The CERTS microgrid test bed at AEP’s Dolan Technology Center includes a semiconductor static switch as a paralleling device. This equipment, along with a DSP controller, offers sub-cycle connection and disconnection between the microgrid and utility. However, similar equipment is not easily duplicated and has a high cost. Therefore, a mechanical switch alternative was produced that offers comparable performance and IEEE 1547 compliance at a reduced price.
The mechanical switch and its associated microprocessor relay used for the project were chosen from major manufacturers of utility equipment. An ABB SACE Emax UL low voltage AC circuit breaker was selected, along with a Schweitzer SEL-700GT intertie relay. Existing cabinet space and wiring was utilized to install this equipment in the test bed. However, the cost of a new installation is estimated $25,000, which compares to the estimated $150,000 cost for a semiconductor switch.
Prior to installation in the test bed, a series of IEEE 1547 compliant tests were run on the SEL-700GT. This was done using an Omicron 256 test set in a laboratory environment. Taking into account a 60ms maximum operating time of the mechanical switch, the tests demonstrated the relay’s ability to operate within the specifications of IEEE 1547. With the mechanical switch and SEL-700GT relay installed in the test bed, another series of tests were run. These tests included synchronized closing, loss of utility anti-islanding, three phase and single phase reverse power, and dead bus close.
In order to improve power quality during testing, the relay’s voltage protection elements were made more stringent. This was done as an attempt to improve the mechanical switch’s ability to operate on abnormal voltage conditions and allow the microgrid generator to remain online. As an alternative to adjusting these settings, additional voltage elements can be added to the relay’s logic that focus on power quality tripping and could ignore the IEEE 1547 reconnection timer.