Series Resonance Withstand Voltage Test for 35kV Cables in a Prefabricated Substation (Box Transformer)

The series resonance withstand voltage test is a critical step in ensuring the insulation performance of high-voltage equipment such as 35kV cables used in prefabricated substations (commonly referred to as “box transformers”). This test verifies that the cables and associated components can withstand operating voltages as well as transient overvoltage conditions, such as lightning strikes or switching surges. This article outlines the testing process, key equipment involved, and proper handling of associated devices like current transformers (CTs), voltage transformers (PTs), and circuit breakers.

Test Objective

The primary purpose of the test is to evaluate the insulation integrity of the 35kV cable and its connections between the box transformer and other associated equipment (e.g., transmission towers or control boxes). The test ensures that:

1. The cable can endure high-voltage stress during actual operation.
2. Connections and accessories, such as cable joints and terminals, meet the required insulation standards.
3. Adjacent devices, such as CTs, PTs, and circuit breakers, are properly protected during the test.

Test Setup and Key Components

The setup involves the following major elements:

1. Series Resonance Test Device:
   • Generates the required high voltage by tuning the resonant frequency of the test circuit to match the system’s natural frequency.
   • Ensures efficient voltage amplification with minimal power consumption.
2. 35kV Cable:
   • The test object that connects the high-voltage side of the box transformer to transmission towers or other equipment.
3. Box Transformer:
   • The high-voltage winding (primary) is subjected to the test voltage.
   • The low-voltage winding (secondary) is grounded to provide a zero-potential reference point.
4. CTs and PTs:
   • CTs measure current on the cable, while PTs step down voltage for monitoring or protection.
   • These components require specific handling during the test to prevent damage.
5. Control Box:
   • Used for monitoring and controlling the test process, including voltage ramp-up and discharge procedures.

Handling Key Equipment During the Test

Proper handling of associated equipment ensures the test’s safety and accuracy:

1. High-Voltage Side (Primary Winding)
   • PT Disconnection: Voltage transformers (PTs) are disconnected to protect their windings from the high test voltage. PTs have a lower voltage tolerance and may fail if left in the circuit.
   • CTs Left in Circuit: Current transformers (CTs) on the high-voltage side can typically withstand the test voltage. However, their secondary windings must be short-circuited and grounded to avoid dangerous open-circuit conditions.
2. Low-Voltage Side (Secondary Winding)
   • CTs’ Secondary Winding Short-Circuiting:
     • The secondary terminals (e.g., 1S1 and 1S2) of CTs must be shorted and grounded. This prevents induced high voltage on the secondary side, which could damage the CT or pose safety risks.
   • Grounding the Low-Voltage Side:
     • The low-voltage winding of the transformer and the associated control circuits are grounded to create a stable reference point and ensure the safety of the test personnel.
3. Circuit Breaker Handling:
   • Disconnect Rectifier Modules:
     • If the circuit breaker includes rectifier modules for control circuits, their connections must be removed to protect sensitive components like diodes or thyristors from overvoltage damage.
   • Adjacent Equipment Grounding:
     • All adjacent devices, including breaker frames and metallic components, are grounded to eliminate potential hazards from induced voltages.
4. Grounding Adjacent Areas:
   • During the test, nearby structures (e.g., transmission towers or enclosures) must be grounded to prevent induced voltage accumulation and ensure a safe working environment.

The Role of Temporary Grounding

If the box transformer’s grounding system is incomplete or has high resistance (a common scenario during commissioning), temporary grounding measures are essential:

1. Temporary Ground Rods:
   • Ground rods are installed to provide an effective and low-resistance grounding path.
   • Proper grounding reduces the impact of induced voltages and ensures that any stray currents are safely discharged into the earth.
2. Grounding Verification:
   • The resistance of the grounding system should be verified using a ground resistance tester, with a target resistance value of less than 4Ω.

Safety Measures During the Test

1. Short-Circuit and Ground All Secondary Windings:
   • Prevents secondary windings from developing dangerous induced voltages during the test.
2. Disconnect Sensitive Equipment:
   • Devices with low voltage tolerance, such as PTs or rectifier modules in breakers, must be isolated from the test circuit.
3. Monitor Adjacent Devices:
   • Ensure that all adjacent equipment, including control boxes and auxiliary devices, are grounded or adequately insulated to prevent accidental damage.

The series resonance withstand voltage test for 35kV cables in box transformers is essential for verifying insulation reliability and ensuring safe operation. Proper handling of CTs, PTs, circuit breakers, and grounding systems is critical to achieving accurate test results and protecting sensitive equipment. By adhering to these procedures, engineers can ensure that the cable and transformer system meet operational and safety standards.