The Status of DC Micro-Grid Protection

Robert M. Cuzner DRS Power and Control Technologies

Milwaukee, WI

Giri Venkataramanan University of Wisconsin-Madison

Madison, WI Abstract: AC microgrids are a convenient approach to integrating distributed energy systems with utility power systems. On the other hand, DC micro-grids can lead to more efficient integration of distributed generation. They are the preferred topology for present shipboard, aircraft and automotive power systems and hold promise for future environmentally friendly office buildings, homes, rural areas and industrial power parks. However, standards, guidelines, practical experience and cost effective implementations for DC system protection are well behind practices in AC system protection. This paper presents a comprehensive overview of the body of research on protection of DC micro-grids, presented with a goal of identifying and advancing the field. The paper presents a discussion of the current status of dc micro-grid protection, including the use of electro-mechanical circuit breakers, solid state circuit breakers, protective system design, ground fault location and fault isolation.

I. INTRODUCTION Installations of micro-grids continue to proliferate as a

viable solution to the problems of greenhouse gas, energy growth demand and the depletion of energy resources through enabling the use of renewable and distributed generation systems. From a practical standpoint, micro-grid implementations are AC systems. Arguably, DC micro-grids present an effective means of distributing high quality power more efficiently to residential, urban and rural areas and to commercial facilities. They will enable Distributed Energy Resources (DER) usage worldwide at various levels of power delivery and increase the efficiency with which multiple renewable energy sources such as photo-voltaic cells, fuel cells and wind power can provide aggregate power to a group of loads. Because both loads and sources can interface to a common DC bus with fewer redundant stages of power conversion, the result is less waste heat and potentially lower cost than AC based implementations of DER. However, AC systems continue to be preferred as opposed to DC systems due to the lack of economically realizable DC micro-grid protection means, standards and technologies.

This paper presents an overview of the body of research and applications of DC micro-grids protection systems to date. The intended contribution of this work is to identify the most viable approaches and to point out the technological gaps that exist in an effort to highlight opportunities for further research and, more importantly, facilitate the future development of commercially viable approaches.

II. MICRO-GRIDS A working definition of micro-grid is a distribution-

level network of generators and loads that can exchange power with other networks, each through a single gateway. The concept of the micro-grid began as a solution to meeting energy demands while avoiding unpopular and costly expansion of centralized utility power generation and distribution capacity [1]-[3]. The idea is to augment the capacity of the utility grid by allowing distributed power generation sources, installed locally to sub-stations or at large commercial facilities, in order to contribute power to loads connected to a local grid. Micro-grids are a keystone of the movement towards environmentally friendly power delivery and meeting the needs of the growing power market in the third world because: (a) they enable the use of renewable resources; and (b) they are better suited for the electrification of rural areas [2]. Several micro-grids have been or are currently being installed worldwide [4].

A typical micro-grid structure is shown in Fig. 1 which connects to the main utility grid through a controllable switch S1. Additional power sources connect to the micro-grid through controllable switches S2-S5 while the loads connect to the micro-grid through circuit breakers. The micro-grid can be installed with lower risk and reduced installation costs if over-laid on a conventional AC distribution system. In this way the renewable energy sources are purchased as turn-key systems and their interface to the utility (S1-S5) is through AC contactors. Continuity of power to the loads on the micro-grid may be ensured by a centralized control which manages the connection of more than one redundant power sources to the grid along with the power delivered to the grid. The system relies upon proven conventional AC 60Hz protective relaying practices for fault protection

III. DC MICRO-GRIDS Fig. 2 shows a notional representation of a DC micro-

grid, which interfaces all sources and loads through a single DC bus, or DC micro-grid. The protective devices in the system are represented in the same way as for the AC micro-grid in Fig. 1 with the exception that the more generic block “PD” is utilized for protective devices in the DC bus, since these are not likely to be circuit breakers.

On contrast, the most obvious advantage to DC micro-grid is the reduction in the number of power electronic converters required. Other key advantages are the ease with which multiple sources can parallel to a single bus, allowing for a more robust de-centralized approach to power flow management using voltage droop, and improved ride-

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through capability when energy sources drop off of the bus [5], [6].

