Step-Up Transformer Protection Relay | Manufacturer for Power Station

This Step-Up Transformer Protection system serves as the core protecting transformer device, delivering full-range transformer secondary protection to secure the unit against various electrical faults.

Protection Composition of Step-up Transformer

Category	Protection Name	ANSI Code	Function Description
Main Protection	Transformer Differential Protection	87T	Rapid isolation for internal faults
	Instantaneous Differential Trip	87T-Instant	Instant trip for severe internal faults with no time delay
Backup Protection	Compound Voltage Overcurrent Protection	51V/50V	Backup protection for phase-to-phase faults
	Neutral Overcurrent Protection	50N/51N	Backup protection for earth faults
	Overcurrent Protection	50/51	Simple general backup protection
Earth Fault Protection	Zero-sequence Current Protection	50N/51N/64N	Protection for earth faults in neutral grounded systems
	Gap Overcurrent / Neutral Overvoltage Protection	50N/59N	Dedicated protection for ungrounded or gap-neutral systems
Non-Electrical Protection	Heavy Gas Protection	—	Trip action for severe internal tank faults
	Light Gas Protection	—	Alarm only for minor internal tank faults, no trip
	Pressure Relief Protection	—	Protection against excessive tank internal pressure
	Oil Temperature / Winding Temperature Protection	49/63W	Alarm or trip for overheated oil and windings
Abnormal Operation Protection	Overload Protection	49	Alarm for long-term equipment overload
	Overexcitation Protection	24	Protection against core overexcitation caused by abnormal voltage or frequency
	Cooling System Fault Protection	—	Monitoring and alarm for failed cooling fans and oil pumps

Transformer Protection schemes

Transformer differential protection theory

It follows Kirchhoff’s Current Law. Current transformers (CTs) are installed on the primary and secondary sides of the transformer. The secondary currents of CTs are connected in a differential loop.

Under normal load or external short-circuit faults: The vector sum of two-side CT secondary currents is nearly zero, no differential current flows through the relay, and the protection stays inactive.

When internal faults occur inside the transformer windings, bushings or leads: The balance of two-side currents breaks, obvious differential current appears, the protection trips instantly to isolate the faulty transformer.

Transformer differential protection calculation

Transformer differential protection setting calculation computes key parameters to guarantee reliable operation for internal faults and prevent maloperation for external faults.

Transformer differential protection setting calculation example

Three-Winding Transformer Parameters Table

Parameter Item	Value & Description
Rated Transformer Capacity Se	31500 kVA
Transformer Winding Connection Group	Yn, y, d11
Transformer Rated Voltage Ratio Ue	110kV / 35kV / 10kV
CT Ratio on 110kV Side nTA	300/5
CT Ratio on 35kV Side nTA	1000/5
CT Ratio on 10kV Side nTA	2000/5
CT Connection Mode	External Phase Compensation Method
Primary Circuit Arrangement	Two parallel branches on the 10kV side
Voltage Regulation Range ΔU	±8×1.25%
CT Connection Coefficient Kjx	Equals 1 for Y connection; equals 3 for Δ connection
Maximum External Three-Phase Short-Circuit Current Ik(3)	Assumed as 1000A (actual value shall be calculated based on site conditions)

Calculation:

• Secondary Rated Current of High-Voltage Side

• Secondary Rated Current of Medium-Voltage Side

• Secondary Rated Current of Low-Voltage Side

Differential Operating Threshold

Krel — Reliability coefficient, take 1.3~1.5

Kap — Aperiodic component coefficient, take 2

Kct — CT homogeneity coefficient; 0.5 for same CT model, 1 for different CT models

fi — Maximum relative error of CT, take 0.1

ΔU — Relative error caused by transformer tap changer adjustment, take half of the total regulation range, i.e. 8×1.25%

Δm — Error from mismatch between setting and calculation tap positions, generally take 0.05

Recommended setting value: 0.4Ie

Knee Point Current

Recommended setting value: 1.0Ie

Percentage Restraint Slope Coefficient

Calculation shall be based on the high-voltage side as the reference side.

ΔU — Relative error caused by transformer tap changer adjustment, take half of the total regulation range, i.e. 8×1.25%

Δm1 — Error from mismatch between setting tap and calculation tap, take 0.05; adopt Δm1 for two-winding transformers.

Δm2 — Error from mismatch between setting tap and calculation tap, take 0.05; adopt Δm1+Δm2 for three-winding transformers.

Restraint characteristic slope K:

Second Harmonic Restraint Coefficient

Recommended value: 0.18

CT Broken Circuit Release Current

Recommended value: 1.2Ie

Instantaneous Differential Trip

Recommended value: 7.0Ie

Transformer overcurrent protection table

No.	Parameter Item	Description
1	HV Side Overcurrent Setting	Operating current values for Stage I (Instantaneous), Stage II (Short Time Delay) and Stage III (Definite Time Delay) (Unit: A or multiple of rated current In)
2	MV Side Overcurrent Setting	Operating current and corresponding time delay of each overcurrent stage (applicable to three-winding transformers)
3	LV Side Overcurrent Setting	Parameters for LV backup overcurrent protection and residual overcurrent protection
4	Protection Operating Time Delay	Operating time of each protection stage (unit: second), e.g. Stage I: 0s, Stage II: 0.5s, Stage III: 1.0s
5	Compound Voltage Block Setting	Low voltage setting (V) and negative-sequence voltage setting (V), used to block overcurrent protection for better selectivity
6	Residual Overcurrent Setting	Operating current and time delay of 50N/51N residual overcurrent protection for single-phase earth fault protection
7	Time & Current Margin for Coordination with Upstream & Downstream Protection	Time margin (normally 0.3~0.5s) and current margin between transformer protection, upstream line protection and downstream feeder protection

Transformer overexcitation protection

It monitors the V/Hz ratio of the transformer. When the voltage is excessively high or the system frequency is too low, the core magnetic flux will exceed the rated limit, triggering the protection to operate. This prevents overheating of the iron core and insulation deterioration, and is mainly applied to large main transformers.

