Utility Interconnection With On-Site Generation
So, you want to generate your own power, for emergencies and/or for everyday use. For the sake of your design/budget/schedule, learn what the utility might expect from you — and what you should expect from it.
Should the utility care about your standby power system? Many health care facilities today are not satisfied with standby power capacity for code-mandated life safety, critical, and equipment loads, installing enough on-site generation to allow them to continue normal or near-normal operation during utility outages.
In these larger systems, generators not only operate in parallel with each other but invariably operate in parallel with the utility. Parallel or “interconnected” operation is defined as having the generator and the utility connected to the facility power system at the same time with each source providing part of the power consumed. Generators may be interconnected to the utility briefly to transfer load without a “blink” or for extended periods of time for testing, peak shaving, or cogeneration. This article will introduce you to some of the regulatory and technical issues to keep in mind when planning, designing, and installing such an interconnected power system.
The Utility Perspective
Electric utility systems in the United States were intended for power flow in one direction, from generators at the power plants, through the bulk transmission network to substation transformers, and then through the local distribution network to loads at the customer’s site. While renewable energy sources have led to many applications of generation at the customer end of the system, most utility distribution networks predate this trend and were not designed to readily accommodate this.
When you broach the subject of operating generators in parallel, the utility’s primary concern will be safety. They require assurance that your generation cannot supply voltage back to the utility system during an outage; this would present a hazard to their crews working on lines or equipment they assume to be dead, and also to the general public if it were to re-energize a downed overhead line conductor. The utility will have technical requirements for your installation, including devices to detect utility failure while in parallel and automatically disconnect the generation, and means to positively isolate the generation from the utility system that can be locked open by line crews.
In addition to safety concerns, generators at customer facilities can adversely affect operation of the power system or power quality. The additional energy contributed by parallel generators when there is a short circuit or fault on the utility system may cause the ratings of protective devices like fuses or circuit breakers to be exceeded. Sudden transfer of large blocks of load between the utility and generators can cause voltage dips or rises that may cause lights to flicker or affect equipment at other customer sites. Finally, harmonic voltages and currents produced by customer generation may cause overheating of equipment or voltage distortion that can affect other customers.
The Interconnection Application Process
Most utilities have a defined process to review proposed customer generator interconnections and address the concerns discussed above, a process which starts with an interconnection application. The information that must be submitted with the application varies, but it typically includes the location, quantity and ratings of the proposed generators, a schematic drawing called a single line diagram that shows how the generation will be electrically interconnected to the utility system, and a description of the intended operation of the generators whether for standby only, peak shaving, or cogeneration. There is usually a nominal fee assessed to cover the utility’s costs of reviewing the application.
Upon receiving the application, the utility will conduct a preliminary review to determine whether the installation has the potential to adversely affect their system. If not, it will be approved along with identification of any technical requirements you must include in the design and any restrictions that will be placed on operation. In many cases, these have been pre-determined by the utility for different types and ratings of generators and can be found in published interconnection guidelines or interconnection requirements. The utility will also identify the costs of any modifications they must make to their system to accommodate the generation, which will be the customer’s responsibility.
Alternatively, they may determine that further engineering studies are needed to assess the impact on their system and identify measures to mitigate it. In this case, rather than receiving approval after the preliminary review, you will receive a description, schedule, and cost estimate for those studies, which are also the financial responsibility of the customer. In some cases, your utility may not be the only stakeholder in the process; for large installations, the regional independent system operator (ISO), an entity that manages the transmission system for reliability across multiple utilities, may also be required to review the application.
If required, engineering studies and/or ISO review can take many months, and it is very risky to order generators or switchgear prior to their completion, so it is advisable to submit the interconnection application as soon as you are able to define your project at the level of detail required by your utility’s process. After approval, and before the generation can be put into operation, an interconnection agreement is usually executed between the utility and the customer defining the rights, responsibilities, and liabilities of both parties.
Time is of the Essence
The technical requirements for an interconnection depend not only on the size but on the length of time the generation will be in parallel with the utility. Many small- and medium-sized health care facilities operate standby generators in parallel, perhaps unknowingly, through the use of closed-transition automatic transfer switches. Such a transfer switch has two independent sets of contacts, one of which connects the load to the normal power (utility) source and one of which connects the load to the emergency (generator) source.
When the utility fails, there is an interruption of the load as the switch disconnects from the utility source, starts the generator, and then connects to the generator when it reaches rated voltage and frequency. When the utility returns, the switch closes the contacts that connect to the utility first and then opens the contacts that connect to the generator, returning to normal conditions without a second interruption of the load. The generator and utility are paralleled only momentarily, typically 180 milliseconds or less. This is the simplest form of paralleling device because there is no active control of the generator; the transfer switch controls monitor both voltage sources and simply wait until the generator voltage “wanders” into synchronism with the utility to make the parallel connection.
