The Connection of Green Energy Generation Plants to Power Systems: Key Factors to Consider

To prevent negative effects on the operation of the power system and equipment, a number of factors must be considered when connecting green energy generation plants to these systems. The following should be factored in:

• The rated power of the equipment in the power system must be sufficient enough to enable the transfer of the produced power from the generation plant into the power system and to the consumers.
• The short-circuit current of the power system must not be inadmissibly increased by the power plants.
• The voltage rise at the connection point i.e. the point of common coupling (PCC) and in the power system must remain below the permissible limits.
• The power quality with respect to harmonics, inter-harmonics, asymmetry and flicker must be properly maintained and not degraded by the connection of the generating system.

With the increased development of green energy generation plants, special aspects need to be considered such as the use of power electronics for the connection of the AC system (e.g. with photovoltaic and wind energy), the abrupt increase or decrease in generation (a common occurrence in the operation of wind energy plants), and distributed generation (decentralized) with small generation units (such as co-generation units producing heat and electricity, biogas power plant).

If a great number of generation units are connected to one power system level e.g. the connection of photovoltaic or co-generation units to the low-voltage system or connection of wind sparks to the high-voltage transmission system, the assessment of the connection and the operation of the generation units will need to be carried out differently, i.e. the assessment differing from a typical connection of one unit or a limited number of generation units to the power system. The increased use of small co-generation units and the connection of photovoltaic installations in low-voltage systems require new considerations regarding the system protection, system operation, power quality and limitation of short-circuit currents concerning these voltage levels. For high-voltage transmission systems the connection of wind sparks is achieved either directly by three-phase AC connection or in case of ‘offshore-plants’ by means of HVDC.

The connection of large generation plants is determined for instance, by the contribution of the generation to the short-circuit current. In medium-voltage and low-voltage systems, the short-circuit power is lower than in high-voltage systems. The acceptable share of the short-circuit level due to the connection of generation is therefore lower as well.
It is important to keep in mind that the contribution to the short-circuit current of generation units connected to the system through power electronics is lower; for instance, in the range of the rated current of the generation unit.

In small hydro plants up to rated power of 500 kW, asynchronous generators are used; for generation with higher rated power, synchronous generators are applied. The structure of connection of small hydro power plant therefore does not differ much from the schemes used for other conventional forms of generation. Small hydro units are connected to LV and MV systems in remote areas, having low short-circuit power.
Wind energy plants are equipped either with asynchronous generators (also with static frequency converter in the rotor circle, so-called static inverter cascades) or with synchronous generators with or without intermediate DC circuit, provided to control the rotor speed independently of the frequency of the power system. Reactive power is required both for the magnetization of asynchronous generators and for the commutation of static frequency converters. Power electronics device generate harmonic and inter-harmonic currents, which need to be considered for the assessment of the power quality. Generally, generation units can cause voltage fluctuations due to the fluctuating generation. This fact must be given special attention in the case of fluctuating wind power in terms of voltage flicker. Wind energy plants and wind sparks with total power above ~40 MW must be allocated to the high-voltage transmission system.

Photovoltaic installations are used either with comparatively low rating of a few kilowatt installed locally on building roofs and connected to the LV system, or as large-scale plant in the capacity range above some hundred kilowatts, to be connected through transformers to the MV system. The electrical energy produced as DC voltage is converted into the desired AC voltage by an inverter, adjusting frequency and voltage level. DC/AC inverters emit harmonic and inter-harmonic currents. In case of the connection of fuel-cells, where the electricity is produced at DC voltage level, the same technical conditions apply as for photovoltaic installations.
The production of electricity using biomass can be executed by means of conventional steam turbines, screw expansion machines or micro gas-turbines using predominantly synchronous generators or inverter installations. Asynchronous generators may also be used.

Related articles:

• Major Technologies for Smart Grid Implementation
• Overhead vs. Underground Power Distribution Systems
• What Is the Difference between Photovoltaic Panels Parallel & Series Connection?
• DC to DC Power Converters: Function, Types, Operation & Applications
• Types of Busbar Arrangements in Grid Stations and Substations
• What Are The Benefits Of Installing A Solar Panel System In The Home?
• AC to AC Power Converters with Intermediate DC Link: Types, Features & Applications