energy meter in the age of the photovoltaic

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Energy Meter in the Age of the

Photovoltaic

Grid connected Solar PV system configuration

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Energy Meter in the Age of the Photovoltaic

DC to AC Converter and the Meter

3Converts DC to AC, the result is not a pure Sine wave and therefore contains Harmonics


Harmonics in electrical Grid

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Harmonics Impacts on the Meter

Displacement of Power Factor (PF) Current in the Neutral wire (for 4 wire system) DC-Component in the Current flow Impact on Current & Voltage Sensors Impact on Power Supply

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Power measurement using 1 Element

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Power (P) = Voltage (V) * Current (I)

Blondel's theorem (named after André Blondel), states that:In a system of N wires, needs (N-1) wattmeter to measure the electrical Power.

For example: 3 phase meter , 3 wire needs 2 wattmeters, but a 3 phase meter, 4 wire (with a neutral) needs 3 wattmeters.

1 Element Single Phase Meter needs

Wattmeter


Power measurement using 1.5 Element

1.5 Element

Current in the Neutral wire, we need to add Current-Meter


1 Element – Fraud cannot be detected

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1.5 Element - Fraud Detection

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How it looks like?

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Power measurement - Voltage/Current some phase

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in this case Pave is MaximumM

AX


Phase shift between Voltage and Current

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Reactive energy due to phase shift

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(1) ø = 00 , only Active Power

(2) ø ≠ 00 , phase shift between Voltage and Current, portion of the Power curve going negative Reactive Energy

(2) Pav is decreased

Reactive Energy

(1) (2)


The meaning of Active and Reactive Energy

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Active Power (W) produces real work Reactive Power (var) is the power required to produce

the magnetic fields to enable the real work to be done Apparent Power (VA) is considered the Total Power that

the Electric company supplies to produce the required amount of Active Power

Active Power

Reactive Power

Apparent Power

√

?

??


Analogy to Active and Reactive Energy

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What is Power Factor (PF)?

PF = Ratio of Active Power to Apparent Power (W/VA)

PF depends on the phase angle between Voltage and Current waves

A low PF indicates a low energy efficiency (losses)


for pure Sine wave PF calculated as follows

Power Space Vectors

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Harmonics from large number of Photovoltaic converters cause Displacement of Power Factor (PF) hence increase

Reactive Energy


Effects of harmonics on wave

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3rd Harmonic anti-phase 3rd Harmonic


Effects of harmonics on Power

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Displacement of PF PF

produces Reactive Energy

PAV1 = VRMS1 * IRMS1 > PAV2 = VRMS2 * IRMS12

P(W)

S(VA)

S(VA)Q(var)


PF for Distorted wave

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for distorted Sine wave PF calculated as follows

Energy of the harmonics is mainly Reactive energy


Fourier Transform

No Harmonics, Balanced 3 phase 4 wire system

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The special about the 3rd Harmonics

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Harmonics can cause current on the Neutral Wire in 3 Phase 4 wire system


Current Sensors are strongly affected by Harmonics

Toroid for CT meters up to 15A (no DC-Tolerance) Toroid for Direct Connection up to 200A

No DC tolerance (needs strong shielding), problem with DC-Component

DC-Tolerance (large Core, needs strong shielding – Expensive)

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Shunt current up to 120A No problem with DC component


Distorted wave: “Fourier Transform” shows DC component DC Component impact on the Energy measurement

DC Component Impact on the Current Sensors

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Shunt


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DC Component Impact on the Current Sensors (cont.)

Magnetic Hysteresis

DC component

DC component impact on CT Linearity


Harmonics Impact on Meter’s Power Supply

"Linear-Power-Supply" and “Switched-Mode-Power Supply”, using a step-down transformer. Usually when correctly designed using high quality capacitors, these types do not “suffer” much from harmonic distortion

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Harmonics Impact on Meter’s Power Supply

“Capacitor-Power-Supply” using a Voltage Dropping Capacitor. The capacitor can be damaged significantly by harmonics

The problem is more difficult when the Meter is installed outside under a burning sun

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Energy Meter in the Age of the Photovoltaic Summary 1/2

Meter cannot resolve the problems caused to the Voltage and Current waveforms, the meter will use those waves to measure energy quantities (P, Q, S)

Voltage and mainly Current Sensors are affected by Harmonic distortion

Photovoltaic Energy supplier’s billing should be based on three electrical quantities: Active power (P), Reactive power (Q) and Apparent power (S)

Meter should be with measurement of P, Q and S. The difference in kVA meter measurement can be as large as 30% when the waveform is distorted

For PF displacement, it is necessary to install “PF-Correction -Capacitors”

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Energy Meter in the Age of the Photovoltaic Summary 2/2

Energy of the harmonics is mainly Reactive energy, even though there are no requirements in the IEC standards for testing Reactive Energy under the effects of harmonics??

3 phase 4-wire system, Meter should be of type “1.5 Element”, (measurement of the Neutral wire current)

CT current sensor cannot detect DC-Component, and also affected by DC-Component. So it is a question if a meter uses CT as a current sensor can be used?

Shunt current sensor can detect DC component . Can be used, provided that the Meter can measure DC-Component (“All-Pass”)

Capacitor-Power-Supply: only with high quality capacitors, that can withstand harmonic distortion

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Accuracy Stability Over Years

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31Energy Meter in the Age of the Photovoltaic

Thank you for your attention!

SEEEI 2012

By: Avi Lugassi

energy meter in the age of the photovoltaic

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