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ASL-441XLH line differential protection relay
Overview
As a numerical line differential protection relay and protective relay for transmission line, the line differential protection relay — line optical fiber differential protection and control unit — can be installed locally in switchgear cabinets or in a centralized panel configuration. Suitable for high‑voltage applications, it features elaborate hardware and software design, enabling long-term reliable operation even in harsh environments.
Function of Line Differential Protection Relays
87L/67/51/67N/51N/51N/50/79/81U/59/27/60
Communication Mode:Optional: RS-485, CAN bus, Ethernet, IEC 60870-5-103 (IEC-103)
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Description
Overview
- As a numerical line differential protection relay and protective relay for transmission line, the line differential protection relay — line optical fiber differential protection and control unit — can be installed locally in switchgear cabinets or in a centralized panel configuration. Suitable for high‑voltage applications, it features elaborate hardware and software design, enabling long-term reliable operation even in harsh environments.
Function of Line Differential Protection Relays
| Category | Function Description | ANSI Code |
|---|---|---|
| Protection Function | Fiber Current Differential Protection | 87L |
| Protection Function | Two-stage Low-voltage Blocked Interphase Directional Overcurrent Protection | 67/51 |
| Protection Function | Inverse-time Interphase Directional Overcurrent (Normal / Very / Extreme) | 67/51 |
| Protection Function | Three-stage Zero-sequence Directional Overcurrent Protection | 67N/51N |
| Protection Function | Inverse-time Zero-sequence Directional Overcurrent (Normal / Very / Extreme) | 51N |
| Protection Function | Three-phase Single-shot Auto-reclosing (Non-synchronism / Synchronism Check / Dead Voltage Check) | 79 |
| Protection Function | Acceleration Protection (Front Acceleration / Rear Acceleration) | 50/79 |
| Protection Function | Overload Protection | 51 |
| Protection Function | Under-frequency Islanding Protection | 81U |
| Protection Function | Overvoltage Protection | 59 |
| Protection Function | Undervoltage Protection | 27 |
| Protection Function | Voltage Transformer Disconnection Monitoring | 60 |
| Protection Function | Control Circuit Disconnection Monitoring | — |
| Data Acquisition | Current, Voltage, Active Power, Reactive Power, Power Factor, Frequency | — |
| Data Acquisition | 13-channel Remote Signals | — |
| Event Record | Protection Event Record | — |
| Event Record | Alarm Event Record | — |
| Event Record | Fault Wave Recording | — |
| Event Record | Remote Signal Change Event | — |
| Event Record | Operation Record Event | — |
| Control | Local / Remote Trip and Close Operation | — |
| Control | Remote Setting Modification | — |
| Control | Remote Protection Enable / Disable | — |
| Operation Box | Trip Position and Close Position Indication | — |
| Operation Box | Self-holding and Anti-jump Function | — |
| Operation Box | Trip / Close Current Adaptation | — |
| Operation Box | Control Circuit Disconnection Alarm | — |
| Communication | Ethernet (Recommended) | — |
| Communication | RS485 | — |
| Others | IRIG-B(DC) (Optional), IEC61850 (Optional) | — |
line differential protection relay setting
| No. | Setting Name | Range | Unit | Remark |
|---|---|---|---|---|
| 1 | Control Word 1 | 0000～FFFF | — | Refer to control word description |
| 2 | Control Word 2 | 0000～FFFF | — | Refer to control word description |
| 3 | CT Ratio Compensation Coefficient | 0.025～40 | — | Compensate inconsistent TA ratios on both sides |
| 4 | Differential Current Setting | 0.2～40.0 | A | |
| 5 | Stage II Current Setting | 0.2～100.0 | A | |
| 6 | Stage III Current Setting | 0.2～100.0 | A | |
| 7 | Stage I Current Time Delay | 0.0～5.00 | S | |
| 8 | Stage II Current Time Delay | 0.1～20.00 | S | |
| 9 | Stage III Current Time Delay | 0.1～20.00 | S | |
| 10 | Stage I Zero-sequence Current | 0.1～20.0 | A | |
| 11 | Stage II Zero-sequence Current | 0.1～20.0 | A | |
| 12 | Stage III Zero-sequence Current | 0.1～20.0 | A | |
| 13 | Stage I Zero-sequence Time Delay | 0.0～5.00 | S | |
| 14 | Stage II Zero-sequence Time Delay | 0.1～20.00 | S | |
| 15 | Stage III Zero-sequence Time Delay | 0.1～20.00 | S | |
| 16 | Current Acceleration Stage Setting | 0.2～100.0 | A | |
| 17 | Current Acceleration Time Delay | 0.0～5.00 | S | |
| 18 | Zero-sequence Acceleration Stage Setting | 0.1～20.0 | A | |
| 19 | Zero-sequence Acceleration Time Delay | 0.1～5.00 | S | |
| 20 | Current Protection Blocking Voltage | 1.0～120.0 | V | Line voltage |
| 21 | Inverse-time Current Reference | 0.2～100.0 | A | |
| 22 | Inverse-time Current Time | 0.005～127 | S | |
| 23 | Inverse-time Zero-sequence Reference | 0.1～20.0 | A | |
| 24 | Inverse-time Zero-sequence Time | 0.005～127 | S | |
| 25 | Inverse-time Exponent | 0.01～10.0 | — | Set to 0.02, 1 or 2 |
| 26 | Overload Current Setting | 0.5～10.0 | A | |
| 27 | Overload Alarm Time | 6～9000 | S | |
| 28 | Overload Trip Time | 6～9000 | S | |
| 29 | Reclosing Synchronism Check Angle | 10～50 | ° | |
| 30 | Auto-reclosing Time | 0.2～20.0 | S | |
| 31 | Under-frequency Element Frequency | 45.0～49.5 | Hz | |
| 32 | Under-frequency Element Time Delay | 0.1～20.0 | S | |
| 33 | Under-frequency Blocking Voltage | 10～120 | V | Line voltage |
| 34 | Under-frequency Blocking Slip | 0.5～20.0 | Hz/S | |
| 35 | Overvoltage Setting | 30.0～120.0 | V | Line voltage |
| 36 | Overvoltage Time Delay | 0.0～100.0 | S | |
| 37 | Undervoltage Setting | 10.0～100.0 | V | Line voltage |
| 38 | Undervoltage Time Delay | 0.0～100.0 | S | |
| 39 | Undervoltage Blocking Current | 0.2～100.0 | A | |
| 40 | PT Ratio | 1~9999 | — | |
| 41 | CT Ratio | 1~9999 | — |
line differential protection relay working principle
- As a line differential protective relay, fiber-optic differential current protection operates on the principle of phase current differentials, and it mainly provides protection for phase-to-phase faults within the protected area as well as single-phase metallic ground faults in low-resistance grounding systems. Operating equation:
- Equation (2) is the main criterion, where M and N represent the two sides of the circuit.
