Three-phase relays continuously monitor and detect voltage faults to protect equipment in both Wye and Delta systems. They offer protection from a variety of conditions, including phase loss, phase reversal, phase unbalance, overvoltage and undervoltage. These versatile relays are available in plug-in and DIN-rail enclosures and feature one or two single-pole double-throw (SPDT) contacts.
China Three-phase relays have been designed to meet a wide range of applications. These include mercury relay replacement, professional food-service equipment, kilns and ceramics, HVAC and refrigeration, petrochemical plants, plastic extrusion/thermoforming machinery, lighting systems, UPS systems and motor-switching equipment. They provide a reliable, economical alternative to expensive mercury or contact relays, and they can be installed in existing equipment without requiring modifications.
Unlike other relays, these units use true RMS voltage measurement to provide more accurate detection of phase imbalance, which is often the cause of equipment damage or malfunction. The units can be set to trigger when the three-phase system voltage drops below or above predetermined threshold values, which prevents phase imbalance damage.
They also have an internal transient protection circuit that eliminates the need for external MOVs, and they are ideal for replacing mercury relays in applications where high-speed current surges can occur. These relays feature three LEDs to indicate status: green, yellow and red.
The relay priority logic selects the optimal directional element to run in each cycle. The GDR uses the status of the 32IE, 32QE and 32VE Enable bits in this logic to determine which directional element is most likely to be the best choice.
If the directional element selected by the priority logic is not appropriate to the detected condition, it will be ignored. This prevents the relay from generating unnecessary alarms. The relay’s internal logic will then select the next best directional element to run. The GDR will continue this process until it finds a directional element that is suitable for the detected condition.
When selecting the directional element to run, the GDR considers the currents I0 and I2 of the protected line. The directional element must be capable of detecting both of these currents, so the relay will only select the zero-sequence current-polarized directional element when the currents I0 and I2 are above their respective sensitivity thresholds.
The GDR will then compare the current-to-residual impedance magnitude Z2F and Z2R to make a fault direction declaration. The relay will declare a forward fault when the Z2F threshold is greater than Z2R and a reverse fault when the Z2R threshold is less than Z2F.
The relay will also detect a ground fault when the negative-sequence current I2 is above its impedance threshold. The directional element will compare the magnitude of this current to its own sensitivity threshold and decide whether the fault is forward or reverse. This is the same way that the relay declares a normal fault.