The DC micro-grid is an outgrowth of the concept of multi-terminal DC (MTDC) systems first proposed as distributed sub-stations for HVDC power transmission [7]. The earliest body of research on the subject was motivated by an immense interest in the early 1990s in exploiting high temperature super-conducting for HVDC transmission [8] and for LVDC Super-conducting Magnetic Energy Storage (SMES) [9].

The idea that Low Voltage DC (LVDC) micro-grids may become common place for residential homes, hospitals, businesses and factories began with the recognition of the trend towards DC fed loads [11] in the early 1990s and the need pursuit of more effective ways to provide battery back-up to sensitive loads. The DC micro-grid has gained popularity with the growing trend toward addition of fuel cell and photovoltaic energy sources to buildings and facilities [12], [13] in the late 1990s.

Multiple source/load DC distribution systems are being implemented into Navy shipboard power systems because of the need and opportunity for redundantly fed power distribution architectures, power system automation and resultant reduced manning requirements and the potential ease of integration with electric propulsion [14]. Aircraft and automotive systems are also trending towards similar DC distribution systems in order to replace mechanical, hydraulic and pneumatic loads with electric ones and to realize a significant potential for increased fuel economy and performance [15], [16].

Starting in the late 1990s there has been a resurgence of research in MTDC systems—where power is taken from more than one independent source and distributed to multiple loads from a DC bus or mesh—including applications to urban areas for large cities [17], premium power quality parks [18], offshore wind power [19], industrial systems [20] and office buildings [21], [22].

IV. DC MICRO-GRID PROTECTION All of the examples cited in Section III may be classified as DC micro-grids and can be notionally represented by Fig. 2. DC voltages for the applications mentioned are in the range of 270V to 1000V. So far, most DC micro-grid implementations have been developmental or for research purposes with each project having its own set of specifications—without recommended guidelines or definitive industry standards. Remarkably, DC traction systems operate under the same paradigm, even though they have been in existence for many decades [23]. In these circumstances, it should be noted that since an overriding operational requirement of any electrical system is the safety [24] and DC micro-grids will not become viable on a large scale until the lack of standards and guidelines is addressed. The safety of the electrical system is translated into appropriate grounding practices, and protection of equipment from electrical faults. The largest safety concerns in DC systems are arcing, and the potential for fire, human touch potential and down-stream loads.

Fig. 1 Micro-Grid Structure

Fig. 2 DC Micro-Grid Structure

The challenge facing DC micro-grid protection is not only the lack of standards and guidelines, but also the relative lack of practical experience. For this reason it is important to understand the work has been done in the area, identify what principles for AC system protection may be applied and where conventional AC system protection falls short, and to apply lessons can be learned in the DC traction industry.

Key design criteria for any protective system [26] are: Reliability—Predictability of the protective system

response to faults and dependability to not trip spuriously on transients or noise

Speed—Fault is removed from the system and normal operating voltages is rapidly restored.

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Performance—Continuity of service to the loads; where a lower performing system loses a significant portion of its loads when a fault occurs

Economics—Installation and recurring costs; generally in opposition to performance, i.e. a good performing system generally costs more.

Simplicity—Quantity of parts, zones of protection, level control de-centralization needed to ensure its reliability

In this context, the remainder of this paper documents the status of DC micro-grid protection as reported in available literature and then to evaluate feasible approaches on the basis of the above criteria. The intended contribution is to help identify the issues and solutions in an attempt to find a direction towards commercially feasible implementation of DC micro-grid protection and in the future. In particular, three aspects of micro-grid protection are considered: (A) protective devices, (B) protective system design and (C) grounding and ground fault isolation. A. Protective Devices

Commercially viable approaches to the protection of DC distribution systems utilize fusing [27] and circuit breakers [28] both of which begin the process of opening a current flow path upon an over-current condition with an arc. The devices are designed so that the length of the arc increases as the over-current condition is sustained until metal is melted or contacts are opened—so that current is forced to zero (and the arc is extinguished) as the arc voltage builds up. Commercially available fuse and circuit breaker designs are optimized by the assumption that the zero current condition will be aided by the natural zero current condition of an AC system, since the vast majority of power distribution systems are AC. Therefore, these devices are sub-optimally applied to DC systems at best and, in general, appropriate de-ratings combined with a good understanding of the L/R characteristics of the application are required [27]-[29]. Inevitably, the practical and successful application of these devices to DC systems is limited by cable inductance and resistance, the DC voltage level and any line to line capacitance that is installed upstream and downstream of the circuit breakers.

