Name / Alessandro Agustoni / Session / 2
Company / Politecnico di Milano / Block / 3
Address / Piazza Leonardo da Vinci 32 / Question n°
20133 Milano / Language used on the floor / English
Phone / +390223993742 / Accompanying visuals on file ? / AGUSTONI_IT_author_ALPHA2_BLOCK3.ppt
e-mail /
PROPOSAL FOR A HIGH QUALITY DC NETWORK WITH DISTRIBUTED GENERATION
The need of a high quality of the electricity supply has induced many end users to equip itself with appliances (UPS) that have inside a dc section in which is inserted a storage system. This necessity is due to the spreading of sensitive loads and, at the same time, to the diffusion of distorting loads that often have an ac/dc converter as input stage. Moreover the technological progress has recently made available on the market various small power generators that often generate electricity in direct current. Therefore it can be thought to integrate the various users and generating devices by means of a local dc distribution system. This solution could simplify the present system and would give to it the characteristics peculiar to UPS by increasing quality level, from the point of view both of the end users and of the distribution network. Furthermore, by means of suitable control strategy, it would be possible to share the load among the various sources of supply – utility, generators and storage systems embedded in the dc local network – in such a way as to optimize both the use of primary energy and the power exchanges with the public network.
In this work it has been considered the use of a dc low voltage local network intended for supplying non-industrial utilities, such as, for example, business centres, residential complexes or office blocks, equipped with their own substations. In the presence of sensitive loads, equipments that have ac/dc input converters and distributed generators, the use of a dc distribution system may make it possible to obtain advantages over the present ac system.In the event of malfunctioning of the public network, the local dcgrid is decoupled from the ac network by the interface converter and can continue to be supplied by the local generators without disturbances.On the contrary, in the ac network is also possible the islanding operation only if the local grid has been disconnected by means of an intertie protection; in this case temporary disturbances occur to the loads. In the ac distribution system, the total number of converters is greater compared with the dc solution, even if it is necessary to have the ac/dc converter, which must be designed for the total power of the loads. Moreover, ac/dc converters are more complex than the dc/dc ones, from the point of view both of their structure and, if they are of the forced commutation type, of the control system. A large number of ac/dc converters is a disadvantage regards the quality of absorption from the public network, since very often, especially in the case of electronic equipment, the input rectifier consists of a diode bridge, which involves high harmonic distortion. In the dc system, the interface converter can be of the forced commutation type and controlled in such a way as to ensure absorption of sinusoidal current in phase with the supply voltage, thus containing the harmonic distortion and the reactive power absorbed and minimizing transmission losses.In this case, guaranteed quality for the loads would be higher, since it would be possible to have a dc section with stabilized voltage irrespective of supply voltage fluctuations.
In the proposed circuital schemes,the ac/dc converter located between the dc bus and the public network has been chosen to be of the forced commutation type, to ensure a sinusoidal absorption with unity power factor and to permit a bi-directional power exchange.In order to alter as little as possible the present distribution system, it has been decided to keep the present MV/LV substation transformer unchanged. Under these constraints,a value considered suitable for the dc section voltage level is 800 V. For supplying ac loads, a 4-wire system has been provided, so that both three-phase and single-phase loads can be used. For supplying dc loads, allowance has been made for the possibility of direct connection to the dc bus bar, or else by means of a step-down chopper; in the first case, to ensure lower voltage values, arrangements have been made for supplying loads with voltages equal to half those of the dc section. For this purpose, the neutral conductor must be kept at an intermediate potential between that of the positive conductor (+400 V) and that of the negative one (-400 V). Two solutions, that follow a typical TN-S layout, have been therefore elaborated, that provide to connect or not respectively the neutral to the grounding system of the transformer substation. In both solutions is necessary to use a 4th inverter leg, whose function is to avoid dc current injection in the MV/LV transformer or to keep the neutral conductor at the intermediate potential.
The control of different converters is based on feedback from the dc bus voltage: if low, this would indicate the need to supply power to the dc network, drawing it from local generators (renewable, if any, or non-renewable), from the storage system, or from the public network; conversely, a high voltage would mean the presence of a power surplus, and therefore the possibility of feeding it back to the network or using it to recharge the storage system. The control philosophy proposed is based on the allocation to each converter of a different reference value for the dc bus voltage. In order to optimize plant management, it ispossible to use an inter-converter communication system; nonetheless, converter-control has to ensure the safety and correct operation of the whole plant, even if not optimized, in the absence of the communication system in order to prevent the latter from becoming a "single point failure" for the whole system.