Integration of Distributed Generation into Grid

Aníbal T. de Almeida and Pedro S. Moura

5.2.1 Introduction

Connecting a distributed power system to the electricity grid has potential impacts on the safety and reliability of the grid, which is one of the most significant barriers to the installation of DG technologies. Electric utilities have understandably always placed a high priority on the safety and reliability of their electrical systems. Faced with the interconnection of potentially large number of distributed generators, utilities have perceived DG as a threat. This has led some utilities to place overly conservative restrictions on interconnected systems, causing added costs that may make an installation economically unfeasible. Several techniques may reduce adverse network impacts allowing DG connection, but those techniques can be project specific and may be expensive, and adversely affect project economics.

Connection of DG fundamentally affects the operation of distribution networks with changes and impacts like:

• Voltage fluctuations

• Increased fault levels

• Degraded protection

• Bidirectional power flow

• Altered transient stability

To reduce the impact in the power grid several requirements are needed. Typical requirements include equipment that prevents power from being fed to the grid when the grid is de-energized, manual disconnects and PQ requirements such as limits on the interconnected system's effects on "flicker," harmonic distortion, and other types of waveform disturbance. Systems may also be required to automatically shut down in the event of electrical failures, to provide an isolation transformer for the system, as well as to provide liability insurance.

Up to recently the lack of a well-defined interconnect standard and failure to adhere to a standard can add considerably to engineering and equipment costs, making process planning difficult. Many interconnection requirements were drafted and adopted without understanding of the protection capabilities of modern DG equipment. As a result, these requirements often unnecessarily burden projects with redundant studies and hardware.

Interconnection requirements for large DG installations (~ 10 MW) are well understood because they are very similar to the interconnections required for central power stations. Interconnection requirements for smaller installations are more difficult because the utility must balance the desire for a safe interconnection with the plant owner's desire to have a "quick and easy" interconnection design to get the DG up and running. Interconnection complexity generally increases with project size and is technology dependent.

Grid interconnection is important for three reasons:

• The number of small generators seeking interconnection to the grid will increase in the future.

• Distributed generation advocates contend that the current interconnection requirements and processes are effectively increasing costs unfairly and pricing DG out of the market.

• Distribution companies are concerned that DG will negatively impact the safety and reliability of the grid and unfairly increase the distribution companies' costs.

5.2.2 Power Distribution

Electric grid is broadly divided into two systems: the transmission system that transfers bulk power at high voltages from power plants to utility-owned substations and a few very large customers, and the

FIGURE 5.10 Radial distribution system. (From California Energy Commission, California interconnection guidebook: a guide to interconnecting customer-owned electric generation equipment to the electric utility distribution system using California's electric rule 21, California Energy Commission, Sacramento, CA, 2003. http://www.energy. ca.gov/reports/2003-11-13_500-03-083F.PDF. With permission.)

FIGURE 5.10 Radial distribution system. (From California Energy Commission, California interconnection guidebook: a guide to interconnecting customer-owned electric generation equipment to the electric utility distribution system using California's electric rule 21, California Energy Commission, Sacramento, CA, 2003. http://www.energy. ca.gov/reports/2003-11-13_500-03-083F.PDF. With permission.)

distribution system that delivers power at medium and low voltages from the substation to the majority of customers.

The utility distribution systems can be categorized as either radial or networked. Power system design is a tradeoff between complexity and cost to maximize economy and reliability. As a result, the general structure of the power delivery system has a networked nature at the transmission level and a more radial nature at distribution level.

Radial distribution refers to a system where the power lines extend from a common substation to the customer loads coming off at single nodes along the line (Figure 5.10). In these distribution systems, power can only flow in one direction: from the substation to a load. Although there can be many radial distribution lines emanating from a substation, each load is typically served by only one line. A disruption to that feed or substation will typically affect all customer loads on that line. A radial system generally offers a less reliable power source than a networked system because it lacks redundancy. However, the radial system and its protection equipment are less complex and less expensive than the networked system.

The introduction of an energy source such as DG within the radial distribution system will affect the load distribution in the system, and may even cause reverse power flow if it is large relative to the load. Introduction of a sufficiently large power source within the radial distribution normally requires some modification to the protection system.

Networked distribution refers to a system where numerous separate lines form a grid so that customer loads can tap-off of multiple independent feeds, which are then tied to a common bus on the secondary side of the transformers (Figure 5.11). These can be separate lines from a common substation or they can be from independent substations.

