The importance of preliminary interconnection studies

Electricity supplier standards and requirements

When building a new plant, mine or hospital, or even when integrating distributed generation, a critical step in a project’s preliminary engineering phase is designing the power system that will supply the loads through the customer’s facilities, regardless of the load being connected.

There are many important decisions to make during the preliminary engineering phase, including which voltage levels to choose, which operating strategies to foster and which equipment, lines and distribution cables to select. At this stage of the design, it’s important not to overlook the public utility’s technical requirements that the customer must meet at the point of common coupling (PCC). In fact, in Canada, each of these electricity providers (Hydro-Québec, Hydro One, BC Hydro, etc.) has its own interconnection requirements tailored to their systems.

In general, these requirements are designed to prevent cross-interference among customers. In fact, the public utility must guarantee the same power supply quality to all its customers across its system. Customers must demonstrate that they comply with these requirements to get the green light to connect their installation to the utility grid. The interconnection study includes the following elements:

• A general description of the load (MW, power factor, specific load cycle)
• A general description of the main electrical equipment
• A single-line diagram of the customer substation
• A description of the protection systems and grounding method
• The characteristics of disturbing equipment
• A power quality study (harmonics, flicker, rapid voltage change); for example, for specific Hydro-Québec load interconnection requirements:
  
  - at 120 kV or more: emission limits for disturbances on the transmission system
  
  - at 25 kV or less: standards C.25-01 and C.22-03

Although most utility requirements are followed in the industry, the power quality study within the customer’s facilities is all too often overlooked or even ignored during the preliminary engineering phase.

Planning rather than correcting: a worthwhile investment

Unfortunately, although these studies are required by the utility before the customer is interconnected, they are often performed too late in the project, after all the equipment has been purchased and, in some cases, already delivered to the site.

It may seem difficult to perform such a study early on in the project because many inputs have yet to be defined. However, early analyses can help guide decisions during the preliminary engineering phase, while the detailed study can be performed later in the project’s engineering phase.

Examples of what can be achieved by performing the preliminary analysis include:

• Predicting optimal equipment selection such as variable frequency drives (VFDs) or transformer connection/phase shift. This will proactively lessen the impact on power quality and any potential mitigations that may need to be applied.
• Where necessary, providing for applicable power quality mitigations that will ensure compliance with the utility’s power quality requirements.
• Planning for the plant’s reactive power compensation requirements to avoid billing penalties due to a low power factor.
• Providing for additional space and equipment in electrical rooms to accommodate added mitigation.

Investing in this thoughtful planning at the beginning of the project ensures you make the right choices at the design stage. This way, you can plan the necessary budgets to add mitigation measures and avoid unpleasant surprises at the end of the project when the utility requests the complete interconnection study, which is essential to power up the customer’s facilities.

Existing facilities

This approach works just as well for a plant expansion as for an electrical equipment upgrade in an existing facility. In fact, it’s important to know that most utilities require a study when the customer makes major changes to its facilities or activities that could affect the disturbances observed at the PCC.

Modifying electrical facilities as part of a modernization project can cause power quality issues that were not present before the project. For example, a plant that replaces its motors (powered direct online DOL or by soft starter) with VFD technology introduces harmonics into its power system, in addition to modifying its reactive compensation needs to maintain the power factor. Very often, when this reactive compensation isn’t tuned, it causes a very common incompatibility, including excess reactive power on the system as well as excessive harmonic emissions that can lead to the utility refusing to supply power. Also, failing to tune the customer’s capacitor banks can cause the new VFDs to trip unexpectedly during switching line events or by its own capacitor bank switching.

Performing a harmonic study beforehand, in conjunction with an analysis of the plant’s reactive power compensation, can help guide you in selecting VFD technology (6-pulse, 12-pulse, AFE, etc.) to comply with interconnection requirements.

Reliable expertise

With over 40 years of experience, BBA has become a leader in performing interconnection studies in the mining, industrial and energy sectors, including wind and solar farms. BBA’s highly specialized expertise is recognized by Canada’s major utilities.

In addition to carrying out interconnection studies, BBA is a partner of choice for engineering, construction supervision and commissioning, which greatly increases project success.