Industrial sectors including iron and steel, mining and chemical plants feature heavy power loads and round-the-clock production, which require high stability and safety of power supply. As the core of factory power systems, factory substations rely heavily on a reliable Relay Protection System to ensure safe and steady operation of the entire power network.
Substation relay protection is categorized by protected equipment, covering line protection, transformer protection relay, busbar protection relay, capacitor protection and motor protection relay. Currently, newly built and upgraded industrial substations widely deploy digital protection relays.
These all-in-one units integrate protection, measurement, control and communication functions, and are equipped with built-in overvoltage protection relay modules to fully satisfy the operational requirements of the factory substation relay protection system for industrial power supply.
Common Protection Relay Types for Factory Substations
Line Protection
The 10kV ~ 35kV outgoing feeders of factory substations are generally equipped with two-stage or three-stage current protection.
| Protection Type | Function | Operating Time Limit |
|---|---|---|
| Instantaneous Overcurrent Protection (Stage I) | Protect faults at the line head end | Instant operation |
| Time-lagged Overcurrent Protection (Stage II) | Cover the entire line and extend to adjacent equipment | Short time delay (0.3s ~ 0.5s) |
| Overcurrent Protection (Stage III) | Backup protection for adjacent components | Long time delay |
For double-source circuits and critical lines, optical fiber pilot differential protection can be installed. Single-phase earth fault protection (zero-sequence protection) is also required for low-resistance grounding systems.
Transformer Protection
Transformers are core equipment of factory substations with comprehensive protection configurations.
Main Protection (Trips Instantaneously)
Differential Protection Relay 87t
Mandatory for standalone transformers with capacity ≥ 10,000 kV·A, and parallel-operated transformers with capacity ≥ 6,300 kV·A.
Instantaneous overcurrent protection relay
Applied to small-capacity transformers.
Gas Protection
Required for oil-immersed transformers of 800 kV·A and above. Light gas operates alarm; heavy gas triggers tripping.
Backup Protection (Time-delayed Operation)
| No. | Protection Type | Function |
|---|---|---|
| 1 | Overcurrent Protection | Protects against external phase-to-phase short circuits. |
| 2 | Composite Voltage Initiated Overcurrent Protection | Adopted where high sensitivity is required. |
| 3 | Overload Protection Relay | Installed on transformers of 400 kV·A and above, operates alarm only. |
| 4 | Zero-sequence Current Protection | Equipped for systems with solidly grounded neutral points. |
Busbar Protection Relay
Factory substations normally operate with sectionalized busbars.
The section circuit breaker is fitted with time-delayed instantaneous overcurrent protection and overcurrent protection.
The instantaneous overcurrent protection for the section circuit breaker is activated only at the moment of closing and automatically deactivated afterwards.
For double busbars or sectionalized busbars in critical substations, dedicated busbar differential protection shall be installed to clear busbar faults quickly.
Capacitor Protection Relay
Factory substations are commonly equipped with shunt capacitor banks for reactive power compensation, with the following protection devices configured:
| No. | Protection Type | Function |
|---|---|---|
| 1 | Overcurrent Protection | Guards against short circuits in capacitor banks and connecting lines. |
| 2 | Overvoltage Protection | Prevents capacitors from being damaged by excessive operating voltage. |
| 3 | Undervoltage Protection Relay | Avoids overvoltage caused by closing no-load transformers with capacitors connected. |
| 4 | Zero-sequence Voltage Protection | Detects internal faults of capacitors. |
| 5 | Fuse Protection | Fitted individually for each capacitor unit. |
Large motor protection relays
Protection schemes for high-voltage motors in factory substations are as follows
| No. | Protection Type | Function |
|---|---|---|
| 1 | Instantaneous Overcurrent Protection | Protects against phase-to-phase short circuits. |
| 2 | Overload Protection | Guards against prolonged overload operation. |
| 3 | Single-phase Earth Fault Protection | Prevents earth faults in stator windings. |
| 4 | Undervoltage Protection | Avoids hazards caused by motor auto-restart upon voltage recovery. |
Busbar Automatic Transfer (BZT)
It automatically switches to the standby power supply when the operating power loses voltage, ensuring stable power supply.
Selection Methods of Relay Protection
Determined according to voltage level and equipment importance
| Voltage Level | Protection Configuration Requirements |
|---|---|
| 110 kV and above | Dual configuration with two independent sets of protection, including main protection and complete backup protection |
| 35 kV | Single set configuration; directional protection is adopted as required by the system |
| 10 kV and below | Simplified configuration, mainly adopting current protection |
Select according to equipment capacity
Larger transformer capacity requires more comprehensive protection configuration. Dual protection shall be equipped for circuits with critical loads.
Prefer Microcomputer Protection
Newly-built and renovated substations shall adopt integrated microcomputer automation systems to integrate protection, control, measurement, signal and communication functions.
Installation of Relay Protection
Installation Positions by Voltage Level
| Voltage Level | Installation Position |
|---|---|
| 66kV ~ 110kV and above | Centralized panel mounting in the main control room |
| 6kV ~ 35kV | Local mounting on switchgears |
Requirements for Current Transformer Configuration
Current transformers for protection shall be configured to eliminate protection blind zones. Protection devices and measuring instruments shall not share the secondary windings of the same set of current transformers. The steady-state error of current transformers shall not exceed 10%.
Requirements for Voltage Circuits
Where disconnection of the voltage transformer secondary circuit may cause maloperation of the relay protection system, a circuit break blocking device shall be installed with alarm signals triggered.
Firm and Reliable Wiring
Ensure firm connections for backplanes, terminal blocks, tripping plates and connectors. Take protective measures for optical cables and network cables against external mechanical damage. Strictly control key quality points during installation.
DC Power Supply
The rated power supply for the relay protection system devices shall be uniformly DC 220V (or DC 110V). Power is supplied by DC panels equipped with battery banks to guarantee continuous power supply during outages.
Protective Relay testing
New Installation Inspection & Test Items
| No. | Test Item | Main Content |
|---|---|---|
| 1 | Visual Inspection | Device appearance, wiring, marking and identification, etc. |
| 2 | Insulation Resistance Test | Insulation resistance of each circuit to ground and between circuits. |
| 3 | Power Supply Performance Test | The device operates normally under 80% of the rated voltage. |
| 4 | Input System Test | Acquisition accuracy of analog signals and digital signals. |
| 5 | Output System Test | Correct operation of outlet relays and signal relays. |
| 6 | Logic Function Test | Simulate various faults to verify the protection logic. |
| 7 | Integral Trip Test | Complete transmission test from fault occurrence to circuit breaker tripping. |
Protection Relay testing equipment
Operator Training for the Relay Protection System
Conduct protective relay training to help staff master the principles and operation methods of protection devices.
Strengthen on-site training for operating personnel, including setting relay protection values and viewing fault information. Protection relay setting calculation shall be carried out by professionals.
Be proficient in operating the SCADA workstation to realize intelligent operation and maintenance of the entire power station’s relay protection system.
