NWEMS Primary Metering August 22, 2018 Bill Unbehaun, Tacoma Power Exchanging Expertise Since 1893 What is Primary Metering? Metering energy flow past a point at high voltage above 600v Both PTs and CTs Meter enclosure with test switch 1
Why Primary Meter? Utility substation or generation metering Intertie metering for wheeling or purchase 2
Distribution Sub Industrial end user may need high voltage or want a bulk rate 3
How is Primary Metering Accomplished? PTs lower voltage to useable level and isolate CTs isolate current signal from high voltage, and may lower it to a useable level Test switch provides safe means to work on meter when necessary Meter senses and records energy consumption /flow Instrument Transformers Provide isolation of metering equipment from high voltages to increase safety of personnel Reduce voltage or current to easily measureable levels Should be insulation (high pot) tested Understand polarity markings Follow utility practice on grounding 4
Grounding Grounding Meter Cabinet 5
Potential Transformers Potential Transformers Can also be called voltage transformers (VTs) Provide isolation for safety and reduction for easier measurement One or two bushings CCVTs Optical VTs 6
Primary Voltage Rating Basic Impulse Level Accuracy Class Burden rating Thermal rating Potential Transformers Voltage class (kv) what voltage is the PT designed for Accuracy class how accurate is the device at replicating the primary at the ratio stated ratio correction / phase angle correction Burden rating how large a load can you put on the secondary and maintain accuracy in VA Basic Impulse Insulation Level (BIL) Thermal rating limit before overheating 7
Ratio Frequency Potential Transformers Ratio secondary is 120v nominal Frequency device is designed specifically for 50 or 60 Hz 8
Potential Transformers Devices in the secondary circuit are wired in parallel Electronic meter sensors impose minimal impedance, but meter power supply transformer may be a significant load with long secondary lead voltage drop Other devices powered off PT may cause imbalance don t exceed burden rating Fusing Potential Transformers Primary Secondary None Can protect equipment and personnel Follow your utility standard 9
Potential Transformers Current Transformers 10
Current Transformers Provide isolation for safety and reduction for easier measurement Bar type, wound, or bushing style (similar to a window CT) Accuracy class Ratio Insulation class BIL Rating Factor Burden rating Frequency 11
Current Transformers Ratio secondary is 5 amps in U.S. Insulation class voltage rating BIL Frequency Rating factor capacity to exceed nominal ratio in multiples Accuracy class Burden rating in ohms Current Transformers 12
Current Transformer Safety Never open circuit the secondary of a loaded CT Potentially deadly voltage develops across the secondary Voltage waveform has very high brief spikes that may not show up on an RMS meter From electrical4u.com 13
Current Transformer Safety Tracking on dirty test leads for HV ratio checks All equipment should be rated for the application Years ago, someone installed 600v CTs in a primary cabinet on 7200v cables During an audit of the service, fortunately no injuries, but approach distance violations caused by visual dissonance of secondary CTs Heavy load or fault current could have damaged 600v CTs Combined VT CT 14
Combined VT CT C1 & C2 V1 & V2 CT ratio Combined VT CT RF VT ratio CT accuracy VT accuracy RVF = rated voltage factor or % overvoltage, here 190%, probably for 30 sec. 15
Selection and Maintenance Test the instrument transformers on a regular schedule billing accuracy begins with the least accurate device, and errors compound On LARGE accounts, try to match PT and CT so ratio correction factors net to 0 and phase angle corrections match Document it s a pain to get a serial number after it s in service Connections 16
Connections Wye 9S meter Delta 5S meter Overhead to overhead Overhead to underground Pad mount for underground Indoor rack Overhead Primary Rack 17
Padmount Primary Cabinet 4 Wire Padmount Primary Cabinet 3 Wire 18
Padmount Primary Cabinet 3 Wire Indoor Primary Rack 4 Wire 19
Indoor Primary Rack Mounted How do we connect the meter? 20
Wiring Secondary conductor between ITs and meter should balance cost and handling with signal loss Voltage and current circuit should be kept separate don t bond PT return with CT return (unless it s your standard) IT circuits should be grounded in 1 place consider grounding at the rack, but follow your utility standards Test Switch 21
Test Switch Inspect test switches carefully before installing there may be defects Examine old installations carefully they may not meet your current standard There are several configurations of test switches voltages on the left, voltage return on the far right, currents in between; OR voltages and currents alternating, etc. Test switch may open up or down And now THE METER (Be sure you ve installed the correct socket for the meter type) 22
23
24
Primary Meter Meter must be accurate with large multipliers inaccuracies can become huge billing errors Multiplier must be correct PTR x CTR (x RCF) :: PTR 1000 x CTR 160 = 160,000! Dropping or adding a zero creates major customer relations problems DOCUMENT! Record test results and multiplier Programming the Meter Modern metering offers a wide variety of metering possibilities: Kwh, Kvrh, Kw Time of use PQ sags, swells, spikes, harmonics KYZ pulses to customer equipment Program the meter as you need Document the meter and software 25
Checks Verify wiring before putting in service be sure phase voltage matches phase current at meter, second eyes are good When loaded, perform an audit with a circuit analyzer or at least use the meter and test switch to check phasing and load Unshort CTs when in service and be sure none are open circuited DOCUMENT! Who built it, who checked it, who installed it Ferroresonance Ungrounded systems, usually 3 wire Caused by line capacitance, PT inductance Usually when line is lightly loaded Triggered by switching or other transient Voltage spikes cause PT to fail catastrophically Resistors in PT secondary seem to correct problem 26
Failure from ferroresonance Failure from ferroresonance 27
Resistors Meter Cabinet Resistors Below Test Switch 28
Maintenance Test CTs and PTs ratio, angle and burden Test meter Verify wiring Justify by ramifications of error or malfunction This is probably a large customer Errors can be large Relationship with customer important Additional Information ABB Instrument Transformers Technical Information and Application Guide ABB Instrument Transformer Reference GE Electrical Utilities Reference Guide for Instrument Transformers GEC Durham website Ritz website EEI Handbook for Electricity Metering 29