Providing Stakeholders with Effective Options

What is Distributed Generation?

Distributed Generation (DG) uses small-scale power generation technologies to generate electricity in close proximity to its utilization point. DG technology portfolios typically include small or micro hydro plants, wind turbines, photovoltaics, fuel cells, reciprocating engines, combustion gas turbines and micro turbines. Plant capacities range in size from a few kW to as high as 20 MW.

While DG technologies have gained significant popularity worldwide in recent years for different reasons, reduction in smoke stack emissions, lower power costs, improved reliability or expansion of energy options remain common motivating factors. DG technology offers customers a number of important options: generation of electricity while serving thermal or cooling needs, electricity generation for peak shaving, provision of standby power, or sale of excess power to the grid. Although successful DG applications can involve stand-alone configurations, their economic viability is significantly enhanced through interconnection with the utility grid.

Due to their small capacities, DG plants are typically connected with electricity Distribution Systems, rather than Transmission Systems. Lack of common interconnection standards is a major barrier to the wide acceptance and installation of DG technologies. Streamlining of relevant regulations and procedures will greatly expedite this process.

Successful interconnection configurations

DG interconnection with distribution systems may involve one of the following operating configurations:

• Use of synchronous generators at DG plant, synchronized with the utility’s power supply system and its frequency, phase and magnitude of output voltage
• Use of induction generators at DG plant operating in asynchronous mode i.e., in step with the utility systems’ voltage but not its frequency and phase
• Use of dc battery sources connected to the utility’s Distribution System through inverter stations offering the most flexible mode of operation.

Regardless of defined mode, the most important element to ensure successful operation of an interconnected DG plant is a comprehensive planning study to validate benefits and, identify and mitigate any adverse effects. Planning studies typically involve the following tasks:

Determination of maximum power ratings

The maximum DG plant that can be successfully connected to existing distribution facilities may be restricted by power transmission capacity of lines related to stability or voltage regulation parameters.

Power Quality Impact Assessments

Power quality assessment studies include determination of DG plant impacts on distribution feeder power quality and impact assessment of the distribution feeders’ power quality on DG plant operation. Investigation of power quality parameters includes temporary over-voltages and under-voltages, unbalanced phase voltage and currents, and wave shape distortions (harmonic content). Harmonic content evaluations are particularly important for DG plants employing dc battery and inverters.

Reliability Evaluations

Connection of the DG plant to the Distribution System may impact positively or negatively on supply system reliability, requiring thorough investigation. Most adverse impacts on reliability can be mitigated using proper protection and control devices.

Protection and Control Evaluations

Coordinated operation of protection and control devices at the interface is mandatory to achieve the required level of reliability, selectivity and speed for detection and clearance of system faults. Interconnection of DG plant may increase the short circuit current on the Distribution System to levels beyond the interrupt rating of existing fault clearing devices. Transformer configurations and grounding techniques at the interface significantly impact the magnitude of fault current contributions from the DG plant. Interface protection and controls must be designed to prevent islanding of the distribution feeder energized solely from the DG plant.

Emission Evaluations

Where DG plant fuelled by renewable energy sources is proposed, to produce environmental benefits, smokestack emission evaluations based on anticipated operating conditions may be necessary to quantify such benefits.

Access, Metering and Dispatch

To maintain balance in stakeholder interests, the standard terms and conditions of power supply may require appropriate modification. This will properly define the rights and obligations of electric utility and DG plant owners with respect to site access, metering and dispatch of real and reactive power.

Economic and Financial Viability Studies

Economic and financial viability studies involve identification and quantification of individual cost elements and returns. Potential benefits include deferral of new Transmission and Distribution capital investments, reduction in electrical line losses, lower incremental costs for peak demand, reduction in energy and demand charges used in combined heat and power applications. Potential benefits include improvements in power quality through voltage support, reactive power support, power factor correction and enhanced reliability through the addition of a backup power source.

For more information, contact:

Steve Cress, 416.207.6000 x6557, steve.cress@kinectrics.com

Related links:

Distributed Generation Interconnection - Projects

Case studies:

Penetration Limits of Distributed Generation (DG)

Remote Generation Dispatch Demonstration Project

Building Integrated Distributed Power Systems