FTU power solutions in the electric power industry

In the electric power industry, equipment such as RTUs (remote terminal units), DTUs (data transfer units), FTUs (feeder terminal units) and TTUs (distribution transformer supervisory terminal units) are switch monitoring devices installed in the substation close to the feed breaker - a disconnector which is located in series at the end of a feeder, within a substation bay, in order to isolate the feeder from the system - which is widely suitable for use in automation engineering applications, urban, rural or enterprise power distribution networks.

These can be used for automated functions such as monitoring, protection and communications in a ring network cabinet in the distribution substation and master station with the implementation of monitoring and fault identification, isolation and power recovery of the fault in the distribution network. The internal structure and design is very similar to power supplies.

Typical FTU system structure

The chart above shows that an FTU monitors three-phase current, voltage and binary signal by way of data analysis. The power supply requirement is as follows:

• Employing a UPS (uninterruptible power supply) by inserting a dual power supply for switched power and automatic battery switching;
• A DC-DC converter with the ability to work within a wide input voltage range caused by the charge and discharge from battery banks;
• The DC-DC converter must be compatible with both 24 and 48 V battery voltage input;
• High 3 kV isolation voltage of the power supply can isolate the interference away from the AC end of the primary power source;
• System power break time remains more than 500 ms when the power supply and the battery power supply disconnects.

Recommended power supply solutions

To satisfy the above requirement for a power supply, Mornsun has released the URF_LP-20W series - a 20 W, 3000 VAC high-isolation range of DC-DC converters. The devices feature the following: 

• High efficiency;
• Wide operating temperature between 40 and 85°C;
• Wide 4:1 (18-75 V) input which suits both 24 and 48 V battery input voltages;
• Isolation voltage up to 3000 VAC can isolate interference introduced from the battery charging side;
• Design meets EMC - CISPR22/EN55022 Class A.

A datasheet on the product can be found at www.mornsun.cn/UploadFiles/pdf/URF_LP-20W%20CN.pdf.

Design reference

• Power-off delay: the extension of power-off time is necessary for the system to save data during a power outage to the main power supply and battery. The most common method to do this is to add some high-value capacitors (could be as high as several thousand µF). Capacitors with a higher capacitance will be needed for the input, especially in situations where the input voltage drops away, so the power off time delay will also be greatly reduced.
• For a higher level of EMC requirement, refer to the datasheet.

DLPC is the Master Distributor in Australia and New Zealand for Mornsun Power products, with official distributors for Victoria and South Australia (Fairmont Marketing) and Auckland (Fero).