1) Fuses The application of fuses for DC systems is principally in

the areas of traction, mining, battery protection, auxiliary low voltage power supplies and power circuit protection and for voltage levels up to 4200V. Fuses are ideally applied to DC systems having a high di/dt (or low inductance) keeping the time for the fuse to reach its melting point to a minimum [27]. From the standpoints of reliability and simplicity, fuses are not a good solution to DC micro-grid protection because of constraints that would have to be placed on the distribution cable length and difficult to predict transient effects of fuse opening on the micro-grid voltage.

2) Circuit breakers Two excellent papers [21], [22] present case studies on a

DC distribution within a commercial facility. Although such systems have yet to be implemented the papers point out that a conventional distribution system utilizing molded

case circuit breakers for protection is feasible for a 325VDC micro-grid. The inherent arc interruption voltage developed by AC circuit breakers can be exploited for current interruption in DC systems by connecting contactors in series until sufficient voltage blocking capability is achieved.

The circuit breaker approach may be economically feasible for DC micro-grids of less than 600V; however, there are additional factors that must be considered. Referring to Fig. 2 some of the sources interfacing to the micro-grid are necessarily conditioned by power electronic converters, namely the utility feed itself and any variable frequency source such as a wind power generation system or a micro-turbine. Additionally, DC to DC power converters may be inserted between the micro-grid and load or groups of loads in order to produce varying levels of voltage or to provide high frequency isolation. These power converters usually require low inductance, line to line and line to ground capacitive filtering as shown in Fig. 3 in order to maintain other important performance criteria at the micro-grid or at points of common coupling with other loads, such as low voltage ripple, stiff voltage response to load shifts and low conducted EMI. When a short circuit is applied downstream of the converter the capacitors rapidly discharge into the fault resulting in a current surge with amplitudes anywhere from 10,000 to 50,000A (see Fig. 4), depending on the filter design, the location of the fault with respect to the converter source and generally related to the installed capacity of the converter.

The impact of capacitive discharge on protection schemes using circuit breakers has been identified specifically for telecommunications applications of DC systems [30] and Navy shipboard implementations of DC micro-grids [31]. The main impacts of capacitive discharge are that both the circuit breaker from the feed and the circuit breaker closest to the fault may trip and the potential for damage to circuit breakers—or worse, the upstream circuit breaker may trip while the circuit breaker closer to the fault does not. Additionally, because the loads on a DC micro-grid generally have significant input capacitance, capacitor discharge from loads adjacent to faults contributes to this problem and inrush current from loads being applied to the system can potentially cause unwanted circuit breaker trips.

The problem of potential circuit breaker damage is illustrated by considering the process by which standard current limiting molded case circuit breakers open when their instantaneous trip is exceeded. An initial force blows open the contacts, an arc develops between them and an electromagnetic coil forces the contacts to open completely. The current peak starts off the process, but the final opening of the contacts does not occur if the peak current is not sustained at a high enough current level for a long enough period of time. The danger is that the initial opening and arcing occurs rather easily, but sufficient force to guarantee that the contacts completely open cannot be guaranteed. Especially for highly inductive systems there is a potential for the contacts to weld closed during the fault.

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Fig. 3 Typical output filter of upstream DC to DC

power converter

Fig. 4 Typical capacitive discharge current profiles resulting to fault downstream of DC to DC converter

Assuming that the circuit breaker(s) open safely during the fault, there is a high likelihood that both upstream and downstream circuit breakers will trip [30]. When this occurs the protection scheme loses its potential for selectivity during fault isolation and a larger portion of loads on the micro-grid are lost. Because of this fact, circuit breaker time/trip coordination techniques are virtually impossible in DC micro-grids unless low voltage power circuit breakers, which will ride through the initial capacitive discharge, are used for the upstream feeds [32]. These devices typically cost more and are significantly larger than equivalent molded case circuit breakers.

DC circuit breakers also limit the speed of fault isolation and recovery, which has a detrimental effect on system performance. This limitation has been addressed through the use of vacuum circuit breakers and hybrid solid state/vacuum interrupters which interrupt over a shorter time span and tend to limit fault current in the process [33].

