The AREVA MVAX91W1DD0754A is a highly specialized and critical component within the AREVA (now part of Framatome/Schneider Electric) protection and control ecosystem for high-voltage electrical power systems. This code designates a specific Winding 1 (primary side) Restricted Earth Fault (REF) Protection Relay Module, designed for integration into complex electromechanical or static relay protection panels for power transformers. The module’s core function is to provide extremely sensitive and fast protection against earth (ground) faults that occur within the protected zone of the transformer’s primary (high-voltage) winding. Unlike overall differential protection, which covers the entire transformer, Restricted Earth Fault protection is specifically tuned to detect low-magnitude internal ground faults close to the transformer’s neutral point—a zone where differential protection may have reduced sensitivity. The MVAX91W1DD0754A module continuously monitors the current balance between the phase current transformers (CTs) on the winding and a dedicated current transformer in the transformer’s neutral-to-earth connection. Any imbalance indicates a fault current flowing to earth within the protected zone, prompting the relay to issue a trip command to isolate the transformer, thereby preventing extensive damage, fire hazards, and ensuring system stability.
This module represents the application of specialized, high-impedance REF protection principles. Its alphanumeric code signifies its precise role (MVAXfor protection function, 91often indicating REF, W1for Winding 1, DDfor specific characteristics or version, 0754Afor detailed ordering/design code). Its value lies in its selectivity; it remains stable for external faults outside its zone while operating decisively for internal ground faults. In large power transformers—critical and expensive assets in transmission, generation, and heavy industry—the AREVA MVAX91W1DD0754A provides an indispensable layer of protection, complementing overall differential, overcurrent, and Buchholz (gas) relays to form a comprehensive transformer protection scheme. It is engineered for maximum reliability and precision, as its correct operation is vital for preventing catastrophic transformer failures and minimizing outage times.
Technical Specifications
Parameter Name
Parameter Value
Product Model
AREVA MVAX91W1DD0754A
Manufacturer
AREVA (now part of Framatome, under Schneider Electric’s Energy Automation portfolio)
Primary (High Voltage) Winding of a Power Transformer
Inputs
Current from phase CTs (typically connected in parallel) and a dedicated Neutral CT (NCT)
Operating Principle
High-impedance circulating current principle; operates on voltage developed across a stabilizing resistor.
Operating Setting
Highly sensitive, typically adjustable in the range of 5-40% of the rated neutral CT current or a specific voltage threshold.
Stabilizing Resistor
External resistor (calculated and selected based on CT parameters and system fault levels) is required to be connected in series with the relay.
Operating Time
Very fast, typically less than 20-40 milliseconds for internal faults.
Output Contacts
Dependable, heavy-duty output relay contacts (usually 2 or more) for tripping the circuit breaker and providing alarms.
Auxiliary Supply
48/110/125/250 V DC or 110/240 V AC (specific to order variant, denoted by suffix).
Case/Form Factor
Designed for panel mounting, typically in a standard 19” rack or customized panel layout.
Standards Compliance
IEC 60255, IEEE C37.90, and other relevant protection relay standards.
Key Features and Advantages
The AREVA MVAX91W1DD0754A is defined by its exceptional sensitivity and stability in high-impedance REF applications. Its primary advantage is the ability to detect low-level ground fault currents that may not be sufficient to operate the main differential or overcurrent protection. This sensitivity is crucial for catching incipient winding faults before they escalate into major inter-turn or phase-to-phase faults, potentially saving the transformer from severe damage.
MVAJ205
High stability during external faults is a cornerstone of its design. The high-impedance principle ensures that during external (through) faults or CT saturation conditions—which can cause significant spill currents in the differential circuit—the voltage across the relay circuit remains below its operating threshold due to the stabilizing resistor. This prevents maloperation (false tripping), a critical requirement for maintaining power system reliability. The module is designed for simplicity and robustness, using proven analog or static electronic components (depending on the era of manufacture) that offer high reliability and long service life with minimal maintenance.
Clear and definitive operation is another key feature. When an internal ground fault occurs, the imbalance creates a voltage high enough to operate the sensitive relay element. This operation is fast and decisive, triggering the output contacts to isolate the transformer. The module often includes flag indicators (targets) that mechanically drop or LEDs that illuminate to provide a clear, persistent visual indication of which relay operated—an invaluable aid for post-fault analysis. As part of AREVA’s legacy protection systems, it is designed for easy integration into existing protection panels and is supported by well-established application engineering guidelines for calculating the required stabilizing resistor and setting the sensitivity.
Application Areas
The AREVA MVAX91W1DD0754A is exclusively applied in the protection of high-voltage power transformers within electrical transmission and distribution networks, as well as in large industrial power systems. Its primary application is for protecting the primary (HV) winding of:
Generator Step-Up (GSU) Transformers in power plants, where it protects the transformer connecting the generator to the grid.
Transmission & Grid Interconnection Transformers at substations, crucial for network stability.
Large Industrial Plant Service Transformers (e.g., for steel mills, chemical plants, data centers), where transformer failure would cause catastrophic production loss.
Auto-transformers used in high-voltage networks.
It is employed in systems where the transformer neutral is solidly grounded or grounded through a low impedance. The MVAX91W1DD0754A is always part of a comprehensive transformer protection scheme, working in conjunction with:
Overall Differential Relay (e.g., AREVA Px4x series): For phase-to-phase and high-current ground faults.
Overcurrent and Earth Fault Relays: For backup protection.
Buchholz Relay: For faults generating gas inside the oil.
Winding Temperature and Oil Temperature Indicators.
This relay is essential in any setting where the cost of an unplanned transformer outage and repair is extremely high, and where system reliability is paramount.
Related Products
AREVA MVAX92W2DDxxxx: The corresponding Winding 2 (secondary side) Restricted Earth Fault Protection module for the transformer’s LV winding.
AREVA Px4x Series (e.g., P441, P442): Microprocessor-based transformer differential protection relays that provide the main protection and may integrate REF functions in newer digital models.
AREVA / Alstom MCAG / MCGG Relays: Electromechanical overcurrent and earth fault relays used for backup protection on the same transformer.
Stabilizing Resistor: An external, high-power precision resistor that is an absolutely critical external component for the MVAX91W1DD0754A‘s operation. It must be sized based on CT parameters and fault calculations.
Metrosil (Non-linear Resistor): Often connected across the relay circuit to limit peak voltage during severe CT saturation under external faults, protecting the relay and CTs.
AREVA / Schneider Electric VAMP Arc Protection Relays: A modern solution for switchgear protection, representing a newer technology line.
AREVA Case / Panel: The standardized enclosure or custom panel in which the MVAX91 module, along with other relays, meters, and controls, is mounted.
Current Transformers (CTs): The specific phase CTs and the Neutral CT (NCT) that provide the input signals to the REF protection scheme.
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