How to install a DTU on an N2 Insulation ring main unit?

DTU (Distribution Terminal Unit), a substation terminal in distribution automation systems, is secondary equipment installed in switching stations, distribution rooms, N2 Insulation ring main units (RMUs), and box-type substations. It bridges primary equipment and the distribution automation master station. Older N2 Insulation RMUs without DTUs cannot communicate with the master station, failing to meet automation requirements. While replacing entire RMUs with new DTU-integrated models would solve this, it requires significant investment and causes power interruption. Retrofitting existing RMUs with DTUs offers a cost-effective solution. Based on practical experience, here's the process for retrofitting N2 Insulation RMUs with sheltered upright and outdoor upright "three-remote" (telemetry, teleindication, telecontrol) DTUs.

1 Key Survey Points for N2 Insulation RMU Retrofit

(1) Check primary equipment defects: Inspect for severe corrosion, mechanism jamming, or deformation. If equipment is too old, retrofitting isn't warranted. 

(2) Verify electric operating mechanisms: Non-electric mechanisms only support telemetry/teleindication without telecontrol capability. Retrofit decisions should consider company requirements.

(3) Confirm secondary wiring terminals: Without accessible terminals, DTU wiring is impossible. RMUs with internally enclosed wiring (requiring bolt removal for access) aren't suitable for retrofitting. (4) Determine RMU configuration: N2 Insulation RMUs typically include incoming cabinets, outgoing cabinets, and voltage transformer cabinets. 2-in/4-out units have 7 bays; 2-in/2-out units have 5 bays. Common DTU configurations include 4, 6, 8, or 10 channels (typically not exceeding 10). Channel count determines DTU dimensions.

(5) Assess installation space: After determining DTU size, verify if the RMU interior can accommodate it. Sufficient horizontal space allows sheltered upright installation; otherwise, outdoor upright is needed. For sheltered upright installation, also consider side cabinet door availability. If the DTU fits only sideways but no side door exists, cabinet modification is required. Outdoor upright installations require an additional external cabinet, increasing costs, affecting aesthetics, and requiring foundation work. Foundation placement should consider environmental impact, proximity to voltage transformer compartments (shorter cables with closer placement), and cable routing options.

(6) Check voltage transformer availability: Current transformers provide measurement current to protection devices and DTUs. While most RMU bays include current transformers, voltage transformers aren't always present. Voltage transformers supply power to devices (line loss modules, power supplies, etc.) and instruments (voltage meters, power meters), provide 220V AC, zero-sequence voltage, and DTU measurement voltage. Through power modules, they indirectly supply operating power, DTU power, teleindication power, and communication power. RMUs without voltage transformers (relying solely on current transformers for protection device power) aren't recommended for retrofitting. Some RMUs have voltage transformers with only 10/0.22 ratios requiring replacement with 10/0.22/0.1 ratio units. Additionally, verify if existing voltage transformer capacity is sufficient for the added DTU load (typically ≤40 VA).

(7) Verify bay equipment types: Electric-operated circuit breakers and load switches use similar control cables (load switches simply lack the "energy stored" signal wire). Manual load switches only require position signals and measurement lines connected to DTU terminals.

(8) Identify safety hazards: Survey potential construction hazards and develop appropriate safety measures.

2 Material Preparation

(1) DTU selection: Post-survey, determine appropriate DTU model (channel count). For common 2-in/4-out configurations, 6-channel or 8-channel DTUs are suitable.

(2) Control cables: These connect RMU terminals to DTU terminals, forming various circuits:

• Signal circuits: Transmit switch positions (closed/open position, energy stored, remote/local status, etc.). Typically use 12×1.5 mm² control cables. Position signals for voltage transformer compartment switches have limited value and are generally not installed.

• Measurement circuits: Include voltage and current measurement (load current and zero-sequence current). Monitor grid parameters to calculate power values and detect abnormalities (phase loss, imbalance, overload). These activate DTU protection functions (three-stage current protection, voltage protection, zero-sequence protection). Typically use 3-4 cores of 6×2.5 mm² cables connecting phase current transformers (UVW three-phase or UW two-phase) to DTU terminals. 2-in/4-out configurations require six 6×2.5 mm² cables. An additional 6×2.5 mm² cable connects voltage transformer 100V terminals to DTU terminals. Many RMUs lack zero-sequence transformers since cable networks have low ground fault probability.