The traditional, low-cost arcing circuit breakers are inadequate for application to higher DC voltage systems or as static switches S1-S5 in Fig. 2, protective devices between the micro-grid main power converter feeds in the system, or to isolate buses or protective zones. Krstic, et al [34]provide an overview of arc stretching and splitting approaches to increase the application to higher voltages. A

number of devices are available commercially but at greater cost and limited current ranges.

Means for extinguishing the arc or providing a bypass path for arc current through external snubbing, resonant circuits and hybrid solid state approaches have also been explored. These are reviewed by Meyer et al. [35] with the conclusion that speed of response is compromised while a significant amount of let-through current is passed on to the system during fault interruption.

3) No load switches An alternative approach to brute force interruption of

current through an electro-mechanical switch has been proposed by Tang, et al for MTDC systems [36] and is also used in Navy shipboard DC Zonal Electric Distribution (DCZED) systems [14]. Fast no load switches are used to isolate portions or zones of the system. Power converter(s) feeding the system de-energize the bus(s) where a fault is detected, isolate the fault with no load switches and then re-energize remaining healthy system. The approach is inherently slow because of time required to de-energize and re-energize buses.

4) Solid state switches An entirely solid state equivalent to a circuit breaker has

the benefit of being able to interrupt current on the order of μsec. Such devices are interchangeably referred to as active current limiting switches or active current interrupters because they autonomously sense an over-current and either hold current to a set limit or drive the current immediately to zero. Active current limiters and current interrupters have been accomplished in a myriad of ways [37], [34].

Navy shipboard applications have been a significant impetus for the development of solid state current interrupters. Presently L-3 Power Paragon Inc. offers high speed solid state static transfer switches for AC systems that can be converted to current interrupters suitable for 700V DC micro-grids, and capable of interrupting 500A with the addition of energy and voltage suppressors [38]. It is important to note that a self-contained solid state circuit breaker (notionally represented in Fig. 5) that can be applied in the same way as its electro-mechanical counterpart must carry with it sufficient current limiting inductance and voltage clamping circuitry to handle the displaced stored energy during interruption. The size, weight and cost of the solid state circuit breaker offerings presently fielded are not insignificant, especially when one considers that the circuit of Fig. 5 is normally required in each power line of the feed. These drawbacks have limited the application of solid state current interrupters and kept them from penetrating industrial and commercial markets.

More recent trends exploit the use of new power semiconductor developments in order to accomplish more power dense and potentially cost effective solid state interrupters. An IGCT-based current limiter proposed by Meyer, et al [39] has the potential for greater simplicity and robustness compared to prior GTO- and IGBT-based counterparts.

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Even more compelling is the recent development of the Super GTO by Vic Temple and Silicon Power Corp. [40], which is extremely power dense and has been demonstrated in a 10kV, 80kA pulsed power application [41]. It is easy to see how this device could be utilized in a current interrupter such as the one shown in Fig. 5.

Load protective current limiters as shown in Fig. 6 have been proposed as a method for protecting for faults at the load [42]. Multiples of these devices could be deployed downstream of the DC micro-grid or a bulk DC to DC converter feeding a cluster of loads such that a single load fault does not cause the bus feeding other loads to collapse. For very low current feeds the current limiting inductor could be wiring to the load and the voltage clamp could be a simple MOV. This approach has been proposed for aircraft and space station applications [15], [43].

For clusters of higher current loads a solid state switchboard as proposed by Krstic, et al [34] contains multiples of the solid state current interrupters of Fig. 5 in the same way as circuit breakers are in a load center or power panel, only there is a single shared voltage clamp for all devices. The voltage clamp is sized with the assumption that not all of the devices will interrupt a fault at once.

Fig. 5 Solid State Current Interrupter

Fig. 6 Load Protective Current Limiter

B. Protective System Design It has long been recognized that the power conversion

components that necessarily make up a DC micro-grid already contain protective capabilities which could be exploited, rather than proliferating the number of additional protective devices and their associated losses and costs [36], [37], [44], [45]. This is a paradigm shift from AC system protection. DC micro-grid designers will need a good understanding of the interactions between power conversion and protective modes of operation, operating scenarios and system responses in order to design a reliable system.

