| Reason: |
The existing language ("¦four wire wye connected meterin€¦"…three wire connected meterg†"…four wire wye grounded source at a Transmission level voltage two element meting…") does not correctly distinguish between the power system characteristics and the appropriate metering configuration. The EEI "HANDBOOK FOR ELECTRICITY METERING" (9th Edition) has a section titled "MULTI-STATOR METER APPLICATIONS WITH INSTRUMENT TRANSFORMERS" which describes "four-wire wye", "three-wire secondary without ground" and "three-wire wye secondary circuit with neutral grounded." The ERCOT transmission system is a three-wire wye neutral grounded transmission system. Only three power carrying conductors are provided on transmission circuits. However, the ERCOT transmission system is an effectively grounded system (as defined in IEEE Std. C69.92.5-1992 "IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Pt V – Transmission Systems and Subtransmission Systems"). The basic factors considered when using this type of grounding system are overvoltage control, ground fault-current magnitude and protective relaying seltion – the ground connection is not used for actual transfer of power. The ERCOT transmission system is not "a four wire" system and is not a delta system. The last paragraph of this section in the EEI "HANDBOOK FOR ELECTRICITY METERING" substantiates this fact. Also, the EEI "HANDBOOK FOR ELECTRICITY METERING" characterizes the meters themselves as either single-stator, two-stator or three-stator.
The question of should the metering practicbe to ‘meterhe ld’ or ‘meterhe source’ for transmissions systems is best answered by applying the information given in the EEI "HANDBOOK FOR ELECTRICITY METERING". The EEI "HANDBOOK FOR ELECTRICITY METERING" contains extensive application information pertaining to low and medium voltage self-contained metering but only has a small section titled "MULTI-STATOR METER APPLICATIONS WITH INSTRUMENT TRANSFORMERS" which addresses metering transmission circuits. However, this section es state" …the three-wire wye connection with neutral ground is frequently encountered. With voltages in the order of 24,000 volts the designer is reasonably certain that customer loads will not be connected line to ground and two-stator metering will be correct. However, this may not always be the case." But for three phase power transformers with delta or ungrounded wye winding connection, phase-to-phase connected single phase power transformers, etc. where energy is supplied only through the three phase wires, then it is always correct metering as defineby "Blondel’s Theorem" and three-stator metering is not any better. This theorem is based on the number of conductors transporting energy and therefore multiple power transformers or metering of generation connected to these type power transformer connections does not affect the decision to utilize this type of metering. The application of current transformers in two phases and two-stator metering will be correct for metering a facility connected to a three-wire wye neutral grounded transmission system if the facility has only power equipment that does not provide a connection for ground path power current (e.g. three phase power transformers with delta or ungrounded wye winding connection, phase-to-phase connected single phase power transformers, etc.) because energy is supplied only through the three phase wires to the power equipment.
The application of current transformers in two phases and two-stator metering is a standard in the utility industry for a facility that has only power equipment that does not provide a connection for ground path power current.
The current that may flow in the ground path power at a facility connected to a three-wire wye neutral grounded transmission system due to insulators, surge arresters, and voltage transformers connected phase-to-ground (which are typically applied on each phase and therefore predominately balanced) is negligible (i.e. less than the error that exists in the instruments transformers alone).
Harmonics are generally not a concern in the transmission system current or voltage. However, any effect on metering performance of harmonics that may be present in the transmission system current or voltage would be similar whether the metering is two-stator or three-stator metering when applied to a facility with only power equipment that does not provide a connection for ground path power current (e.g. three phase power transformers with delta or ungrounded wye winding connection, phase-to-phase connected single phase power transformers, etc.). Any concern that harmonics may be present in the transmission system current or voltage should be handled on a case by case basis.
The EEI "HANDBOOK FOR ELECTRICITY METERING" discusses the fact that "When voltage transformers are connected in wye-wye there is a third-harmonic voltage generated in each primary winding." This issue will be the same whether the metering is two-stator or three-stator metering when applied to a transmission facility. This issue is addressed by ensuring that each voltage transformer is connected to the transmission facility grounding systemnbsp; This "… provides a path a path for third-harmonic current flow, thereby keeping the third-harmonic voltage at a low value."
As long as it can be verified that the facility being metered has only three phase power transformers with delta or ungrounded wye winding connection, phase-to-phase connected single phase power transformers, etc. where energy is supplied only through the three phase wires then the length of line should not affect the decision to utilize this type of metering.
With this understanding and verification of the circuit connections, two-stator metering will be correct and this application should not be restricted to only existing sites (grandfathering).
Requiring the use of three current transformers where only two are necessary for metering and the current transformer in the third phase is not needed for some other reason (power quality monitoring, etc.) is not efficient use of resources (cost of current transformers, stands, foundations, conduit, wiring, installation and ongoing maintenance). Also, the additional high voltage current transformers can fail disruptively, potentially damaging other |
