CIRED Workshop - Helsinki 14-15 June 2016 Paper 0337- Paper No 0337 Page 1 / 4 Investigation of Resynchronisation Process and its Influence on Microgrid Components Simon EBERLEIN Krzysztof RUDION University of Stuttgart – Germany University of Stuttgart – Germany Simon.eberlein@ieh.uni-stuttgart.de rudion@ieh.uni-stuttgart.de ABSTRACT One of the most challenging aspects regarding the control of microgrids is the reconnection with the bulk power system. During this process, high stresses can occur for the microgrid components, depending on the alignment of the voltages of the two systems. How deviations in frequency, amplitude and angle of the voltages, as well as a time delay in the switching of the control of the grid-forming generators in the microgrid to grid-feeding control affects these stresses is investigated in this work. Simulations are carried out in two real MV networks with power electronic-based and synchronous generators. It is shown that an angle deviation can strongly affect the stresses of synchronous generators, while delays in the switching of the control mode result in high loads for inverter-based generators. The results can be used to determine the dimensioning of generators in microgrids as well as the layout of the protection system. INTRODUCTION Microgrids are a viable solution for future smart grids to avoid outages or to enhance power quality. With the increasing penetration of the distribution system with distributed generators (DGs), it has become possible to operate parts of the grid in islanded mode, independent from the bulk power system. If a disturbance occurs, the network dissociates into cells in order to isolate the faulty element and to keep as many customers supplied as possible [1]. Table 1: Microgrid reconnection requirements [2] Avg. rating of DR/MVA |∆f| /Hz |∆  |/p.u. |∆ |/° 0-0.5 0.3 0.1 20 0.5-1.500 0.2 0.05 15 1.5-10 0.1 0.03 10 A critical aspect of microgrid operation is the transition between the islanded and grid-connected mode after the fault has been remedied and the bulk power system is able to supply the loads again. During this transition, high stress of the microgrid equipment can occur. These transient states, with possibly high overcurrents or generator torques, can trigger the protection system and affect the ageing of the components. In this paper it is investigated to what extent an imperfect synchronization of a microgrid with the bulk power system can influence the stress of the system components, such as inverter- based and synchronous DGs. This is of interest because the frequency, amplitude and phase of the voltage in the microgrid are usually not exactly aligned with the bulk power system due to measurement errors, fluctuating loads and generators in the microgrid, fluctuating frequency of the bulk power system or the response time of circuit breakers. They can even deviate immensely from the values required (see Table 1, where ∆f, ∆ and ∆  are the differences of the voltages of the microgrid and the bulk power system at breaker closing in frequency, angle and amplitude) in case of faulty operation of the synchro-check. Moreover, it is required to reconnect the microgrid as fast as possible sometimes, for example, due to looming stability problems in the microgrid. Then a compromise needs to be found between the speed of the resynchronization and the voltage alignment at the breaker closing. Numerical simulations are carried out using dynamic power system simulation software. The focus in this work is on the stress after the circuit breaker closing. Hence, a simple approach is chosen for the resynchronization control before the breaker closing as its impact on the behavior after the breaker closing is minor. Moreover, protective devices that would trigger under certain circumstances during the resynchronization process are not considered. Table 2: Characteristic data of test systems Rural grid Urban grid R/X 1.39 1.58 Total/average line length (km) 274/0.78 175/0.5 Max./min. load (MW) 43.4/12.6 34/10.3 Generation capacity (MW) 51.8 24 Number of distributed Gen. 82 176 Number of substations 223 294 Power of the two grid-forming units (MW) 9.1 and 3 1.33 and 0.64 MODELLING AND CONTROL Medium voltage power systems For the simulations two test systems based on real German MV distribution system parts, one urban with high and one rural with low load density, are used. The parameters characterizing the considered grids are shown in Table 2. The MV power systems were adjusted to allow for islanded operation. In the islanded grid, there need to be some units that provide the voltage reference for the other units i.e. that control their voltage amplitude and frequency (grid-forming) while the other units control their output real and reactive power (grid-feeding) [3]. The two largest DGs of each grid are chosen as grid-forming units. The rest of the DGs, which are aggregated at the substations, are modelled as grid-feeding units. The loads are modelled as static PQ-nodes and are scaled accordingly to obtain the balance between generation and load, which is needed in the islanded mode.