Reliable implementation of micro-grids depends on two key concepts: peer-to-peer and plug and play for each component of the micro-grid [46]. Peer-to-peer means that faulted components are located and isolated without centralized control or inter-module communications. Plug and play means components installed such that their removal does not affect the behavior of other components.

A review of the literature on micro-grids reveals the following fundamental requirements for a safe, reliable and well-performing DC micro-grid [2], [5], [6], [14], [36], [46]-[48]:

1. No single point of failure 2. Redundantly fed electrical zones or buses that can

be rapidly separated from the bulk power system 3. Ability to rapidly re-configure the system, island the

system and re-direct power away from a fault while minimizing interruption of power to non-faulted portions of the system

4. Ability to locate and isolate faults without the need for inter-component communications

5. Ability to recover from a fault to a reliable configuration and mode of operation

6. Load prioritization into sensitive vs. non-sensitive 7. Minimize the effect of a faulted load on other loads 8. Redundant feeds to sensitive loads 9. Ability to quickly shed non-sensitive loads 10. Load and power flow management from a

centralized control 11. Transient operational capability in degraded

modes, such as loss cooling and loss of power flow management

12. Condition-based maintenance The degree to which these requirements are met will be

a matter of cost and the performance needs of the system. However, the future need is for the adoption of accepted standards in meeting these requirements, which will motivate economic solutions. The current practice for utility-connected DER systems is to return the utility systems to its original configuration with all DER units de-energized when an unexpected disturbance occurs [48]. This level of system dependability is a long way from a micro-grid protection system adequacy that is comparable to AC systems conventions.

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ZONE 3ZONE

3

BUS 3

BUS 2

Fig. 7 Multi-Terminal DC System

Fig. 8 DC Zonal Electric Distribution System

Fig. 9 Micro-Grid Electrical Zone

Two examples of DC micro-grid systems presently being researched and implemented represent examples of how some of the above DC micro-grid requirements are can be met: the MTDC system proposed by Tang and Ooi [36], shown in Fig. 7 and the DCZED system [14] shown in Fig. 8. Both of these systems have architectures which allow them to meet requirements 1 through 5.

For example, the MTDC of Fig. 7 is sub-divided into three or more zones and utilizes a handshaking method to isolate a fault on any of the buses associated with the zones—Bus 1, Bus2 or Bus 3—each fed by a voltage source converter (VSC) through a cluster of two or more switches tied to a common feed point. When a fault occurs on Bus 1 an over-current is detected by all three the voltage source converters, VSC 1, VSC2 and VSC3 so all three rapidly bring their outputs voltages to zero. Each VSC opens one of the three no load switches associated with it that has largest positive (outgoing) magnitude—in this S11, S21 and S33

are opened. The result is that each non-faulted bus has a static switch open on only one end, therefore, each VSC is ramped back up to full voltage and voltage is measured on the output of each of its switches. No voltage will be sensed on the faulted bus, so the output switch is not closed. If a voltage is sensed then this means the bus is healthy and its output switch can be re-closed. In this way a faulted bus has been isolated without any of the three VSCs communicating with each other. When the fault occurs, loads on the healthy buses encounter a small interruption in power but are quickly restored to an operative state once the handshaking process is complete.

The DCZED system of Fig. 8 is sub-divided into two or more zones, each having two redundant DC buses, which can be isolated by static switches on each end and can receive power from a VSC in its associated zone or from a VSC in another zone via its connection to adjacent buses. The VSC in each zone can supply power to one of the redundant buses in each zone. Assume initially that the system is in a split bus condition, with VSC 1 feeding Bus 11 and VSC 2 feeding Bus 22, with all inter-zonal static switches close. If a fault occurs on Bus11, the condition will be sensed at the output of VSC 1 and at the switches S15, S13 and S22. VSC 1 will rapidly bring its output to zero while VSC 2 is unaffected. VSC 1 will know immediately that the fault is on either Bus 11 or Bus 21, so VSC 1 will open its associated S01 and can quickly ramp up its output, synchronize with the voltage sensed on S02 and close into Bus 12 and Bus 22. S15 will automatically open after a sensed over-current condition and subsequent removal of voltage on both sides and it will remain open. Only the switches on Bus 11 and Bus 21 that have positive (outgoing) sensed over-current will open after subsequent removal of voltage. This selection ensures connections to healthy adjacent buses are not erroneously severed. The response of this system is potentially more robust than that of the MTDC because voltage is never completely removed from the healthy redundant bus in the zone. It is possible that the system will be in a temporary overload condition while the VSC that was feeding the faulted bus is re-configured and paralleled into its healthy bus.

