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Solar Utility Interconnection

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Presentation on theme: "Solar Utility Interconnection"— Presentation transcript:

1 Solar Utility Interconnection
Photovoltaic Systems Solar Utility Interconnection Distributed Generation • Generators • Inverters • Interconnection Codes and Standards • Interconnection Concerns • Point of Connection • Metering • Utility Interconnection Policies • Public Utilities Regulatory Policy Act (PURPA) • Interconnection Agreements Arizona Solar Power Society

2 With distributed generation, utility customers are served by both the centralized power plants and the power exported from interconnected distributed generators. Distributed generation is a system in which many smaller power-generating systems create electrical power near the point of consumption. Operating these systems in parallel with the utility’s distribution grid makes these systems interactive. See Figure 12-1.

3 Most power is generated by rotating generators, which require precise interconnection procedures to avoid damage to the equipment and the utility. Current is induced in a conductor as it moves through a magnetic field. If the movement is from rotation, the current varies sinusoidally, producing AC power. If a set of coils rotates through a stationary magnetic field, the design is known as a generator. With three sets of equally-spaced coils, three-phase power is produced. See Figure Alternatively, a design with a magnetic field rotating around a set of stationary coils produces the same output. The resulting voltage is proportional to the strength of the magnetic field, and the frequency is proportional to the rotational speed.

4 Inverters must be identified as interactive and listed to required standards before being interconnected. For interactive inverters, synchronizing functions are performed automatically and internally. Since inverters have the ability to monitor and regulate system output directly through microprocessor controls, synchronizing can be done cost-effectively and directly. Inverters can also incorporate additional protective and safety features that may otherwise be required as external equipment on generators. Since the interactive inverter is the primary utility interface device, it must meet all requirements for utility interconnection and be listed and identified for use in interactive PV systems. See Figure 12-3.

5 Interactive-only systems are the simplest way to interconnect a PV system with the utility.
The most common type of interactive PV system is one that does not use energy storage. The array is connected to the DC input of an interactive-only inverter. The inverter output interfaces with the utility, typically at a site distribution panel or electrical service entrance. See Figure 12-4.

6 The inverters in bimodal systems can continue to supply power to certain loads in the event of a utility outage. Bimodal systems are interactive systems that include battery storage, so they can operate in either interactive or stand-alone mode, providing a back-up power supply for critical loads such as computers, refrigeration, water pumps, or lighting. Unlike interactive-only inverters, the DC input of the inverter is connected to the battery bank, not the array. The array charges the battery bank, through the charge controller. See Figure 12-5.

7 Several codes and standards address specific interconnection issues with PV systems.
In the United States, the technical requirements for PV systems are established through national codes and standards published by the Institute of Electrical and Electronics Engineers (IEEE), Underwriters Laboratories (UL), and the National Fire Protection Association (NFPA). These organizations work collectively to ensure electrical equipment and installations are safe, through the combination of standards, equipment testing and certification, and enforceable codes. As a PV system is an electrical system, any general electrical codes and standards apply, such as the National Electrical Code®. Additional standards dealing specifically with the interconnection of PV systems also apply. See Figure 12-6.

8 During a utility outage, an islanding inverter can energize the utility lines around the PV system, potentially damaging equipment and creating a serious safety hazard. Islanding is the undesirable condition where a distributed-generation power source, such as a PV system, continues to transfer power to the utility grid during a utility outage. See Figure Islanding is a serious safety hazard for utility lineworkers working to restore power after an outage. Since the workers expect the grid to be de-energized, they may be shocked by the power present in the system from an islanding inverter. Also, islanding can damage utility equipment by interfering with the utility’s normal procedures for restoring service following an outage, primarily because the islanded electrical system is no longer in-phase with the utility system.

9 When a single-phase inverter is added to a system with more than one ungrounded (hot) conductor, the neutral conductor can become overloaded. Inverter and utility service types must be matched to prevent overloading. If a two-wire, single-phase inverter is connected to the neutral and one of the ungrounded conductors of a split-phase 120/240 V service or a three-phase, wye-connected service, the return current on the neutral (grounded) conductor will not balance and may overload the conductor. See Figure Therefore, the neutral conductor must be oversized. NEC® Section requires that the sum of the maximum load between the neutral and ungrounded conductor and the inverter’s current output rating must not exceed the ampacity of the neutral conductor. Some inverters avoid this problem with two ungrounded outputs. Alternatively, multiple inverters can be installed, one for each ungrounded conductor.

10 Adding single-phase output from an interactive inverter to a three-phase power system can result in unbalanced voltages between the phases. Section does not allow the connection of single-phase interactive PV inverters to three-phase power systems unless the interconnection can be designed to minimize unbalanced voltages between the phases. See Figure One option would be to use three small inverters and connect each inverter to a different phase. A better solution would be to use a single three-phase inverter. With a three-phase inverter, all phases must automatically de-energize when voltage becomes unbalanced or any phase is lost completely.

