Analele Universităţii din Oradea
Fascicula de Energetică, Vol. 15
2009
ASPECTS REGARDING DC DISTRIBUTION SYSTEMS C.O. GECAN, M. CHINDRIŞ, G.V POP Technical University of Cluj – Napoca, 15 C. Daicoviciu
[email protected]
Abstract - The paper presents some general considerations regarding the energy distribution in DC networks as an alternative to the well known AC distribution systems. Nowadays, the DC systems can be found essentially in special applications, i.e. telecommunication, electrical traction systems. Some aspects regarding these systems are presented in the paper. In most cases, the DC systems are supplied from the existing public distribution networks. As a result, some energy conversion processes occur at the interface between the two systems. In the paper several methods of achieving this interface, based on the performances required on the both AC and DC sides, are presented. In order to avoid the conversion processes, carried out with a specific efficiency, questions arise regarding the DC supply for various suitable applications. In the context of the new worldwide energy policy, which imposes measures to produce electricity using renewable energy sources, Low and Medium Voltage DC networks can interconnect different distributed generation units. Besides Low and Medium Voltage DC networks, some aspects regarding High Voltage DC networks are also presented. Keywords - renewable energy, distribution systems, DC networks, AC-DC interface, applications
1. INTRODUCTION The demand for reliable energy is growing while society relies more and more on electricity. The occurring outages have more negative effects to the customers and outage costs increases. The attention of the end users to the electric power quality problem, the widespread use of the electronic equipments and the need to integrate the new distributed generation units have increased the importance of considering a distribution networks in direct current (DC). In the early days the first electricity distribution systems were based on DC technology but were rapidly replaced with AC systems due to its benefits compared to DC. Since many years now, DC has been used for transmitting power over long distances in High Voltage DC systems (HVDC). With HVDC, the power I.S.S.N. 1224 – 1261
transmitted over a given distance is largely increased as compared to an AC line. Despite the losses in the converter stations at the ends, losses are also greatly reduced. This is mostly due to the absence of the reactive current component, which in the AC systems increases the current magnitude and thus the losses. From technological point of view the DC distribution system is a new concept in electrical distribution and it generates a new area of business to power electronic device manufacturers. The main idea is to extend the DC section, nowadays present in many electric devices, distributed generation systems and uninterruptible power supplies, at the level of DC distribution system. DC systems at lower voltage levels are today used in very specific applications, among which telecommunication equipment, hybrid vehicles and traction systems. However, most equipment in use today in offices and households is consumer electronics that uses low DC voltage. In the present paper some considerations regarding existing DC systems are presented. Remarks concerning costs, applications fields, structure and different advantages are pointed out for High-, Medium- and Low- DC systems.
2. DC SYSTEMS APPLICATIONS
IN
SPECIAL
Nowadays DC systems can be found in specific applications: telecommunications and electric traction systems. Some aspects regarding these applications are presented below.
2.1. Telecommunications The telecommunication system uses a Low Voltage DC power system, and it was developed when the centralized battery system was built. The system is powered by the public network through rectifiers. Batteries are used to feed the system in case of a fault in the AC network. The batteries are sized for an operation time of 3 to 8 hours, depending on the loads sensitivity and other existing back-up systems (i.e. a diesel generator). The nominal DC systems voltages used by modern telecommunications facilities are either +24 V or -48 V. Figure 1 presents a typical DC distribution system for telecommunication.
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2009
The DC systems are classified by means of voltage level in High Voltage DC systems (30 kV < U ≤ 1500 kV), Medium Voltage DC systems (1500 V < U ≤ 30 kV) and Low Voltage DC systems (U ≤ 1500 V). The following presentations are brief descriptions for each DC system mentioned above.
3.1. High Voltage DC systems Fig. 1 - DC distribution system for telecommunications [1]
2.2. Electrical vehicles and traction systems DC systems are also used in electric propulsion based vehicles and ships. Besides the combustion engine, hybrid electrical vehicles use an electric system witch supply power to the wheels when the car accelerates and stores the power produced in deceleration periods in batteries. The electric power is also used to supply the other loads in the car. A 300 V DC voltage level is suitable for full hybrid vehicles. A layout of a hybrid electrical vehicle is presented in Figure 2.
Fig. 2 - DC distribution for electrical hybrid vehicles [2] DC has been used for a long time in traction systems, and the reason why is that DC machines are easy to speed control by simply changing the series resistance. Standard voltages in traction systems are 600 or 750 V for urban metros and 1.5 up to 3 kV for regional systems. Usually these systems are supplied from the public network using rectifiers.
3.