Designed for overhead and transmission line protection, this relay adopts two proportional restraint coefficients: K1 = 0.5 and K2 = 0.7.
- ICD: Minimum operate current, i.e. the pickup setting of differential current.
- IINT: Inflection point current, fixed at 4× rated current.
Differential Overload Alarm
A differential overload alarm will activate with a 2s delay once the measured differential current exceeds 0.5 times ICD.
CT Broken Circuit Blocking
The relay supports CT disconnection blocking logic. Phase-wise CT disconnection is confirmed only when both conditions below are met simultaneously:
- Differential current ＞ 0.5 × ICD
- No current detected at the local terminal, while remote terminal current ＜ 6 A
2nd Harmonic Restraint
The differential protection is blocked if the second harmonic component accounts for more than 20% of the fundamental current.
Outline and Installation Dimensions
FAQ
Q: ANSI code of line differential protection?
A: ANSI standard code is 87L.
Q What is line differential protection relay (ANSI 87L)?
It compares synchronized current phasors at two terminals of the protected line via communication channels. Based on Kirchhoff’s Current Law, it trips both breakers instantly once internal fault generates obvious differential current, with absolute protection selectivity.
Q What are its core merits vs distance (21) protection?
Immune to power swing, load encroachment, arc resistance and parallel line mutual coupling; no complicated zone setting; ultra-fast tripping for internal faults. Shortcoming: Must rely on fiber/communication links, cannot serve as remote backup for adjacent line faults.
Q Common communication media for 87L?
Mainly OPGW fiber optic cable (most stable); also SDH/MSTP optical transmission, dedicated pilot wire for short lines.
Q What happens if communication link breaks down?
Relay triggers channel fault alarm, automatically switches to backup distance & overcurrent protection to avoid protection loss.
Q Why strict time synchronization required?
Asymmetric sampling time between two ends creates false differential current, leading to maloperation. Most devices adopt GPS/IRIG-B or internal clock synchronization.
Q Why CT polarity must be strictly checked?
Reversed polarity directly produces huge differential current under normal load, causing unnecessary trip. The standard rule: CT secondary current direction shall point into the protection zone at both line ends.
Q How to suppress unbalanced differential current?
Set restraint current threshold to counter CT saturation error, line distributed capacitance current and ratio mismatch of two-side CTs.
Q Is CT secondary opening allowed?
Absolutely forbidden. Open CT secondary induces extremely high voltage, damaging relay modules and endangering personal safety.
Q Typical application scenarios?
Long-distance EHV/UHV transmission lines, important tie lines between power grids; distance protection acts as backup protection for 87L.
Q Main adjustable parameters?
Differential operate current threshold, restraint coefficient, capacitance current compensation coefficient, channel delay compensation, fault detection delay.
Q Protection false tripping without actual fault?
Main causes: wrong CT ratio/polarity, channel jitter/sync deviation, excessive line capacitive current, external severe CT saturation.
Q Protection refuses to trip during internal fault?
Check whether 87L function is enabled; verify channel connection, CT circuit integrity, and whether differential current exceeds setting value.
Q How to handle CT saturation on through external faults?
Modern relays embed CT saturation identification algorithm; increase restraint coefficient during parameter configuration.
Q Necessary commissioning tests?
Single-terminal injection test, end-to-end joint debugging, channel delay test, whole-group trip interlock test for both substations.
Q Daily routine maintenance items?
Monitor channel link status via SCADA; regularly check communication optical modules; perform periodic secondary circuit insulation inspection during power outage.
Q Can it support multi-terminal line protection?
Yes, mainstream 87L relays support 3-terminal / 4-terminal differential protection for multi-branch transmission lines.
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xiao zhang –
Line differential protection relay (ANSI 87L) features outstanding anti-CT-saturation performance and high sensitivity for internal short-circuit faults. Real-time bidirectional current data exchange ensures reliable discrimination between in-zone and out-of-zone faults; multiple communication interfaces available for global transmission line projects.
Jack –
Accurate protection settings, clean trip operation — no misoperation or failure to operate.