Potential Transformers (PTs) are critical components in power systems, designed to step down high system voltages to manageable levels for metering, protection, and control devices. Their accuracy and performance depend heavily on the burden—the total electrical load imposed on their secondary winding by connected devices (e.g., meters, relays, transducers) and the connecting leads. Exceeding a PT’s rated burden can cause significant measurement errors, compromising system reliability.
This guide breaks down the burden calculation process into clear, actionable steps, with explanations of key concepts, formulas, and practical considerations.
Burden (measured in volt-amperes, VA) refers to the total apparent power consumed by all devices connected to the PT’s secondary winding, including:
Active devices (e.g., wattmeters, relays)
Passive devices (e.g., voltmeters, fuses)
Losses in the secondary connecting leads (due to resistance and reactance).
PTs are rated for a specific burden (e.g., 50 VA, 100 VA) at a specified power factor (typically 0.8 lagging). Calculating the actual burden ensures it stays within this rating to maintain accuracy.
The first step is to list every device connected to the PT’s secondary winding. These may include:
Metering devices: Energy meters, voltmeters, power factor meters.
Protection devices: Overvoltage relays, undervoltage relays, distance relays.
Control devices: Indicating lamps, transducers for SCADA systems.
Auxiliary components: Fuses, terminal blocks, switches.
Note: Even small devices (e.g., indicator lamps) contribute to the burden and must be included.
For each device, obtain its rated burden (in VA) from manufacturer datasheets or nameplates. This value represents the apparent power the device consumes at its rated voltage (typically 110 V or 220 V for PT secondaries).
Key details to record:
Example:
A digital energy meter: 10 VA, pf = 0.8 lagging.
An overvoltage relay: 20 VA, pf = 0.7 lagging.
A voltmeter: 5 VA, pf = 0.9 lagging.
Each device’s burden is its rated VA at the PT’s secondary voltage. For most devices, this is directly provided by the manufacturer (e.g., “10 VA at 110 V”).
If a device’s rating is given at a different voltage, convert it using the formula:VAPT secondary=VArated×(Vdevice rated voltageVPT secondary)2
Example: A relay rated 15 VA at 220 V, connected to a 110 V PT secondary:VA110V=15×(220110)2=15×0.25=3.75VA
The connecting wires (leads) between the PT secondary terminals and the devices introduce resistance (R) and reactance (X), which consume additional VA. This “lead burden” must be added to the total.
Resistance depends on wire material (copper/aluminum), length (L, in meters), and cross-sectional area (A, in mm²):R=ρ×AL
For low-voltage secondary circuits (PT secondaries: 110–220 V), reactance is often negligible at 50/60 Hz, but for long leads, use:X=2πfL×10−3
Simplification: For short leads (< 50 m), reactance is often ignored, and only resistance is considered.
The total current (I) in the secondary circuit is determined by the total device burden (from Step 3):I=PT Secondary Voltage (V)Total Device VA
Lead power loss (VA) is:Lead VA=I2×(R+X)
Example:
Total burden is the sum of:
Total Burden (VA)=∑(Device VA)+Lead VA
Note on Vector Summation: For high-precision applications (e.g., revenue metering), device burdens should be summed as vectors (considering phase angles from power factors). For protection or general purposes, arithmetic summation is acceptable (conservative, as it overestimates slightly).
PTs are specified with a rated burden (e.g., 50 VA, 100 VA) and a maximum burden (often 125% of rated). The total calculated burden must be ≤ rated burden for accuracy, or ≤ maximum burden for short-term operation.
If total burden exceeds rated burden:
Errors in voltage measurement increase (PTs are calibrated for rated burden).
For metering PTs, this can lead to incorrect billing.
For protection PTs, relay operation may be delayed or inaccurate.
Include a safety margin (typically 10–20%) to account for:
Future additions of devices.
Variations in device load (e.g., relays drawing more VA during operation).
Adjusted Total Burden=Total Burden×(1+Safety Margin)
Let’s walk through a practical example:
Device burdens: 10 + 20 + 5 = 35 VA (arithmetic sum).
Lead resistance:R=1.72×10−8×(40/2.5×10−6)≈0.275Ω.
Secondary current: I=35/110≈0.318A.
Lead VA: I2×R=(0.318)2×0.275≈0.028VA.
Total burden: 35 + 0.028 ≈ 35.028 VA.
PT rated burden: 50 VA (assumed).
Result: 35.028 VA ≤ 50 VA → Acceptable.
Power Factor: For critical applications, sum burdens using vector addition (Stotal=(Ptotal)2+(Qtotal)2, where P=VA×pf, Q=VA×1−pf2).
Temperature Effects: Lead resistance increases with temperature (use 20°C resistivity for calculations, then adjust if operating temp is known).
Multiple Circuits: If PT secondary feeds multiple parallel circuits, calculate each circuit’s burden and sum them.
Burden calculation ensures PTs operate within their rated limits, maintaining accuracy in metering, protection, and control. By systematically accounting for device loads, lead losses, and safety margins, engineers can optimize PT selection and system performance. Always cross-verify with manufacturer specifications and relevant standards (e.g., IEC 61869-3, ANSI C57.13).
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