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Large scale integration of micro-generation to low voltage grids (MICROGRIDS)


Generally, the present regulatory practices have addressed sensibly the technical requirement for connecting DGs to distribution systems in order to maintain safety and power quality. This includes the development of new standards associated with DG technologies, connection practices, protection schemes, ancillary services and metering. Administrative processes such as application for new connection have also been standardised and disseminated with an aim to make the process transparent for DG developers and investors while maintaining DSO’s monitoring ability to ensure that the necessary connection standards are not compromised. Besides connection policies and development of technical standards, various policies were also designed to provide financial supports and favourable market environments to stimulate DG growth such as financial incentives for small generators in the forms of government grants, exemption of transmission use of system charges and transmission losses charges, climate change levy exemption, and Renewable Obligation. Incentives are also given to the DSO to facilitate more small-scale generation including micro generation in their network. However, it is evident that DG especially micro generation still face a number of barriers such as: -A relatively low electricity prices, which discourage new investment, -A relatively high capital cost for renewable technology, -A relatively high connection cost due to deep connection charges policies, -Lack of market support mechanisms which: --Allow DG to access freely electricity market, -- Reward DG according to its service to the network, -- Minimise the financial penalty for imbalances due to the use of intermittent RES. - Lack of incentives/force for DSO to change their passive operation philosophy, which limits the amount of DG that can be connected. Therefore, commercial questions such as creation of a level playing field, development of market for aggregators, and cost reflective network pricing to acknowledge the costs and benefits of distributed generation to the networks in addition to the key technical questions such as active management of distribution networks, coordination of the operation between microgrids and public electricity systems, and islanding mode operation are still required to be addressed and solved. More radical changes may be necessary to facilitate efficient integration of the operation and development of microgrids in the systems, in order to extract the additional value of micro grids in terms reducing of the overall system operating costs, network investment deferral, service quality and reliability improvements and provision of a variety of services to support network operation during various disturbances.
Two possible options had been investigated to overcome the absence of high short circuit levels required for conventional over current sensing in a MicroGrid. - Increasing the fault current contribution from converters at a reasonable cost - Finding alternate means of detecting an event within an isolated MicroGrid A detailed analysis of the probable protection schemes for the MicroGrid was performed. - For a fault on the distribution system, the MicroGrid will continue to operate in an island by opening the circuit breaker upstream of the main distribution transformer. - For a fault on the MicroGrid network, the whole MicroGrid will discontinue its operation. Sectionalising of a general MicroGrid is not advised as the cost involved could not be justified for the benefits gained. The decision would be mainly dependent on the identified needs of the MicroGrid users. - For a fault at a residential consumer, the relevant consumer would be disconnected from the MicroGrid, and normal operation would continue. Over current protection relay, distance protection relay, Residual Current Device (RCD) and Short Circuit Protection Devices (MCCB or fuse) have been identified as the possible solutions to be used in the protection of a MicroGrid. INESC have carried out a study to evaluate the discriminating capability of zero sequence protection to be used in the MicroGrid. The fault analysis had been performed assuming grid-connected operation of the MicroGrid. In the near future, fault analysis during islanding operation of the study case MicroGrid will be performed.
- There are several methods to allocate losses that have been investigated. The methods based on: -- Proportional allocation; -- Marginal allocation; -- Unsubsidised marginal allocation; -- Proportional sharing allocation; -- Allocation using the impedance matrix; -- Incremental allocation of active losses; --Loss allocation based on the results of OPF problems. - A mechanism for neutralising the impact of choice of reference node on the magnitude and the polarity of loss allocation factors by apportioning total losses equally between generators (including the reference node) and loads has been investigated. - A pricing methodology for distribution network with distributed generation has also been developed. The method can be used to calculate the charges for demand customers and distributed generators to recover the network capital investment. The method is based on the Long Run Marginal Investment Cost pricing concept on the reference network. - The methodology can take into account annual loading and generation profile on the network, network characteristics (topology and network design considering security requirement). - The results of the methodology are the time of use and location specific entry and exit DUoS tariffs for power injected or taken from the network. - The tariffs need to be adjusted using the developed revenue reconciliation methods in order to generate adequate allowable revenue to recover the investment, and O& M costs.
