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Relay protection field
Current Transformer (CTs) feed signals to protective relays in factories. When abnormal currents occur, relays trip breakers, preventing motor burnout and costly equipment damage.
Relay protection devices (protection relays) play a critical role in ensuring stability, safety, and regulatory compliance of power generation and distribution systems. With renewable energy deployment, distributed generation, and stringent grid codes, relays are central to fault detection, grid disconnection, frequency and voltage enforcement, and overall system reliability.
Germany’s regulations for grid connection of generation systems set legal requirements for protection relays. Two key technical connection rules are:
VDE-AR-N 4105: governs low-voltage connections (≤100 kW, <1,000 V AC).
VDE-AR-N 4110: for medium-voltage connections (>100 kW or ≥1,000 V AC).
In both cases, grid-feeding or grid-network protection relays are required to monitor voltage, frequency, outages, and related phenomena; to disconnect generators under out-of-limit or unsafe conditions; and to prevent islanding (i.e., unintentional separation of a portion of the grid).
1. Hydroelectric Plants and Distributed Generation
A concrete case is Lukas Anlagenbau GmbH, a company in Bavaria, which operates many hydroelectric power plants (capacities from about 5 kW to 10 MW). Due to the VDE-AR-N 4105 regulation (and its revisions), its plants must comply with grid-feeding monitoring requirements: detecting over-/under-voltage, over-/under-frequency, and reacting when parameters deviate.
To satisfy these demands, Lukas Anlagenbau has deployed ABB’s CM-UFD.M31 grid feeding monitoring relay in over 60 hydroelectric plants. This relay continuously monitors relevant parameters and trips the interface switch (disconnects the plant from the public grid) when limits are exceeded. The device also displays error messages, supports precise thresholds (frequency and voltage), and offers good usability (DIN-rail installation, readable display).
This case illustrates how relay protection ensures both operational safety and regulatory compliance, while allowing distributed generation to participate in the grid.
2. Solar Photovoltaic Systems and “NA protection”
Another key example is in solar PV deployment, especially in the low-voltage sector. Under VDE-AR-N 4105, any generation plant between about 30 kW and 135 kW must have Network and System Protection (NA protection) installed.
The NA protection combines a monitoring relay and redundant interlocking switches. The relay monitors the grid’s voltage and frequency, and when deviations exceed tolerances, it disconnects the PV system from the grid within a specified time (often within 0.2 seconds). The interlocking switches are arranged so that two switching elements in series ensure fault safety—if one fails, the other still ensures disconnection. The switching devices also feed back status to the monitoring relay, ensuring correct operation.
3. Devices / Relays In Use
German manufacturers such as Ziehl produce protection relays tailored to these standards. For example, the UFR1001E and UFR1002IP devices monitor voltage, frequency, vector shift, and rate of change of frequency (ROCOF), among others, meeting the requirements of VDE-AR-N 4105:2018-11 and VDE-AR-N 4110:2018-11.
These relays are used in generation plants (solar, hydro, wind) and provide crucial functionalities: detection of abnormal grid conditions, safe disconnection, remote control/standby, display and logging of faults. The Ziehl relays are designed to be single-fault-safe (i.e., if one component fails, the safety feature still works), which is a requirement under German grid rules.
From these applications, we can see several categories of problems relay protection devices help with:
Grid Stability and Safety
Renewable energy sources are variable; sudden changes in generation (e.g. due to clouds over PV or wind speed drops) can cause frequency and voltage fluctuations. If not managed, these fluctuations can propagate and destabilize the wider grid. Relays ensure that generation plants disconnect when parameters exceed safe tolerances, protecting both the plant and the network.
Preventing Islanding
Islanding (a generation unit keeping running when the grid is disconnected) is dangerous for maintenance crews and can lead to equipment damage. Protection relays with the correct detection (voltage loss, frequency drift, vector shift, etc.) ensure that when grid connection is lost or faults arise, generation is safely shut down. Solar systems in the NA protection regime are required to include anti-islanding protection.
Compliance with Grid Codes and Legal Requirements
Regulatory standards like VDE-AR-N 4105 / 4110 in Germany impose specific protection, safety, and performance requirements. Operators must use relays certified for these standards, with correct settings. Compliance avoids legal penalties and ensures that plants can remain connected and remunerated. The case with the hydro company adopting ABB’s CM-UFD.M31 to meet VDE code is an example.
Fault Detection and Fast Disconnection
Faults in lines, over-voltage, under-voltage, over-frequency, or under-frequency conditions must be detected quickly to prevent damage to components and prevent cascading failures. Relays provide this fast detection and actuation (switching off) to isolate faulty parts.
Quality of Power and Grid Integration
As renewable energy increases, power quality (voltage fluctuation, flicker, harmonics) and coherence (phase, frequency) become more challenging. Protection relays help monitor and enforce limits that maintain acceptable power quality for all users. They also support functions like vector shift (to detect aberrant phase relationships) and ROCOF (rate of change of frequency), which are important for stability when large amounts of distributed generation are online.
Device usability matters: The real-world case in Germany shows that relays which are easy to configure, with clear displays and simple setup processes, are much more likely to be properly installed and maintained. The hydroelectric plant operator budgeted for ease of configuration and precision.
Redundancy and fault-safety: Two independent switching elements in series, monitoring of switch positions, dual-channel relays, fail-safe design are not just nice features but regulatory requirements in many German grid codes.
Thresholds and timing: Settings such as voltage/frequency limits, delay before disconnection, switching capacities, etc. must be tailored to local grid operator requirements. German rules define time limits (how quickly a generator must disconnect when the voltage/frequency is out of range). The sensitivity, precision, and speed of relays are decisive.
Standard evolution: The VDE-AR-N 4105 and 4110 standards have evolved. Manufacturers and operators must keep up with revisions. Protection relays often provide preset threshold values for current versions of these standards.
Relay protection devices are a cornerstone of the safe, compliant, and stable integration of renewable and distributed energy sources. Through enforcement of regulatory standards (VDE-AR-N 4105 / 4110), deployment of advanced relay devices (ABB, Ziehl etc.), and real case work in hydroelectric and solar PV systems, relays address key issues:
disconnecting in unsafe voltage/frequency conditions
preventing islanding
protecting equipment and grid stability
ensuring compliance and avoiding penalties
As Germany's energy transition (“Energiewende”) continues, with more decentralized generation, energy storage, microgrids, and smart grids, protection relays will grow in importance—not only for safety and regulation, but also as enablers of reliability, resilience, and flexible operation.