11 Interactive inverters can be connected to either the load side or the supply side of the main service disconnect. Section permits the output of interactive PV inverters to be connected to either the load side (customer side) or supply side (utility side) of the service disconnect. See Figure For many smaller systems, the point of connection is usually made on the load side, usually at a circuit breaker in the distribution panel. When the requirements for a load-side connection are not possible (such as due to the size of the PV system), interactive systems may be connected to the supply side. In cases of very large PV installations, existing service conductor ampacity may not be sufficient and separate services may need to be installed.

12 Interconnection on the load side of the service disconnect is done through back-fed circuit breakers. Each source interconnection must be made at a dedicated circuit breaker or fused disconnect. See Figure Multiple inverters are considered multiple sources. Each requires a dedicated interconnection device, unless their outputs are first combined at a subpanel.

13 Back-fed circuit breakers are circuit breakers on the load side of the main service disconnect that supply PV power to the busbar. A back-fed circuit breaker in a panelboard shall be positioned at the opposite (load) end from the main circuit location. See Figure A permanent warning label must be applied by the back-fed breaker with the following or equivalent marking: “WARNING: INVERTER OUTPUT CONNECTION, DO NOT RELOCATE THIS OVERCURRENT DEVICE.”

14 Interconnection on the supply side of the service disconnect must include a separate service-rated fused disconnect or circuit breaker. The NEC® requires this new service to have disconnects and overcurrent protection as described in Article 230, “Services.” See Figure A service-rated circuit breaker or fused disconnect meets these requirements and may also satisfy a utility’s requirement for an accessible, lockable safety disconnect that clearly indicates open status. Because a service disconnect with at least a 60 A rating must be used (per Article 230), even for lower-output-rated PV systems, smaller fuses and adapters may be required. Because services and taps are unprotected on the line side up to the fuse on the primary side of the distribution transformer, this new service disconnect must also have appropriate interrupting ratings consistent with potential utility fault currents and other service equipment. Fused disconnects with 200,000 A interrupting ratings are available to meet this requirement.

15 Interconnected PV systems must include labeling that clearly identifies the disconnects and point of interconnection. Regardless of the point of connection, NEC® Article 705, “Interconnected Electric Power Production Sources,” requires that a permanent directory be placed at each service location showing all power sources for a building. See Figure If the PV system is connected on the load side, this labeling may be in addition to labeling at the panelboard. If the point of connection is on the supply side, this labeling may serve the purposes of a supply-side label near the service disconnects.

16 With a net metering arrangement, exported power makes the electric meter run backward, crediting the PV system owner for power supplied to the utility grid at the retail rate. Net metering uses one meter that can operate in both directions, effectively subtracting exported electricity from imported electricity. See Figure This assigns them the same, full retail value, which is most advantageous to the customer. Sometimes a customer’s existing meter is capable of operating backwards without any modifications. If a different meter must be installed, it is the responsibility of the utility to do this, though it may charge the customer a fee.

17 Dual metering can be accomplished with two separate meters or with one meter that can measure and record energy flow in both directions separately Two-meter arrangements are more common for larger independent power producers, though they are also used for a variety of PV systems in states that have not yet mandated net metering rules. Two unidirectional meters, or a single multi-register meter, record exported and imported energy separately. See Figure

18 PURPA defines the entities that can contribute to the collective energy supply.
PURPA defines a class of IPPs known as qualifying facilities. See Figure A qualifying facility (QF) is a non-utility large-scale power producer that meets the technical and procedural requirements for interconnection to the utility system. PURPA mandates that utilities purchase power from QFs at the utility’s avoided cost. Avoided cost is the cost that a utility would normally incur to generate a given amount of power, often synonymous with the wholesale market value of electricity. When purchasing this energy, the utility “avoids the cost” of generating it themselves.

19 Utility interconnection agreements commonly require outside disconnects for PV systems so that the system can be isolated in the event of an outage or emergency. Utilities may require that PV systems have disconnects that are outside the building and accessible by utility personnel. See Figure The utility may retain the right to disconnect the system, without prior notice to the customer, if work is necessary on the utility’s part of the system or the customer fails to comply with the interconnection agreement.

20 Net metering policies vary by state and sometimes also by utility.
If the utility is in a state that mandates metering requirements, the utility must abide by those rules. In 2009, policies on net metering exist in 44 states and the District of Columbia. See Figure However, not every state has established rules regarding interconnections and metering. In states without established policies, utilities may choose to offer metering programs, though the implementation and requirements will vary between utility companies.


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