A MicroGrid is subject to the same safety requirements as any other utility electric power system. The earthing system of a MicroGrid must be able to deal with both grid-connected and islanded operation. An extensive literature review of the earthing systems and practices applied today in Low Voltage (LV) power systems worldwide was carried out. The objective of this work was to determine and describe the state of the art in LV grounding practices in various countries, in order to select the most appropriate earthing system/ combination of systems for a MicroGrid. The main earthing systems, TN (TN-C, TN-S and TN-C-S), TT and IT have been analyzed, with respect to their performance and effectiveness, as well as to the specific protection and equipment requirements associated with their application. According to this review, the most suitable earthing systems for a MicroGrid are identified as follows in the order of their suitability. - TN-C-S - TT - IT As per proposed protection guidelines, the MicroGrid would not lose the source earth at the distribution transformer in any event. Accordingly, the micro-sources do not require to be earthed. At present, the performance of each of these earthing systems in a MicroGrid is investigated via simulation. UM are using the specialist grounding software tool CDEGS for this purpose while NTUA are using EMTP.
- Different types of distribution networks were analysed and simulated under different operating scenarios, considering different power factors. The impact of Microgeneration was evaluated through the changes experienced in voltage profiles, power losses and branch congestions in these networks, assuming that the equivalent load consumption was reduced due to the connection of Microgeneration units to the LV grids. - The analysis on the impact of Microgeneration was based on a methodology characterized by: -- Simulation of Microgeneration connection to the network by reducing the value of active power at each node. -- System evaluation, concerning voltage profiles, loss estimation and branch congestion analysis. -The work focused mainly on the analysis of the influence of Microgeneration in energy distribution networks. -The results obtained indicate that Microgeneration brings some interesting socio-economical and technical advantages for distribution system operation. - It was seen that active power losses decrease significantly with the growth of Microgeneration penetration and consequently the corresponding cost is also strongly reduced. Concerning network congestion issues, it was observed that branch load can be significantly reduced considering a high percentage of Microgeneration penetration. In addition, a reduction in CO2 emissions can be achieved with high Microgeneration percentages.
- A methodology to quantify the reduction in carbon, SO2 and NOx gas emission obtained from reduction in energy supplied by hydrocarbon based power plants - A method based on Linear Programming problem to maximise the value of micro CHP by optimising the schedule of electricity and heat generation taking into account seasonal variations in domestic heat and electricity demand, electricity prices, gas prices, room insulation, and ambient temperature. - The method is used to quantify the economic benefits of replacing old conventional boilers with new domestic CHP, and the break even period of investment. The case studies demonstrate promising results as the break even can be 5-10 years and it can be expected to be lower as the price of dCHP falls. - The method can also be used to quantify the impact of increased efficiency in the domestic utilisation of gas and electricity on the reduction of CO2 emission. It is demonstrated that 0.3 0.5 kT CO2 reduction can be achieved with 1 M installation of dCHP. - A generic urban and rural distribution network has been developed to provide high-level details on distribution network. The model can incorporate typical characteristics of the networks such as length, impedance, capacity, yardstick costs, and network loading, demand and generation distribution for various voltage levels. - An optimisation methodology and load flow calculation have been developed to calculate losses, power flows, voltages and DG curtailment required to satisfy network limits. -Various case studies have been performed on the generic network to quantify the benefits of microgrids in terms of: -- Losses reduction; -- Ability to correct power factor by providing reactive support; -- Increasing spare capacity, deferring new network investment. -The result of the studies demonstrate different value of implementing microgrids in rural and urban network, different value for dCHP and PV technology - A methodology to calculate the benefits of enabling islanded operation in Microgrids based on reliability assessment.
The understanding of the dynamic behaviour or micro generation sources and storage devices is crucial to identify the right decentralized control strategies to be installed in a MicroGrid. For this purpose, a simulation platform for the analysis of the dynamic behaviour of a microgrid under three phase-balanced conditions was implemented in the Matlab/Simulink environment Specific issues like the response time constant of each microsource, voltage and frequency control were addressed.
