33kV protection relay schemes at a distribution substation are built from six ANSI functions: 50/51 phase overcurrent, 50N/51N or 67N earth fault, 87T transformer differential, 27/59 under- and overvoltage, 81 frequency and 50BF breaker failure. The relay reads current from CTs and voltage from VTs, compares every sample against the set thresholds, and trips the breaker once a threshold is exceeded for the set time.

Function numbers follow ANSI/IEEE C37.2-2022. Relay performance requirements come from the IEC 60255 series and CT accuracy from IEC 61869-2:2012. Those three references are the ones a relay specification should cite by name.

Table of Contents

At a Glance: What a 33 kV Scheme Covers

50/51 phase overcurrent: phase-to-phase and three-phase faults, trips on magnitude and time
51N earth fault, non-directional: single phase-to-earth fault on a radial feeder
67N earth fault, directional: earth fault on a parallel or meshed feeder, trips for the forward direction only
87T transformer differential: internal winding fault, compares currents instead of reacting to magnitude alone
87N restricted earth fault: earth fault inside the transformer zone, sees currents the 51N element misses
27/59 under- and overvoltage: busbar voltage outside the band, alarms or trips per the grid code
81 under- and overfrequency: islanding and load-shedding logic, not a fault-clearing function
50BF breaker failure: the breaker does not open on a trip command, backs up to the upstream breaker

A 33 kV relay and an 11 kV relay are frequently the same hardware. What changes is the setting calculation: fault level, earthing arrangement and CT ratio differ by site, so the same device ships with a different setting file. This page is scoped to 33 kV. For the generic workflow across voltage classes, see how to choose a protection relay for a medium voltage substation; for the neighbouring voltage class, see power protection relays for 22 kV substations.

The sections below cover where these relays sit in a substation, the function set by ANSI code, how the trip decision is made, how to shortlist a model, what to test before energisation, the standards that apply and how to evaluate a supplier.

33kV protection relay front panel mounted in a metal-clad switchgear cubicle (illustrative)

What Is a 33kV Protection Relay?

Definition and Measured Inputs

A 33kV protection relay is a numerical microprocessor-based protection device designed for medium voltage systems rated at 33kV. It acquires real-time current signals from current transformers (CT) and voltage signals from voltage transformers (VT/PT).

After continuous data processing and algorithm calculation, the relay executes predefined actions including local alarm, circuit breaker tripping and remote signalling once electrical parameters deviate from normal operating thresholds.

Why Is Protection Required in 33kV Systems?

Medium voltage networks operate under variable load conditions and are exposed to insulation ageing, mechanical damage and operational misoperation. Common faults and consequences are summarised below:

Fault Type	Potential Impact
Short circuit	Severe thermal and mechanical damage to cables, transformers and switchgear
Earth fault	Personnel safety hazards, progressive insulation breakdown
Overload	Accelerated insulation ageing, permanent thermal damage
Voltage abnormality	Malfunction of motors, generators and sensitive power equipment

Where 33 kV Relays Are Used

Three installation types account for most 33 kV schemes: utility distribution substations stepping down to 11 kV, industrial intake substations feeding large drives or process plant, and collector substations on solar, wind and storage projects. The protected objects are similar in all three. What differs is the fault level, the earthing arrangement and the grid code that governs the interconnection.

33kV Substation Protection Scheme

These relays form the core protection system within distribution, industrial and renewable energy substations. Typical protected objects include:

Incoming and outgoing feeders
Busbar sections
Step-down transformers (33kV/11kV, 33kV/0.4kV)

33kV Feeder Protection

Feeder protection relays are installed on each outgoing circuit to isolate line faults rapidly, and most projects use a multifunction feeder protection relay so one device carries overcurrent, earth fault and reclosing. Standard protection functions:

Overcurrent protection (ANSI 50/51)
Non-directional / directional earth fault protection (ANSI 50N/51N, 67N)
Auto-reclosing function for transient overhead line faults

33kV Transformer Protection

For 33kV side of distribution transformers, protection packages combine main protection and backup protection:

Transformer differential protection (ANSI 87T, main protection)
Overcurrent backup protection
Restricted earth fault protection (REF)
Winding over-temperature protection

Renewable Energy Projects

33kV collection systems are common in large-scale solar farms, onshore wind farms and BESS stations. Protection requirements focus on:

Grid interconnection compliance
Fast fault isolation to avoid islanding risks
Continuous remote condition monitoring

Main Protection Functions by ANSI Code

Modern numerical relays at 33 kV integrate multi-function protection complying with ANSI / IEEE protective relay numbers.

Overcurrent Protection (50/51)

Instantaneous overcurrent (50) and time-delayed overcurrent (51) detect excessive load or short-circuit current. Widely used as primary or backup protection for feeders, transformers and large medium-voltage motors.

Earth Fault Protection (50N/51N)

Monitors residual current to identify cable insulation failure and earth faults. Critical for underground cable networks where gradual insulation degradation often occurs without obvious external symptoms.

Under Over Voltage Protection Relay (27/59)

The over voltage and under voltage protection relay protects transformers and rotating machinery against sustained voltage deviation, preventing voltage collapse and equipment dielectric stress.

