Tests short-circuits at hundreds of kVolts using PXI

Author(s):
Aart Jurriens - KEMA Nederland B.V

Industry:
Energy/Power

Products:
PXI-6133, Embedded, PXI-5122, PXI-1042Q

The Challenge:
Upgrade an existing test system for use in safety and certification tests, for equipment to be used in the distribution and transmission network of utilities all over the world. These devices are tested for faults under high power conditions, up to 8000 MVA 3 phase 50 Hz (1060 kV peak at 63 kA).

The Solution:
Using four PXI-6133 boards and two PXI-5122 high-speed digitizers, that are synchronized, we developed a transient recording system that measures simultaneously 3 phase AC currents and voltages on multiple locations during making and breaking high-power circuits.

Tests at the KEMA High Power Lab

At KEMA we have multiple High-Power and High-Voltage laboratories to conduct medium and high voltage (from 10 kV up to 1.1 MV) testing and certification. For manufacturers from all over the world KEMA is an independent organization that certifies and affiliates test reports that are globally accepted. The components that are tested at KEMAs High-Power Lab are for example high voltage circuit-breakers, switches, transformers, fuses, and various other power transmission and distribution components. These components are tested at high voltage and high currents (up to 360 kA-0.5 second).

High-power testing is necessary to test objects to real-life fault conditions. For example circuit breakers are tested to determine if they are capable to interrupt a short-circuit current.

Energy network topology

In most countries in Europe there is a main 380 kV national power grid. This grid connects all the power plants together, and is coupled with the national power grid of neighboring countries. Provincial distribution centers transform this down to 150 kV, and in urban distribution centers down to 50 kV. Further on this is transformed to 10 kV to power the local nets. In the streets transformer cabinets are used to supply houses with 230 Volt mains power and 380 Volt power current.

All these values can be different by country, just as the certification requirements are. For example a 380 kV circuit breaker is tested for a short circuit in the Netherlands at 63 kA, but in Germany it is tested at 80 kA, since the distance between power plants  in certain areas is smaller. At KEMA we have our own generators to provide exactly the current, voltage and frequency that is needed for each of these certification tests.

The high-power lab is capable of performing controlled short-circuit three-phase tests at power levels of up to 8000 MVA (largest direct power in the world). Through synthetic testing we are able to test objects under real life condition with (ultra) high voltage (1000 kV) and high current (1000 kA peak and 360 kA for 0.5 second) during breaking and making a contact. These expensive tests are commonly part of the certification of energy power generation, power transformation and transportation equipment.

Examples of common tests:

-        Make test: the goal of this test is to show that a switch or circuit breaker is capable of making (switching on) a short-circuit current (from a faulted circuit). When the contacts are open there is a high voltage between the contacts, and after making, when the contacts are closed, there is a high asymmetrical current, later this current will settle at a symmetrical value. The behavior of the switch has to be recorded very accurately for certification of the object.

-        Circuit breaker test: Circuit breakers should be able to interrupt a short circuit current in a high voltage environment. After breaking in real world a high transient recovery voltage will occur, while at the same time the current stops (called “current zero”). In our test lab we can provide the same environment to the circuit breaker (for very high short circuit ratio), using synthetic testing, a method through which we couple in pre-charged capacitors and ignite them just before current zero. At current zero the transient behavior of the voltage has to be recorded accurately. The figure shows a typical circuit breaker test.

Transient Measurement System

Our test system contains four PXI-6133 boards and two PXI-5122 boards. All these boards are triggered simultaneously and are synchronized using the PXI backplane and it’s triggering and synchronization features.

The PXI-5122 14-bit 100 MS/s 2 channel digitizers are used specifically to record the transient recovery voltage behavior, when we break a high voltage circuit. Frequencies can go up to 500 kHz, and we want to record this behavior very accurately.

The four S-series PXI-6133 data-acquisition boards each contain 8 analog inputs that are sampled simultaneously at 14-bits. So in our test setup we have 32 simultaneous channels available. The PXI‑6133 is capable of recording at a rate of 2.5 MS/s on each channel, though because of PCI-bus bandwidth limitations we cannot run all 32-channels at full speed at the same time. In our test setup we sample at 10 µs (100 kS/s for LF signals). STL (Short Circuit Testing Liaison) requires sampling at 100 µs, so we are 10 times above the specification. During a typical test we acquire around 250 MB of data. After acquisition this data is decimated to a lower sample rate. Tests usually take between 100 ms and 3 seconds.

The currents and voltages at all 3 phases for supply and load side of the unit under test are recorded. We have developed custom voltage/current signal conditioning devices to isolate and match to the input levels of the data-acquisition devices. Shielding is another important factor in these tests, since during a test the ground potential in a very well grounded area can still be in the magnitude of kVolts.

Besides measuring voltages and currents, we can also attach other sensors, such as pressure sensors, and encoders to measure the position of a switch/circuit breaker (travel recorder).

The PXI system is embedded in a system cabinet. On the front there is a universal panel through which the test operator can connect all channels that need to be recorded. Inside the cabinet there is fixed cabling to the PXI system. The PXI system is not directly attached to a user interface. All interaction to the recording system is through TCP/IP commands, to setup the system, trigger and start an acquisition. Test data is first stored on local hard drive, and connected to an Oracle database, on TCP/IP request. To eliminate grounding problems we also used fiber-optic network cabling

Conclusion

Our previous system was limited in acquisition rate to 1MS/s and was only able to measure for a short period (0.5 seconds with 1 µs sampling time). With our new system we can acquire at a higher rate, we have less external connections and we have a lower system price. Another benefit is the modularity of the system and the improvement of testing speed.

For more information, contact:
Aart Jurriens
KEMA TDT/HPL
KEMA Nederland B.V.
P.O. Box 9035,
6800 ET Arnhem, The Netherlands

Tel: +31 26 356 2419
Fax: +31 26 351 1468
E-mail: hpl@kema.com – attn. Aart Jurriens
Web: www.kema.com

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