A cluster of microsources operating in an isolated low voltage grid brings some problems, mainly in load-following situations, due to the slow response of microgenerators to control signals and due to the reduced inertia of the system. In order to allow islanded operation, such a system requires forms of energy storage (batteries, flywheels, supercapacitors) to ensure initial energy balance. Frequency droop control for fast load tracking was tested. The control strategy adopted combined this frequency control droop modes with the storage device response and load shedding possibilities, in a cooperative way in order to ensure successful overall operation. The simulation platform developed within the result "Analysis of Dynamic Behavior of Micro Sources" was further upgraded in order to evaluate the dynamic behaviour of several microsources operating together in a LV network under pre-specified conditions including interconnected and autonomous operation of the MicroGrid. The simulation platform based on Matlab/Simulink allows the simulation of moving to islanded operation and load following during autonomous operation mode. Since the MicroGrid is expected to be an inverter dominated grid, its control mode was derived based on two possible schemes for controlling inverters: "PQ control: the inverter is used to supply a given active and reactive power set point; "Voltage Source Inverter (VSI) control logic: the inverter is controlled to feed the load with pre-defined values for voltage and frequency A critical issue for MicroGrid islanded operation is the presence of an energy storage device to provide balance between load and consumption during transients. One can fulfil this requirement by establishing one of two operation modes for a cluster of microsources connected to an electric grid: "Single Master Operation: A VSI (or a fully controllable synchronous machine directly connected to the grid) can be used as the reference voltage when the main power supply is lost; all the other inverters can be operated in the PQ mode; "Multi Master Operation: Two or more inverters are operated as a VSI (no synchronous machine is needed). Controllable loads play an important role under some MicroGrid operating conditions, namely those concerning a large imbalance between load and generation (load larger than generation). In order to deal with this problem, a load-shedding scheme was implemented to aid frequency restoration to its nominal value after the islanding of the microgrid and to avoid storage devices overload during the initial transient period. A secondary load-frequency control was also implemented to avoid storage devices to keep injecting (absorbing) power whenever frequency differs from the nominal value (storage devices inject power proportionally to frequency deviation). Simulation studies were performed in different study case low voltage networks including several micro generation technologies, in order to evaluate the quality and performance of the developed control solutions. Different types of loads and load/generation scenarios were also considered. MicroGrid islanding was tested in two different conditions: forced islanding following a short circuit in the upstream medium voltage network or intentional islanding due to maintenance needs. In both cases, the proposed control strategies proved to be feasible.
The main objective was to analyse the dynamic behaviour of a MicroGrid in interconnected (with a main Medium Voltage network) mode and in emergency (islanded) mode under fault conditions, i.e. in case of short-circuit, for instance. Several simulations were performed using the MatLab/Simulink simulation platform developed. For simulation purposes, two different operating scenarios were considered: when the Microgrid is connected to the main MV network and when the Microgrid is operating in islanded mode. Plus, two different fault locations were considered: a fault on the main MV network and a fault inside the MicroGrid network. Several fault elimination times and fault resistance values were considered for the presented situations. Also, the impact of load types on the dynamic behaviour of the MicroGrid was evaluated. In addition, for the situations described, a comparison between the dynamic behaviour of the MicroGrid with and without the possibility of load-shedding was tested. Special attention was paid to the influence of motor loads in the dynamic stability of the MicroGrid in fault situations.
The Microgrids simulation tool developed in Fortran has the following features and capabilities: - It contains the complex modelling of: -- Transmission lines. -- Synchronous generators (active and reactive generation can be controlled and/or fixed separately). -- Non controllable generators (PV,wind) can be modelled as a fixed PQ load for a snap shot condition. -- Tap changers. -- Shunt reactive compensators. -- Fixed and controllable loads. -- Active frequency droop control characteristics. -- Reactive voltage droop control characteristics for a 3-phase balanced system. - Load flow analysis provides the conditions of voltages (magnitude and angle), power flows, and losses in the system. All possible types of sources can be included (PV, PQ, sources with no control capabilities). - An advanced solution technique for optimising operation in Microgrids has been developed. The objective is to minimise the operation costs incurred by utilising micro sources, trading with public grids and shedding loads. Operational constraints and physical limits of individual devices are taken into account. The decision variables are the despatch of active and reactive power generation from micro sources, positions of tap changers, reactive compensators, controllable loads. The secondary outputs are the nodal prices for active and reactive power injection. The prices can be used as market signals to the despatchable micro sources and loads to stimulate optimal allocation of resources in microgrids. - The tool was used to simulate a real time power exchange market operated by MGCC either in grid connected or in islanded mode. A combined active and reactive energy markets have been successfully simulated on a MV and various LV test systems. The simulation can demonstrate trading interaction among micro sources, loads and the public grid. - The simulation can also demonstrate the ability of the proposed decentralised control approach using a closed loop price signal to handle small and large scale disturbances inside and outside microgrids. - The tool provides concurrently active management capability, automation and optimisation for operating Microgrids.
