Power Transformer Protection and Faults

Power Transformer Protection and Faults . Power transformers are crucial components in electrical power systems, enabling efficient energy transfer across varying voltage levels. Ensuring their protection from faults is vital for uninterrupted power supply and system stability. This article delves deep into power transformer protection and faults, highlighting essential mechanisms, common challenges, and best practices.

Introduction to Power Transformer Protection

Power transformers play a pivotal role in managing and distributing electricity. These transformers step up or step down voltage to appropriate levels for transmission or consumption. Despite their robust design, transformers face potential faults that can disrupt the energy flow, damage equipment, or even lead to catastrophic failures.

Effective protection systems mitigate these risks, ensuring reliable operations and safeguarding the equipment.

Read More : Maintenance of Transformer

Transformer Protection for Different Types of Transformers

The protection system used for a power transformer depends on the transformer’s categories. A table below shows that,

Category	Transformer Rating – KVA
Category	1 Phase	3 Phase
I	5 – 500	15 – 500
II	501 – 1667	501 – 5000
III	1668 – 10,000	5001 – 30,000
IV	> 10,000	>30,000

Transformers within the range of 500 KVA fall under (Category I & II), so those are protected using fuses, but to protect transformers up to 1000 kVA (distribution transformers for 11kV and 33kV) Medium Voltage circuit breakers are usually used.
For transformers 10 MVA and above, which falls under (Category III & IV), differential relays had to be used to protect them.

Additionally, mechanical relays such as Buchholtz relays, and sudden pressure relays are widely applied for transformer protection. In addition to these relays, thermal overload protection is often implemented to extend a transformer’s lifetime rather than for detecting faults.

Importance of Transformer Protection

Transformer protection is critical for several reasons:

System Reliability: Protecting transformers prevents disruptions in power supply, ensuring continuous electricity distribution.
Equipment Longevity: Proper protection reduces the risk of damage, prolonging the transformer’s operational life.
Safety Assurance: Protection mechanisms minimize hazards such as electrical fires or explosions.
Economic Benefits: Avoiding costly repairs and downtime saves operational expenses.

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

Transformers overheat due to the overloads and short circuit conditions. The allowable overload and the corresponding duration are dependent on the type of transformer and class of insulation used for the transformer.

Higher loads can be maintained for a very short amount of time if it is for a very long, it can damage the insulation due to temperature rise above an assumed maximum temperature. The temperature in the oil-cooled transformer is considered maximum when its 95*C, beyond which the life expectancy of the transformer decreases and it has detrimental effects in the insulation of the wire. That is why overheating protection becomes essential.

Large transformers have oil or winding temperature detection devices, which measure oil or winding temperature, typically there are two ways of measurement, one is referred to hot-spot measurement and second is referred to as top-oil measurement, 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.

Thermometer-with-Temperature-Controll-Box

The box has a dial gauge which indicates the temperature of the transformer (which is the black needle) and the red needle indicates the alarm set point. If the black needle surpasses the red needle, the device will activate an alarm.

If we look down, we can see four arrows through which we can configure the device to act as an alarm or trip or they can be used to start or stop pumps or cooling fans.

As you can see in the picture, the thermometer is mounted on the top of the transformer tank above the core and the winding, it’s so done because the highest temperature is going to be at the center of the tank because of the core and the windings. This temperature is known as the top oil temperature. This temperature gives us an estimate of the Hot-spot Temperature of the transformer core. Present-day fiber optic cables are used within the low voltage winding to accurately measure the temperature of the transformer. That is how overheating protection is implemented.

Overcurrent Protection in Transformer

The overcurrent protection system is one of the earliest developed protection systems out there, the graded overcurrent system was developed to guard against overcurrent conditions. power distributors utilize this method to detect faults with the help of the IDMT relays. that is, the relays having:

Inverse characteristic, and
Minimum time of operation.

The capabilities of the IDMT relay is restricted. These sorts of relays have to be set 150% to 200% of the max rated current, otherwise, the relays will operate for emergency overload conditions. Therefore, these relays provide minor protection for faults inside the transformer tank.

Differential Protection of Transformer

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.

Read More : What Is an Ideal Transformer?

Restricted Earth Fault Protection

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.

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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. Because the positive sequins components are displaced by 120*, so at any instant, the sum of all the currents will flow through the protection relay. So, the sum of their currents will be equal to zero, as they are displaced by 120*. 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.

Buchholz (Gas Detection) Relay

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.

Read More : What is Potential Transformer (PT)?

When a fault condition occurs, we have sparks between the windings, or in between windings and the core. These small electrical discharges in the windings will heat the insulating oil, and the oil will break down, thus it produces gases, the severity of the breakdown, detects which glasses are created.

A large energy discharge will have a production of acetylene, and as you may know, acetylene takes a lot of energy to be produced. And you should always remember that any type of fault will produce gases, by analyzing the amount of gas, we can find the severity of the fault.

Over-fluxing Protection

A transformer is designed to operate at a fixed flux level exceed that flux level and the core gets saturated, the saturation of the core causes heating in the core that quickly follows through the other parts of the transformer that leads to overheating of components, thus over flux protection becomes necessary, as it protects the transformer core. Over-flux situations can occur because of overvoltage or a reduction in system frequency.

To protect the transformer from over-fluxing, the over-fluxing relay is used. The over-fluxing relay measures the ratio of Voltage / Frequency to calculate the flux density in the core. A rapid increase in the voltage due to transients in the power system can cause over fluxing but transients die down fast, therefore, the instantaneous tripping of the transformer is undesirable.

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, this is done by a microcontroller-based relay which measures the voltage and the frequency in real-time, then it calculates the rate and compares it with the pre-calculated values. The relay is programmed for an inverse definite minimum time (IDMT characteristics). But the setting can be done manually if that is a requirement. In this way, the purpose will be served without compromising the over-flux protections. Now, we see how important it is to prevent the tripping of the transformer from over-fluxing.

Common Causes of Transformer Faults

Electrical Causes

Insulation Failure: Leads to short circuits or arcing.
Overloading: Prolonged excess current flow damages windings.
Harmonics: Nonlinear loads cause overheating and efficiency loss.

Mechanical Causes

Loose Connections: Create hotspots and uneven current distribution.
Vibration: Causes displacement or loosening of components.

Environmental Factors

Moisture and Contaminants: Degrade insulation quality.
Extreme Temperatures: Cause thermal stress, leading to faults.

Steps to Minimize Transformer Faults

Regular Maintenance Practices:
- Routine inspections for wear and tear.
- Testing insulation resistance and oil quality.
Use of High-Quality Components:
- Employing premium materials reduces the likelihood of premature failure.
Real-Time Monitoring Systems:
- Installing advanced monitoring tools for early fault detection.

Economic and Operational Impact of Transformer Faults

Transformer faults can lead to severe consequences, including:

Operational Downtime: Halting energy distribution affects businesses and homes.
Repair Costs: Significant expenses for component replacement or repairs.
Revenue Loss: Prolonged outages result in financial losses for utility providers.

Investing in effective protection mechanisms minimizes these impacts.

Power Transformer Protection and Faults

FAQs About Transformer Protection and Faults

What are the common types of transformer faults?

The most common faults include winding short circuits, core overheating, overloading, and external overvoltage.

How does a Buchholz relay protect a transformer?

A Buchholz relay detects internal faults by monitoring the accumulation of gases and oil movement, triggering alarms or circuit breakers.

Why is differential protection critical for transformers?

Differential protection ensures that only faults within the transformer trigger a response, avoiding unnecessary shutdowns during external disturbances.