What are the steps and methods for troubleshooting secondary circuit faults in switchgear?

Switchgear is very common in substations. Different specifications of switchgear vary in size and shape, but their operating principles are basically the same. Switchgear often contains microprocessor-based protection devices, intelligent instruments, and intelligent operator displays. These devices are connected via secondary wiring. The microprocessor-based protection device collects analog and digital signals, controls the opening and closing of switches, and trips circuit breakers promptly and accurately in case of faults. Intelligent instruments display current, voltage, power factor, and other parameters. Intelligent operator displays allow manual control of circuit breaker opening and closing, and locally display the status of relevant equipment within the switchgear.

Fault inspection of switchgear secondary circuits is a rigorous, meticulous, and systematic task. Safety procedures must be followed to ensure the safety of personnel and equipment. The following are detailed inspection steps and methods:

Core Principles:
Safety First:Follow the "two-ticket, three-system" approach, confirming that relevant primary equipment is de-energized or has implemented safety isolation measures to prevent accidental contact or malfunction.

Step-by-Step Approach:Proceed from simple to complex, from easy to difficult, from phenomena to essence.

Logical Analysis:Analyze based on drawings and principles, avoiding blind testing.

 

I. Preparations Before Fault Inspection

 

Safety Measures:

Issue a work permit and implement safety measures such as power outage, voltage testing, grounding, and signage.

Confirm that primary equipment related to the faulty circuit is reliably isolated.

When working on secondary circuits, prevent open circuits in current transformers (CTs) and short circuits in current transformers (TVs).

Technical Documentation Preparation:

Schematic Diagram/Expanded Diagram: Understand the logical relationship and working principle of the entire circuit.

Installation Wiring Diagram/Terminal Block Diagram: Used to locate specific wiring positions on site.

Equipment Manuals: Understand the functions and parameters of protection devices, relays, instruments, etc.

Tool and Instrument Preparation:

Common Tools: Screwdriver, wire strippers, multimeter (high internal resistance, preferably with continuity testing and voltage/current ranges), clamp ammeter, insulation resistance meter (megohmmeter), low-voltage test pen.

Specialized Tools: Relay protection tester (for analog input and protection calibration), portable waveform recorder.

Personal Protection: Insulating gloves, insulating mat, safety helmet.

 

II. General Steps and Methods for Fault Inspection

 

Step 1: Information Collection and Phenomenon Analysis

Inquiry: Obtain detailed information from operating personnel regarding the phenomena observed during the fault (e.g., alarm sounds, indicator lights, protection action information, circuit breaker tripping status, presence of unusual odors or noises, etc.).

Observation:

Check the event logs, fault reports, and SOE (Sequence of Events) displayed on the protection devices and control equipment screens. This provides the most direct clues.

Observe the status of all indicator lights and instruments.

Check for any obvious signs of burning, discoloration, loosening, or broken wires.

Signal Reset: After recording, attempt to reset the signals and observe which signals cannot be reset. These are usually the key fault points.

 

Step 2: Drawing Analysis and Fault Location

Refer to Drawings: Based on the fault symptoms (e.g., a "control circuit open" alarm), locate the corresponding circuit on the schematic diagram (e.g., the circuit breaker opening/closing circuit).

Narrowing the Scope: Based on logical relationships, narrow down the possible fault scope to a specific local circuit (e.g., the closing circuit, tripping circuit, signal circuit, measurement circuit, etc.).

 

Step 3: Layered Inspection and Testing

1. Appearance and Foundation Inspection

Tightness Check: Gently pull the wiring by hand or with a tool to check if the terminal blocks, relays, and back panel wiring are loose. (Note: This must be done with the power off or with safety measures in place.)

Component Inspection: Check if fuses and circuit breakers are blown or tripped; check relay contacts for burning or sticking; check if push buttons and changeover switches are in the correct positions.

Preliminary Insulation Assessment: Observe for signs of moisture, dust accumulation, or damage from small animals.

2. Electrical Quantity Measurement (Core Component)

Voltage Method (Most Commonly Used):

Prerequisite: Ensure the control power supply (DC110V/220V or AC220V) is normal.

Method: Use a multimeter in DC voltage mode, with the common negative terminal as the reference point, and measure the voltage point by point along the circuit.

Example (Finding a Break in the Closing Circuit):

Measure the voltage of the small busbar (+KM, -KM) of the closing circuit power supply. It should normally be the rated voltage.

Measure the voltage at the front end of the closing button/closing relay contacts.

Press the closing button or simulate closing conditions, and measure the voltage at the rear end of the contacts, the circuit breaker auxiliary contacts (normally closed), and both ends of the closing coil.