The Microgrids simulation tool developed in MatLab-Simulink has the following features and capabilities: - Steady state analysis in MatLab code provides initial conditions of the state variables of the connected sources. For the load flow analysis all possible types of sources can be included (PV, PQ, sources with no control capabilities). - The dynamic analysis part is based in both MatLab and Simulink. Data handling, assignment of the initial values and output of results is performed in the main program coded in MatLab, whereas for the numerical integration Simulink is used. - For the dynamic analysis, microsources are generally represented as balanced EMFs behind an impedance, neglecting stator transients. - Several microsources can be integrated in the dynamic simulation with their respective electronic interfaces, like -- Induction machines -- Synchronous machines -- Micro-turbines -- Photovoltaic Systems -- Fuel Cells -- Wind Turbines -- Batteries -- Flywheels -- Super capacitors, etc. - It can be used for the dynamic simulation of LV microgrids either in grid-connected or in islanded mode of operation. - LV networks with resistance comparable to or greater than reactance can be handled. - Unbalanced network conditions, due to unsymmetrical sources, loads, series elements, faults etc. can be modelled and simulated. - All basic neutral earthing schemes can be represented (TN-C, TN-S, TT, IT). - Frequency domain representation (phasor approach) has been adopted for both the network and the sources to increase the simulation efficiency. Natural phase quantities (a-b-c) are used.
Dynamic security assessment provides a way to evaluate the robustness of the MicroGrid to survive a sudden disconnection from the MV network. Such a sudden change on the system’s operating conditions must be quickly and efficiently compensated by the micro sources in order to avoid large frequency excursions, which may trigger the existing frequency relays, causing the system to collapse. Since the MicroGrid dynamic behaviour analysis is a complex and computational burden task, the evaluation is performed exploiting functional knowledge, generated off-line, and using machine-learning techniques. The generation of functional knowledge requires the analysis of the dynamic behaviour of the micro sources operating together in the Low Voltage network under several different predetermined operating conditions, for the situation - passage to islanded operating mode i.e. disconnection from the upstream Medium Voltage network. In order to generate a representative learning set, a large number of dynamic simulations was performed in order to generate a sufficient amount of data to cover the set of possible operation points (different scenarios of load and distributed generation) for the MicroGrid. For each operating condition the security index maximum deviation in frequency is evaluated and kept.
If a system disturbance provokes a general black out such that the MicroGrid is not able to separate and continue in islanded mode, and if the MV system is unable to restore operation in a specified time, a first step in system recovery will be a local Black Start. The strategy to be followed involves a software module to be installed in the MGCC responsible for controlling the MC and the LC. Two types of black-start functions were developed: local black-start of the MicroGrid after a general system blackout and MV grid reconnection during black-start. The strategies to be followed use the hierarchical control system of the MicroGrid and its communication facilities, namely LC, MC and the MGCC. The main problems to be dealt with include building the LV network, connecting micro-generators, controlling voltage, controlling frequency and connecting controllable loads. During the restoration of the LV network, load-tracking problems will arise, since some micro generators (fuel-cells, micro turbines) have slow response and are inertia-less. Such a system requires some form of storage to ensure a fast energy balance between local generation and consumption. A sequence of actions to be carried out during the MicroGrid Black Start was developed and tested through numerical simulation. For this purpose, a simulation platform under the MatLab/Simulink environment was developed in order to evaluate the dynamic behaviour of several micro sources and the corresponding power electronic interfaces operating together in a LV network. In order to analyse in a more detailed way the fast transients associated with the initial stages of the restoration procedure, an EMTP-RV software tool was used.