Non-conventional instrument transformers
and power quality aspects – an overview
Erik P. Sperling
Representative Meeting / Switzerland June 7th. 2017
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Presentation overview
1. History
2. Instrument transformer overview
3. Power quality aspects
4. NCIT’s for voltage measurements
5. NCIT’s for current measurements
History
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• Since 120 years, inductive instrument transformers are used for billing, metering and protection purposes
• British patent from 1887, voltage transformer from 0.1 V up to 10 kV
• Beginning of the 1930s, a changeover from 110 kV to 220 kV system voltage network
• In the 1950s, a second important changeover from 300 kV to 420 kV system voltage network
• Current topics today UHV with voltage levels up to 1.2 MVAC and ±1.1 MVDC
1936 220 kV Cascade voltage transformer of Emil Pfiffner.
Instrument transformer overview
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Conventional type
Inductive voltage transformers Capacitive voltage transformers
Inductive current transformers
AC AC
AC
IEC standard IEC 61869-1 & IEC 61869-2/-3/-5
Non-conventional type
Divider (C/RC/R - type) Pockels/Piezo effect
Zero-flux Rogowski coils Faraday effect Shunts
AC (DC)
DC AC AC DC
AC DC
IEC standard IEC 61869-1 & IEC 61869-6 IEC 61869-7/-10/-11/-14/-15
AC
AC DC
System requirements – frequency content in a network
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• Rated frequency
• Sweeps, dips, swells, flicker, ferro-resonance
• Power quality ranges
• Transient impulses LIWL, SIWL, chopped, chopped under SF6
• DC components
| Defined as measuring range in standard
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Definition:
It is the comparison between current existing values, measured in the power network, and agreed characteristics of the energy by the supplier.
Main quality criteria are:
1. Voltage/current magnitude
2. Fundamental frequency
3. Wave shape (voltage/current)
4. availability
Definition of power quality criteria in:
Standard EN 50160
System requirements – Power quality
EMC – standards: (HV- and UHV networks)
(Limits)
IEC 61000–3–6 harmonics
IEC 61000–3–7 Flicker
IEC 61000–3–13 Unbalance
(Test technics)
IEC 61000–4–7 General Guide
IEC 61000–4–30 PQ measuring methods
IEEE 519-2014 ” IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems”
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System requirements – PQ examples
Range of frequency DC (0 Hz) up to < fr
DC offset in a AC network
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System requirements – PQ examples
Voltage dips
0 0.02 0.04 0.06 0.08 0.1 0.12-150
-100
-50
0
50
100
150test signal: dip 60 %, 2.5 cycles
Range of frequency DC (0 Hz) up to < fr
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System requirements – PQ examples
Swells in voltage systems
0 0.02 0.04 0.06 0.08 0.1 0.12-300
-200
-100
0
100
200
300test signal: swell 200 %, 2.5 cycles
Range of frequency DC (0 Hz) up to < fr
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System requirements – PQ examples
Range of frequency DC (0 Hz) up to < fr Unbalance in voltage systems
Flicker 1 Hz to 70 Hz
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System requirements – PQ examples
Range of frequency DC (0 Hz) up to < fr Single phase ferro-resonance oscillations Characteristic: sub-harmonics 1/3; 1/5; 1/7 of fr
Three-phase ferro-resonance oscillations Characteristic: sub-harmonics 1/2 of fr
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System requirements – PQ examples
Range of frequency fHF > fr
f = h fr; h: even-whole-numbered f = h fr; h: uneven-whole-numbered
f h fr; inter-harmonics
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System requirements – PQ examples
Range of frequency fHF > fr
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System requirements – PQ examples
Range of frequency fHF > fr Voltage sag with 400 Hz superimposed signal
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System requirements – PQ examples
Range of frequency: transient Phenomena >> fr
Voltage interuption
Switching operation in parallel switchyard
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System requirements – PQ examples
Range of frequency: transient Phenomena >> fr
Few impulses Very fast du/dt Duration: approx. 0.3ms
Many impulses High-frequency content Duration: approx. 1ms
Power Quality – Possible causes
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DC
• No. of DC systems increasing
• Coupling between AC/DC systems (kV up to MV)
• Solar activity (natural phenomenon)
• Earthing strategy
SHR TFR
• Increasing voltage fluctuation due to load variation
• Increasing ferro-resonance oscillations
• Unbalances load
• Fluctuate production of energy because of renewable sources
• Significant increasing of switching operations
• Coupling between AIS/GIS systems
• Increasing of natural phenomenon effects because of AIS area expandings
• Equipment or system failures
HFR
• Electric energy feed-in by power electronics
• Non-linear loads
• Frequency converter for traction and drives
• Coupling between different networks with converters
• Electric-arc furnaces
Power Quality – Possible impact
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increasing of power losses within the network
increasing electric stresses within the HV insulation system
thermal stresses within the connected equipment due to harmonic currents
increased sound noise emission (transformers, coils, capacitors etc.)
incorrect control of equipment.
faulty activation of protection equipment (old protection system)
Question: how suitable are inductive instrument transformers for PQ-measurements ?
