The Canadian Electrical Code, Rule 14-612 Transfer Equipment for Standby Power Systems prohibits the simultaneous connection of two or more power supplies to electrical equipment and facilities.
There is an obvious exception. Rule 14-612 does not apply to parallel generation systems covered by Section 84 — Interconnection of Electrical Power Production Sources. Section 84 does apply when an electrical utility customer opts to connect and operate its own generation equipment in parallel with the electrical utility network.
No doubt you have noticed that the Scope paragraph of Section 84 contains the words: “This Section is supplementary to or amendatory of the general requirements of this Code——.” This is always interpreted to mean that the rules found in this special section of the code always override any similar applications that you may find in the general sections of the code. Here are a few of those special rules.
To avoid equipment damage and power outages, Rule 84-006 requires that privately-owned parallel generators must always be synchronized with the electrical utility’s supply voltage before interconnection, and that synchronization must be maintained continuously. With one exception, this requires that the owner must install and maintain suitable control equipment.
Induction generators are an exception to this requirement. This type of machine depends on the regular supply voltage to create the magnetic flux in the generator’s squirrel cage rotor, to induce current flow in the stator windings. Induction machines automatically operate in synchronism with the main supply voltage because they cannot induce current flow when not interconnected with a separate voltage source. They also stop producing power and shut down after a few cycles as the magnetic flux decays to zero when main power supply fails or is disconnected. Although this rule does not appear to distinguish between the two generator types, no doubt some practical interpretation may be necessary.
Rule 84-008 states that when power is lost in one or more phases of the main supply, a parallel power source must be automatically disconnected and not reconnected until power has been restored. This is a precaution to prevent “”islanding””, a condition that may arise when the parallel supply continues to generate power into the electrical utility grid, thereby creating an electrical shock hazard to people working to restore the power.
To prevent equipment damage after an electrical utility outage, the parallel generator must always be shut down and re-synchronized with the utility supply before reconnection. An induction generator would already have shut down automatically since it cannot operate on its own, and so there would be no concern about a lack of synchronization on startup.
From time to time, an electrical utility may need to maintain its lines without incurring the risk of voltage feedback from its customer. For this reason, Rule 84-024 requires the owner of parallel generation equipment to provide a disconnect switch accessible to the electrical utility for safety isolation purposes.
Personnel must be able to verify that cables and equipment are de-energized and cannot be re-energized before work begins. People must also remain confident that electrical equipment can be operated safely under load and closed safely under fault conditions.
With parallel supplies, switchgear is often energized from both directions. To minimize the potential electrical hazards to operating and maintenance personnel, Rule 84-026 specifies that a disconnection means must:
- have visible contacts so that personnel can verify whether the contacts are open or closed
- have multi-phase operation, capable of opening safely under load
- be capable of being closed with an inadvertent fault on the system without risk of injury to the operator
- be capable of locking open
- display a warning sign to indicate that both sides may still be energized when the disconnect is in the open position
- be accessible in case of emergency
Rule 84-026 also requires that fuses be isolated from both directions to ensure that fuses are not handled nor replaced when live.
Because they are working in an environment that contains more that one electrical power source, operating and maintenance personnel must be able to correctly identify all electrical supplies and the means to disconnect cables and equipment when necessary. Rule 84-032 ensures that people have up-to-date knowledge with the requirement that a notice and single-line diagram be posted at the main electrical service and at each generator, explaining the switching, isolation and interlocking arrangement for each supply and their methods of operation.
As with earlier articles, you should check with an electrical inspector for a more exact interpretation of any of the above in each province or territory as applicable.