Earth fault current at 33 kV depends on the earthing arrangement: solid, resistance or resonant (Petersen coil) earthed. That choice decides whether the 51N element sees enough current to operate at all, so confirm it before fixing the pickup. A relay set for a solidly earthed network under-reaches on a resistance-earthed one, and a directional 67N element adds nothing where there is only one source and one direction of fault current.

Frequency Protection (81)

Mandatory for grid-connected generators and renewable power plants. It detects abnormal frequency drift triggered by power imbalance and initiates disconnection according to grid codes.

Differential Protection (87)

High-speed selective main protection. The protection and control relay is widely adopted for transformers and busbars. It distinguishes internal faults from external through-faults and achieves rapid fault clearance to minimise equipment damage.

How the Relay Detects a Fault

Protection Relay Operating Principle

CT and VT Inputs at 33 kV

Two input choices decide whether a scheme works, and neither appears on the relay datasheet. The first is the CT ratio: it sets the primary current behind 1 A or 5 A at the relay, and a ratio picked from the load current rather than the fault level leaves the pickup near the bottom of the relay range, where accuracy is worst. The second is the VT connection. A three-phase four-wire VT gives phase voltages, while a broken-delta VT gives the residual voltage that a 59N element needs. Confirm both against the single-line diagram before the panel is built, because changing a CT ratio after installation means re-terminating the secondary wiring.

Burden is the third item, and the one most often missed. Add the lead resistance over the actual run from the switchyard to the relay, then compare the total against the CT rated burden. Long cable runs between the switchyard and the control room are the usual reason a correctly specified CT still saturates.

Substation Communication Integration

Numerical relays at 33 kV support these industrial protocols for connection to SCADA and substation automation systems:

IEC 61850 (preferred for modern digital substations)
IEC 60870-5-103
Modbus RTU / Modbus TCP
Ethernet communication

Operators can remotely view fault records, adjust protection settings and retrieve disturbance recordings.

Relay Types by Protected Object

Numerical Protection Relay

Microprocessor-based multi-functional relays. Replace older electromechanical relays. Advantages: flexible setting, fault event recording, built-in communication and multi-protection function integration. This is the mainstream selection for new 33kV substation projects.

Feeder Protection Relay

Optimised for overhead lines and underground cables. Core focus: overcurrent, earth fault, auto-reclosing and optional feeder differential protection relay. Suitable for most distribution feeder applications.

Transformer Protection Relay

Integrates differential protection, thermal overload monitoring, earth fault protection for transformer, and restricted earth fault protection, specially designed for power transformer protection and fault isolation.

Transformer schemes at this voltage class usually add restricted earth fault to the differential element. The differential element covers phase-to-phase and three-phase faults. REF covers earth faults close to the star point, where the differential current stays below pickup. On a 33/11 kV transformer with a delta-connected 11 kV winding, an earth fault on the 33 kV star side produces no matching zero-sequence current on the delta side, which is exactly the case REF is there for.

Medium Voltage Motor Protection Relay

Deployed for large 33kV drive motors such as water pumps and compressors. It covers winding over-temperature, current unbalance, locked rotor and overload protection.

How to Select the Right 33kV Protection Relay?

Project engineers and relay and protection engineers need to confirm application scenarios before finalising model selection. The table below provides quick reference:

Protected Equipment	Recommended Core Protection Functions
Distribution Feeder	Overcurrent + Earth Fault + Auto-reclosing
33kV Step-down Transformer	Differential + Overcurrent + REF + Over-temperature
Medium Voltage Motor	Thermal protection + Overcurrent + Current unbalance

Beyond protection functions, evaluate the following factors:

Communication compatibility: Confirm matching protocols with the existing SCADA system; prioritise IEC 61850 for digital substation upgrades
Auxiliary supply: DC 24V / 48V / 110V / 220V options to match panel power design
Environmental parameters: Operating temperature range, altitude tolerance for outdoor or highland substations
Certification requirements: Check project tender requirements for IEC test certificates

Time grading between the 33 kV incomer and the 11 kV outgoing feeder has to cover breaker operating time plus relay overshoot. Where the margin is too small to hold, change the curve or the CT ratio rather than shaving the time multiplier, and record the result in a coordination study. Settings derived without short-circuit data are the most common rework item on site.

👉 ASL-441H line protection relay: send the single-line diagram and the CT ratios, and the application engineers return a function list and a setting file for the 33 kV feeders in scope.

Testing and Commissioning on Site

Reliable protection performance relies on complete testing before formal energisation.

Factory Acceptance Test (FAT)

Conducted at the manufacturer’s workshop before shipment. Key test items:

Protection characteristic curve verification via secondary injection test set
Communication function verification
Dielectric insulation test
Input/output contact test

Site Acceptance Test (SAT)

Standard on-site commissioning workflow:

Commissioning flow for a 33 kV feeder relay, from factory acceptance to site acceptance (illustrative)

Common Application Problems

Incorrect Protection Coordination Settings

Mismatched time grading or wrong CT ratio settings are the most frequent root causes of unwanted tripping or failure-to-trip. All protection settings should be calculated based on system short-circuit current data via formal protection coordination study.

