Application Instructions for 50kA Current-Limiting Devices on Offshore Oil Platforms

1. Operation of the System With and Without CLiP (Current-Limiting Device)

Under normal operating conditions, the switchboard operates as follows:

• All bus tie breakers are closed, connecting the three bus sections in parallel;
• Two generators are online and supplying power to the switchboard.

Under this configuration, the prospective fault current at the switchboard is less than 50kA. Therefore, the current-limiting device (CLiP) is not inserted into the circuit.

During maintenance operations involving the opening of one generator (taking it offline) and closing of another (synchronizing and connecting), the system operates as follows:

• All bus tie breakers remain closed, keeping the three bus sections interconnected;
• Three generators are temporarily connected to the system (for a limited duration during the generator changeover).

Under this condition, the system short-circuit capacity increases, and the prospective fault current exceeds 50kA. Since the switchboard’s short-circuit withstand rating is 50kA, the current-limiting device must be inserted into the circuit to ensure equipment safety.

The CLiP monitors the rate of current rise over time. When the current exceeds a preset threshold, the device activates and interrupts the bus connection by melting an internal fuse element. This limits the actual fault current to below 50kA, ensuring it remains within the safe design limits of the switchboard.

This process enables fault isolation without causing a blackout of the entire eHouse power distribution system.

Summary:

• When the prospective fault current > 50kA (all bus ties closed and three generators online), the CLiP must be in the circuit. This condition occurs only during the transitional phase of single-generator maintenance.
• When the prospective fault current < 50kA (only two generators online or two of the three bus ties open), the CLiP should be disconnected from the circuit.

2. Operation and Maintenance Requirements

The facility owner shall approve the proposed alternative operating arrangements. Decisions should also be based on additional data related to the current-limiting fuse, including maintenance requirements, estimated service life, and the capability of personnel performing equipment maintenance. These actions shall be incorporated into the operation and maintenance manual.

3. Design and Testing of the Current-Limiting Fuse

The current-limiting fuse shall be designed and tested in accordance with recognized standards such as IEC 60282-1:2009/2014 and IEEE C37.41 series, and shall be suitable for the intended application and environmental/operational conditions. Only a single current-limiting fuse shall be used; any combination of current-limiting devices requires special consideration and evaluation.

The CLiP has obtained KEMA type test reports covering breaking capacity, temperature rise, and insulation tests, along with calibration records for measuring equipment. Testing has been performed in compliance with IEC 60282 and ANSI/IEEE C37.40 series standards.

4. Insulation Level of the Fuse Holder

• The fuse holder has a rated impulse withstand voltage of 110kV BIL;
• The isolation transformer has passed a 150kV BIL test and can be used in 27kV-class systems;
• Each isolation transformer undergoes a 50kV AC dielectric test during production.

5. Verification of Fuse Suitability for Operating Temperature

The current-limiting fuse has been manufactured and tested in accordance with IEC 60282-1 or IEEE C37.41 series standards.

IEC 60282-1 specifies a maximum ambient temperature of 40°C, while the classification society standard SVR 4-1-1, Table 8, requires 45°C. Evidence compliant with Appendix E of IEC 60282-1 (or equivalent standards) must be provided to demonstrate that the fuse is suitable for the maximum expected ambient temperature of 45°C.

Testing covers the requirements of IEC 60282-1 and ANSI/IEEE C37.41. The Series II interruption test is more stringent than IEC requirements, as it demands 100% test voltage (IEC allows 87%). G&W tests Series I duties at 100% voltage and 100% current—exceeding all standard requirements. The actual project uses a 4000A rated device.

For a 5000A switchgear without forced cooling, the temperature rise margin is 5K at 40°C ambient, enabling it to carry 5000A at 40°C and 4000A at 50°C ambient.

6. Time-Current Characteristics and Current-Limiting Performance

This type of device does not have a conventional time-current curve (TCC). Its operation is completed within 0.01 seconds—well before the starting point of typical TCC curves—effectively making it an instantaneous device.

In practice, each application is evaluated on a case-by-case basis, using worst-case scenarios (fully asymmetrical faults). System currents are plotted with appropriate time resolution to clearly illustrate all interactions. This approach is superior to the potentially misleading use of peak let-through current curves.

7. Peak Overvoltage and Power Dissipation During High Fault Current Operation

• According to IEC and ANSI/IEEE requirements for 15.5kV-rated equipment, the peak voltage during operation (maximum measured 47.1kV) remains within the 49kV range, and does not involve the release of large amounts of heat or steam associated with expulsion-type interruption.

• The CLiP's heat dissipation structure is essentially a busbar with machined current-limiting sections.

• The total heat dissipation of a three-phase CLiP system at 4000A is approximately 500W.

8. Short-Circuit Study and Validation of Cascading Protection

The short-circuit study shall demonstrate the function of the current-limiting device and how it reduces the symmetrical fault current below the switchboard’s rated withstand level. If the proposed arrangement is intended to operate as "cascaded protection," compliance with the conditions specified in classification society standard SVR 4-8-2 / 9.3.6 must be verified. The selection of the triggering point and determination of let-through current in each direction shall be clearly defined.

9. Calculation of Busbar Withstand Capability for Maximum Short-Circuit Current

Calculations shall be performed in accordance with IEC standards to verify the busbar’s ability to withstand the mechanical and thermal effects caused by the maximum prospective short-circuit current.