Types of Transformer.
Transformers can be categorized in different ways, depending upon their purpose, use, construction etc.
(A) On the basis of construction, transformers can be classified into two types
(i) Core type transformer
When winding surround the core, the transformer is core form
(ii) Shell type transformer
When winding are surrounded by the core, the transformer is shell form
Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 KV or 75 MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra high voltage and higher MVA applications because, though more labor-intensive to manufacture, shell form transformers are characterized as having
Inherently better KVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.
(B) On the basis of their purpose.
1. Step up transformer:
When a transformer is used to “increase” the voltage(with subsequent decrease in current) on its secondary winding with respect to the primary, it is called a Step-up transformer. 2. Step down transformer:
When it is used to “decrease” (with subsequent increase in current) the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.Voltage decreases at secondary.
(C) On the basis of type of supply.
1. Single phase transformer
2. Three phase transformer
(D) On the basis of their use.
1. Power transformer: Used in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.
3. Instrument transformer: Used in relay and protection purpose in different instruments in industries.
Current transformer (CT)
Current transformers are generally used to measure currents of high magnitude. These transformers step down the current to be measured, so that it can be measured with a normal range ammeter. A Current transformer has only one or very few number of primary turns. The primary winding may be just a conductor or a bus bar placed in a hollow core (as shown in the figure). The secondary winding has large number turns accurately wound for a specific turns ratio. Thus the current transformer steps up (increases) the voltage while stepping down (lowering) the current. Generally, current transformers are expressed in their primary to secondary current ratio. A 100:5 CT would mean the secondary current of 5 amperes when primary current is 100 amperes. The secondary current rating is generally 5 amperes or 1 ampere, which is compatible with standard measuring instruments.
Potential transformer (PT)
Potential transformers are also known as voltage transformers and they are basically step down transformers with extremely accurate turns ratio. Potential transformers step down the voltage of high magnitude to a lower voltage which can be measured with standard measuring instrument. These transformers have large number of primary turns and smaller number of secondary turns. A potential transformer is typically expressed in primary to secondary voltage ratio. For example, a 600:120 PT would mean the voltage across secondary is 120 volts when primary voltage is 600 volts.
(E) On the basis of cooling Method.
Types of Transformer.
Transformers can be categorized in different ways, depending upon their purpose, use, construction etc.
(A) On the basis of construction, transformers can be classified into two types
(i) Core type transformer
When winding surround the core, the transformer is core form
(ii) Shell type transformer
When winding are surrounded by the core, the transformer is shell form
Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 KV or 75 MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra high voltage and higher MVA applications because, though more labor-intensive to manufacture, shell form transformers are characterized as having
Inherently better KVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.
(B) On the basis of their purpose.
1. Step up transformer:
When a transformer is used to “increase” the voltage(with subsequent decrease in current) on its secondary winding with respect to the primary, it is called a Step-up transformer. 2. Step down transformer:
When it is used to “decrease” (with subsequent increase in current) the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.Voltage decreases at secondary.
(C) On the basis of type of supply.
1. Single phase transformer
2. Three phase transformer
(D) On the basis of their use.
1. Power transformer: Used in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.
3. Instrument transformer: Used in relay and protection purpose in different instruments in industries.
Current transformer (CT)
Current transformers are generally used to measure currents of high magnitude. These transformers step down the current to be measured, so that it can be measured with a normal range ammeter. A Current transformer has only one or very few number of primary turns. The primary winding may be just a conductor or a bus bar placed in a hollow core (as shown in the figure). The secondary winding has large number turns accurately wound for a specific turns ratio. Thus the current transformer steps up (increases) the voltage while stepping down (lowering) the current. Generally, current transformers are expressed in their primary to secondary current ratio. A 100:5 CT would mean the secondary current of 5 amperes when primary current is 100 amperes. The secondary current rating is generally 5 amperes or 1 ampere, which is compatible with standard measuring instruments.
Potential transformer (PT)
Potential transformers are also known as voltage transformers and they are basically step down transformers with extremely accurate turns ratio. Potential transformers step down the voltage of high magnitude to a lower voltage which can be measured with standard measuring instrument. These transformers have large number of primary turns and smaller number of secondary turns. A potential transformer is typically expressed in primary to secondary voltage ratio. For example, a 600:120 PT would mean the voltage across secondary is 120 volts when primary voltage is 600 volts.
(E) On the basis of cooling Method.
Types of Transformer.
Transformers can be categorized in different ways, depending upon their purpose, use, construction etc.
(A) On the basis of construction, transformers can be classified into two types
(i) Core type transformer
When winding surround the core, the transformer is core form
(ii) Shell type transformer
When winding are surrounded by the core, the transformer is shell form
When winding are surrounded by the core, the transformer is shell form
Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 KV or 75 MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra high voltage and higher MVA applications because, though more labor-intensive to manufacture, shell form transformers are characterized as having
Inherently better KVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.
1. Step up transformer:
When a transformer is used to “increase” the voltage(with subsequent decrease in current) on its secondary winding with respect to the primary, it is called a Step-up transformer. 2. Step down transformer:
When it is used to “decrease” (with subsequent increase in current) the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.Voltage decreases at secondary.
(C) On the basis of type of supply.
1. Single phase transformer
2. Three phase transformer
(D) On the basis of their use.
1. Power transformer: Used in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.
3. Instrument transformer: Used in relay and protection purpose in different instruments in industries.
Current transformer (CT)
Current transformers are generally used to measure currents of high magnitude. These transformers step down the current to be measured, so that it can be measured with a normal range ammeter. A Current transformer has only one or very few number of primary turns. The primary winding may be just a conductor or a bus bar placed in a hollow core (as shown in the figure). The secondary winding has large number turns accurately wound for a specific turns ratio. Thus the current transformer steps up (increases) the voltage while stepping down (lowering) the current. Generally, current transformers are expressed in their primary to secondary current ratio. A 100:5 CT would mean the secondary current of 5 amperes when primary current is 100 amperes. The secondary current rating is generally 5 amperes or 1 ampere, which is compatible with standard measuring instruments.
Potential transformer (PT)
Potential transformers are also known as voltage transformers and they are basically step down transformers with extremely accurate turns ratio. Potential transformers step down the voltage of high magnitude to a lower voltage which can be measured with standard measuring instrument. These transformers have large number of primary turns and smaller number of secondary turns. A potential transformer is typically expressed in primary to secondary voltage ratio. For example, a 600:120 PT would mean the voltage across secondary is 120 volts when primary voltage is 600 volts.
(E) On the basis of cooling Method.
Transformers can be divided in two types depending on cooling method.
1. Oil immersed transformers
Oil Natural Air Natural (ONAN)
Oil Natural Air Forced (ONAF)
Oil Forced Air Forced (OFAF)
Oil Forced Water Forced (OFWF)
2.Dry type Transformer
Air Natural (AN)
Air Blast
Oil Natural Air Natural (ONAN)
For oil immersed distribution transformers most common cooling method is ONAN. In this method, the heat generated in the core and winding is transferred to the oil. According to the principle of convection, the heated oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air flow around the transformer. In this way, the oil in transformer keeps circulating due to natural convection and dissipating heat in atmosphere due to natural conduction.
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