IEC 60076-24-2020 Power transformers – Part 24: Specification of voltage regulating distribution transformers (VRDT)

IEC 60076-24-2020 Power transformers – Part 24: Specification of voltage regulating distribution transformers (VRDT). 4 Environmental conditions Service conditions for VRDTs are the same as those specified for power transformers in IEC 60076-1 and IEC 60076-1 1 . The purchaser shall identify in the enquiry to the manufacturer any service conditions not covered by the normal service conditions as specified in IEC 60076-1 and IEC 60076-1 1 and, for the control unit, the normal service conditions as specified in IEC 60068-2 (all parts). Any different applications shall be agreed between manufacturer and purchaser. NOTE 1 Transformer with regulating unit: Application liquid immersed transformer: indoor or outdoor use and storage. Application dry transformer: indoor use and storage. NOTE 2 Control unit: Application is indoor use and storage. 5 Electrical characteristics and general requirements 5.1 General As per IEC 60076-1 :201 1 , 5.3 for transformers connected to generators, the value of the rated voltage is as given in IEC 60076-1 :201 1 , 5.4.3, the 1 1 0 % of overvoltage is at no load and limited at 5 % at rated power. For specifying the rated low voltage, the purchaser shall take into account the possible over- excitation which can increase no-load losses, depending on the technology of the VRDT. 5.2 Regulation steps Unless otherwise specified, there are 3 to 1 1 on-load steps. The regulation can be symmetric or asymmetric. Unless otherwise agreed, step sizes are to be uniform to give constant flux voltage variation (CFVV). 5.3 Voltage steps Unless otherwise specified, voltage steps are 1 ,0 % up to 3,0 % and shall be agreed between manufacturer and purchaser. 5.4 Regulation range Unless otherwise specified, the on-load regulation range is ±5 % up to ±1 0 % of the rated voltage ratio for symmetrical regulation. 6 Service life Loading guides (IEC 60076-7 and IEC 60076-1 2) are applicable. Deviations shall be agreed between manufacturer and purchaser. The manufacturer shall indicate the number of operations for which the VRDT is designed. The manufacturer shall indicate a future need for firmware updates of the control unit. 7 Parallel operation of the VRDT For parallel operation of the VRDT, special precautions should be taken to avoid high circulating currents. NOTE Depending on the impedance of the connections between the VRDTs, significant circulating currents are possible (IEC 60076-8). A typical solution can be a master/follower arrangement or a single grid controller. 8...