the definition of protective action sequence
diagrams for each PoC. The timing of all
protective actions is also considered in order to
guarantee proper interplay.

On component level, protective devices are then
placed at PoCs to realize their previously defined
protective action sequences. Finally, protective
devices are dimensioned and trigger thresholds
are selected in order to enable the required
reaction of all components in case of a fault.

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