Effects of the distributed generation on the power grid
The optimal connection of a production plant to the public network has to allow the transit of both active and reactive energy and the "cut-off" in case of abnormal operating conditions, both in the public and the private networks. In Italy, the connection between distributed generators and distribution system is regulated by the CEI 11-20, while the IEEE 1547 standard of July 2003 is in force in the United States, that indicates new guidelines for the optimization of the interconnection between the distributed generation and the network.
The technical aspects to be evaluated during the connection phase of a distributed generation unit to the distribution network are: regulating the voltage, the short circuit current and the problems of protection selectivity.
The distribution networks that are primarily selected to accept the vast majority of distributed generation are medium and low voltage networks, which are therefore subject to the greatest impact.
The current power grid has been designed and managed in accordance with the concepts of passive network and radial operation: from a technical point of view, these features appear as barriers to the penetration of the distributed generation. Protections, automatisms and control and regulation systems are the parts of the network affected by the introduction of distributed generation, so the connection of a new distributed generation plant needs a number of technical issues to be taken into account. They are defined by the AEEG Resolution 160 / 06 and summarized as follows:
management of transients resulting from start-up, synchronization and paralleling of the production equipment: rigging connection / disconnection of distributed generation plants, especially in the case of asynchronous generator, involve voltage changes, therefore voltage holes that last until the activation of the automatic voltage regulation procedure, which reports such values in the range of proper functioning. To avoid such problems, it is necessary to carry out the maneuvers of disconnection and re-synchronization of the generators in accordance with appropriate procedures defined by the standard. Instead, the procedure of insertion and disconnection of generators with static interface does not exhibit any critical component because the characteristics of static converters allow to perform such maneuvers with gradual transitions from the load condition to the shutdown.
Voltage profiles and voltage regulation in the network. The number of distributed generation units connected to the network can be limited due to the problems related to voltage regulation. Distributed generators can distort this adjustment through power inputs that mislead calibration systems and by increasing the band of uncertainty of the voltage. Due to the current passive configuration of network, the logic of voltage regulation, without generation plants connected to the network itself, involves the need to adopt a relatively high voltage value in proximity to the primary station, thus providing the relative voltage losses along the network and allowing users away from the primary station to be served by a voltage level within the limits of service quality. The presence of distributed generation plants connected to the distribution network changes the voltage profiles: in particular, an increase of voltage profiles can be noted in the point of connection of the plant to the distribution network, in proportion to to the power supplied; anyway, changes in the voltage profile occur along the entire feeder,sometimes even above the tolerable levels.
However, producer can not always guarantee his provision, so, in addition to voltage surges, voltage drops may also occur; the allowable voltage variations along a distribution line are fixed in the order of 4-6% of the value of the rated voltage. The CEI 11-20 allows the dealer to agree on a plan of reactive power exchange compatible with the characteristics of the generators in the system: agreements for the sale of such energy between the network operator and the producer are then possible, provided that the group of generation has suitable technical features. For the connection of a generation plant to both medium and low voltage network, actions must be taken on size factors, power and location of the generator along the line and suitable generators have to be arranged to allow the control and adjustment of the power itself.
changes in the levels of short-circuit currents and associated thermal / dynamic stress of power lines and components: the presence of plants connected to the grid, at worst even without the interposition of static converters, may result in an increase of short-circuit currents within the network, so that the protection devices tend to disrupt the normal exercise and create, even without a real fault, a malfunction of the network and a reduction of its quality. It is therefore necessary to check that the contribution to the total short-circuit current supplied by distributed generation groups preserve cut-off ability as well as selectivity of the protections. If the insertion of a generation plant determines an increase of the short circuit current on the MV network that does not preserve the correct functioning of circuit breakers, a technical-economic estimate is needed to assess the extent of the relative costs and therefore the convenience of the plant inserted. The problem of the increase in short-circuit currents arises when its value exceeds the cut-off limit of the devices installed for the network protection. In addition, these changes cause both thermal and dynamic stresses of the components subject to short-circuit current which, if significant, can lead to damage of the plant. For this reason, the installation of a new generation plant must be preceded by a careful check of the increments of short circuit current related to the installations located in a specific area: currently, they must remain below 12.5 kA, that is the cut-off limit of the circuit-breakers protecting MT networks. In addition, in order to take into account the problem of both thermal and dynamic variations, it is necessary to take into account the maximum limits of the overcurrent that the conductor can tolerate without damage, depending on the type of conductor and duration of the short circuit current. In the near future, the increased use of distributed generation will require strengthening the distribution network as it is subject to not negligible power flows.
