This paper first explains the required feed-in of voltage supporting currents during grid faults in a simplified case. Secondly, resonance points of the grid conection point are calculated analytically. The influence of weak grid conditions and large numbers of inverters is analysed. To underline the theoretical discussion on test conditions, measurements concerning Low Voltage Ride Through (LVRT) and stable operation of PV plants are presented. They comprise measurements from single inverters, as well as from large PV plants in the megawatt range.
In many electrical grids worldwide, the rising amount of installed PV power entails a considerable influence of PV systems on grid quality and stability. All indicators point to an increasing trend towards power electronic generation units. Both wind and solar plants have reached power levels comparable to conventional power plants. Considerable shares of renewable generation occur quite regularly throughout the world, not only in the German grid (e.g. 50% at 2l.July 2013, ). The ongoing substitution of synchronous generators by power electronic driven generation units require that those generation units guarantee the same level of stability and grid control functions. At Fraunhofer ISE's Megawatt Inverter Laboratory, several dozens of large PV inverters, with a nominal power of up to 1 MY A, have been characterised during the past four years. These measurements were expanded to fault ride through field tests in a 5 MW solar test plant. Solar power stations can be built up out of hundreds to even thousands of inverters running in parallel; therefore high requirements apply to the inverter's control algorithms. This paper discusses the various characteristics of grid connection points in large scale PV power plants and their impact on the system stability. Critical issues of the Fault Ride Through Test also examines the inverters performance in weak grid conditions. We present measurement results of both Low Voltage Ride Through (L VRT) field and laboratory measurements. To summarise, the performance of today's inverters will be compared with the needs of the future energy generation system and with the requirements of currently applicable international grid codes.
II. NECESSITY OF DYNAMIC GRID SUPPORT
A. Fault handling in today's electrical energy system
The transition of the electrical energy system towards a grid dominated by inverters requires a shift of responsibility and grid control capability towards PV inverters and wind turbines. The basis of the present protective system is a high short circuit power at the transmission level combined with a cascaded and staggered arrangement of protection devices at significant knots. This allows the grid protection device, which is the closest to the fault, to react and clear the fault. Due to the size of conventional power plants, the feed-in is at the highest voltage level and the energy is transferred to the consumers via the lower voltage levels.
B. Fault handling in power electronic dominated electrical energy systems
The inverters capability of supplying reactive current allows supporting of the grid voltage during the fault. First this reduces the impact of the fault onto nearby power consumers and secondly the inverters participate in the provision of short circuit current. Grids with a high share of power electronic loads and generation units will have a highly decentralised structure. Renewable energy sources usually form blocks in a smaller power range and require higher numbers of energy sources. They often feed into low and medium voltage levels. Any power electronic based generator has no or very limited over load capability unlike rotating machines or transformers. Power electronic generators also do not have an intrinsic inertia. Today's grid protection system is based on high fault currents; fault currents slightly higher than the nominal current are difficult to detect by the protection devices. To ensure a quick clearing of faults by grid protection devices each generation unit, including the distributed ones, need to provide dynamic grid support by feeding fault current into the grid.
The following calculation shall show how effective the feed-in of capacitive fault current is. A simple grid is presented in Fig. 1. The ideal grid is represented by a voltage source. The fault, represented by a variable load, is located at the end of a long inductive transmission line. This end is also fed by a PV inverter .
In a fust case, the PV inverter is disabled. The resulting voltage VFAULT at the end of the transmission line is now defined by the line impedance and the fault impedance.
The variation of the fault impedance is shown in the dotted yellow line in Fig. 2. The power transferred over the transmission line (or the power dissipation in the fault) reaches a maximum when the impedances have the same absolute value.
When the inverter is enabled the current Ipv influences the voltage VFAULT at its terminals. If the inverter feeds active current into the grid fault, the influence on the grid voltage is almost negligible (red line compared to yellow line in Fig. 2). In the considered model, the transmission line is dimensioned to reach a short circuit power of 40 times the inverter power at its end. This can also be expressed in a short circuit ratio of Sshort circuit! Pinverter = 40. The blue and the green line in Fig. 2, show the influence of the phase angle of Ipv onto the voltage VFAULT and the power dissipation of the fault. Bearing in mind that a high fault current is needed to clear the fault as fast as possible, it is made visible in the green line that only capacitive fault current helps to support the grid voltage in inductive grids.
If a power electronic dominated grid consists of a large number of generation units, one can assume that many of them operate in part-load mode and can deliver higher fault currents (nominal current) than their present/actual current at the particular moment. This can be compared to a limited over-load capability. In micro grids with only few generation units, the voltage stability (voltage support corresponds to capacitive current) needs to be balanced against the frequency stability and the need for active current.