One of the most important and costly parts of any distribution system is the transformer. Since it is a sealed, inert device that is typically doused in oil, the number of problems that can arise with it is constrained. However, even an uncommon fault can have serious consequences for the transformer, and the lengthy turnaround time for repairs or replacement only makes the situation worse. This makes safeguarding electricity transformers an absolute necessity.
There are two major categories of transformer faults: external faults and internal faults. To prevent damage to the transformer, an external fault is quickly and reliably cleared by a sophisticated relay system. Sensors and measuring devices are primarily responsible for detecting issues on the inside. Later in the piece, we’ll delve deeper into those procedures. Although this article will focus primarily on power transformers used in distribution systems, it is essential to note that there are many other types of transformers. To get your feet wet with how electricity transformers function, you can read up on their inner workings.
Relays and current transformers provide comprehensive protection for transformers used in mission-critical settings, while more basic protection features like overexcitation protection and temperature-based protection can identify conditions that ultimately lead to a failure condition.
In light of this, this piece will discuss the most widely-applied principles for safeguarding transformers.
Transformer Protection for Different Types of Transformers
The power transformer’s category determines the type of protection mechanism used. As can be seen in the chart below,
- Fuses are used to secure low-voltage (Category I & II) transformers up to 500 kVA, while medium-voltage (MV) circuit breakers are used to safeguard high-voltage (11 kV and 33 kV) distribution transformers up to 1000 kVA.
- For transformers 10 MVA and above, which falls under (Category III & IV), differential relays had to be used to protect them.
Transformer safety also makes extensive use of mechanical relays like Buchholtz and sudden pressure switches. Thermal overload protection is frequently applied in addition to these relays to increase the lifespan of a transformer rather than for fault detection.
Common Types of Transformer Protection
- Overheating protection
- Overcurrent protection
- Differential Protection of Transformer
- Earth Fault Protection (Restricted)
- Buchholz (Gas Detection) Relay
- Over-fluxing protection
Overheating Protection in Transformers
The overloads and brief circuits cause the transformers to overheat. The type of transformer and insulation class used for the transformer determine the maximum allowable overload and the associated duration.
While sustained higher loads for brief periods are possible, doing so for too long can cause insulation damage due to an increase in temperature above a safe threshold.
When the temperature inside an oil-cooled transformer reaches 95 degrees Celsius, the transformer’s life span drops and the insulation of the wire begins to deteriorate. This highlights the significance of safeguards against burning.
The below image shows a typical thermometer with a temperature control box from reinhausen used to measure the temperature of a liquid insulated conservative type of transformer.
A dial gauge inside the case displays the transformer’s temperature (represented by the black needle) and the threshold at which an alert will sound (represented by the red needle). An alert will sound if the black needle rises above the red one.
The picture shows a thermometer mounted on the top of the transformer tank above the core and the winding; this is done because the middle of the tank is where the transformer’s core and windings generate the most heat. The highest safe operating temperature for gasoline is this number. The Hot-spot Temperature of the transformer core can be roughly calculated from this reading. Modern low voltage windings incorporate fiber optic lines for precise temperature monitoring. That’s how you set up safeguards against burning.
Overcurrent Protection in Transformer
The overcurrent protection system, which includes the graded overcurrent system, was created to prevent damage caused by electrical currents that exceed safe levels. The IDMT relays are used by electricity distributors to employ this technique for fault detection. which means these channels have:
- Inverse characteristic, and
- Minimum time of operation.
The IDMT relay’s powers are limited. In order to prevent the relay from operating in an emergency overload state, it must be adjusted to a current between 150% and 200% of its maximum rated current. As a result, the minimal protection offered by these relays against faults within the transformer tank.
Differential Protection of Transformer
One of the most widely used and, arguably, best-protecting transformer protection systems is the Percentage Biased Current Differential Protection. Transformers with ratings higher than 2 KV require these safeguards.
One half of the transformer has a star connection, and the other has a delta connection. The CTs on the star side have delta connections, while the CTs on the delta side have star connections. Each transformer’s neutral has been connected to ground.
The rectifier uses two coils, one for operation and one for containment. The restraining-coil is responsible for creating the restricting force, and the operating-coil generates the operating force, as their respective names imply. Current transformers (CTs) have a restraining-coil connected to their secondary winding and an operating-coil linked midway between their equipotential point.
Transformer Differential Protection Working:
In a normal power transformer setup, there is no current flowing through the operating coil because the current is balanced on both sides of the transformer. However, if a fault develops within the windings, the differential relay’s working coils will begin to generate a current imbalance. As a result, the primary transformer is safeguarded by the relay, which trips the circuit breakers.