implementation of the search procedure for faulty sections: in case of fault on the power grid, a series of automatic procedures are implemented to identify and isolate the faulty section. Its identification is based on the presence or absence of voltage along the line on which the fault occurs. The presence of distributed generation plants do not involve alterations to this procedure if, at the time when the fault occurs, the system is automatically disconnected from the public grid in a shorter time than what is required for the first closure or "fast closure" (300 seconds approximately), so as to avoid the failure of the procedure and any damage to the generators that may be caused by the rapid reclosing of the circuit breaker. If the distributed generation is not disconnected from the network, abnormal behavior of the system may occur, as the distributed generation itself would "power" the fault and supply current to some sections of the line, in relation to the short circuit impedance.
proper functioning of the protection systems. The presence of active installations may cause an additional problem for both medium and low voltage distribution network: line protections may lose selectivity and untimely tripping of circuit breakers may occur. This can happen, in case of fault on a feeder and if the delay of the overcurrent protection placed in the faulty section is high, when the distributed generation plant located on a working feeder powers the fault and the value of this current exceeds the intervention limit of the overcurrent protections; this may cause the opening of the protections also in the working feeder of the plant, despite this not being subject to failure, thus creating an undesired "out of service" of a working section of the line.
For this reason, the distributed generation plant has to be disconnected before fast reclosure occurs, in order to prevent the failure of the procedure and any damage to the generators; also, the contribution of the active elements of the network (ie asynchronous motors , synchronous machines and converters) to the short circuit current in the feeder must be taken into account. The sum of these components must be less than the the overcurrent protection limit. It is therefore necessary to revise the delay times of protection, due to the possible non-compliance of the minimum time of intervention to the safety of the plant and of the people. One solution would be to replace all protections with switches and relays directional relays able to recognize the contribution given by local generators: however, such a choice would be uneconomic and not completely safe.
unwanted island operation of portions of the network. The unwanted island operation occurs when, during a fault, a portion of the distribution system is still fed by the local generators, even if the lines have been cut off by the operator. This system is to be avoided as it results in a series of problems:
- The safety of the staff responsible for troubleshooting, who may find portions of the network still energized even if disconnected from the system itself;
- The premature wear of electrical installations owned by the users, which continue to run at very low voltages and away from the nominal voltage;
- Difficulty in restarting the network portion that caused the unwanted island. For these reasons, it is necessary to provide for communication devices and remote control between the generators, so as to prevent distributed generation plants from powering the lines when they are not powered by the public network or when small variations of the electrical parameters occur in the point of connection of the plant to the grid. The rules, including the Italian CEI 11-20, provide for the automatic disconnection of the generator in case of failure in the network.
The existing electricity grids create therefore an impediment to the diffusion of distributed generation. The current development of distributed generation technologies require two priority solutions:
- How to encourage the generation of electricity from renewable sources that are typically characterized by discontinuity of production, small size and dislocation throughout;
- How to involve small plants on the balance between supply and demand of energy.
On the basis of these priorities, a number of studies and research focus on a single procedure, that is transforming the old electricity grid into a smart grid.
In fact, smart grids consist in a system of power transmission that allows to avoid deficits and surpluses: energy is transported from areas with an excess of production, compared to actual consumption, to areas with a deficiency of production, thus compensating the difference. This is achieved by means of sensors and chips scattered in the system in such a way as to constantly detect data on the amounts of energy that has been used / produced at each point of the network and, according to the needs, to properly direct the flows of energy.
Finally, the smart grid allows a more dynamic and flexible use of the energy, also thanks to the contribution of renewable energy sources.