By Donald C. Vanderbrook
POWER GENERATION: The Right ConnectionsEngineering
The popularity of on-site generators is increasing, but engineers who specify these systems must be careful to consider how they will connect to the utility grid.With the deregulation of power utiliti...
The popularity of on-site generators is increasing, but engineers who specify these systems must be careful to consider how they will connect to the utility grid.
With the deregulation of power utilities taking place in North America comes growing uncertainty about the availability of electricity. Consequently, commercial, industrial and institutional users are turning to distributed generation (DG) solutions, with on-site generating equipment providing power on demand.
On-site generation gives users emergency back-up power in the case of an outage, and also allows the facility to produce its own power when needed under rate agreements with the utility. Power companies typically offer a lower year-round rate to large power users in return for the right to temporarily curtail or interrupt their supply during peak periods of demand. With generating equipment on-site, those facilities can continue to operate by providing some or all of their own power.
With a distributed generation system, the facility’s generator is connected to the utility’s distribution grid via an electrically operated breaker.
Since the generator is connected or paralleled with the utility’s distribution system, defining protection requirements becomes an important, but sometimes overlooked issue.
In considering any type of closed transition generator interconnect with the grid, i.e. in a “make before break” arrangement where the generator connects before the utility is disconnected, some basic considerations need to be explored. Too often, the design team has a generator and associated paralleling switchgear installed, only to find that the equipment does not meet the utility’s requirements for grid interconnection.
When considering a distributed generation project that would involve operating in a base-load, peak-shave or soft-load manner (where the facility is combining its own power with that supplied from outside), the designers should consult the local electric utility. Typically, there are specifications or guidelines for interconnection to the grid, as well as special rate structures that apply. These can range from stipulations regarding the most basic protective relaying, to highly structured rate agreements or contracts. There may be associated incentives, which can provide significant cost benefits, and punitive tariffs if the terms of the contract are not met.
Care should be taken when selecting protective relaying to assure that each device meets or exceeds the utility’s requirements. Common devices required for paralleling with a utility’s distribution system, as defined by the Institute of Electrical and Electronics Engineers (IEEE), include:
synchronization check device used to permit the paralleling of two circuits when these circuits are within specified phase angle, slip frequency, and/or voltage parameters;
undervoltage relay allows for operation of a feeder via an electrically operated circuit breaker within operating parameters of nominal to minimum voltage;
directional power relay used to prevent power flow in a given direction;
instantaneous overcurrent relay functions to open the circuit on an excessive instantaneous overcurrent value, or excessive rate of rise, which indicates a fault in the circuit;
AC time overcurrent relay used to control the opening of a circuit when the current exceeds a predetermined setpoint;
overvoltage relay functions (signals) on a set value of overvoltage;
frequency relay can be either over-frequency or under-frequency sensitive, or both, depending on features.
It’s important to note that the higher the voltage, the more stringent the relay and protection requirements become for the grid interconnect. The precaution is understandable, as there is less direct protection between the primary distribution point and the point of generation.
Questions often arise over what a utility-grade protective relay is. A utility-grade relay is one recognized by the specific utility for the application being considered, based on their own experience and testing. The use of such relays shows again how important it is to consult the utility early in the project so that there are no delays and costly changes later on. In addition to protective relays, the utility may require the use of surge arresters, grounding resistors, etc. to satisfy their requirements for protection and fuse coordination.
From a business perspective, the utility typically requires customers with distributed generation systems to participate in a program the utility has designed — often known as becoming a power pool participant.1 The utility will usually assign an account manager to work through the details of its contract with the customer. The contract will provide the facility with benefits in the form of lower electrical rates in exchange for it providing power when called upon by the utility. The downside to this agreement is that the failure to bring the genset on line when required can have significant penalties or tariffs.
The facility must supply either its own bi-directional meter or contract with the utility for the necessary equipment. If the facility intends to supply its own metering, the equipment must be approved by the utility as suitable in terms of accuracy and performance. The designers need to review the entire design of the distributed generation system with the account manager and any other specialists the utility deems necessary, such as relay protection or distribution engineers.
The above is by no means a complete and standard protocol for any distributed generation application. Consultation with the utility early in the planning process (preferably before the system has been quoted) will help the designer determine the exact installed cost so that an accurate cost/benefit analysis can be performed. Provincial and local electrical and building codes must, of course, be considered as well.
Canadian Electricity Association (CEA), tel. 514-886-5364, www.canelect.ca
IEEE, tel. 800-678-IEEE, www.ieee.org
Electrical Generating Systems Association (EGSA), tel. 561-750-5575, www.egsa.org
Electric Utilities Act, tel. 780-427-4952, www.gov.ab.ca/qp
Donald C. Vanderbrook is engineering project manager, special engineered projects, with Generac Power Systems of Wisconsin.
1 “Technical Guideline for Interconnection of Generators to Distribution Systems,” 16 December 1999, ATCO Electric, p. 7. www.atcoelectric.com/powerprod/PowerProducers/Interconnection_Manual.html