Wednesday Writeup: Primary Current Injection Testing
Many conversations the team at OneStep Power have with people interested in our technology include other technologies needed to verify remote power systems. One of the testing methods we advocate heavily is Primary Injection testing.
For electrical engineers, the Electrical Engineering Portal has a great and in-depth article describing the outcomes, benefits and methods for primary injection testing. For people who just need the basics, the following will help understand the importance of this test method.
Primary Injection Basics
In high-reliability systems, such as dynamically positioned vessels, trains, and power sub-stations, the order in which protection devices operate is critical to the delivery of safe and reliable power: if the protections operate incorrectly a fault can be propagated, causing the outcome to escalate rapidly. Full system blackouts, significant equipment damage, fire, and explosions may result from incorrect protection systems.
Primary current injection testing (or "primary injection") is the protocol of inducing a programmed sequence of overload and fault magnitude currents to flow in a distribution system and measuring the periods of time for the trip devices to activate - and the order they open in.
On-site commissioning and maintenance tests are performed using a primary current injection test set that is specifically designed to be lighter in weight and more portable than factory test equipment. The test set has a built-in high-current transformer that supplies a low-voltage overload or fault current.
Testing consists of a pre-planned series of combinations to ensure the ability to identify the fault and to confirm the directional sensing of the current transformers.
Primary Injection Applications
The correct functioning of the trip devices of low-voltage power circuit breakers can be tested using primary current injection. However, because primary injection is a relatively time-consuming service, it is usually performed only on circuit breakers that are components of a critical process or engineered safety system.
“Primary injection testing is, however, the only way to prove correct installation and operation of the whole of a protection scheme." Electrical Engineering Portal
Circuit breakers that have thermal-magnetic or electro-pneumatic trip devices are more likely to be tested using primary current injection because it is the only means available for verifying their correct functioning.
Circuit breakers that have solid-state trip systems can be tested using secondary injection, which is less expensive and faster.
Since secondary current injection test cannot verify the correct functioning of the current sensors of a solid-state trip system, primary current injection is used during commissioning (start-up) to supplement a program of periodic secondary-current testing.
Primary injection tests are always carried out after secondary injection tests, to ensure that problems are limited to the VTs and CTs involved, plus associated wiring, all other equipment in the protection scheme having been proven satisfactory from the secondary injection tests.” Electrical Engineering Portal
A complete description of the methods and interpretation of field testing of molded-case circuit breakers (MCCBs) can be found in NEMA standard AB-4-2003.
Without primary injection the level of assurance that can be gained on a remote power system's protection systems is limited. Directional current sensing, which is vital to modern protection systems, can easily be installed incorrectly and will only be identified through primary injection testing - or when a fault occurs.
Primary injection testing is critical to safe operations in closed bus and battery-hybrid systems offshore.
Note: This information is provided only as a guide and detailed research should be completed prior to decisions about the application of primary injection testing to specific systems. OneStep Power can assist with supporting a robust power system testing regime, including the provision of primary injection testing support.