As an example of how a system would meet requirements 6 through 9, consider Fig. 9, which is a forward looking architecture for the electrical system within a zone based on the DCZED system of [14]. Here all of the loads within a zone are fed from the two redundant buses. For the fault scenario described above, there is continuity of power to the AC and DC power supplied to the loads during the event because the protective devices feeding the DC to AC inverter and DC to DC converter (PD) are essentially auctioneering diodes in the forward direction with the additional ability to interrupt current if a fault occurs downstream. Loads are also partitioned into sensitive and non-sensitive loads with the non-sensitive loads fed through PDs that are fast solid state switches. This fast load shed may be necessary in the event of certain catastrophic events, such as the severe reduction in power source capability, in

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order to keep the buses feeding sensitive loads from collapsing. In this notional system all other inter-zonal protection is accomplished with conventional circuit breakers. C. Grounding and Line to Ground Fault Isolation

The final area in micro-grid protection is grounding and ground fault isolation. DC micro-grids need to be floating systems—i.e. there are very high DC and AC impedances between any line and ground for the following reasons:

o Off-shore wind power systems require underwater cabling which need to be ungrounded to avoid corrosive effects

o Navy shipboard systems are floating so that if a ground fault occurs on anywhere continuity of service can continue to essential loads [49].

o Industrial systems have a large number of DC to AC inverters and variable frequency drives which create common mode voltage reflected as “neutral shift” voltages at the neutrals of three phase loads [50], [51]; grounding would cause a DC offset, increased neutral currents and resultant stresses to the loads.

o A similar phenomenon will occur when multiple generation sources feed the grid unless the paths for neutral currents are broken through transformer isolation [51].

Ground fault detection and location will be a necessary element of DC micro-grid protection. Although many applications can operate with a ground fault indefinitely, the incident of a second ground fault to the system could lead to undesirable effects. One particular area of concern is a DC micro-grid having a DC to AC inverter with downstream molded case circuit breakers. If simultaneous DC and AC ground faults down steam circuit breakers may begin to open but the arc may not extinguish due to persistent DC currents leading to equipment damage and possibly fire. It will be a necessity for power conversion or fast electronic protective devices upstream to detect a dual ground fault and isolate it from the system.

Detecting a ground fault within the system is generally easy because of voltage offsets. However, locating the ground fault is a more difficult problem because the voltage offset is constant throughout the system. The way to locate the ground fault is to create a path for common mode current to flow into the ground fault and then determine into which part of the system the ground current is flowing. This can be especially challenging when a high impedance ground fault is to be located and isolated—which will be a necessity in order to ensure the safety of DC micro-grids installed in offices and homes.

Although there has been considerable research and practical experience with ground fault detection and isolation in AC systems, the same cannot be said about DC systems, especially those having the particular characteristics of DC micro-grids. Perhaps the only area to draw from is DC traction systems, which have to float because of the requirement for underground cabling. Still, DC traction ground fault isolation relies on creating a

resistive path for current to flow through [23], which is problematic for DC micro-grids because of neutral shift.

Although there is some published work on DC micro-grid ground fault isolation based on capacitive current sensing and superposition of DC offsets [51], this is an area where more research and innovative ideas are required.

V. CONCLUSIONS This paper has presented a comprehensive overview of

the body of research in the area of DC micro-grid protection. The overwhelming conclusion is that cost effective and reliable DC micro-grid protection is in an infant state compared to AC system protection.

Power dense and lower conduction loss devices are required, with an emphasis on packaging, higher levels of integration with the surrounding system or with self-contained energy absorption components and lower cost. Solutions for low cost down stream solid state DC circuit breakers in order to overcome the complications to circuit breaker coordination introduced by capacitor discharge effects.

Protective system design requires taking advantage of the protective capabilities of power conversion components. High speed, high power AC to DC converters that interface power sources to the grid and include the functionality of the static switch between source and grid would enhance speed and simplicity. Intelligent system interruption, fault isolation and rapid recovery as opposed to reliance upon external protective devices.

Effective means for ground fault detection and isolation are required in order to ensure system safety.

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