Hong Kong – Fronting the Le Diamant Building in the heart of buzzing Nathan Road, Kowloon, is a newly erected LED Billboard. The advertising billboard is the centrepiece of the project towering 8 storeys. The billboard uses the latest LED technology to bring a new generation of marketing media to life. Despite a successful testing and commissioning, the LED billboard was faced with severe issues of nuisance tripping. The tripping incident was most frequent when the LED displayed light in white. Frequently, the billboard LED display encountered lengthy down time and could not be turned back on.

Fig 1. LED Display Billboard (switched off) at Le Diamant, Hong Kong

With the initial suspicion that the tripping were caused by power quality issues, the Mun Hean Hong Kong team were invited to investigate and resolve the problem.

The LED billboard measures 28m height and 8m width and is powered by its own proprietary switched mode PSU. Its electric supply is connected via a dedicated 800A MCCB at the building main DB panel. Diagnostic by the MH team found that THD-I of over 30% with an average Harmonics current of 200A. With predominantly 5th and 7th order harmonics presence. The harmonics level is below acceptable levels as stipulated by local utility regulations on harmonics pollution limit. More importantly, given the intense usage of LED lighting, a non-linear load, the resultant harmonics caused sever exertion in the power quality of the facility, resulting in frequent tripping of the system.

The solution proposed by Mun Hean was the installation of a 200A Active Harmonic Filter System. The immediate problem in this installation was the limited space in the switch room to install a rack mounted system. A wall-mounted Sinexcel AHF system was specially designed for this project. Being a 4x50A modular system, the AHF solution was compact and allows for easy upgrading should it be required in the future.

Result of the installation was a success. The AHF compensation reduced the THD-I to below 5%, complying with local regulations. The nuisance tripping problem was resolved. Further, it was reported that the brightness of the Billboard was 50% brighter than initially installed.

As the usage of non-linear loads such as Inverter Drive & Switching Mode Power Supplies proliferates, harmonic problem will become a prominent problem in power quality. Whereas harmonics resolution can be a complex and specialised area, with the right solutions, there is tremendous cost savings in terms involved.

Location
– Le Diamant, 703-705 Nathan Road, Mong Kok, Kowloon, Hong Kong

Challenge
– Frequent nuisance tripping of circuit breaker of a large LED billboard display system
– The switch room space is very tight and lack of space in existing panel to install rack component

Customer requirements
– To eliminate nuisance tripping of the electric supply to their LED Display Billboard
– To meet the Hong Kong Electric Co. Ltd requirement of harmonic limit

Facts in brief
– LED billboard producing over 30% Current Harmonics
– The highest amount of harmonics injected from the billboard occurred when the scene contain mostly white colour graphics

Solution applied
– Data logging at load MCCB point pre-AHF solution
– Assessment of the THD-I and Harmonics Current Loading
– Installation of Active Harmonics Filter at the main DB where the load is connected

Engineering services
– Data logging of the load post-AHF solution
– Report on Harmonics Power Quality

Fig 2. Harmonics Spectrum before AHF solution
Predominant 5th and 7th Order Harmonics

Fig 3. Harmonics Spectrum after AHF solution with less than 5% THD-current