There are only a few systems that use DC voltage for transmission and distribution of electricity. In most cases, electricity is transported and distributed through AC networks. In the second part of the 20th century, transportation systems based on HVDC networks has become financially feasible. In the literature, there are studies regarding the cost of a HVDC transmission system. The cost depends on many factors, such as power capacity to be transmitted, type of transmission medium (underground, by air, undersea), environmental conditions and other safety and regulatory requirements. It is impossible to make a general statement regarding the most financially efficient solution (HVDC or HVAC) and a technical and financial study is recommended to be performed for each situation. Usually a HVDC transmission line costs less than an AC line for the same transmission capacity but the terminal stations are more expensive in the HVDC case due to the fact that it must perform the conversion from AC to DC and vice versa. Above a certain distance, the HVDC alternative will always give the lowest cost due to the lower cost of the overhead lines and cables [3]. A comparison between the two solutions is presented in Figure 3. A HVDC network can also be found in applications in witch connects two HVAC networks. In this case the HVDC network is called back-to-back station and it is used for coupling of AC networks with different frequencies and / or different phase numbers [4].
DC DISTRIBUTION SYSTEMS
Quick development of semiconductors made electronic equipments to become dominant as a share in applications for residential buildings and offices; because they use a different voltage level from the network, both in frequency and amplitude, the arising issue is the change of the voltage level, respecting the quality requirements. These efforts involve costs and energy losses. Since most electronic equipment used DC voltage, the questions arise regarding the use of DC distribution systems instead of AC distribution systems. I.S.S.N. 1224 – 1261
Fig. 3 - HVDC/HVAC costs [3] The main advantage of the HVDC networks is that it is technically feasible to achieve a substantial power upgrading of existing AC lines through their conversion to DC, by using the same cables and conductors [3, 4].
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3.2. Medium Voltage DC systems Medium Voltage Direct Current (MVDC) network is an energy distribution system that usually supplies the LVDC networks. A MVDC system can also interconnect distributed generation units (i.e. connection of the offshore wind farms to the AC network). These networks can provide the energy needed in residential or industrial areas or in different industrial plants. Figure 4 presents a MVDC network as described above. Fig.5 - LVDC Network with distributed generation units [5] There also exists Ultra Low Voltage Direct Current (ULVDC) networks witch are characterized by a voltage level up to 120 V. In practice, this system is used only in case of electronic equipment for offices and residential buildings. Most of electronic equipments have in their configurations a transformer followed by a rectifier which transforms and converts the AC voltage to DC voltage at the necessary operating level. The transformer is supplied even when the equipment is in stand-by; therefore, losses occur during the period the equipment is not on. Increasingly fewer equipment used in offices and residential applications are subject in operation to AC voltage (Figure 6).
Fig. 4 - MVDC network for residential and industrial areas The most important equipments in these networks are the rectifiers and inverters witch are carrying out the conversion process. In operation, they must remove or fight different disturbances that may occur in the network.
3.3. Low Voltage DC systems [5, 6, 7] Equipments such as computers, fluorescent lamps with electronic ballast, or TVs use DC voltage. They have in their configurations a rectifier, which converts the AC voltage into DC voltage. The conversion process introduces harmonics in the AC network, which have different negative effects (currents in the null conductor, inadequate protection operation). Such equipment can be supplied directly by DC voltage. Problems arise for the electrical machine with rotating magnetic field, AC motors or other equipment, witch, for normal operation need to be supplied in AC voltage. The supply of the AC loads will be achieved in this case through an inverter witch can provide AC voltage to the bus where more AC loads are connected. The new worldwide energy policy seeks to take measures to produce electricity using renewable energy sources. In this context, a Low Voltage DC network can interconnect distributed generation units. DC voltage can be obtained directly by using renewable energy sources. A diagram of a LVDC (Low Voltage Direct Current) network witch interconnects distributed generation units is shown in Figure 5.
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Fig. 6 - AC voltage supply [5] One solution proposed for DC and AC voltage distribution is shown in Figure 7.
