Power Distribution Systems - Part - 1

Basic Principles

The best distribution system is one that will, cost-effectively and safely, supply adequate electric service to both present and future probable loads—this section is intended to aid in selecting, designing and installing such a system. The function of the electric power distribution system in a building or an installation site is to receive power at one or more supply points and to deliver it to the lighting loads, motors and all other electrically operated devices. The importance of the distribution system to the function of a building makes it imperative that the best system be designed and installed. In order to design the best distribution system, the system design engineer must have information concerning the loads and a knowledge of the types of distribution systems that are applicable. The various categories of buildings have many specific design challenges, but certain basic principles are common to all. Such principles, if followed, will provide a soundly executed design. The basic principles or factors requiring consideration during design of the power distribution system include:

■■ Functions of structure, present and future

■■ Life and flexibility of structure

■■ Locations of service entrance and distribution equipment, locations and characteristics of loads, locations of unit substations

■■ Demand and diversity factors of loads

■■ Sources of power; including normal, standby and emergency

■■ Continuity and quality of power available and required

■■ Energy efficiency and management

■■ Distribution and utilization voltages

■■ Busway and/or cable feeders

■■ Distribution equipment and motor control

■■ Power and lighting panel-boards and motor control centers

■■ Types of lighting systems

■■ Installation methods

■■ Power monitoring systems

■■ Electric utility requirements

Trends in Systems Design

There are many new factors to consider in the design of power distribution systems. Central and state legislation has been introduced to reduce the output of carbon emissions into the environment; the intent being the reduction of their impact on climate change. In order to address the subsequent need for clean power, there has been an accelerating trend toward the incorporation of solar and other sustainable energy sources into existing and new building designs. Energy storage systems (ESS) are now making renewable energy a more viable option by helping to stabilize power output during transient dips or interruptions to power production. Utility deregulation has also provided financial incentives for building owners and facility managers to participate in peak demand load shaving programs. These programs are intended to reduce load on the utility grid in response to a 1 hour or 1 day ahead signal from the utility. The users shedding or cycling of nonessential loads is generally initiated by a building management system (BMS) in conjunction with power monitoring and lighting control equipment. To ensure uninterrupted operation of key customer loads, incorporation of other types of distributed generation such as fuel cells and diesel or natural gas fired reciprocating generator sets may be desired or required. Hospital complexes and college campuses are increasingly adopting the design of central utilities plants (CUPs). In lieu of a separate boiler plant, cogeneration is used to produce electricity and the wasted heat from the combustion process is recaptured to provide hot water for the campus. Large co-generation plants (3 MW and above) often include large turbines or reciprocating engines as their prime movers for the generators. To enhance service continuity, these generators use a continuous source of natural gas as their fuel supply. Cogen plants generally have higher power conversion efficiencies and produce lower carbon emissions. The growing impact of adverse weather conditions such as hurricanes and flooding is now driving incoming service and distribution equipment rooms to be located out of basements and other low lying areas. Regions prone to these storms often experience downed utility power lines and/or flooded manholes, resulting in a loss of power to thousands of customers. In order to quickly return power to these facilities, additional on-site backup generation is being included in both new designs and as upgrades to existing sites. This trend for resiliency is increasing among grocery stores, large chain stores and other distribution facilities requiring refrigeration to keep products from spoiling as well as large multifamily dwelling complexes in low lying flood plain areas. Building costs continue to rise and rent-able or usable space is now at a premium. To solve both problems, many design and construction firms are looking at off-site prefabrication of key elements. Electro Centers or Integrated Power Assemblies (IPA) can be fitted out with a variety of electrical distribution equipment and shipped to the site in preassembled modules for mounting on elevated foundation piles, building setbacks or rooftops. Finally, the need to have qualified building electrical operators, maintenance departments and facility engineers has collided with growing expectations for improved productivity and reduced overall operating costs. The increasing proliferation of smart devices and enhanced connectivity with power distribution equipment has expanded facility owner’s options. These capabilities allow for automated communication of vital power system information including energy data, equipment wellness and predictive diagnostics, and electrical equipment control. The future “Internet of Things” promises to add millions of more sensors and other devices to collect operational data and send it through the Internet to “cloud-based” computing services. There, information from multiple devices can be analyzed and actions can be taken to optimize performance and reduce downtime. Various sections of this guide cover the application and selection of such systems and components that may be incorporated into the power equipment being designed.