Ground-fault overvoltage on the subtransmission system was identified as a potential issue, because the substation transformers are delta connected on the 46-kV side (Fig. 6), so DG feeds into the 46-kV system as an ungrounded source with respect to that system. EMTP-RV simulations show that the overvoltage could fail surge arresters on the subtransmission system under certain conditions: a ground fault, light loading and the DG still feeding into the system after separation of the 46-kV line from the utility system “grounding source” transformer. Under these conditions, a ground fault of even just a few cycles' duration could damage surge arresters.

Simulations showed that loading on the subtransmission system of at least 2.5 times the rating of the DG machines was needed to adequately suppress the overvoltage. All but one of the DG sites had either sufficient loading or an inseparable grounding bank transformer nearby that mitigated the overvoltage. At the site where that was an issue, load was transferred onto the subtransmission circuit to satisfy the minimum loading requirement.

The DG plants are proving very successful. Already, the first set of three DG plants, commissioned in late 2005, has successfully provided system support for two peak load seasons. The existing plants have collectively logged well over 1000 hours of system support operation.

From a control and protection standpoint, the performance of the units has been as predicted and without major problems. Only minor issues have occurred such as the need to fine-tune the LTC setting at one site. Another minor issue is that there have been rare occasions when, due to power system disturbances, the DG have tripped off-line. However, analysis finds the trips were warranted and consistent with relay settings. This shows that, while DG can be successfully applied for system support, because it is located at the distribution system level, it is exposed to a higher rate of disturbance events that may lower its dispatch reliability.

From the perspective of integrating the DG equipment to the power system, this project illustrates the complexity of connecting large generators to distribution systems. For the purposes of safety, reliability and power quality, several power system changes and safeguards beyond the typical smaller DG system requirements were implemented. In addition, these systems had an impact on the subtransmission system, which is an often-overlooked area when it comes to DG integration. The major changes/upgrades needed to make the systems work included:

• DTTs, which were added between the distribution feeder circuit breaker and DG plants to mitigate the danger of distribution feeder islands. DTTs were also implemented on 46-kV line circuit breakers to the DG where appropriate.

• Voltage check relays to block reclosing into live islands. Such relays were employed at the 46-kV and distribution feeder breaker locations at several sites.

• Adjustments to automatic reclosing dead-time duration on the 46-kV system. These were made to coordinate with DG anti-islanding protection.

• Adjustment to LTC control settings at the substation transformers: elimination of line-drop compensation, higher set voltage and reverse-power detection disabled.

• Load being added to a potential 46-kV island area at one of the six sites to help suppress ground-fault overvoltages on that island to avoid damaging surge arresters.

• A large grounding reactor was employed at the DG to help mitigate generator neutral currents (both steady-state and fault-related currents).

• Slow ramping of the generator power during starts and stops. This was recommended to minimize feeder voltage excursions and the generators operate in fixed power-factor mode to avoid fighting the LTC.

Brian Bui is a lead protection engineer for Hawaiian Electric Co., where he has 19 years of experience working in the system protection area. He received his BSEE and MSEE degrees from the University of Hawaii (Honolulu, Hawaii, U.S.) in 1984 and 1987, respectively. brian.bui@heco.com

Alan Hirayama is a senior technical services engineer for Hawaiian Electric Co., where he has worked for 17 years. He worked as a distribution planning engineer for 16 years and is currently in the Energy Projects department working on various distributed generation projects. He received his BSEE degree from Santa Clara University (Santa Clara, California, U.S.) in 1987. alan.hirayama@heco.com

Phil Barker is the founder of Nova Energy Specialists, LLC, a Schenectady, New York, U.S.-based consulting firm providing analytical services regarding power systems, distributed generation and related energy technologies. He received his BSEE and MSEE degrees from Clarkson University (Potsdam, New York). He has authored more than 30 technical papers and articles, and is a senior member of the Institute of Electrical and Electronics Engineers. pbarker@novaenergyspecialists.com