Fig. 7 - AC and DC voltage supply[5] The number of converters is reduced compared with the classical solution and the power losses of the electronic
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equipment are reduced accordingly by waiving the process of conversion and use a scheme such as that proposed in Figure 7. Basically, only one rectifier is needed, located before the DC bus. Power sensitive loads can be supplied also through a battery storage system. Similarly with the AC Low Voltage systems, the EU low voltage directive defines the limits for voltages used in the DC Low Voltage systems. The EU directive [8] covers equipment designed for use with a voltage rating between 75 and 1500 V DC. This includes also the distribution systems. The above mentioned directive allows using higher voltage rating in DC systems than in AC systems. Because of the higher voltage rating and higher rms value of DC voltage, compared to corresponding AC voltage, the transmission capacity of a Low Voltage DC system is higher than the corresponding Low Voltage AC system [9]. Different possibilities, concepts and efficiency analysis have been made in the literature to investigate the opportunities and challenges associated with adopting a LVDC distribution system. To avoid the risk of damage to the cable insulation, the DC voltage Udc can be taken equal to 2 ⋅ U ac for a single-phase AC load and
2 / 3 ⋅U ac for a three-phase
AC load [10]. In other publications [9, 11] greater DC voltage was adopted and a comparison with LVAC systems was made in respect of transmitted power. Higher voltage rating in the DC distribution system leads to smaller currents and therefore smaller power losses than in the AC Low Voltage system. The transmission power can be considered with respect to the thermal limit and to the voltage drop of the cable. More power can be transmitted with DC distribution system compared to the AC systems as a result of DC connection type (unipolar, bipolar) and the DC voltage. Besides the aspects regarding the power transmitted, LVDC networks used at the customer end present several others advantages: - safety: the DC voltage is not as dangerous for human body as the AC voltage, because it does not lead to involuntary muscle contractions. The DC voltage must be less than 120 V to avoid this danger to the human body. Moreover, a completely safe system may be realized by choosing a sufficiently small value for the voltage level; - magnetic fields are reduced; - application of the DC system reduces voltage fluctuations at the customer’s end and the operating voltage can be kept nearly constant; - since for the whole system just one rectifier is needed, we can chose a better quality one (a IGBT based converter) in witch case with a proper control, the impact of electronic equipment in the AC network can be reduced by lowering the harmonic content; - a converter that allows a bidirectional flow can introduce energy into the AC network when there is a extra energy produced in the DC network due to increased potential of renewable energy sources.
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LVDC SUPPLY
A LVDC network may be supplied in different ways depending on the source nature. Usually a LVDC distribution system can be supplied by the AC network. Sometimes, renewable energy resources can provide a highly reliable DC voltage to supply the LVDC network, otherwise different AC/DC interfaces needs to be used to connect the energy source to the LVDC network. Some aspects regarding LVDC supply sources and AC/DC interfaces are presented bellow.
4.1. LVDC Supply Sources The number of alternative generation sources connected to the distribution system increases. Some of them, like photovoltaic and fuel cells, produce DC voltage, and can be easily connected to a DC distribution system directly, or through a DC/DC converter. Wind turbines are generating high-frequency AC voltage witch has to be synchronized to the AC network. To satisfy the quality requirements of the AC distribution system, the energy produced by the wind turbines is improved by using two conversion steps: an AC/DC conversion followed by a DC/AC conversion. An AC/DC converter may be used to avoid this expensive solution and, in this way, a connection with a DC network can be established. Fuel cells produce DC voltage outputs, and they are connected to the AC networks through power conditioning units such as DC/AC and DC/AC inverters [12]. In case of a DC network the DC/AC conversion step would be avoided. Renewable energy resources integration in a LV public distribution network represents the aim of the new worldwide energy policy. Issues regarding integration of alternative sources in DC networks are well represented in literature [13, 14].
4.2. AC/DC Interface A DC network can be supplied by the AC network or by generators using renewable energy and providing AC voltage. In all these cases the AC/DC interface has a particular importance in the operation and performance of the DC distribution system. The interface may provide voltage level control on the DC side, high power quality, bidirectional power flow, protection in abnormal operation conditions (faults, disturbances, etc), low losses, etc. Depending on the required performance, there are several methods to create the AC/DC interface [2]: Diode Rectifier, Voltage Source Converter (VSC), Three Level Voltage Source Converter, Voltage Source Converter with DC/DC Buck Converter a) Diode Rectifier A diode bridge is a very cheap device for rectification of AC to DC, and it can be designed for single-phase and three-phase connection. A three-phase Diode Rectifier scheme is presented in Figure 8.
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Fig. 8 - Three-phase Diode Rectifier [2] The output DC voltage is uncontrolled and decreases if the load current increases. The rectifier consumes symmetrical AC currents and produces a high amount of odd low-frequency harmonics. The power can flow only from the AC side to the DC side and the Diode Rectifier provides no galvanic isolation. The diode bridge can be modified by adding some components which will give it Buck or Boost characteristics with controllable DC voltage and power factor correction (PFC). This is obtained by controlling the current through an inductor by opening and closing a transistor. The PFC Diode Rectifier has no galvanic insulation.