Transformer overvoltage protection

Suppresses lightning surge and switching overvoltage on transformer windings to avoid breakdown of winding insulation.

Transformer overheating protection

It monitors the transformer oil temperature and winding temperature in real time. An alarm will be triggered when the temperature exceeds the limit; if the temperature stays over the threshold continuously, the protection will trip to prevent accelerated insulation aging and winding burnout.

Common temperature measuring components: oil temperature thermometer, platinum resistance winding temperature sensor, thermal sensitive element.

Transformer thermal overload protection

Thermal Overload

It calculates winding equivalent temperature rise via thermal accumulation model, different from simple current-based overload protection.

Core Principle

It simulates winding heat buildup with current, cooling mode and temperature rise curves, distinguishes short surge loads from persistent overheating, and features inverse time characteristic: alarm first, trip on sustained overtemperature.

Difference from Regular Overload Protection

• Transformer overload protection: Basic overload protection, alarm only by current threshold
• Transformer thermal overload protection: High-precision protection based on thermal effect modeling, standard for large oil-immersed and dry-type transformers

Buchholz relay protection of transformer

Minor gas protection: Generates gas from minor internal faults and only triggers an alarm. Major gas protection: Trips immediately upon oil surge caused by severe short-circuit faults.

Pressure relief protection

The pressure relief operates when abnormal oil pressure occurs due to faults.

Transformer primary and secondary protection

Transformer restricted earth fault protection

Application Scenario: Special protection for internal earth faults of transformer windings, often used in conjunction with differential protection.

Function: Accurately identify internal winding earth short-circuits, avoid maloperation caused by external earth faults, and feature high sensitivity.

Transformer surge protection

Transformer surge protection is installed on primary side to suppress lightning overvoltage.

Conclusion of Step-Up Transformer Protection

This Step-Up Transformer Protection scheme covers comprehensive transformer primary and secondary protection for the step-up transformer, including differential, overcurrent, zero-sequence and thermal overload protection to safeguard the high-voltage primary and low-voltage secondary windings against all typical faults.

Differences among distribution transformer protection, oil transformer protection and step-up transformer protection

Distribution transformer protection targets small grid distribution transformers with basic overload & ground fault protection.

Oil transformer protection refers to oil-filled unit-specific protection including Buchholz and oil temperature monitoring.

Step-Up Transformer Protection is full high-grade main & backup protection for power generation boosting transformers.

How to Select Relays for Step-Up Transformer Protection

Relay selection directly affects the safety and stability of Step-Up Transformer Protection, which shall consider voltage level, transformer capacity, communication standards and O&M demands comprehensively.

Voltage Level Matching

Select relays suitable for 35kV, 66kV, 110kV and 220kV to satisfy grid insulation and parameter standards for Step-Up Transformer Protection.

Capacity Matching

Transformers of 5MVA, 10MVA, 50MVA, 100MVA and above adopt different sensitivity settings in Step-Up Transformer Protection. Large-capacity GSU transformers need high-precision multi-functional relays.

Communication Compatibility

Supports mainstream protocols including IEC 61850, Modbus RTU/TCP, IEC 60870-5-103 to enable seamless SCADA connection for Step-Up Transformer Protection.

Recording & Self-Diagnosis

Opt for relays with SOE, fault/disturbance waveform recording and self-diagnosis to simplify post-fault analysis and daily maintenance of Step-Up Transformer Protection.

FAQ

What is the main protection of transformer?

Transformer main protections are longitudinal differential (87T) and differential instantaneous overcurrent (87T-I). Per Kirchhoff’s Current Law, differential current stays near zero in normal operation but spikes instantly for internal faults to trip the transformer. It is the sole protection to quickly clear winding inter-turn and internal faults.

What is transformer differential protection?

Transformer differential protection (87T), core transformer short circuit protection, detects internal faults via current comparison and trips instantly with rising differential current.

What functions does earthing transformer protection have?

Earthing transformer protection covers zero-sequence overcurrent, neutral overvoltage, interphase short-circuit and anti-resonance functions to detect and isolate ground and internal faults.

What are the protection used in transformer?

Various electrical and mechanical protection devices including differential, short-circuit, overload, earth fault and oil monitoring protection are adopted for transformers.

Conclusion

Choose our reliable and fully functional Step-Up Transformer Protection system to deliver comprehensive fault prevention and stable operational safety for all your boosting transformers in power generation projects.

No matter you have technical consultation, parameter customization or bulk purchasing demands, please feel free to send us your inquiries anytime, and our professional engineering team will offer you tailored solutions promptly.