M. García-Gracia

Integrated Control Technique for Compliance of Solar Photovoltaic Installation Grid Codes

Energy policies along with technological advancements in power electronics have allowed a high penetration of photovoltaic installations. To normalize grid connections, in particular for solar photovoltaic (PV) installations, an arrangement of requirements have been set as a grid code to prove that this penetration can provide support and fully control during both steady-state and transient conditions of the grid. In this paper, a control system for a PV installation connected to the grid is developed to accomplish the requirements raised in the 2010 draft version of the Spanish grid code P.O. 12.2. Control loops for steady-state conditions as well as transient conditions (voltage dips, swells, and frequency control) have been implemented. Simulations have been performed using PSCAD/EMTDC™ program and supported by field tests of an existing PV installation in Spain using the voltage dip generator.

Protection in Distributed Generation

Most protection systems for distribution networks assume power flows from the grid supply point to the downstream low voltage network (Jenkins et al, 2000). Protection is normally based on overcurrent relays with settings selected to ensure discrimination between upstream and downstream relays. A fault on a downstream feeder must be cleared by the relay at the source end of the main feeder. It must not result in the operation of any of the relays on an upstream feeder unless the downstream relay fails to clear the fault. This will result in a blackout in a part of the network that should not have been affected by the fault.

Wind Farms and Grid Codes

All customers connected to a public electricity network, whether generators or consumers, must comply with agreed technical requirements. Electric networks rely on generators to provide many of the control functions, and so the technical requirements for generators are unavoidably more complex than for demand customers. These technical requirements are termed ‘Grid Codes’. The technical requirements governing the relationship between generators and system operators need to be clearly defined. The introduction of renewable generation has often complicated this process significantly, as these generators have physical characteristics that are different from the directly connected synchronous generators used in large conventional power plants. In some countries, a specific grid code has been developed for wind farms, and in others the aim has been to define the requirements as far as possible in a way which is independent of the power plant technology. The technical requirements within grid codes and related documents vary between electricity systems. However, for simplicity the typical requirements for generators can be grouped as follows: • Tolerance the range of conditions on the electricity system for which wind farms must continue to operate; • Control of reactive power often this includes requirements to contribute to voltage control on the network; • Control of active power often this includes requirements to contribute to frequency control on the network; • Protective devices; and • Power quality. It is important to note that these requirements are often specified at the Point of Common Coupling (PCC) between the wind farm and the electricity network. In this case, the requirements are placed at wind farm level, and wind turbines may be adapted to meet these requirements. It is also possible for some requirements to be met by providing additional equipment, as for example for FACTS devices. One of these new connection requirements regarding wind energy is fault ride-through capability. In the past, wind generators were not allowed to remain connected to the utility when voltage at the PCC fell below 85 %, forcing their disconnection even when the fault happened far from the wind farm (Jauch et al, 2007; Rodriguez et al, 2002). That is the reason

Harmonic Distortion in Renewable Energy Systems: Capacitive Couplings

Renewable energy systems such as wind farms and solar photovoltaic (PV) installations are being considered as a promising generation sources to cover the continuous augment demand of energy. With the incoming high penetration of distributed generation (DG), both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric network (Dugan et al., 2002). This latter issue is an umbrella concept for a multitude of individual types of power system disturbances. A particular issue that falls under this umbrella is the capacitive coupling with grounding systems, which become significant because of the high-frequency current imposed by power converters. The major reasons for being concerned about capacitive couplings are: a. Increase the harmonics and, thus, power (converters) losses in both utility and customer equipment. b. Ground capacitive currents may cause malfunctioning of sensitive load and control devices. c. The circulation of capacitive currents through power equipments can provoke a reduction of their lifetime and limits the power capability. d. Ground potential rise due to capacitive ground currents can represent unsafe conditions for working along the installation or electric network. e. Electromagnetic interference in communication systems and metering infrastructure. For these reasons, it has been noticed the importance of modelling renewable energy installations considering capacitive coupling with the grounding system and thereby accurately simulate the DC and AC components of the current waveform measured in the electric network. Introducing DG systems in modern distribution networks may magnify the problem of ground capacitive couplings. This is because DG is interfaced with the electric network via power electronic devices such as inverters. These capacitive couplings are part of the electric circuit consisting of the wind generator, PV arrays, AC filter elements and the grid impedance, and its effect is being appreciated in most large scale DG plants along the electric network (Garcia-Gracia et al., 2010).

Simulation of surges on power lines using SPICE and EMTP: a comparative study

Overvoltages in power systems can be very destructive. The way one protects power equipment against surges is important from an economic point of view. In power systems, the insulation level is analysed for several cases: lightning surges, and switching operation. In this paper the simulation of these transient phenomena is achieved using SPICE and EMTP, and they are matched. For this purpose, several SPICE models have been developed: lightning arrester, insulator, power line with coupling, steel towers, and transformers. Several application examples which illustrate the performance of these models are also discussed.

Non-dissipative snubber design for AC/DC converters by using resonant techniques

One of the main objectives of the design of power converters working at high frequencies is the reduction of the switching losses in the semiconductor devices, looking for a better efficiency or a further increase in the working frequency. The resonant techniques have been developed as a way of reducing these losses. This paper describes a non-dissipative snubber applied to a 5 kW AC/DC converter, controlled by means of an IGBT, working at a frequency of 20 kHz and using quasi-resonant switching techniques. The paper analyses the working conditions of the switching device, explaining the special features of the control strategy, derived from the resonant behaviour of the converter.