Restricted Earth Fault Protection
When a fault develops at the bushing of a transformer, a large fault current can flow. If that’s the situation, the problem must be fixed immediately. To ensure that a ground fault outside of the transformer’s zone only triggers the appropriate relay, all other relays should remain inoperable until the fault in the designated zone has been isolated. The relay is designed to safeguard against limited earth faults, hence the name.
The image above shows the Protection Devices installed on the transformer’s safe side. Let’s pretend this is the main side of the transformer and that the secondary side has a ground fault. Because of the ground fault, a Zero Sequence Component will be present if there is a fault on the ground side; this component will only move on the secondary side. And the transformer’s main winding will show no effect.
When a fault happens in this relay, all three phases will be affected: the positive sequence, the negative sequence, and the zero sequence. At any given time, the sum of all currents will run through the protection relay because the positive sequins components have been rotated by 120 degrees. Since they are separated by a distance of 120*, the total of their currents will be zero. The converse is also true for the negative parts of the series.
Let’s pretend for a moment that a flaw has occurred. The CTs will pick up on the fault because of the zero-sequence component of the current, and the protection relay will trip to safeguard the transformer.
Buchholz (Gas Detection) Relay
A Buchholz cycle is depicted in the image to the right. When a fault arises in the transformer, the resolved gas is detected by the Buchholtz relay, which is installed between the main transformer unit and the conservator tank and uses a float switch.
There shouldn’t be any gas inside the transformer, but if you look carefully, you can see an arrow indicating that gas is flowing out of the main tank and into the conservator tank. Dissolved gas accounts for the vast majority of the gas, with nine distinct gas types possible based on the nature of the fault. This relay has two valves at its apex; one is used to release excess gas, and the other is removed to collect a sample of the gas.
Sparks can be seen between the windings or between the windings and the core whenever a fault situation exists. The insulating oil is heated by the small electrical discharges in the windings, and this breakdown creates gases; the intensity of the breakdown indicates what glasses are formed.
As you may be aware, the production of acetylene requires a great deal of energy. This means that a big energy discharge will result in the production of acetylene. Remember that all faults release gases; by measuring the volume of these emissions, we can gauge the extent of the damage caused by the flaw.
How Buchholz (Gas Detection) Relay Works?
Gas flows through the pipe after an electrical fault causes the baffle plate to tilt, which in turn causes the lower float to fall, creating a situation in which the top float is higher than the lower one and the baffle plate is angled. The presence of both of these factors points to the presence of a major flaw. which triggers an alert and turns off the transformer.
However, the relay’s utility extends beyond this application; for example, suppose that minor arcking is occurring inside the transformer, and that this arcking is producing a small amount of gas. This gas produces a pressure inside the relay, and the upper float falls, dislodging the oil inside. What you see in the picture below is an example of
We have identified a problem; next, we will drain off some of the gas via the valve above the relay and examine it chemically to determine what caused the gas buildup.
If the transformer’s insulating oil level decreases due to leaks in the chassis, the upper float will fall, the lower float will fall, and the baffle plate will remain in its original position, triggering the relay’s detection mechanism. Under these circumstances, a separate alarm goes off. The process is depicted in the picture below.
The Buchholz relay can find problems by using these three techniques.
Over-fluxing Protection
Over flux protection is required because a transformer can only safely function at a certain flux level; above that level, the core becomes saturated and begins to overheat, which quickly spreads to the rest of the transformer and causes it to overheat as well. Both excessive power and decreased system frequency can lead to over-flux conditions.
The over-fluxing relay shields the generator from damage caused by excessive current flow. The flux density in the core is determined by the over-fluxing relay by measuring the voltage/frequency relationship. The instantaneous tripping of the transformer is undesirable because over fluxing can be caused by a rapid rise in the voltage due to transients in the power system, but transients die down quickly.
A microcontroller-based relay measures the voltage and the frequency in real-time, calculates the rate, and compares it to the pre-calculated values; this is done because the flux density is directly proportional to the ratio of voltage to frequency (V/f), and the instrument should detect the ration if the value of this ratio becomes greater than unity. Inverse absolute minimum times are what the relay is set to (IDMT characteristics). If necessary, however, the configuration can be handled directly. The goal will be achieved without jeopardizing the over-flux safeguards in this manner. Keeping the converter from over-fluxing is obviously crucial now that we know that it can happen.