Specs for Medium Voltage Systems

A medium voltage system is defined as having a power output range between 1,001 and 72,500 volts. It is used where a considerable amount of voltage is required, such as electric-driven crude oil pumps at pump stations. The medium voltage system is made up of a variable frequency drive (VFD) that regulates and distributes the output power.

Modules

• The average VFD module consists of three major parts: a switchgear, a transformer and the VFD itself. The switchgear controls the power distribution within the module. The transformer is based on an adjoining platform and functions by isolating the VFD from the main power system and limiting the voltage, while the VFD varies the power frequency.

Switchgear

• Switchgears contain a circuit breaker, control wiring, and a multifunction protective relay computer that regulates the circuit breaker. Switchgears safeguard against power overloads, similar to the action of electrical breaker panels in a household. All switchgears require inspection and quality assurance in accordance with the National Electric Code guidelines. The different types of switchgears include metal clad, metal enclosed, vault and padmounted.

Transformers

• Prior to reaching the VFD, the voltage needs to be reduced, which is done by the transformer. Modules may also contain a smaller transformer, located on the same platform as the large transformer. Transformers are filled with cooling oil, and are thus fixed in a spill containment tank to protect the environment from any possible spillage.

Variable Frequency Drive (VFD)

• VFDs generate a sgnificant amount of heat, and thus require a liquid cooling system. They also contain a DC link, capacitators, terminations and controls for the power cables, and the electronics needed for power management. VFDs vary the frequency of the output power, meaning it varies the rate of alternating current.