With the increasing popularity of on-site power, it's important for those interested in distributed generation to know how to connect their system to the utility grid.
You may think that utility companies have a universal standard for allowing interconnection of on-site power with the utility, but this isn’t the case. Each utility is independent and has its own set of interconnection standards, and in some cases no interconnection standard exists at all.
Most utility companies will have different interconnect requirements based on the size of the connected generation in the kW or kVA rating and/or the voltage at which the connection is being made.
Although there are no hard and fast rules for interconnecting a non-utility generator (NUG) with the utility, you need to adhere to certain technical requirements. This will help you ensure the safety of your personnel and equipment and provide for the reliability of both systems.
The issues that you need to consider when paralleling with the utility include costs, contracts, environmental impact, and mode of operation. Along with reviewing the main transformer and generation characteristics like voltage, you also need to consider four main issues: the utility metering section, utility disconnect device, protective relaying, and interlocking and control.
Utility metering section. The utility metering section pertains to revenue metering. The service entrance sections house the utility instrument transformers and metering devices, which are sometimes owned by the utility companies. In other cases, they’re furnished as part of the paralleling switchgear for the project.
The standards by which many utilities will adequately define encompass mechanical construction requirements like bus bar sizes, clearances, and the location of components. The overall dimensions of the metering section are also taken into consideration.
Utility metering section standards also deal with the type of CTs and PTs that are required. In most cases the utility will be responsible for furnishing the CTs, PTs, and revenue meters. When the metering section is built into the owner’s furnished switchgear line-up, it’s essential to determine whether the utility has a requirement for “hot” or “cold” metering, or where the metering section is located with relation to the utility disconnect device.
Utility disconnect. A disconnect device is required to disconnect the on-site generation device from the utility source. In some cases this may be the breaker used to parallel with the utility. However, in many cases, the utility will want full access to the disconnect device and the capability to lock out the on-site generation. This device could be a pole-mounted utility disconnect or an owner-furnished disconnect switch. The main requirement is that the utility has a means of disconnecting and locking out the onsite generation device to ensure the safety of the workers during maintenance calls.
Protective Relaying. Although utility-grade relaying schemes aren’t typically required for use with closed transition transfer switches, a full relaying package will be necessary in all cases of extended utility parallel operation. These relaying packages consist of a protective relay scheme that uses utility-grade protective relays to accomplish a number of functions. The utility’s primary concern is that the on-site generation system doesn’t cause any adverse effects on the system. Therefore a relay scheme is required to disconnect the generation system from the utility in the event of abnormal conditions during parallel operation.
The primary protective functions that should be provided for all systems that parallel with the utility, for the utility point of connection are:
Synchronizing—Insure in phase closing of both sources together
Over/undervoltage—Protect against system overvoltage and undervoltage conditions that cause damage to equipment on either side of the interconnection point.
Over/under frequency—Protect against over and under frequency conditions
Phase balance—Protect against phase voltage imbalance and phase rotation.
Phase-to-phase and ground fault—Protect against phase-to-phase faults and phase-to- ground faults both on the utility grid and within the NUG facility.
Current balance—Protect against for current imbalance.
Some of the additional relaying that may be necessary is reverse power, under power, and directional current.
Interlocking and Control. Some utilities will require a review of the control and interlocking scheme used to control the parallel operation. This will help ensure that out-of-phase paralleling isn’t possible. The interlocking logic will prevent out-of-phase closing and may allow open- and closed-transition operation. With the use of PLCs, it isn’t uncommon to control these breakers with interlocking for both manual and automatic operation.
All utility companies are independent, and their requirements differ from one another. A complete interconnect package that includes protective relay drawings and detailed metering section arrangements should be submitted to the utility for approval prior to construction. This will hopefully avoid unnecessary and costly field changes.