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Ind. voltage transformer EOF 72 Ind. current transformer EJOF 72
Frequency response measurement
How to measure PQ parameters ?
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First resonance peak of conventional VT’s depending on the system voltage Vm
Source: IEC/TR 61869-103, 2012-05
Characteristics and measurement results @ f < fr & f > fr
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Frequency response measurement
Frequency response of ɛU(f) for 36kV-VT(light
green), 72.5kV-VT(dark green), 123kV-VT(blue),
245kV-VT(purple),420kV-CTVT(red) and 420kV-RC-
divider (yellow), as measured at PFIFFNER
Frequency response of Δ(f) for 36kV-VT(light
green), 72.5kV-VT(dark green), 123kV-VT(blue),
245kV-VT(purple),420kV-CTVT(red) and 420kV-RC-
divider (yellow); as measured at PFIFFNER
Non-conventional measuring devices – voltage
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High voltage terminal
Metallic expansion bellows (hermetically sealed)
RC-divider primary active part
RC-divider secondary active part
Secondary terminal box
AC voltages: 72.5 – 800kV DC voltages: ±50 – ±500kV
Insulator (Porcelain or composite)
ROF 420, Germany Type ROF/RGF
Oil- or SF6-gas impregnated solutions
Non-conventional measuring devices – voltage
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HV terminal
Primary active part R1 & C1
Secondary active part R2 & C2
Secondary terminal box
Ground terminal
Insulator
Type RGK
RGK 400DC, Switzerland
AC voltages: 72.5 – 500 kV DC voltages: ±50 – ±500kV
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Non-conventional measuring devices – voltage
Frequency dependent ratio
𝑍2𝑍𝑔𝑒𝑠
=𝑅2
𝑅2 + 𝑅11 + 𝑗𝜔𝐶2𝑅21 + 𝑗𝜔𝐶1𝑅1
𝑓 → 0 𝑈2𝑈1
= 𝑟𝑅 =𝑅2
𝑅2 + 𝑅1
𝑈2𝑈1
= 𝑟𝑅 =𝑅2
𝑅2 + 𝑅1
𝑓 → ∞ 𝑈2𝑈1
= 𝑟𝐶 =𝐶1
𝐶1 + 𝐶2
𝑈2𝑈1
= 𝑟𝐶 =𝐶1
𝐶1 + 𝐶2
𝑈2(𝑗𝜔)
𝑈1(𝑗𝜔)=
𝐼 ∙ 𝑍2𝐼 ∙ 𝑍𝑔𝑒𝑠
=𝑍2𝑍𝑔𝑒𝑠
Complex transfer function
𝑍2𝑍𝑔𝑒𝑠
=𝐶1
𝐶1 + 𝐶21 + 1
𝑗𝜔𝐶2𝑅2
1 + 1𝑗𝜔𝐶1𝑅1
Equivalent circuit diagram
Non-conventional measuring devices – voltage
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Frequency response of an RC-divider (NCIT)
Type ROF 420 Upr: 400 kV Um: 420 kV UT: 630 kV ULIWL: 1425 kVpeak USIWL: 1050 kVpeak Uchop.: -1640 kVpeak Usr: 100/√3 class: 0.2% cable length: 270 m 𝒇 → 𝟎 𝒇 → ∞
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Non-conventional measuring devices – voltage
ECD secondary terminals connected in series
ECD secondary terminals connected in parallel
Non-conventional measuring devices – voltage
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RC-Dividers Pockels/Piezo-Effect (electro-optical effect)
Source: FastPulse Technology, Inc.; Electro-Optic Devices in review , Figure 2, Laser & Applications April 1986
Basically, no magnetic iron core is used
Frequency response performance of NCIT voltage measurement systems
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Non-conventional measuring devices – current Faraday effect
(magneto-optic current measurement) Rogowski coil (magnet field effect)
Source: IEC TR 61869-103, figure 40 𝑢𝑖 𝑡 ~𝑑𝑖(𝑡)
𝑑𝑡
Basically, no magnetic iron core is used
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Non-conventional measuring devices – current
Shunt (resistance current measurement)
Zero flux (magnet field saturation effect)
𝑢 𝑡 = 𝑅 ∙ 𝑖(𝑡)
CT
Integrator
Amplifier
Saturation
detector
Peak
detector
Relay
Oscillator
Amplifier
Amplifier
Amplifier
Contact of
K1 relay
Detect residual flux, its
direction and value
Cancel the oscillator’s effect
Resistance
Output the polarity and value
of the voltage in the case that
residual flux is not zero
K1 relay operates if Ip
become over the limit of
amplifier output,
Electrical circuit
Energize the cores
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Frequency response performance of NCIT current measurement systems
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Conclusions
• Frequency content in a network can be divided into 4 ranges: DC, sub-harmonic, harmonic & transient voltages
• The conventional instrument voltage transformers have a limited bandwidth depending on system voltage level
• For voltage PQ measurement, RC-dividers have the highest potential due to wideband characteristic and direct secondary voltage analysis
• For current PQ measurement, different measuring applications are available.
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Thank you very much !