What are the common faults encountered during the operation of power transformer longitudinal differential protection？

Transformer Longitudinal Differential Protection: Common Issues and Solutions

Transformer longitudinal differential protection is the most complex among all component differential protections. Misoperations occasionally occur during operation. According to 1997 statistics from the North China Power Grid for transformers rated 220 kV and above, there were 18 incorrect operations in total, of which 5 were due to longitudinal differential protection—accounting for approximately one-third. Causes of maloperation or failure to operate include issues related to operation, maintenance, and management, as well as problems in manufacturing, installation, and design. This article analyzes common field-related issues and presents practical mitigation methods.

1. Unbalanced Current Caused by Transformer Inrush Current

During normal operation, magnetizing current flows only on the energized side and creates unbalanced current in differential protection. Typically, magnetizing current is 3%–8% of the rated current; for large transformers, it is usually less than 1%. During external faults, voltage drops reduce magnetizing current, minimizing its impact. However, during energization of an unloaded transformer or voltage recovery after an external fault clearance, a large inrush current can occur—reaching 6–8 times the rated current.

This inrush contains significant non-periodic components and high-order harmonics, predominantly the second harmonic, and exhibits current waveform discontinuities (dead angles).

Mitigation methods in longitudinal differential protection:

(1) BCH-type relays with fast-saturating current transformers:
During external faults, the high non-periodic component quickly saturates the core of the fast-saturating transformer, preventing unbalanced current from being transferred to the relay coil—thus avoiding false tripping. During internal faults, although non-periodic components exist initially, they decay within ~2 cycles. Thereafter, only periodic fault current flows, enabling sensitive relay operation.

(2) Microprocessor-based relays using second-harmonic restraint:
Most modern digital relays use second-harmonic blocking to distinguish inrush from internal faults. If misoperation occurs during external fault clearance:

• Switch from phase-by-phase ("AND") restraint to maximum-phase ("OR") restraint mode.

• Reduce the second-harmonic restraint ratio to 10%–12%.

• In systems with large capacity where fifth-harmonic content is also high after fault clearance, add fifth-harmonic restraint.

• For transformers equipped with dual differential protections, consider using waveform symmetry principles to identify inrush—this method is more sensitive and reliable than harmonic restraint alone.

2. Incorrect Wiring in CT Secondary Circuits

A recurring cause of misoperation is reversed polarity of current transformer (CT) secondary terminals—a result of inadequate training, deviation from design drawings, or insufficient commissioning checks.

Preventive practice:
Before putting longitudinal differential protection into service—after new installation, periodic testing, or any secondary circuit modification—the transformer must be loaded, and the following checks performed:

• Measure the unbalanced voltage in the differential loop using a high-impedance voltmeter; it must comply with code limits.

• Measure magnitude and phase angle of secondary currents on all sides.

• Construct a hexagonal vector diagram to verify that the vector sum of same-phase currents is zero or near-zero, confirming correct wiring.

Only after these verifications should the protection be formally commissioned.

3. Poor Contact or Open Circuit in Secondary Circuits

Misoperations due to loose connections or open circuits in CT secondary loops occur annually.

Recommendations:

• Strengthen real-time monitoring of differential current during operation.

• After relay installation/commissioning or major transformer overhauls, inspect all CT secondary connections.

• Tighten terminal screws and use spring washers or anti-vibration clips.

• For critical applications, use two parallel cables for the differential secondary wiring to mitigate open-circuit risk.

4. Grounding Issues in Differential Protection Secondary Circuits

Some sites violate anti-accident measures by having two grounding points—one in the protection cabinet and another in the switchyard terminal box. The resulting ground potential difference, especially during lightning or nearby welding, can induce spurious differential current and cause false tripping.

Solution:
Strictly enforce single-point grounding. The only reliable ground point should be located inside the protection cabinet.

5. Insulation Degradation of CT Secondary Cables

Insulation failure in CT secondary cables—often due to poor construction practices—also leads to misoperations. Common causes include:

• Cable sheath damage during laying,

• Splicing two cables when length is insufficient,

• Welding cable conduits with cables inside, causing thermal damage.

These create hidden risks to protection reliability.

Preventive measures:

• During major equipment maintenance, periodically test insulation resistance between each core-to-ground and core-to-core using a 1000 V megohmmeter; values must meet code requirements.

• Keep exposed wire ends at terminals as short as possible to prevent accidental grounding or phase-to-phase short circuits due to vibration.

6. Selection of Current Transformers for Longitudinal Differential Protection

Differential protection involves CTs across different voltage levels, with varying ratios and models, leading to mismatched transient characteristics—a potential source of misoperation or failure to operate.

• 500 kV side: Use TP-class CTs (transient-performance class), whose gapped cores limit remanence to <10% of saturation flux, greatly improving transient response.

• 220 kV and below: Typically use P-class CTs, which have no air gap, higher remanence, and poorer transient performance.

Selection guidance: While TP-class CTs offer superior technical performance, they are expensive and bulky—especially on the low-voltage side, where installation in enclosed bus ducts is difficult. Therefore, unless special system requirements exist, P-class CTs should be preferred if they satisfy actual operational needs—avoiding unnecessary cost and installation challenges.

Additionally, secondary cable cross-section must be adequate:

• For long cable runs, use ≥4 mm² conductor size to minimize burden and ensure accuracy.