Current Transformer Saturation

Severe short-circuit current may lead to CT saturation, distorting secondary current signals and affecting differential protection operation. CT accuracy class and burden must be correctly sized during design.

Specify a protection class, not a metering class: 5P or PX to IEC 61869-2:2012. Check the knee-point voltage against the maximum through-fault current and the connected burden, which includes the lead resistance out to the relay. A metering CT saturates on a bus fault, and a saturated CT turns into a false differential current that the 87T element reads as an internal fault.

Communication Interruption

Common triggers include damaged fibre cables, inconsistent network IP addressing and incorrect protocol parameter configuration. Field technicians should verify physical connections first during troubleshooting.

Standards That Apply at 33 kV

Product design, testing and application follow internationally recognised standards:

IEC 60255-1:2022: measuring relays and protection equipment, general requirements
IEC 61850: communication networks and systems for power utility automation (a series, no single edition year)
IEC 62271-1:2017+AMD1:2021: AC switchgear and controlgear above 1 kV, common specifications

How to Choose a Reliable 33kV Protection Relay Supplier?

EPC contractors and asset owners carry the risk when a device fails on site. Ask a 33kV protection relay supplier for three deliverables before awarding the order: a setting calculation, factory test reports and documentation in the project language. Most MV protection solutions list the same ANSI functions; what separates them is the engineering that comes with the relay.

Technical Support Capacity

Professional suppliers offer end-to-end engineering support:

Assistance on relay model selection according to single-line diagrams
Guidance for protection setting calculation
Technical consultation during design phase

Standardised Project Execution Workflow

Project execution workflow for relay supply: enquiry, setting file, factory test and shipment (illustrative)

Overseas Project Service

International projects require complete documentation and after-sales support:

English versions of datasheets, wiring diagrams and operation manuals
Remote technical guidance during on-site commissioning
Spare parts supply guarantee for long-term operation

FAQ About 33 kV Relays

1. What is a 33kV protection relay?

It is a numerical multi-function protection device applied in 33 kV power networks. It monitors CT and VT signals, detects electrical faults and sends trip or alarm commands to protect feeders, transformers and other primary equipment.

2. Which protection functions are required for a typical 33kV distribution feeder?

The minimum package includes time-delayed and instantaneous overcurrent protection, earth fault protection. Auto-reclosing is recommended for overhead feeders.

3. What is the main difference between 33kV and 11kV protection relays?

Hardware voltage rating of CT/VT input circuits is not always different. The main difference lies in protection setting calculations based on system short-circuit levels, and project-specific grid code requirements. Many universal numerical relays support both voltage levels via software configuration.

4. How is the relay tested before the feeder is energised?

On-site testing mainly adopts secondary injection test equipment. Engineers simulate fault current and voltage to verify operating thresholds, operating time and breaker trip loop.

5. Can the relay connect to a SCADA system?

Yes. Modern numerical relays support mainstream communication protocols including IEC 61850, 103 and Modbus, enabling real-time data upload, fault message reporting and remote parameter access within the SCADA platform.

6. What is a breaker protection relay?

A breaker protection relay is a numerical protective device used to monitor circuit breaker operating conditions and implement fault protection logic. It detects abnormal system current/voltage signals and sends trip commands to the circuit breaker to isolate faults. It can also supervise breaker control circuits, spring charging status and breaker failure conditions.

Conclusion

A 33kV protection relay is specified by function list first and by model second. Fix the earthing arrangement and the short-circuit levels before anything else, because those two decide whether 51N operates and whether the CT stays linear. Then check the CT class, the grading margin and the communication protocol against the existing SCADA.

Three items decide whether a scheme survives its first fault: a setting calculation based on real short-circuit data, a protection-class CT, and a trip-circuit check carried out with the breaker in place. Skip any one of them and the relay will be blamed for a fault it was never able to see.

Technical References and Sources

Source	What it covers	Type	Edition
IEC 60255-1:2022	General requirements for measuring relays and protection equipment	Standard	2022
IEC 60255-151:2009	Functional requirements for over/under current protection (ANSI 50/51)	Standard	2009
IEC 61869-2:2012	Current transformers: accuracy class, burden and rated output	Standard	2012
IEC 60255-187-1:2021	Functional requirements for differential protection (ANSI 87T, 87N)	Standard	2021
IEC 61850-7-4:2010+AMD1:2020	Logical node and data object classes for substation communication	Standard	2010+AMD1:2020
IEC 62271-1:2017+AMD1:2021	Common specifications for AC switchgear and controlgear above 1 kV	Standard	2017+AMD1:2021
ANSI/IEEE C37.2-2022	Device function numbers and acronyms used on the scheme drawings	Standard	2022
IEEE C37.112-2018	Inverse-time characteristic equations for overcurrent relays	Standard	2018

Written by Leon Zhang, Protection Application Engineer · Technical review by Li Ming, Electrical Engineer · Last reviewed October 2026

substation protection relays

generator protection relays

Motor Protection Relays

Distribution Automation Systems and Equipment

Renewable Energy Relay Protection

Substation Auxiliary Equipment

Protection & Control Panels