Earth Leakage Relay for Photovoltaic Systems
The application of earth leakage relays in solar photovoltaic (PV) systems can ensure the safe operation of PV systems and prevent electric shock and fire. This article will introduce the causes of leakage current in PV systems, the importance of leakage current protection, and the application of leakage current protection.
Causes of leakage current in photovoltaic systems
The leakage current in photovoltaic (PV) systems is essentially a common-mode current. The root cause is the inevitable parasitic capacitance between the live parts of the photovoltaic system (including photovoltaic modules, cables, inverters, etc.) and the earth.
The generation mechanism of leakage current is as follows:
The generation of common-mode voltage: When the inverter is powered on, especially for transformerless (non-isolated) inverters, the high-frequency switching operation within it generates a common-mode voltage between the PV terminals (photovoltaic array) and the ground.
The role of parasitic capacitance: This common-mode voltage acts on the parasitic capacitance between the photovoltaic system and the earth.
The formation of the loop and the generation of current: Under the action of common-mode voltage and parasitic capacitance, the current will form a complete loop through the PV array-inverter-grid-earth, thereby generating leakage current. Since the loop impedance of the transformerless system is relatively low, this common-mode voltage will induce significant common-mode current.
IEC Standards for PV Leakage Current Detection
The leakage current in a PV system typically consists of two main components: capacitive AC leakage current and resistive DC leakage current. In order to effectively prevent accidental electric shock, international standards (such as IEC 62109-2) stipulate that the leakage protection relay of the photovoltaic system must be able to detect both DC and AC leakage current components.
Earth leakage relay for PV system
In photovoltaic systems, type B earth leakage relays must be used. This is because photovoltaic modules generate direct current, while inverters generate pulsating direct current, high-frequency alternating current, and complex leakage currents of superposition of direct current and alternating current. In practical applications, earth leakage protection solutions are usually deployed at multiple points, covering key locations on the DC and AC sides.
Our type B earth leakage relay can provide DC side leakage current protection, which is mainly used to protect the insulation safety of photovoltaic modules, DC cables, and DC input terminals of inverters, and prevent DC side grounding faults.
String-level/combiner box output: In the DC combiner box of a large power station, the output of each or several PV strings is monitored. By passing the positive and negative conductors of the string through a DC-specific zero-sequence current transformer (ZCT) and cooperating with a type B leakage current protection relay, once string-level leakage is detected, an alarm can be issued or the faulty string can be isolated through a DC fuse/circuit breaker.
Inverter DC input: A DC-specific ZCT and a type B leakage current protection relay are installed at the inverter DC main input cable. This is a more common configuration for monitoring the leakage current of the entire DC circuit from the PV array to the inverter.
After the type B earth leakage relay detects a fault, it outputs a signal to the DC circuit breaker (DC Breaker) or DC contactor (DC Contactor), quickly cutting off the DC circuit to prevent the fault from spreading and arcing.
Our earth leakage protection relay can also perform AC side leakage current protection, mainly protecting the lines on the AC output side of the inverter, the inverter itself, and the grid connection point, to prevent leakage caused by AC side grounding fault or internal fault of the inverter from affecting the grid and personal safety.
Inverter AC output end: Install an AC-specific zero-sequence current transformer (ZCT) in the main circuit after the inverter AC output (all phase lines and neutral lines), and cooperate with a type B leakage current protection relay.
AC junction box/grid connection point: Set up a total AC ZCT and a type B leakage current protection relay at the entrance where the AC sides of multiple inverters are connected to the distribution network or the public grid.
After the earth leakage monitoring relay detects the AC side leakage current, it outputs a signal to the AC circuit breaker (AC Breaker) to cut off the connection between the inverter and the grid.
Why choose earth leakage relay for PV systems?
Type B earth leakage relays can detect both DC and AC (including high frequency) leakage currents, covering all possible leakage types of photovoltaic systems.
The leakage current relay has a communication interface (such as RS485/Modbus), which can upload leakage current data to the central monitoring system (SCADA) to achieve remote real-time monitoring, historical data recording, fault alarm, and diagnosis.
The earth leakage monitoring relay can be used in conjunction with DC insulation monitoring device to provide more comprehensive protection for ungrounded (IT) or high-impedance grounded systems (common on the DC side).
Professional Type B ELR relays usually have good anti-electromagnetic interference capabilities to reduce false operations.
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