Fig. 11 - Three Level VSC [2] d) VSC with DC/DC Buck Converter By connecting two converters in series (Figure 12) the controllability of the output DC voltage increases. Since the output DC voltage is controlled by the Buck converter the voltage between the VSC and the Buck converter is allowed to vary in a wider range. Using a two converter interface increases the possibility to maintain the DC voltage at the reference value. The configuration has bidirectional power flow but no galvanic isolation.
Fig. 12 - VSC with DC/DC Buck Converter [2] Fig. 9 - Single-phase Diode Rectifier with PFC: a) Buck, b) Boost [2]
5.
b) Voltage Source Converter (VSC) This solution provides a bidirectional power flow and a controllable power factor. Due to the fact that the VSC is a converter utilizing high-frequency switching, filters have to be installed between the converter and the AC side. The VSC operated as a rectifier has a Boost output characteristic. The lower level of the output DC voltage is determined by the AC voltage. A transformer used between the VSC and the AC network, as suggested in Figure 10, will serve: - to step down AC voltage; - as galvanic isolation between AC and DC sides; - as a filter.
Fig. 10 - VSC with transformer [2] c) Three Level VSC Compared with the two-level VSC interface it results in two, instead of one, controlled DC links from the same AC supply (bipolar DC system), witch means that loads can be connected to either of the two polarities, and it is possible to maintain a balanced DC voltage.
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CONCLUSIONS
In this paper some aspects regarding DC distribution systems are presented. Besides DC networks used in special applications like telecommunications, electrical vehicles and traction systems, other features are presented for High-, Medium- and Low- voltage DC systems. More aspects are presented in case of the Low Voltage DC networks. Some DC voltage supply sources are presented: renewable energy resources (solar energy, wind energy, fuel cells) or AC network and different AC/DC interface possibilities. A DC supply can lead to big advantages if a proper voltage level is chosen. Higher voltage rating in the DC distribution system leads to smaller currents and therefore smaller power losses than in the AC Low Voltage system. This enables the use of smaller cable cross sections than in the corresponding AC system. With a Low Voltage DC distribution systems higher transmission powers and transmission distances can be achieved when compared to the traditional Low Voltage system. A DC network at the consumer end introduces some advantages: safety (direct current is not as dangerous for the human body as alternate current), reduction of the magnetic fields, improvement of the electric power quality. More and more loads used in offices and commercial facilities are internally based on electronics. Using a DC network, electronic loads can be supplied more efficiently. By using a proper DC voltage level, the conversion process is avoided, and thus the afferent energy losses are reduced. Taking into account of the
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increased number of electronics in different applications, the reduced energy losses and other advantages, a Low Voltage DC network may be a feasible solution to replace the Low Voltage AC network in adequate purposes like offices and households applications.
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of the European Union, 2006, Strasbourg, L374/10 – L374/19 [9]. T. Kaipia, P. Salonen, J. Lassila, J. Partanen , “Possibilities of the Low Voltage DC Distribution Systems”, Stockholm, 2006, available at website www.lut.fi/fi/technology/lutenergy/electrical_engineering/r esearch/electricitymarkets/research/networkbusiness/Docu ments/DCdistribution_Kaipia.pdf [10]. D. Nilsson, A. Sannino, “Efficiency analysis of low- and medium- voltage DC distribution systems”, in IEEE Power Engineering Society General Meeting, Vol.2, Denver, CO, 2004, pp. 2315-2321. [11]. P. Salonen, T. Kaipia, P. Nuutinen, P. Peltoniemi, J. Partanen, “An LVDC Distribution System Concept”, in Poceedings of Nordic Workshop on Power and Industrial Electronics, Helsinki, 9-11 June 2008, available at website http://lib.tkk.fi/Conf/2008/urn011603.pdf [12]. M.Y. El-Sharkh, A. Rahman, M.S. Alam, P.C. Byrne, A.A. Sakla, T. Thomas, “A dynamic model for a stand-alone PEM fuel cell power plant for residential applications”, Journal of Power Sources 138 (2004), pp. 199–204. [13]. A. Agustoni, E. Borioli, M. Brenna, G. Simioli, E. Tironi, G. Ubezio, “LV DC distribution network with distributed energy resources: analysis of possible structures” in Proceedings of 18th International Conference and Exhibition on Electricity Distribution (CIRED 2005), Turin, Italy, 6-9 June 2005, pp. v5-10 [14]. R. Magureanu, M. Albu, M. Priboianu, A.M. Dumitrescu, “A DC Distribution Network with Alternative Sources”, in Proceedings of the 15th Mediterranean Conference on Control and Automation – MED07, 27-30 June, 2007, Athens, Greece, p.1-4.
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