The networked system offers reliability advantages over the radial system because it provides multiple power sources for loads. This multipath design is sometimes referred to as a looped system. These systems use network protectors that quickly isolate faults to protect the grid and shift customer loads onto the remaining feeds. Understandably, utilities are reluctant to allow the interconnection of anything that they feel will endanger the integrity or safety of this system.

System protection in a networked distribution system is more complex and expensive than in the radial distribution system due to the extra intelligence needed for reliable, effective protection.

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FIGURE 5.11 Networked distribution system. (From California Energy Commission, California interconnection guidebook: a guide to interconnecting customer-owned electric generation equipment to the electric utility distribution system using California's electric rule 21, California Energy Commission, Sacramento, CA, 2003. http://www.energy.ca.gov/reports/2003-11-13_500-03-083F.PDF. With permission.)

FIGURE 5.11 Networked distribution system. (From California Energy Commission, California interconnection guidebook: a guide to interconnecting customer-owned electric generation equipment to the electric utility distribution system using California's electric rule 21, California Energy Commission, Sacramento, CA, 2003. http://www.energy.ca.gov/reports/2003-11-13_500-03-083F.PDF. With permission.)

Because the networked system is specifically designed to deliver energy from multiple transformers to loads, it is capable of dealing with reverse power flow.

5.2.3 Types of Grid Connections

The electric power system interface is the means by which the DG unit electrically connects to the power system outside the facility in which the unit is installed. Depending on the application and operation of the DG unit, the interface configuration can range from a complex parallel interconnection, to being nonexistent if the DG unit is operated in isolation (Figure 5.12).

In remote applications, due to the high costs of the power grid expansion to the consumption site, the option of interconnecting with the local grid may be impractical. In these cases, the DG units become the unique means of energy supply at low cost. In this configuration, the DG unit provides power for all loads completely isolated from grid, providing the utility no backup or supplemental power.

In near-to-grid applications, the DG unit owner can opt for interconnection, by several types of connections. Depending on the application and the operation mode of the DG unit, the connection system with the grid can represent a complex parallel interconnection or can be inexistent if the DG is operating isolated from the grid. The complexity of the interconnection systems increases with the required interaction level between the DG unit and the distribution grid.

Interface complexity vs. interaction

Grid interconnection: Bi-directional power flow

Grid interconnection: Bi-directional power flow

Grid interconnection: No power export

Isolated: With automatic transfer

Isolated: With automatic transfer

Isolated - No grid source

Isolated - No grid source

| Requires a closed transition | Utilizes an open transition Does not require a transition

FIGURE 5.12 Complexity vs. interaction. (From Arthur D. Little, Distributed Generation: System Interfaces, An Arthur D. Little white paper, ADL Publishing, Boston, MA, 1999. http://www.encorp.comdwnld/pdf/whitepaper/ ADLittleWhitePaperDGSystemInterfaces.pdf. With permission.)

For most customers, DG systems are most cost-effective and efficient when they are interconnected with the utility grid. In simple terms, "interconnected with the grid" means that both the DG system and the grid supply power to the facility at the same time. Paralleled systems offer added reliability, because when the DG system is down for maintenance, the grid meets the full electrical load, and vice versa.

Distributed generation systems can be designed to keep a facility up and running without an interruption if the grid experiences an outage. Also, grid-interconnected systems can be sized smaller to meet the customer's base load as opposed to its peak load. Not only is the smaller base-load system cheaper, it also runs closer to its rated capacity and, therefore, is more fuel efficient and cost-effective.

Two different types of grid interconnection are possible: parallel or roll-over. With the parallel operation, the DG system and the grid are interconnected and both are connected to the load. In the rollover operation, the two sources are interconnected, but only one is connected with the load.

A typical interconnection system includes three kinds of equipment:

• Control equipment for regulating the output of the DG

• A switch and circuit breaker (including a "visible open") to isolate the DG unit

• Protective relaying mechanisms to monitor system conditions and to prevent dangerous operating conditions

5.2.3.1 Isolated Operation

In remote applications, the DG units become the unique means of energy supply at low cost. In this configuration, the DG unit provides power for all loads completely isolated from grid, providing the utility no backup or supplemental power (Figure 5.13). Isolated operation is also possible in sites that are normally connected to the grid but in which continuous supply is required in the event of an outage. Some generating facilities, such as a hospital emergency generator, power the customer's partial or entire load isolated from the utility.

Utility

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