When things go wrong: A new solution

Authors:
Ulf Gustke - Product portfolio manager Megger - Hagenuk KMT GmbH, Germany
Lothar Koopmann - Managing director CEO Elektro Koopmann GmbH Germany
Oliver Nicolai - Sales director Germany Megger - Seba Dynatronic GmbH, Germany
Sacha Markalous - General manager Megger - Hagenuk KMT GmbH, Germany

In the first part of this article, which was published in the March 2017 edition of Electrical Tester, we looked at the requirements for new cable test technology to address the challenges of testing today’s ever-longer high- and medium-voltage cables efficiently and safely. In this second part of the article, we look at the practical solution developed by Megger, working in conjunction with utility company TenneT and testing service provider Elektro Koopman GmbH. 

The solution – functional design

To ensure that the novel solution devised for testing modern cables would be capable of providing the most reliable results quickly and in the widest possible range of applications, it was decided after careful consideration, to make provision for the 14 operating modes shown in this diagram:

The wide range of operating modes was decided on to give the operator access to a diversity of test approaches, allowing her or him to easily select the mode best suited to the application in hand.

The solution – structural design

The decision was taken to install all of the test equipment in a standard shipping container. The mechanical construction of the equipment and its arrangement within the container were based on Megger’s wide experience of designing and constructing cable test vans, and of developing special-purpose CFL equipment.

From a technology perspective, the key elements of the solution are: 

• HV DC transformer for permanent charging of the DUT which allows, for example, high power burning up to 40 kV with 20 kVA active power
• Various surge generators capable of providing surge energy up to 6250 J
• High power discharge unit, capable of discharging 75 µF of cable capacitance charged to 150 kV within 3 seconds
• Built-in state-of-the-art TDR technology
• System architecture that allows almost all functions to be centrally controlled from the operator’s room

Concept becomes reality

The transition from concept to reality went exactly as planned, and the finished container is shown in the picture below.

The control “cockpit”

The operator’s room provides comprehensive facilities for the control and monitoring of all test procedures.

The cables and accessories room

The cables and accessories room contains all of the equipment needed to connect the container safely to the DUT.

Type and acceptance tests – the high power discharge unit

To confirm the safety margin of the vital high power discharge unit, tests were carried out by a certified external testing body in Germany.

The photograph shows type testing in progress for a key part of the high power discharge unit. This test was successful and confirmed that the capability of the discharge unit was as specified.

Acceptance testing of the complete test container

An important requirement for the HV DC power burner is that it should be able to charge a cable and ignite an arc at the fault location. This is necessary to remove moisture from the insulating material interface areas and the cable system, and/or to fracture the insulating material around the arc channel. This produces a conductive path in the insulation, which changes the impedance of the cable section. This change of impedance is visible in a TDR trace. However, the burning process also reduces the withstand voltage of the cable at the fault location.   

To demonstrate this behaviour, an artificial test setup was created. A cable section with a hole drilled in it was installed in a water basin. The picture below shows the result of using the high power capabilities of the test container to produce an arc in water.  

(Photo shows a powerful standing arc underwater)

Using the container in the field

To provide maximum flexibility and convenience in onshore applications, the container can be mounted on a trailer, as can be seen in the picture. This shows the container/trailer combination in use in one of its first ‘real life’ applications, in December 2016.

The container-based solution is equally convenient for wind farm and offshore platform applications.

Conclusions

The container-based system for maintenance and testing of high-voltage cable feeders and long medium-voltage feeds is the result of close collaboration between TenneT, a leading European electricity distribution operator, Koopmann, a highly respected and very experienced maintenance service provider, and Megger, one the world’s most innovative developers and manufacturers of cable test equipment.

The system is now ready for use in onshore and offshore applications, and is on 24/7 standby. Its principal applications relate to high-voltage cable feeders in DC and AC network cable installations. This novel container-based solution is an important step toward guaranteeing the dependability of new energy generation technologies and it is, therefore, a major contribution to the future security of energy generation.