What is an RMU?
An integral component of electrical distribution networks are ring main units, or RMUs. These small switchgear components regulate and safeguard the power supply in medium voltage (MV) networks. RMUs are usually found in restricted areas such as industrial complexes, metropolitan regions, and other places.
Key Functions of RMU
• Ensures uninterrupted power supply by enabling alternate routing in case of faults or maintenance. • Protects distribution transformers and downstream circuits. • Reduces voltage fluctuations and conductor material requirements compared to radial systems.
Here are some of the primary attributes and capabilities of Ring Main Units:
1). Compact Structure
RMUs are made to be as small as possible while maintaining the necessary protection and switching capabilities. They are made up of a wide range of switchgear parts that are all kept in a small enclosure, including earthing switches, load disconnect switches, & circuit breakers.
2). Ring Configuration
The primary function of RMUs is to distribute power by forming a ring-shaped network, hence their name. They create a closed loop by joining incoming & outgoing power lines, which enables electricity to be provided from several sources. In case of a malfunction or repair, this configuration offers backup supply pathways and improves reliability.
3). Protection & Switching
Circuit breakers, which enable the safe switching & interruption of electrical currents, are a feature of RMUs. The network is shielded from problems such as short circuits and overcurrents by these circuit breakers. In order to isolate & ground the network while repairs or maintenance are being performed, load disconnect switches & earthing switches are additionally included.
4). Remote Control & Monitoring
Advanced technology for remote control & monitoring are common in modern RMUs. This enables operators to monitor power flows & system conditions in real time, remotely operate the RMUs, and swiftly identify any abnormalities or failures. The ability to operate remotely improves operational effectiveness and allows for prompt reaction to network events.
5). Safety Features
RMUs are equipped with a number of safety measures to protect workers and equipment. These could include safety shutters to shield workers from unintentional contact with moving parts, interlocking devices to stop improper operations, and fault indications or alarms to notify operators of abnormal conditions.
6). Easy Installation & Maintenance
RMUs are made to be simply installed; they often come as prefabricated units that are portable and readily network-connected. Additionally, they make simple maintenance easier by enabling the replacement or repair of individual parts without interfering with the operation of the entire network.
In medium voltage networks, RMUs are essential to the effective and dependable distribution of electrical power. They provide a small and adaptable switching, protection, & control solution that minimizes downtime and disturbance during maintenance (or) fault conditions while ensuring the continuous supply of electricity.
Tests Performed on RMU
Testing and commissioning of RMUs are important to ensure their safety, reliability, & performance before being put into service.
The tests are generally categorized as follows:
Factory Acceptance Tests (FAT)
Performed at the manufacturer’s site before dispatch:
- Dielectric Tests
- Primary & Secondary Injection Tests
- High Voltage Test
- Partial Discharge Test
- Gas Insulation Leakage Test
- Contact Resistance Test
- Mechanical Operation Test
- Temperature Rise Test
- Protection Relay Testing
- IP Rating Test
Dielectric Tests : Power frequency withstand and impulse voltage tests to verify insulation strength.
Primary & Secondary Injection Tests : Check current transformer (CT) ratios and protection relay operation.
High Voltage Test : Confirms RMU can handle rated voltage without breakdown.
Partial Discharge Test : Detects insulation defects.
Gas Insulation Leakage Test : Checks SF6 or vacuum insulation integrity.
Contact Resistance Test : Measures resistance across switching contacts.
Mechanical Operation Test : Ensures smooth operation of breakers, isolators, and switches.
Temperature Rise Test : Evaluates heating effects at rated current.
Protection Relay Testing : Verifies correct operation of protection features.
IP Rating Test : Ensures proper ingress protection against dust and water5.
Site Acceptance Tests (SAT)
Performed after installation, before commissioning:
- Insulation Resistance Test
- Circuit Breaker Timing Test
- SF6 Gas Pressure Test
- Earthing & Continuity Test
- Functional & Interlocking Test
- Load & No-Load Operation Test
- VT & CT Ratio Test
- Secondary Wiring & Relay Configuration Check
Insulation Resistance Test: Checks insulation integrity using a megger.
Circuit Breaker Timing Test: Measures opening/closing times of breakers and switches.
SF6 Gas Pressure Test: Ensures correct gas pressure (for SF6-insulated RMUs).
Earthing & Continuity Test: Confirms proper grounding.
Functional & Interlocking Tests: Verifies operation of all interlocks and switching mechanisms.
Load & No-Load Operation Test: Confirms operation under various conditions.
VT & CT Ratio Test: Ensures proper transformer ratios for metering and protection.
Secondary Wiring & Relay Configuration Check: Verifies correct connections and relay settings.