The medium voltage transformer from DAELIM TRANSFORMER

medium voltage transformer

The medium voltage transformer is one of the important equipments in the power system and distribution.

Medium voltage transformers refer to 6-35kV transformers, which are mainly used in commercial buildings, residences, civil infrastructure, IT data and service centers and other fields.

Today, we mainly focus on the introduction of the connection group, test, identification, standard and so on of the medium voltage transformer.

Pad-mounted Transformer

We can provide you single phase and three phase pad mounted transformer

Dry-type Transformer

Type:Cast resin; Rated Capacity: Up to 25MVA; Rated Voltage: Up to 36KV;

Pole Transformer

TypeCSP type Frequency: 50/60Hz; Rated Power: 5~167kva

Oil immersed transformer

Frequency: 50/60Hz Rated voltage:10kv, 20kv,30kv Rated Power: 400~2500kva

Table of Content

The connection type of the medium voltage transformer

At present, among the 6-35kV voltage levels, the distribution transformer with dyn1 and ynyn0 winding connection is the most common. As we all know, the distribution transformer winding with Dyn11 connection has obvious advantages over Ynyn0 connection, which suppresses the third harmonic, reduces the zero sequence impedance, and is more conducive to improving the sensitivity of single phase short circuit current action of the circuit breaker at the low-voltage side, making it widely used in engineering design in recent years.

According to the experience, the over-current protector is also used as the single-phase grounding protection at the low-voltage side. For the transformer with winding Dyn11 (hereinafter referred to as D, yn11 transformer), its sensitivity coefficient can meet the requirements; while for the transformer with winding ynyn0, its sensitivity coefficient can meet the requirements or not.


Try for free information about the power transformer specification for tender in Canada

Secondary distribution system design of the medium voltage transformer

At the secondary medium-voltage level voltage transformer system with compact HV or LV transformer substations are mostly used as shown in Figure 1.Presently,secondary transformer substations are not included in the“substation auto matior system” in most of the cases, and can therefore not be monitored or tele controlled.

Statistics from power supply companies referring to supply interruptions at the end customer show that Germany about 80% of the interruptions are caused by failures in the medium-voltage system.The System Average Interruption Duration Index(SAIDI) describes the total time of all interruptions for customers, divided by the number of customers. Typical values for a German municipal utility are 10 minutes of annual outage per customer.In other regions of the world the outage times reaches from hours to days.

The utilities customers require maximum availability of the electrical power supply.

Daelim single phase pad mounted transformer is designed as per NEMA,IEEE/ANSI C57.12.38 and  IEEE/ANSI C57.12.00, all transformers meets with DOE energy efficiency.


The information of the medium voltage transformer we will show as follow

a) Outline of the transformer showing the dimensions, the location of the accessories and the details of the lifting loops, surge arresters and supports; 

b) Head losses guaranteed in watts at full secondary current at the main branch, reported at an average winding temperature of 5. SL

c) No head losses guaranteed in watts at the rated voltage reported at 25 °C; 

d) Percentage impedance in nominal power

e) Total weight of the transformer; 

f) Weight of oil;  main tap position; 

g) Regulation at full load per unit and power factor of 0.8; 

h) Certificate that all supplied transformers will be free of PCBs; 

i) The following information about HT bushings:

•Name of the bushing manufacturer

• Nominal voltage


• Distance from the leakage point

• Dimensioned drawing of the bushing) The following information about the transformer oil to be supplied: Name of the oil manufacturer of the  transformer

• Certificate of compliance with IEC standard or IEEE standard or CSA standard

Let’s know learn more about the How to purchase a 10 mva substation transformer

The Routine tests of the medium voltage power transformer

The Routine tests as specified in the standard must be performed on each transformer. 

Certified test reports must be provided for each transformer. 
The Routine tests of the medium voltage transfomrer should be carried out in the supplier’s laboratory or in a third-party laboratory, if necessary to achieve the required accuracy.  The instruments used in the tests must be periodically calibrated to standards traceable to national standards and must have valid calibration certificates at the time of testing. 
The test reports must indicate the accuracy, in ± percent of the measurement, based on the accuracy of the test instruments. 
The buyer / employer representative will have the right to be present during the acceptance test of the medium voltage transformer.
A set of routine test records for each individual transformer must be delivered to the buyer prior to delivery. 
Acceptance tests must be performed in the presence of the Buyer’s representative and must consist of repeating routine tests on a sample selected at random from a statistically valid size of the transformers offered for delivery.  The tests of the medium voltage transformer must be carried out in the manufacturer’s laboratory. 

The MARKING of the medium voltage transformer

1. The Capacity marking 
The front part (high voltage side) of the medium voltage transformer must be marked with the power of the transformer in white letters not less than 150 mm high with strokes of paint at least 25 mm wide. 
Each transformer must be supplied with the identification plate, including the information declared in the different standard.  Information will be in Portuguese and English.  A second identification plate must be provided with the same information in Portuguese. 
2. The Nameplate of the medium voltage transformer
The nameplate must be stainless steel, weatherproof, installed in a visible position and showing the information listed below:
The manufacturer’s name(DAELIM BELEFIC) or trade mark and country of origin(China); 
The type of transformer (medium voltage transformer)
The manufacturer’s serial number of the transformer and test number.
The year of production of the medium voltage transformer, and the date of the production of the medium voltage transformer; 
The specification of the medium voltage transformer as the clients requirement based on the different standard(IEEE std, CSA std, IEC std)
The number of phases of the medium voltage transformer, three phase distribution power transformer or single phase distribution transformer.
The rated power (kVA); When 55 ℃ rise the rated capacity, when 65 ℃ rise the rated capacity of the medium voltage transformer. When ONAF, the rated capacity of the medium voltage transformer.  
The nominal frequency (Hz); usually is 60 HZ, or some are 50 HZ. 
The nominal voltage (kV and Volts) for both windings of the medium voltage transformer; 

The rated current (A) for both windings of the medium voltage transformer; 

The Rated impulse withstand voltage (BIL) of both windings (kV) of the medium voltage transformer 

The connection symbol (vector group) – Dyn11 of the medium voltage transformer

The Short circuit impedance, value measured in percentage (Z%) of the medium voltage transformer; 

The resistance (Ohm) of the medium voltage transformer; 

The type of refrigeration (ONAN);  9) max.  of oil and windings of the medium voltage transformer 

The total mass (kg) of the medium voltage transformer ( will also show in the drawing and nameplate); 

The total mass of the core and windings of the medium voltage transformer; 

The volume of oil (liters) of the medium voltage transformer; 

The Connection diagram of the medium voltage transformer; 

The table with the tap voltages and positions of the medium voltage transformer; 

The winding temperature of the medium voltage transformer; 

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About Bin Dong

Hello, I am Bin, General manager of Daelim which is a leading transformer manufacturer. If you have problems when you are looking for the equipment, what you need to do is tell us.