High voltage DC (HVDC) systems have proved their worth in the transmission of electricity over long distances. DC network technology is being extended to medium voltage (MVDC) and low voltage (LVDC) networks, where there are significant advantages for certain types of network. DC is being considered as an alternative to AC in sparsely populated areas, or low load networks. The nature of DC makes it an ideal candidate for energy storage as a feature from the outset.
The DC network concept seeks to leverage the availability and development of high power electronic converters while customers have the choice of either an AC system or a combined AC/DC system. Developments in the field of electronic converters, spurred on by the expansion of renewable energy and smart grid systems, makes it possible to provide economic and safe converters for DC networks, which allow the advantages of DC distribution to be exploited.
In the field of solar power, inverters capable of handling input voltages from 1000 to 1500 V DC and output capacities up to 300 kVA are common and are well developed. FACTS devices and systems also use high power electronics. The smart functionalities of power electronics together with embedded ICT provide the means to meet the stringent demands imposed on the cost efficiency and service quality of electricity transmission and distribution using MV and LV DC networks .
DC transmission and distribution has an application for low load networks, being targeted at remote communities, sparsely populated areas, offshore wind farms and industrial applications. MVDC transmission is a relatively new concept operating in the range of 15 to 50 kV and 30 to 150 MW capacity. MVDC represents a viable alternative to AC interconnection to provide power to remote communities. While these communities typically have low loads (1 to 10 MW), a high voltage AC grid (for example, 138 kV) is required due to long interconnection distances. Such AC systems could transfer 100 to 200 MW of energy, far in excess of what is required. Using MVDC offers a cost-efficient, scalable method of achieving benefits of interconnection .
With DC, energy losses are generally lower, giving a higher power transfer capability at a similar voltage level. Greater distances can be covered and the flow of power can be controlled. This is becoming important since energy generation takes place at all levels, leading to undirected energy flows which jeopardise the stability of power grids. Another advantage of DC networks is that they can be decoupled and connected regardless of frequency, voltage and quality. For instance, a network which is very stable in terms of frequency and voltage and has a low harmonic content, can be coupled to an unstable network using DC .
A DC link is able to transmit a higher capacity than an AC link of the same size. If an AC link is converted to DC, the capacity is improved. Where there is an existing network infrastructure the output of a three-phase network can be improved by approximately 20% by coupling DC links with it. With four- or six-phase systems the performance increase is 60 and 80% respectively. With the existing AC technology the network would have to be upgraded to higher voltage levels .
MVDC has the capability to handle bidirectional flow of electricity. This is particularly important in networks where renewable energy plant is installed. The development of bidirectional converters for low voltage applications is advanced, and the same design principles can be applied to MVDC equipment.
DC networks make use of four quadrant modular multilevel converters (MMC) based on voltage source (VSC) technology. A typical example is shown in Fig. 1. The number of modules are adapted to the DC voltage level used. Earlier units were based on current source converters but were limited in application.
There is an overlap between the MVDC transmission and MVDC distribution sectors, the terms, and technology being used interchangeably. Although distribution is traditionally from a bulk supply point towards the load, and transmission between points in the network.
There are a number of different configuration that could be used;
MVAC-MVDC-MVAC ( Fig. 2)
This is a point to point or DC link connection and may use bidirectional energy flows. The rectifier inverters may incorporate reactive power capabilities. This configuration is usually found in MVDC transmission applications
MVAC–MVDC–LVAC distribution network (Fig. 3)
This replaces an MV network on a lightly loaded route and ends in a radial LVAC network. This provides an alternative to the situation where each consumer requires an Inverter, and will depend on the distance between consumers. The configuration is used where clusters of consumers exist.
MVAC-MVDC-LVDC distribution network (Fig. 4)
This application requires a network DC/DC converter , or multilevel high frequency link transformer (MDCT). These devices are under development.
MVAC-LVDC distribution network (Fig. 5)
This is the area where most of the development is taking place, especially as a replacement for AC distribution in sparsely populated area with low loads. The use of DC/AC inverters at customer premises is made economic by developments in technology. Costs are lower when compared with the situation where each customer requires a transformer, in the MVAC distribution system.
Advantages over AC
By converting a circuit to MVDC, it is in theory possible to increase the specific transfer capacity of that circuit compared to the nominal AC rating . Clearly as this is now fully controllable, it may have additional network capacity increases (i.e. removal of power-flow or voltage limitations). The MVDC capacity increase is based on the following factors :
In contrast to conventional HVDC systems, MVDC requires high voltages levels but very low-current transmission. For example, ±50 kV at 10 A is equal to 1 MW of energy transfer, enough to address the needs of most remote communities.
Protection for DC networks
Protection issues are one of the main challenges in the development of the VSC-based DC networks. Due to the special behaviour of the DC fault currents, it is almost impossible to coordinate the over-current relays based on the time inverse grading used in AC circuits.