Normal Condition: When the circuit is conducting, there should be voltage across the coil. If there is a sudden change in voltage before and after a certain point (e.g., voltage at the front end, no voltage at the rear end), then that point (contact, wiring, component) is the fault point.

Resistance Method/Continuity Method:

Prerequisite: The power supply to the circuit being tested must be disconnected! Very important!

Method: Use a multimeter in resistance or continuity mode.

Application:

Check circuit continuity (e.g., wire and contact continuity).

Measure coil resistance (e.g., opening/closing coil, relay coil).

Check contact resistance (should be close to 0Ω).

Check insulation to ground (megohmmeter required, select appropriate range for rated voltage).

Current Method:

Method: Use a clamp meter (suitable for AC circuits) or a multimeter in current mode connected in series (extra caution required).

Application: Measure the balance of secondary current in a current transformer (CT) and check for open circuits; measure the operating current of signal relays, etc.

3. Segmented Isolation Method

For complex circuits, certain connecting pieces or terminals can be temporarily disconnected to divide the circuit into several segments for troubleshooting.

Common Technique: Temporary Short-Circuit Method (used to determine contact integrity). For example, if a limit switch is suspected of not closing, after power is off and safety is confirmed, short-circuit its two ends with a jumper wire, then power on and test. If the function is restored, it proves that the contact is faulty. (Note: This method is high-risk; the consequences of short-circuiting must be understood, and restoration should be performed as soon as possible.)

4. Component Replacement Method

When a relay, protection module, or instrument is suspected of being damaged, it can be replaced with a spare part of the same model with normal parameters for testing.

5. Simulation Test Method

Using a relay protection tester, standard current and voltage are injected into the protection device to simulate various fault types and verify the correctness of the protection device's logic function, settings, and output circuit. This is one of the most effective methods for verifying protection circuits.

 

Step 4: Fault Handling and Recovery

Handling: Based on the troubleshooting results, tighten wiring, replace damaged components (fuses, relays, coils, etc.), clean the circuit, and reset parameters.

Verification: After fault handling, a transmission test must be performed.

Protection Transmission: Use a testing instrument to apply pressure to verify correct protection operation and reliable circuit breaker tripping.

Operational Test: Perform opening and closing operations both locally and remotely to ensure the control circuit is normal.

Signal Verification: Ensure all signals are correctly uploaded to the monitoring system.

Recording and Reporting: Record the fault phenomenon, cause, handling process, and results in detail, and update relevant drawings and documents.

 

III. Common Secondary Circuit Fault Types and Targeted Inspections

 

Control Circuit Disconnection:

Key Inspection Points: Operating power circuit breaker/fuse, anti-pumping relay, circuit breaker auxiliary contacts (normally open, normally closed), opening and closing coils, circuit terminals.

Method: Measure point by point along the opening/closing circuit using the voltage method.

Protection Device Abnormality or Failure to Operate/False Operation:

Check Power Supply: Is the device's operating power supply normal?

Check Input Quantities: Use a clamp meter or tester to measure the secondary values ​​of the input TA and TV to verify their correctness.

Check Setting Values: Verify that the protection setting values ​​are correct.

Check Output Circuit: Check if the connecting pieces are engaged, the output relays and their contacts, and the wiring to the trip coil.

Simulation Test: Perform a comprehensive verification using a tester.

Signal Circuit Abnormality:

Check Common Terminal: Is the signal power supply normal?

Check Signal Relays/Contacts: Do they operate and reset normally?

Check Indicator Lights/Signal Lights: Are they damaged?

Check communication: For the transmitted signal, check the communication cable, port, and protocol settings.

Measurement circuit malfunction (inaccurate current or voltage):

Check the secondary side of the TA/TV: for open circuits (current) or short circuits (voltage).

Verify the turns ratio: check if the turns ratio set in the device matches the actual ratio.

Measure the input value: verify the accuracy of the analog input at the device's terminal block.

Check the sampling module: replace or calibrate if necessary.

 

Contact Us

 

For power systems, the reliability of switchgear determines the safety of the entire power grid. This reliability is based not only on its robust casing and advanced circuit breakers, but also on its precise and complex internal secondary control system—acting as the "nerve center" of the equipment, directing every precise action and safeguarding safe operation at every moment. At Shaanxi Huadian, we understand that a hasty troubleshooting might reveal a loose terminal block; an unexpected protection failure could stem from an unclear blueprint. Therefore, we infuse the pursuit of excellence in secondary circuits into every manufacturing and inspection stage. We provide full-cycle services, from technical consultation and blueprint refinement to professional training and lifetime spare parts support. Shaanxi Huadian—a trustworthy partner. Please contact us if needed.

Email:pannie@hdswitchgear.com.

Whatsapp/Wechat:+8618789455087