Document information
Tags/Project: Test-Procedures-wiki
| Metadata Field | Document Information |
|---|---|
| Status | Under Review |
| Created | 2026-06-23 |
| Last Updated | 2026-10-01 |
| Author | Davide Cuneo |
| Rev. # | 1.3 |
| Reviewers | Davide |
Introduction and Context
This procedure describes the operative steps necessary to perform an High Voltage (HV) test for superconducting magnets. As an example, the MCBRD corrector magnets at the end of the training testing campaign is examined. In this case the magnet was still cold in the cryostat but the same procedure, with different tresholds for the HV test, must be applied at room temperature. Clearly, the operating steps will have to be adapted to the magnet under test. It is possible that single aperture magnet will be tested, therefore some of the steps described below will have to be skipped or interpreted. The test is explicitly required by the magnet’s test plan and is meant to confirm the retention of the insulation while the magnet is still installed in the cryostat.
This specific test described below was carried out by Davide Cuneo together with Jean Luc Guyon at SM18, CERN. The measured insulation was 22 GOhm, a value indicating very good insulation quality.
Few words about the magnet and test
The MCBRD is an orbit corrector magnet made up of two apertures (APs), AP1 and AP2, built in a Canted Cosine Theta (CCT) configuration (NbTi, two layers, maximum current up to 400 A). On the magnet insert, the four copper leads are identified as follows: A and B correspond to AP1, while C and D correspond to AP2. This naming convention should be kept in mind throughout the test, since every connection described below is given with reference to these labels.
The test is made up of two separate runs, one for each aperture:
- AP1 vs ground, with AP2 grounded
- AP2 vs ground, with AP1 grounded
Required Material
To carry out the test the following instrumentation and items are needed:
- Megger, i.e. the instrument able to generate high voltage and measure the leakage current used to verify the insulation;
- Dedicated software (Power DB Lite) for driving the Megger, which is used both to set the test parameters and to save the collected data;
- PC workstation on which to install and run this software;
- Megger’s terminal cables fitted with crocodile connectors, for quick connection to the leads;
Test preparation
The first phase of the procedure consists in safely disconnecting all the wiring linking the sensors to the acquisition system. The operator must first disconnect all the connectors of the pressure and temperature sensors located on top of the insert flange. Once this is done, the VT connectors coming from the insert tower and going to the magnet are disconnected.
To ensure the highest level of safety in the following phases, in particular at the moment of touching the copper leads, it is essential to press the emergency button (if present) of the system, so as to rule out any possibility of accidentally powering the magnet. This step should be regarded as an indispensable precaution, never to be skipped. It is important to operate with no power in the magnet. It is also important to disconnect the power to the heaters, if present.
Only at this point can the power cables be disconnected from the copper leads. The cables are then set down on the cable rack, which is properly grounded.
Take note of ambient temperature, humidity and magnet’s temperature.
HV Test Configuration
The test is carried out using the Megger, an instrument able to generate high voltages and to check insulation quality by measuring the leakage current between the two environments under test. The Megger is a compact unit, ready for use as soon as the two terminal cables are connected to its terminals.
For the first run, concerning the verification of AP1’s insulation to ground while AP2 stays grounded, the cables coming from the Megger are connected using crocodile connectors, which are quick and easy to install. The positive lead (+) can be connected indifferently to lead A or to lead B, or even to both at the same time, since the result does not change. The negative lead (–) is instead connected to the cryostat which, being itself grounded, acts as the ground reference for the magnet.
It is important to remember that, as specified by the test plan, during this first run AP2 must remain connected to ground for the entire duration of the test.
Megger Parameters & Test Execution
Before starting the test, the voltage level and the number of steps for the voltage ramp must be set on the Megger. In the reference procedure, 5 steps up to a maximum of 1.6 kV were used, with each step held for 1 minute. Such a high voltage value is justified by the fact that the magnet is still at cryogenic temperature; if the test is instead performed at room temperature, the reference voltage drops considerably, down to around 300 V.
During execution, if a leakage current is detected between the negative (cryostat, ground) and the positive (AP1) that goes beyond a threshold on the order of microamps (µA), the test is considered not passed and the Megger stops the measurement automatically. This is the test’s acceptance criterion: no leakage current threshold exceedance throughout the whole ramp.
To drive the instrument and save the collected data, the dedicated software, installed on the PC workstation connected to the Megger, must be used. The complete test setup is shown in Figure 1.
Repeating the Test
Once the first run is complete, the same procedure is repeated identically for the other aperture, naturally swapping the configuration: the Megger’s positive lead is now connected to lead C or D (AP2), while AP1 must be connected to ground. All other parameters (voltage levels, number of steps, duration of each step, acceptance criterion) remain unchanged.
After the test
Once both runs of the test have been completed and the results have been checked against the acceptance criterion, the setup can be restored to its normal configuration. The Megger is disconnected first, together with the banana cable used to connect it to the leads. All the cables previously removed from the insert are then reconnected in their original position, restoring the full instrumentation and power circuit. The cryogenic valves, which had been frozen for the duration of the test, are then un-frozen, allowing normal cryogenic operation to resume. At this point, the heaters and any power supplies that had been disconnected beforehand are reconnected as well.
Operational Checklist
To be filled in during test execution, in order:
- Disconnected all pressure and temperature sensor connectors on the insert flange
- Disconnected the VT connectors coming from the insert tower
- Pressed the system’s emergency button before touching the copper leads and confirm the absence of current in the magnet
- Disconnected the power cables and set them down on the grounded cable rack’
- Take note of ambient temperature, humidity and magnet’s temperature.
- Connected the Megger’s positive lead (+) to lead A or B (AP1 test), with AP2 grounded
- Connected the Megger’s negative lead (–) to the cryostat (ground)
- Set the Megger according to the test plan procedure
- Start the test via the dedicated software
- Verified no leakage current threshold exceedance (µA) throughout the ramp
- Saved the test data via software
- Repeated the whole procedure for AP2 (lead C or D, with AP1 grounded)
- Saved the data of the second run and compared the result against the acceptance criterion
After the test
- Disconnect the Megger and remove the banana cable
- Reconnect all the cables on the insert
- Un-freeze the cryogenic valves
- Reconnect the heaters and previously disconnected, power supplies
Feedback
Found an error or something outdated in this note? Please contact Davide Cuneo (davide.cuneo@physics.uu.se).