Protection Relays for Transformer are critical devices securing power transformers against faults and abnormal operating conditions. They rapidly detect electrical failures, overloads, temperature anomalies and mechanical risks, and trigger tripping or alarm signals timely.
Introduction to Common Functions of Transformer Protection Relays in Power System
87T Protection Relay
Transformer Differential Protection Relay Application Scope
Large and medium-sized transformers: Generally applied to standalone transformers of 10kV and above with capacity over 10MVA. It is also recommended for parallel-operated transformers rated 6.3MVA and higher.
High sensitivity: Quickly clear internal inter-phase and turn-to-turn short circuit faults.
Inrush current suppression: Modern differential relays adopt harmonic restraint to avoid mal-operation during no-load closing.
Transformer Differential Protection Diagram
Relay for transformer protection mainly implements differential protection in transformers. This protective relaying only responds to internal faults and trips, while external faults are handled by other protection devices.
Transformer Differential Protection Scheme
Differential protection schemes differ among transformers. Here is the standard configuration for three winding transformer protection.
Protection Starting Elements
Differential current instantaneous trip: Rapidly isolate severe faults, operating current set to (7~12)Ie Percentage differential protection: Apply to minor internal faults
Percentage restraint characteristic setting
| Setting Parameter | Recommended Value | Description |
|---|---|---|
| Differential threshold | 0.3~0.5 Ie | Avoid normal unbalanced current |
| Inflection point current | 0.8~1.0 Ie | Starting point of restraint characteristic |
| Restraint coefficient | 0.4~0.6 | Ensure stable restraint during external faults |
| Second harmonic restraint coefficient | 0.15~0.20 | Identify magnetizing inrush current |
| TA disconnection blocking | 1.2 Ie | Prevent false operation |
50 51 Protection Relay
Transformer overcurrent protection serves as a common protective function and is essential for all types of transformers.
The logic of three-stage overcurrent detection remains identical for all phases and stages (taking Stage 1 Phase A over current protection relay as an example); its operating conditions are as follows:
50 Protection Relay
| Protection Type | Operating Time | Role in Transformer Protection |
|---|---|---|
| Instantaneous Overcurrent Protection (Instant Trip) | 0s | Main protection for small-capacity transformers |
51 Protection Relay
| Protection Mode | Applicable Scenario | Features |
|---|---|---|
| Compound Voltage Initiated Overcurrent Protection | Step-up transformers, system tie transformers | Higher sensitivity |
| Low Voltage Blocking Overcurrent Protection | Step-down transformers with insufficient overcurrent protection sensitivity | Improve protection sensitivity |
50N Protection Relay
The logic for the first and second stages of zero-sequence overcurrent protection based on zero sequence current transformer protection is consistent (taking the first stage as an example); the operating conditions are as follows:
50G Protection Relay
51N Protection Relay
Normal inverse time
Very inverse time
Extremely inverse time
Logic Diagram of Zero-Sequence Overcurrent Inverse-Time Protection
27 Protection Relay
Protection Logic Diagram for Transformer Under Voltage Protection
59 Protection Relay
An overvoltage protection relay is used to prevent abnormal increases in system voltage from damaging the insulation performance of transformers.
59N Protection Relay
Zero-sequence overvoltage protection is a protection device used to detect system ground faults, and is mainly applied in neutral ungrounded systems or systems grounded via arc suppression coils.
24Protection Relay
Transformer over-excitation protection is primarily used to prevent damage caused by saturation and overheating due to excessive magnetic flux density in the core when voltage rises and frequency drops.
Working Principle
Transformer magnetic flux density B is proportional to the voltage-frequency ratio U/f. When U/f exceeds the permissible range (generally 1.05~1.1 times rated value), core saturation occurs. The magnetizing current surges sharply, causing overheating and vibration, and insulation damage may happen in severe cases.
Overload Protection of Transformer
Transformer overload protection monitors long-term over-rated-capacity operation and prevents accelerated insulation aging caused by overheating. It generally only triggers alarm signals instead of tripping, reminding operators to reduce load.
Summary of Protection Relays for Transformer
Electrical protection of transformer falls into six major types: main protection, phase-to-phase and grounding transformer protection, voltage & frequency protection, overload & thermal protection, non-electrical quantity protection and auxiliary protection.
Main protection covers differential relay protection of transformer including 87T transformer differential protection, 87U differential instantaneous trip protection, 87N earth differential protection, together with 63 gas protection, swiftly cutting off internal transformer faults.
Over current protection of transformer consists of 50 instantaneous overcurrent protection, 51 definite and inverse time overcurrent protection, 50N zero-sequence instantaneous overcurrent, 51N zero-sequence inverse time overcurrent, 67 directional overcurrent protection and 50G/51G gap zero-sequence overcurrent, offering backup safeguard against phase faults and earth fault protection of transformer.
Voltage & frequency protection contains 59 overvoltage protection, 27 undervoltage protection, 59N zero-sequence overvoltage protection, 24 overexcitation protection and 81U/81O under & over frequency protection to maintain stable operating voltage and frequency.
Overload & thermal protection adopts 49 thermal overload protection and 46 negative sequence current unbalance protection, monitoring transformer overload and current unbalance issues related to current transformer protection.
Non-electrical quantity protection includes 63 sudden pressure protection, 26O/26W oil and winding temperature protection, 38/74 cooler failure protection, defending faults of machinery, temperature, pressure and cooling system.
60 PT circuit break alarm serves as auxiliary protection, fully guaranteeing safe and steady operation of transformers.
Types of Electrical Transformer Protection
Differential Transformer Protection
Main functions: Transformer differential protection, transformer differential instantaneous trip protection
According to application requirements, transformer differential protection is divided into two-winding and three-winding transformer differential protection.
Transformer Backup Protection
Differential relay protection of transformer works together with transformer backup protection, which trips faults with delay if differential or gas protection malfunctions or external faults happen.
According to operational requirements, transformer backup protection devices are classified into high, medium and low voltage backup protection units.
Non-electrical protection device
These typically include protection functions such as heavy gas, light gas, high temperature, overtemperature, and pressure relief for transformers.
Transformer measurement and control device
Transformer protection and control serves to measure and control the transformer.
On-site Installation Method of Secondary Protection of Transformer
Generally, transformer relay protection devices are mostly mounted on cabinets on site, namely standard protection panels for transformer. Distribution transformers are primarily installed inside high-voltage switchgears.
| Equipment Name | Quantity | Main Function |
|---|---|---|
| Main Transformer Differential Protection Device | 1 set | Serve as main protection to rapidly clear inter-phase and turn-to-turn short-circuit faults inside transformer windings |
| Main Transformer High-voltage Backup Protection Device | 1 set | Act as backup protection against external inter-phase faults and serve as local backup of differential protection |
| Main Transformer Low-voltage Backup Protection Device | 1 set | Protect low-voltage side busbars and lines, providing backup protection for low-voltage side faults |
| Main Transformer Non-electrical Quantity Protection Device | 1 set | Receive gas, temperature, pressure and other non-electrical signals to trigger alarm or tripping |
| Main Transformer Measurement & Control Device | 1 set | Collect analog quantities such as voltage, current and power as well as switch status, and realize remote switching operation |
It should be noted that non-electrical protection varies between dry-type and oil transformer relay protection. Gas protection is exclusive to oil-immersed transformers.