In addition power electronics cannot provide high short-circuit current traditionally used for fault detection .Unlike AC systems where a natural zero crossing of the current is utilised for opening a circuit breaker and fault isolation, short-circuit currents in DC systems must be interrupted at high values to open the faulted branch. Generally most approaches for DC short circuit protection can be divided into “breaker-less” and “breaker-based” schemes. The former utilises coordinated control of power converters to interrupt first the current and then no-load mechanical contactors to isolate the faulted section, as well as reconfigure and re-energise the system . A breaker-based approach should provide more flexibility because the circuit breaker should isolate the fault but enable continued operation of the non-faulted system.
The principles of protection apply to both MVDC and LVDC networks. Problems with applying conventional protection method exist because of two reasons:
To overcome these problems several unique systems have been adopted based on the DC fault characteristics. Line fault characteristics are affected by the inverter overcurrent characteristic and exhibit the following general behaviour; The overall fault characteristic is that DC voltage drops rapidly to the steady-state value, while DC current increases rapidly and then decreases to the steady-state value.
Fig. 6 shows a typical fault-response curve.
In AC side protection the AC supply to the converter is interrupted in the case of a fault on the DC line, shutting off the DC side, and disabling the whole system. A DC short-circuit is also fed by the AC network through the freewheeling diodes of converter. Hence, the circuit breaker at the AC terminals of this converter will ultimately trip.
MVDC transmission systems
The border between transmission and distribution is somewhat blurred, as the same technology will be used for MVDC systems in both fields. The distinction drawn is the configuration of the system. Transmission systems can be primarily classified as connecting together two AC networks. The DC link has the advantage of being able to connect together networks without issues of phase , frequency variation and strength without. This could be a future application for micro-grids where network stability and strength are not guaranteed.
Example: Siemens MVDC Plus system
Examples are the Siemens MVDC PLUS (MVDC power link universal system),which operates in the range 20 to 50 kV and power ranges from 50 to 150 MW. The system has a claimed range of 200 km. The technology is based on the HVDC PLUS technology used in the Siemens HVDC transmission system, but is reduced to its basic functions. Like the HVDC system, the medium-voltage transmission system operates with voltage-source converters (VSC) in a modular multilevel converter design (MMC) that convert alternating current into direct current and vice versa.
The current on the transmission route can flow in both directions. Thanks to the use of insulated-gate bipolar transistors (IGBT), the commutation processes in the converter run independent of the network voltage. Both converter stations can be operated as a static synchronous compensator (statcom). The extra high-speed control and protection intervention capabilities of the converters ensure the stability of the transmission system, which reduces network faults and malfunctions in the three-phase grid. This significantly improves the security of supply for energy suppliers and energy customers alike .
LVDC distribution systems
LVDC systems have been under development for some time and several trial/pilot networks are in operation. The low voltage option finds application mainly in distribution to residential and small businesses, where the consumption takes place at low voltage levels. DC distribution systems operating at LV differ from transmission systems in that a radial architecture is used rather than a point to point or ring system. The common topology is a bipolar ±750 V DC system.
A typical configuration is shown in Fig. 7.
In the unipolar system, only two conductors are used, one of which may be earthed. The decision to earth either the positive or negative conductor determines which conductor system is used. The unipolar DC system is shown in Fig. 8.
The bipolar system uses three conductors, a positive, negative and a neutral of 0V. Voltages of +750 V DC, -750 V DC giving 1500 V DC between conductors seem to be a common choice. This may be influenced by the limit of 1500 V DC to the low voltage classification of most standards. In the bipolar connection the loads are connected between the positive and negative conductors, with an extra connection to the neutral. In the case of failure of the positive or negative, the load is provided via the remaining conductor and the neutral. In the bipolar system two unipolar systems are connected in series. In the bipolar system customers can be connected between voltage levels with multiple ways. The connection alternatives are:
Bipolar DC system with connection alternatives are shown in Fig. 9.
A large percentage of consumer loads today are DC driven, including IT equipment, lighting, VF motor drives, and electric vehicle chargers, and there is a move towards DC reticulation in residential, commercial, offices and industrial facilities. In addition, much of the distributed generation produces DC power. DC distribution is a natural response to this. MV and LV systems lend themselves naturally to battery based energy storage systems. LVDC is the main application area under consideration, as storage can be installed close to or at the consumer without voltage conversion . There are several DC distribution networks undergoing trails worldwide, and the results are promising.
 Siemens: “Siemens introduces DC transmission system for medium voltage to the market” www.siemens.com/press/PR2017100005EMEN
 A Rentschler: “Autonomous distribution grids in power ranges up to 150 megawatts”, Siemens customer magazine 25 September 2017.
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 Siemens: “MVDC PLUS : Managing the future grid” www.siemens.com/press/pool/de/events/2017/energymanagement/2017-10-euw/presentation-mvdc-plus-e.pdf
 J Partenan: “Research of Low-Voltage Direct Current Distribution”, LUT.
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 G Bathurst, et al: “MVDC – The New Technology for Distribution Networks”, Eleventh IET international conference on AC and DC power transmission.
 B Kweon, et al: “Design and operation schemes for battery storage systems in low voltage DC distribution systems considering voltage control and economic feasibility”, CIRED 23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015.
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