27-06-2012, 05:01 PM
DISTRIBUTION POWER GENERATOR ISLANDING GUIDELINES
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INTRODUCTION
Purpose
The purpose of this report is to outline how BC Hydro will consider the technical attributes of power generator (PG) planned islanding in the integrated distribution system at 35 kV and below. The commercial terms for PG planned islanding are excluded and will be developed separately.
This report focuses on new PG interconnections, but could be applied to the retrofit of an existing PG for planned islanding capability.
The current electric utility practice for PGs on the interconnected distribution system is to de-energize the PG generators when the utility transmission or distribution circuit is out of service. This is to ensure customers’ power quality, field crew safety and to avoid operation and protection complexities.
PG Islanding
BC Hydro has an objective to improve distribution customer service reliability in locations where the reliability is below customer expectations. Customer service reliability may improve if a PG can island with part or all BC Hydro customer loads on the interconnected feeder and/or other feeders from the same distribution substation. Reliability improvements are more likely when the transmission supply to the substation is not secure (single radial supply) and/or the PG interconnecting feeder is long and subjected to trees, storms, motor vehicle accidents, etc. The BC Hydro distribution feeders of interest operate at 12.5 kV and 25 kV.
Benefits may accrue to a PG built for planned islanding, such as selling electricity to BC Hydro during certain outages.
Islanding is defined for this report as the condition when a portion of the BC Hydro system is energized by one or more PG facilities and that portion of the system is separated electrically from the rest of the BC Hydro system. PG islanding may be inadvertent or intentional.
Planned islanding capability is the ability of a PG to continue to supply or electrically energize in a safe, controlled and reliable manner, part of the distribution system, including customer loads, that is separated from the rest of transmission or distribution system.
The discussion below is limited to an island consisting of either a whole or partial distribution feeder. The island does not include any part of the distribution substation or the transmission system. However, the BC Transmission Corporation (BCTC) would consider islanding of a distribution substation low voltage bus for a specific PG project, if requested by BC Hydro.
Current Islanding Practice
Nearly all the BC Hydro distribution feeders are radial, resulting in a power outage for all the load customers connected to the feeder during a feeder outage. A PG without planned islanding capability does not provide increased customer service reliability since the PG must be de-energized during the outage.
The PG interconnection may reduce the reliability of the feeder. For example, auto-reclose of the substation feeder CB and main line recloser may be disabled, and one or more of the existing protection devices in the feeder may be removed to support protection co-ordination between the PG entrance protection and the BC Hydro upstream protection device. Feeder outage restoration time may increase when line crews travel to PG sites to lock open and tag the PG entrance switch or service cut-outs under BC Hydro Safety Practice Regulations. 3
Distribution Power Generator Islanding Guidelines
Customer reliability is an important consideration for customers and BC Hydro. During a power outage, the utility loses customer satisfaction while customers lose business and/or convenience. Different customers have different concerns and different costs due to the outage. The reduction of power outage frequency and outage duration is desirable for society as a whole. If, during a power outage, the PG with planned islanding capability is allowed to form an island with some or all of the customer loads on the BC Hydro feeder and continue to supply the loads, the reliability to those loads could improve.
PG Islanding Issues
To enable PGs to operate in a load island, numerous issues need to be considered and resolved. These include:
• Islanding scenario: inadvertent islanding or planned islanding,
• System reliability: reliability improved or decreased or remains the same,
• Power quality: potential power quality problems,
• Additional BC Hydro/PG equipment and associated costs for planned islanding,
• Additional operation and safety requirements at BC Hydro Control Centre, safety concerns during restoration and possibly grounding and/or protection issues,
• Economic and commercial considerations.
RELIABILITY
Introduction
This section defines the most used distribution reliability indices, and relate these to BC Hydro practice with Customer Based Reliability (CBR).
Reliability Indices
Reliability statistics, based on long-duration interruptions, are the “yardstick” used by most power utilities and regulators to quantify the security of power supply in accordance with national standards. Faults on the distribution system cause most long-duration interruptions, and a fuse, breaker, recloser or sectionalizer locks-out the faulted section.