Monitoring Higher Voltages

 Some building electrical systems just will not cooperate when it comes time to monitor power. You search and search and cannot find a point to monitor power on the low voltage side of the transformer (below 600 volts). Now you must face the fact that only the high voltage side can be used for power monitoring. This involves working with voltages from 5 to 35 kV (kV=1000 volts). You can install new equipment or sometimes use the existing metering equipment. We hope this article will make it a little easier to plan your installation.

 Insulation Rating

 
Devices used in power monitoring must be insulated for the voltage monitored. Most metering is done at lower voltages (between 120-480 V), so these devices are typically insulated for 600 volts.

If it is only possible to monitor power at a distribution voltage, this monitoring is usually done at the switchgear or the substation. The current transformers (CT's) and potential transformers (PT's) used will carry insulation ratings of 5, 15, 25, or 35 kV.

The insulation rating of the device must exceed the maximum (line-to-line) voltage of the circuit on which it is used. Listed below are voltages that require the different insulation ratings:

The insulation rating also lists a "Basic Insulation Level" or B.I.L. rating, which is a rating of the ability to withstand a large voltage impulse or surge caused by lightning (for example, 110 kV B.I.L.)

 Potential Transformers

 Some meters and transducers can use a direct voltage hookup below 600 V. Above 600 V, devices are designed for a 120 V input. Potential transformers (PT's) reduce the system voltage to the useable 120 V. A 2400:120 potential transformer will have a ratio of 20:1 and will have an insulation rating of 5 kV. These 5 kV PT's are available in single phase or three phase. If 15 thru 35 kV PT's are used, they are only available in single phase models that are grouped together for three phase.

If the PT's are connected in a wye configuration (See Fig. 1), only one fuse is required per PT. For safety reasons, it is not recommended to fuse a neutral connection. Common practice is to fuse both terminals on a line-to-line delta configuration. Remember that a line-to-line voltage will be higher than a line-to-neutral voltage. On a 7200/12470 volt system, a 7200:120 PT is used line-to-neutral or a 12000:120 PT is used line-to-line.

Typical wiring diagrams for three-phase potential transformer situations

 Current Transformers

Typically it is acceptable to install a 600 V window type current transformer (CT) over insulated cable at higher voltages (NOTE: some local codes do not allow this). If a CT is used over an insulated cable with a current carrying sheath or concentric neutral, the sheath ground wire or neutral should be taken back through the CT window prior to grounding. Otherwise, the CT will sense the current in the neutral or sheath.

If the current to be monitored on a system above 600V is a bare cable, bare bus, or insulated cable (in an area that does not allow the use of 600 V CT's), an in-line higher voltage CT must be used. Window style CT's are not available in the higher voltages. The conductor must be separated and the CT is inserted in-line. The CT's are physically large, and the connections are made with lugs bolted to bolts or bars on the CT.

 Physical Mounting

Finding a place to mount medium voltage CT's and PT's is usually your biggest challenge. They are heavy and can measure 12 inches or more on each side.

An enclosure, a room, or fenced in area will be required to install the CT's and PT's. Switchgear manufacturers have a pre-assembled enclosure with CT's and PT's mounted and wired. This is called a "metering bay."

 Using Existing CT's and PT's

Often existing CT's and PT's can be used. Care must be taken not to over-burden (over-load) the existing devices. There is an article in this section on burden calculations.

If the existing CT's are supplying a 3 position selector switch for an ammeter, remember that the switch will be shorting out the other two CT's when reading one. This may require some special wiring to bypass the switch.

 Summary

Monitoring voltages above 600 volts is very common. Mounting the CT's and PT's can be a problem. The outputs are 120 V for PT's, and 0-5 amps for CT's just like 600 V devices. When using formulas C and D, for watt transducer calculations (page 129 in the Kele catalog), you will use the ratio of primary to secondary in your calculations. The higher voltage CTW Series CT's and PT Series PT's are included in our current catalog.