Fixed vs remotely adjustable power reduction
Many plant operators in Germany are faced with a choice of installing a fixed or a remotely adjustable power reduction at their PV plant. With a fixed power reduction of the plant’s output, the inverters are adjusted to a certain percentage of the module’s power output. Only this portion is fed into the public power grid. However, as a result, the grid is very inflexible. If the amount of electricity required from the PV plant increases or decreases, the plant operator has to be informed first before the reduction can be adjusted accordingly. Despite this inflexibility, this method does have the advantage of having a relative low amount of technical expenditures required. A modern plant monitoring and management system such as the Solar-Log™ can be used to control the inverters to limit the feed-in, for example, to 70% of the module‘s power output.
In contrast, an adjustable reduction is a bit more technically complex to implement. The idea behind an adjustable reduction is that a central authority like a power company monitors how much electricity is currently needed. Then, thanks to the remotely adjustable power reduction at decentralized generating plants, the power company can request more or less power be fed into the grid depending on the current power needs.
There are various options to implement this solution. One option is to equip the plant with ripple control receivers. The control signals from the grid operator are sent to the ripple control receiver are then relayed via potential-free contacts. Then a control element such as a data logger evaluates the relay contacts and uses these commands to control the inverters.
A distinction is made between active and reactive power. The Solar-Log™ PM+ data logger, for example, has a double interface to distinguish the potential-free contacts. Up to two ripple control receivers can be connected to this interface, one for active power reductions and one for reactive power reductions. An advantage of this solution is that reactive power is configured with the data logger and does not have to be entered for each and every inverter.
Practical implementation of individual requirements
For plants larger than 100 kWp, it also makes sense to require a confirmation of the current amount of feed-in power to have information on the current production levels. For this, remote control technology such as telecontrol systems are used since, unlike with ripple control, technology, bi-directional communication is possible. This is a requirement that requires a considerable amount of time and effort. Christoph Zeitz, responsible for Feed-In Management at Solare Datensysteme GmbH, describes the practical implementation, “The challenge here is to process the analog signals and the confirmation signal. However, since these signals could require different standards, our system has been customized to deal with any variations. The Solar-Log™ data logger with Powermanagement, the Utility Meter measuring device and I/O Boxes can be used for this purpose.”
Three components, one system
“The Utility Meter allows for variable reactive power adjustments via the Q(U) function. It determines the amount of current present in the medium-voltage power grid and submits this value to the data logger. Based on the pre-determined characteristic curve, the data logger constantly calculates the amount of reactive power to supply and relays this requirement to the connected inverters. It also measures the reactive power, current and voltage, which are relayed in parts back to the grid operator,” Zeits adds. The I/O Box is the third component and is installed between the remote control technology and the data logger. It receives the many different signals from the remote control technology and relays them to the logger, which then controls the inverters.