HIGH NEUTRAL EARTH VOLTAGES IN INVERTER AIRCONDITIONER - A CASE STUDY.
The Problem:
We were entrusted to study and analyses the problem and provide solutions. The dominant loads in the bank were all nonlinear loads like led lighting, UPS and Inverter Air conditioners. The Air conditioners and lighting were all single Phase loads, however the input of the ups was 3 phase and hence no imbalance, but the UPS did produce 3rd,5th, and 7th harmonics of 45%THDI.
The harmonics level of air conditioners was also about 40% THDI. Imbalance was seen as some air conditioners in other phases was switched off. The branch experienced repeated failures of the PCB cards.
The Manufacturer (a global Japanese Brand) refused to set them right until the elevated neutral voltage was brought down to < 5V. The electrical contractor tried to improve the grounding, however the problem persisted.
We used our PQ analyzer to study and analyse the problem of the air conditioners, and we observed the following.
a. Neutral-Earth Voltage 13V
b. Load imbalance.
c. Harmonics 3rd??,5th and 7th with other harmonics in smaller quantities.
d. Maximum current when operating with 2nos of airconditioners 10A. Total airconditioners is 13Nos.
When Inductive reactance XL =2πFL and capacitance 1/2πFC, the existence of coupling inductance and capacitance exists , higher harmonics is generated and aggravated by the harmonics generated by UPS, inverters PWM output and led lighting which generates a fair amount of harmonics, the high, frequency current between neutral and earth wire elevated the neutral voltage beyond the unsafe limits of electronic Inverter.
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The load unbalance compounded the problem as the residual currents which do not get cancelled in the neutral, as in the case of balanced linear loads (zero sequence currents) also flowed to the neutral and compounded the problem of high frequency Harmonics, and consequently elevated neutral voltage.
High neutral current can cause hot neutrals, which can result in snapping of neutral wires and causing dangerous high voltages and damaging equipments. Infact they can be a potential fire hazard. Which we have seen many hospitals and commercial buildings fire accidents.
PROBABLE SOLUTIONS:
ISOLATION TRANSFORMER:
As the Isolation transformer output is Isolated and neutral requires special care in designing. The earth is shorted, thereby reducing the neutral earth voltage to zero. However these transformer do not mitigate harmonics, the 3rd harmonics circulates in the delta and cause additional heating and requires the transformer to be K rated (K-13) preferably. This derated transformer has to be oversized by 50% and requires additional iron cores and copper thereby increasing the cost.
TRIPLEN ELIMINATOR:
WAVEFORMS TRIPLEN ELIMINATOR a Harmonic Mitigation Transformer (HMT) with zero sequence impedance is a better alternative as the substantial part of the dominant 3rd harmonics can be cancelled out in the secondary of the HMT. It is also possible to mitigate the 5th and 7th with additional windings. This is a better alternative as it does the dual function of reducing the Neutral voltage to zero and also mitigate harmonics, thus reducing heat considerably and also improving the reliability of the installation and also increases the electrical efficiency as the 3rd harmonics is offloaded in the phases & reduces the current harmonics in the phases and saving energy.
Additional value of this HMT prevents heating of neutral cables thereby preventing the potential of a fire hazard of the electrical installation. Phase current imbalance at the mains was substantially improved.
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1 年Phasor . NFP-R (Neutral Failure Protection) is a compact affordable device intended to be mounted on a standard 35mm DIN rail in any electrical distribution board, providing protection against neutral line disconnection. In electrical systems, neutral failure causes massive damages in equipment and can cause substantial expenses in repairs. The NFP-R is used to monitor the network and alert or deactivate (trip) the monitored electrical network. The device constantly senses the neutral line to the facility. In the event of an unexpected disconnection of the neutral line that causes overvoltage, for any reason at all, the NFP-R detects the malfunction and instantly cuts the power supply to an adjacent connected contactor, which cuts the power supply to the whole facility, thus preventing permanent damage to all connected equipment. Once the malfunction is resolved, the NFP-R counts 10 seconds and automatically reconnects the power supply to the contactor and by that to the whole facility. www.phasor.co.il