Closed-transition transfer switches are readily accepted by most utilities because they are a standard manufactured product with the required controls and safety provisions designed-in and tested by a third party such as Underwriter’s Laboratories (UL). However, there is often a limit on the amount of load that can be transferred using this method because of the voltage dip that may result from near-instantaneous transfer of load to the utility. When this limit is exceeded, multiple transfer switches with staggered timing or switchgear that incorporates soft load transfer capability are required.
Short-Term Parallel for Soft Load Transfer
In a typical soft load transfer scheme, the independent sets of contacts in the transfer switch are replaced by two electrically operated circuit breakers in switchgear. The transfer controls actively adjust the frequency, voltage, and loading of the generator by interfacing to the engine governor and the voltage regulator. Frequency and voltage adjustments bring the generator into synchronism with the utility before establishing the parallel condition and, once in parallel, the controls reduce the load on the generator, transferring it back to the utility gradually at a rate that avoids objectionable voltage dip. When load on the generator has been ramped down, it is disconnected and shut down. Typical parallel times to ramp 100% of rated load from the generator to the utility vary from 20 to 90 seconds.
Requirements to expect from the utility for a short-term parallel installation include automatic synchronization and an independent synchronism-check relay, as well as a protective relay that can detect a failure of the utility while in parallel operation and automatically open the utility circuit breaker. There will often also be a requirement for communication of data such as generator and circuit breaker status to the utility’s control center over a supervisory control and data acquisition (SCADA) system. In some cases, the utility may require the ability to remotely trip the utility circuit breaker or block it from closing through the SCADA system when conditions on their system make it unsafe to parallel. The costs of these measures, even if installed by the utility in the case of the SCADA system, are typically borne by the customer.
Extended Parallel Operation
There may be any number of reasons to design a generator to remain in parallel with the utility longer than required to complete a soft load transfer. The ability to pick up part of the facility load for testing can be more economical than using a load bank for a large machine, and doing so by running in parallel with the utility may entail less risk than transferring that load to the generator in isolation. The utility may offer an economic incentive to operate on-site generation for peak shaving, which also may be more easily accomplished with less risk by operating in parallel than in isolation.
Finally, the generator may be intended for continuous rather than standby operation. On many large health care campuses, engine-driven generators meet the code-required emergency and standby power missions, and steam turbine or gas turbine units in a cogeneration application provide the additional capacity required to support normal operations while providing a constant economic benefit.
Extended parallel operation can usually be approved with the same type of switchgear and controls required for short-term parallel operation, but this may trigger additional utility monitoring and metering requirements. Automatic protection against inadvertently feeding power back into the utility system, a reverse power relay, is also required when there is no power purchase agreement and no intent to sell power to the utility.
If your generation normally serves part of the facility load and you expect the utility to pick this up if the unit trips off line, or is out of service for maintenance, they will generally assess some form of standby power charge. The rationale is that they incur cost to construct and maintain facilities, and a certain amount of generation reserve, adequate to supply the entire load, even though they earn no revenue from the part of it normally supplied by the generation. Standby charges can be significant and may require creative solutions to maintain the economic feasibility of a project. Installing multiple smaller units instead of a single large unit can allow you to contract for standby power equal to the capacity of a single unit rather than the entire installation. Coupling the ability to shed or reduce non-critical loads with bringing standby generators on line may also allow you to reduce the required amount of standby power.
We could devote several articles, if not a book, to the technical aspects of utility interconnection, but from a project planning standpoint, here are some key considerations:
For engine-driven generators, you must answer the old question of whether you need a standby, prime, or continuous rating for the application.
Generators must have winding characteristics, including a parameter called “pitch” that are matched to any other generators they will parallel with and that are compatible with the winding connections of the utility transformer.
Switchgear and protective devices are required to comply with recognized industry standards promulgated by the Institute of Electrical and Electronic Engineers (IEEE) and in many jurisdictions will be required to be listed by UL or another recognized independent testing agency.
If dedicated generator metering is required, many utilities have detailed standards for the construction of the switchgear housing the metering equipment. Initial planning of space in the switchgear room should include utility metering and SCADA equipment to avoid moving walls later.
The utility may want to review and approve the switchgear manufacturer’s shop drawings before fabrication and will certainly require review and approval of the proposed settings of utility-required protective devices. They will also want the opportunity to witness protective device testing during commissioning.
In conclusion, utility requirements for an interconnected generator installation can significantly impact the design, cost, and schedule of a project. An up-front understanding of the technical requirements and early initiation of the interconnection application process improve your chances of reaching the goal of a successful startup on time and without surprises.
Timothy Coyle, an electrical engineer with 32 years of experience, specializes in planning, analysis, and training related to critical power systems in institutional and utility facilities. He is a senior electrical engineer with Karges-Faulconbridge, Inc. Email him at email@example.com.