• Control circuits: Enable remote/local control of circuit breakers or load switches. Typically use 3 cores of 12×1.5 mm² cable.

• Power circuits: Supply power to modules like power supplies. Typically use 2 cores of 6×2.5 mm² cable.

For common 2-in/2-out and 2-in/4-out RMU configurations, required control cable specifications and reference lengths are shown in Table 1.

Among these: 

① For 12×1.5 mm² control cables: One end of the cable cores connects to circuit breaker closing control, circuit breaker opening control, common terminal for opening/closing operations, etc., while the other end connects to the DTU via terminal blocks, forming the remote control circuit. Other cores connect to circuit breaker closed position, circuit breaker open position, disconnector closed position, grounding disconnector closed position, remote position, energy-stored position, common terminal, etc., with the other ends connecting to the DTU via terminal blocks, forming the remote signaling circuit. Electrically operated load switches require the same wiring as circuit breakers except for the absence of the "energy stored" signal wire. Unused cable cores should be kept as spares. A 2-in/2-out configuration requires 4 cables of this type; a 2-in/4-out configuration requires 6 cables. These cables are not needed for the voltage transformer compartment.

② For incoming and outgoing line compartments: 6×2.5 mm² cables connect to the U, V, W three-phase or U, W two-phase current transformers and common terminals for each incoming or outgoing line. Three-phase connections require 4 cores; two-phase connections require 3 cores. Remaining cores are kept as spares. A 2-in/2-out configuration requires 4 cables of this type; a 2-in/4-out configuration requires 6 cables.

③ For the voltage transformer compartment: One additional 6×2.5 mm² cable connects the cabinet's U, V, W three-phase 100V and 220V terminals (requiring 5 cores total) to the DTU terminals. This measured voltage primarily monitors power outages and voltage anomalies within the cabinet, supports power calculation, provides sampling for voltage-based relay protection, and supplies power to the power module (which provides operating power to the DTU).

(3) Auxiliary materials: Prepare fireproof sealant, PVC wire marker tubes, cable identification tags, nylon cable ties, wire wrapping tubes, insulation tape, and other auxiliary materials as needed based on actual conditions.

(4) Installation tools: Prepare cable strippers, screwdrivers, multimeters, and other necessary tools.

3 Construction Procedures

Since DTU installation only requires secondary equipment to be de-energized, primary equipment operation remains unaffected. To prevent accidental power interruption to primary equipment during DTU installation and commissioning, the following must be confirmed in advance:

The remote/local switch is set to "local" or "locked" position All relay protection output hard plates have been withdrawn All air circuit breakers except device power supply and AC power supply are disconnected

(1) First, securely mount the DTU and ensure reliable grounding with ground resistance not exceeding 10 Ω.

(2) Connect one end of the prepared control cables to corresponding DTU terminals and the other end to cabinet terminals. Due to mechanical tension in the cables, maintain adequate slack as reserve length. Cable laying and wiring must comply with secondary cable connection requirements. For example: control cables should be neatly and securely bound with nylon cable ties; both ends of cables must have identification tags; exposed wire cores after removing cable insulation should be wrapped with wire wrapping tubes. Since this is retrofit wiring, both ends of each wire core must be clearly marked with PVC marker tubes. Unused wire cores should be insulated with tape to prevent accidental contact.

(3) After completing wiring, verify all connections again to ensure accuracy. Check that no tools or leftover materials remain at the site.

(4) Conduct joint commissioning of the DTU with primary equipment and the distribution automation master station to ensure accurate "three-remote" (telemetry, teleindication, telecontrol) functionality. After verification, label the corresponding remote control hard plates according to line numbers and directions. Settings can be input during the commissioning process. Since factory testing of DTUs can only verify communication functionality (without wiring, the master station cannot see telemetry and teleindication data), on-site joint commissioning is necessary to confirm proper wiring and "three-remote" functionality.

(5) Seal all cable openings and clean the site.

(6) As needed, energize the appropriate air circuit breakers, plates, and switches. After equipment commissioning, do not arbitrarily change plate and switch positions.