Transformer Backup Protection

Transformers are the primary components of the electrical power networks that ensure the reliable transmission & distribution of electricity. 

Transformers are protected by primary protective relays such as differential protection to ensure system stability & prevent extended outages.

However, if the primary protection fails (or) malfunctions the transformer’s backup protection is essential. 

Backup protection ensures that faults within (or) outside of the transformer zone are made removed which reducing equipment damage & increasing the overall system reliability.

Transformer backup protection schemes must adhere to the IEC 60255 standards which establish the specifications for measuring the relays and protective devices.

Why is Backup Protection Required?

Primary protection devices such as differential relays or Buchholz relays may occasionally fail due to 

• Relay malfunction, 
• CT failure (or) 
• Communication difficulties. 

Without backup protection, the undetected failures could cause severe transformer damage, fire dangers and also prolonged power outages. 

Backup protection serves as the second line of protection against an electrical fault. 

Features of Backup Protection of Transformer

1. Functions when primary protection (e.g., differential relay) fails.
2. Provides fault clearing with a time delay, ensuring synchronization with primary protection.
3. Detects internal (winding, insulation, oil) and external defects (line faults, overloads).
4. Improves system stability & reliability by reducing damage and outage times.
5. Serves as a secondary protection against transformer failures.

Types of Backup Protection for Transformer

There are many different types of backup protection are available to protect the transformer from failure that includes:

1). Overcurrent Protection (OCR)

2). Distance Protection

3). Overflux (Over Excitation) Protection

4). Standby Earth Fault (SEF) Protection

5). Buchholz Relay (Mechanical Backup Protection)

1). Over Current Protection (OCR)

Overcurrent protection is the most utilized type of transformer backup protection. 

It utilizes the overcurrent relays (OCRs) that activate if the current exceeds a predetermined limit leading to faults such as short circuits (or) overloads.

Phase Overcurrent Protection: Prevents phase-to-phase & 3-phase short circuits. It ensures that the transformer is unplugged if the fault current exceeds acceptable limits.

Earth Fault Protection: Earth Fault Protection detects ground faults via measurement of the current in the neutral (or) residual path. 

This helps to prevent insulation breakdowns (or) single-line-to-ground failures.

OCRs are simple, cost-effective & dependable. 

They are typically time-delayed so that primary protection (such as differential relays) can take effect first. 

If the primary protection fails, the OCR will trip after a certain time delay, resolving the error.

IEEE C37.113 suggests relay applications in transformer protection, such as backup overcurrent & earth fault schemes.

2). Distance Protection

Distance relays are mostly utilized on the EHV & HV transformers connected to the transmission networks. 

Unlike overcurrent relays, distance relays also used to assess the impedance between the relay and the failure location. 

Because impedance is directly proportional to distance, the relay can determine the location of the fault.

If a fault occurs within the specified protection zone (Zone 1, Zone 2 or Zone 3), the distance relay sends a trip signal. 

This type of protection is extremely selective, making it perfect for transformers connected to lengthy transmission lines (or) interconnected grids. 

It performs reliably even during faults with small fault current.

3). Overflux (Over Excitation) Protection

Overfluxing occurs when the voltage-to-frequency ratio (V/f) exceeds the transformer core’s safe design limit. 

Overfluxing can occur as a result of system overvoltage or low frequency operation.

The consequences of overfluxing include core saturation, excessive heating, insulation failure & even transformer failure.

Overflux relays continuously check the voltage-to-frequency ratio. If it get surpasses a certain level the relay activates the transformer (or) creates an alert.

This backup protection is particularly necessary in power plants & grid-connected transformers, where anomalous frequency or voltage variations are likely occurs.

4). Standby Earth Fault (SEF) Protection

SEF protection is applied when the transformer’s neutral current is available for measurement. 

It serves as a backup protection for the earth faults that can’t be noticed by the primary protection systems.

In certain conditions, a differential relay cannot operate due to low-level ground faults. 

SEF prevention identifies unprotected ground faults by detecting the imbalanced current in the neutral & disconnects the transformer before they cause significant harm.

This strategy improves reliability, especially in systems with the resistance-grounded (or) solidly grounded neutrals.

5). Buchholz Relay (Mechanical Backup Protection)

The Buchholz relay is a mechanical protection device installed in the oil-filled transformers located across the main tank and the conservator.

Eventhough it is commonly recognized as primary protection it can also act as backup protection against the internal transformer faults.

It identifies potential issues such as winding insulation breakdown, short circuits, overheating & oil leaks.

Faults within the transformer result in gas development or improper oil movement. The relay notices these changes & either sounds an alert (or) trips the transformer.

Thus, the Buchholz relay gives early warning & isolation before a minor defect progresses to a large transformer failure.

Summary

Backup transformer protection is important for ensuring an uninterrupted power supply & protecting valuable transformer equipment. In case of a primary protection failure, utilities can provide a reliable safety net by utilizing 

1. Overcurrent Relays, 
2. Distance Protection, 
3. Overflux Relays, 
4. Standby Earth Fault Relays and 
5. Buchholz Relays. 

Implementing a well-designed backup protection technique methods not only protects the transformers yet also reduces downtime and hence boosting complete system dependability, safety and efficiency.