Therefore, these relays provide minor protection for faults inside the transformer tank. The Percentage Biased Current Differential Protection is used to protect power transformers and it is one of the most common transformer protection schemes that provide the best overall protection.
These types of protection are used for transformers of rating exceeding 2 MVA. The transformer is star connected on one side and delta connected the other side. The CTs on the star side are delta-connected and those on the delta-connected side are star-connected. The neutral of both the transformers are grounded. The transformer has two coils, one is the operating coil and the other is the restraining coil.
As the name implies, the restraining-coil is used to produce the restraining force, and the operating-coil is used to produce the operating force. The restraining-coil is connected with the secondary winding of the current transformers, and the operating coil is connected in between the equipotential point of the CT.
Normally, the operating coil carries no current as the current is matched on both sides of the power transformers, when an internal fault occurs in the windings, the balance is altered and the operating coils of the differential relay start producing differential current among the two sides of the transformer.
Thus, the relay trips the circuit breakers and protects the main transformer. A very high fault current can flow when a fault occurs at the transformer bushing. In that case, the fault needs to be cleared as soon as possible. The reach of a particular protection device should be only limited to the zone of the transformer, which means if any ground fault occurs in a different location, the relay allocated for that zone should get triggered, and other relays should stay the same.
So, that is why the relay is named Restricted earth fault protection relay. In the above picture, the Protection Equipment is on the protected side of the transformer. Let's assume this is the primary side, and let's also assume there is a ground fault on the secondary side of the transformer.
Now, if there is a fault on the ground side, because of the ground fault, a Zero Sequence Component will be there, and that will circulate only on the secondary side. And it will not be reflected in the primary side of the transformer. This relay has three phases, if a fault occurs, they will have three components, the positive sequence components, the negative sequence components, and the zero sequence components.
Similar is the case for the negative sequence components. Now let us assume a fault condition occurs. That fault will be detected by the CTs as it has a zero-sequence component and the current starts flowing through the protection relay, when that happens, the relay will trip and protect the transformer. The above picture shows a Buchholz relay. The Buchholtz relay is fitted in between the main transformer unit and the conservator tank when a fault occurs within the transformer, it detects the resolved gas with the help of a float switch.
If you look closely, you can see an arrow, gas flows out from the main tank to the conservator tank, normally there should not be any gas in the transformer itself. Most of the gas is referred to as dissolved gas and nine different types of gasses can be produced depending on the fault condition. There are two valves at the top of this relay, these valves are used to reduce the gas build-up, and it's also used to take out a gas sample. In these situations the switch-disconnectors must have a tripping coil to allow the action of the built-on protections of transformers.
Figure 2 — Switch-disconnector associated with fuses. Fuses must have a mechanical latch to indicate the fusion and to provoke three-pole opening of the switch-disconnector , to avoid the functioning of the installation only with two phases. Manufacturers provide tables to choose the rated current of a fuse , taking into account the rated voltage and power , like the one shown in the Table 1, according to IEC standards. Tables vary from manufacturer to manufacturer , according to the standards used , being recommended to use the table provided by the selected manufacturer.
Table 1 — Rated current of fuses for power transformers protection. Transformer Rated Power. An external fault in the star side will result in current flowing in the line current transformer of the affected phase and at the same time a balancing current flows in the neutral current transformer , hence the resultant current in the relay is therefore zero.
So this protection will not be actuated for external earth fault. But during internal fault the neutral current transformer only carries the unbalance fault current and operation of the protection takes place. This scheme of restricted earth fault protection is very sensitive for internal earth fault of electrical power transformer.
The protection scheme is comparatively cheaper than differential protection scheme. Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer.
In this scheme the CT secondary of each phase of electrical power transformer are connected together as shown in Figure 3. Figure 3 — Diagram of restricted earth fault protection. Whenever there is an unbalancing in between three phases of the power transformer, a resultant unbalance current flow through the close path connected to the common terminals of the CT secondary. An unbalance current will also flow through the neutral of power transformer and hence there will be a secondary current in Neutral CT because of this unbalance neutral current.
In restricted earth fault scheme the common terminals of phase CT are connected to the secondary of Neutral CT in such a manner that secondary unbalance current of phase CT and the secondary current of Neutral CT will oppose each other. If these both currents are equal in amplitude there will not be any resultant current circulate through the said close path. The restricted earth fault protection is connected in this close path. Hence the relay will not response even there is an unbalancing in phase current of the power transformer.
The basic criterion for transformer loading is the temperature of the hottest spot of the solid insulation hot-spot. It must not exceed the prescribed value , in order to avoid insulation faults , since l oading capability of power transformers is limited mainly by winding temperature. The temperature of solid insulation is the main factor of transformer ageing.
With temperature and time , the cellulose insulation undergoes a depolymerization process. As the cellulose chain gets shorter , the mechanical properties of paper such as tensile strength and elasticity degrade.
Eventually the paper becomes brittle and is not capable of withstanding short circuit forces and even normal vibrations that are part of transformer life. This situation characterizes the end of life of the solid insulation. Since it is not reversible , it also defines the transformer end of life. Transformer overloads can occur during contingency conditions that are the product of one, two, or various system elements being isolated from the power the system. Traditionally, inverse-time overcurrent relays an inverse-time curve is characterized by the inverse variation of current with the time , as shown in Figure 4 for overload protection , but a difficulty is that transformers are usually outdoors where ambient temperature affects their loadability , and hence the optimum pickup settings of such relays.
Figure 4 — Inverse-time characteristic curve. However, for liquid-immersed power transformers , the temperature of the winding hot-spot is the important factor in the long-term life of the transformer.
The insulating oil temperature is dependent on the winding temperature , and is used to indicate the operating conditions of the transformer. Many numerical transformer protection relays available today include protection functions that operate on insulating oil temperatures , calculated loss-of-life due to high oil temperature , and predicted oil temperatures due to load.
These types of functions are not routinely applied , but modern utility operating practices try to maximize the utilization of power transformers , which may increase the occurrence of over-temperature conditions and transformer ageing.
Over-temperature conditions and accelerated aging are adverse system events that must be identified and protected against. The thermal capacity used is calculated according to a mathematical model which takes into account:. The protection gives a trip order when the heat rise E , calculated according to the measurement of an equivalent current I eq , is greater than the set point E s. The protection tripping time is set by the time constant T.
Lightning protection of power transformers is achieved by surge arresters installed in the transformer tank , as shown in Figure 5. Figure 5 — Surge arrester. The most common surge arresters are non-linear metal oxide resistors type in porcelain or silicone rubber housing , and are fitted in parallel with the object protected and connected to the earth grid.
Resistance of non-linear resistors is in inverse proportion to the current , that is to say that the resistance is high for current service values and very low for high lightning discharge currents. Differential protection of the power transformer effect on the different current.
This also should consider while apply a differential protection. The factors can also result in differential current in under balanced power conditions. Following are some of the situations which can occur. Differential relays are used to reduce the mention effects of transformers. Buchhols relay are one of the protection system which is really important in electrical power transformer. Normally there buchholz relay are gas actuated and its installed in oi.
This protection equipment is used to immersed transformer for protection against all kind of faults.
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