Temporary overvoltages can be dangerous for high-voltage DC cables. The new TOV type test can be used to find out whether cables can withstand this stress. HIGHVOLT supplies the appropriate test components for this purpose.
When we draw electricity from the socket at home, it is electricity that changes polarity – in other words, alternating current,” says Dr. Ralf Pietsch, Head of Science and Documentation at HIGHVOLT. Over long stretches of land and sea, however, the electricity flows as direct current and is only converted to alternating current by converter stations near population centers. Pietsch explains, “During these switching processes, however, faults can cause the voltage on the DC cables to double from 525 kilovolts to more than one megavolt for a few microseconds to milliseconds, or to even change polarity.”
This so-called temporary overvoltage (TOV for short) is pure stress for DC cables and can damage their insulation. It can also cause space-charge phenomena inside the cable, which can lead to local field surges in the cable when the polarity of the voltage changes, thereby damaging the cable. If the cable fails, nearby electrical equipment or even people are then at risk.
Manufacturer want stress
To ensure that nothing dangerous happens in the event of an overvoltage or total failure of the cable, more and more cable manufacturers are testing their DC cables with the new TOV type test. “This test does not yet have IEC standards but is already described and recommended in the CIGRE brochure as an optional test method,” Pietsch tells us. The test is carried out at the end of a large test series as the ultimate stress test. For about 100 milliseconds, a megavolt is applied to the cable in a controlled manner. Only when the cable has survived this can it be delivered to the network operator.

A number of components are required for the stress test: a capacitor tower, two DC voltage sources, two sphere gaps, a resistor as well as a voltage divider and the end termination to which the DC cable is connected.
In this 16-meter-high capacitor tower, the voltage rises to over one megavolt before it reaches the test cable via sphere gap.
However, TOV testing is very complex. After all, the test setup fills the area and height of an entire test hall. Ralf Pietsch adds: “In addition, the individual test components must also withstand the overvoltage – and not just once like the cable to be tested, but with each subsequent test. This requires special materials, bushings and insulation.”
The test itself consists of three sub-tests to check all possible types of temporary overvoltage in circuits. In all cases, a DC high voltage of 525 kilovolts is applied via a DC voltage source to an end termination that is connected to the test cable. In addition, a voltage divider is also connected to the end termination, via which the temporary overvoltage can be measured. A second DC voltage source is used in parallel to charge a 16-meter-high capacitor tower (a modified pulse voltage generator) to over one megavolt.
Connected to this is a sphere gap with two opposing spheres made of a tungsten-copper alloy. “This particular alloy is important because on the spheres, the accumulated overvoltage of the capacitor turns into an extremely hot arc, and normal copper would simply melt in the process,” explains Pietsch. “The overvoltage then passes through an integrated resistor to the end termination and ultimately into the cable.”
This is how the TOV test is structured

In the first part of the test, the overvoltage is slowly dissipated via the ground. In the second part of the test, however, the voltage drops abruptly. For this purpose, another sphere gap is connected between the 525-kilovolt DC source and the earth, which abruptly removes the overvoltage from the cable. In the last sub-test, the overvoltage also changes polarity at a high frequency.
“We are seeing more and more network operators requesting TOV testing.”
Dr. Ralf Pietsch, Head of Science and Documentation at HIGHVOLT
Know-how and components
“Since the TOV test is relatively new, it is not yet mandatory for cable manufacturers,” says Ralf Pietsch. “But we see that more and more network operators are demanding this stress test.” HIGHVOLT has been involved here from the very beginning and has already been able to supply components or the complete test setup for cable manufacturers such as Prysmian, Nexans or independent test institutes such as FGH Mannheim
“With these orders, we have been able to gain quite a bit of experience in recent years in order to improve and standardize the testing even further,” Pietsch continues. “Furthermore, I am sure that TOV type testing will become mandatory in the near future. After all, grid operators want to play it safe with cables before they lay them in the ground.” In the context of the energy transition and the expansion of the German power grid with underground cables, it can therefore be assumed that the demand for TOV testing will increase significantly. It’s good that HIGHVOLT already has the expertise and all the components to provide such testing.

YOUR CONTACT
Do you have questions about the TOV-test?
Ralf Pietsch is there for you:
R.Pietsch@highvolt.com