Designing and developing a reliable, real time power frequency meter logging system compliance with IEC 61000-4-30 standard
In this research, IEC 61000-4-30 standard is being executed by using NI LabVIEW 2012 and KEIL Embedded Development Tool to implement a power frequency meter logging system.
Frequency became the indication of the balancing between generation and load. For this reason, it is extremely important to check the frequency of any system consistently and with high accuracy and minimum cost. There must be a standard to follow to ensures all instrument and system are using the same definition and measuring techniques.
【Introduction to frequency shifts:】
Electricity is a fundamental form of energy observable in both positive and negative forms which expressed in terms of movement and interaction of electrons. Its’ production and consumption must matched simultaneously and endlessly. However, not all generators have the abilities of mitigate imbalances between the suppliers and consumers of electricity. The electric power grid, for example, has only the synchronous generators with rotational kinetic energy contributing to the balancing of production and consumption. With too much generation of electricity, the system frequency increases and vice versa. It is an difficult task to maintain a perfect generation and load balance even if the active control systems putting effort to achieve it by continually adjusting the generators power input. Minor mismatches between generation and load will cause minor frequency deviations. The reliability and efficiency does not affected by this small shifts in frequency. Large shifts of frequency, however and unfortunately, will definitely degrade the load performance and interfere the system protection schemes and thus at the very end, leading to the collapsing of the whole system. To wrap things up, maintaining tolerable and up to standard frequency deviation operational limits is unexpectedly easier said than done.
Two standard frequencies, 50 Hz and 60 Hz, governs the electric power supply globally. 50 Hz is the “European” Standard frequency used by four and a half continents of the world which includes Europe, Asia, Africa and Australia, whereas 60 Hz covers one and a half continents which includes North America and part of South America.
No matter which frequency is used, the frequency with zero uncertainty is the most ideal frequency. The nominal frequency is the desired frequency which sometimes is different from the actual output frequency and eventually causes frequency offset. Off-nominal frequency will definitely affect the dependability and efficiency of its uses. One of the impact is that the off-nominal frequency can damage the equipment which includes all the generator and load. Furthermore, the quality of the products may be degraded and even worse, the power system may collapsed due to the triggering of the protective system. More often than not, the generator protective systems will initiate the steps to prevent the damaging effect of off-nominal frequency. However, this protective move will eventually exacerbate the overall generation and load imbalance.
There is difference in power system requirement under normal and contingency circumstances. Under normal situation, less tolerances are maintained, whereas more tolerance are maintained under contingency situation.
In the effort of avoiding mechanical resonances, most of the turbine generators regulates the frequency. This is being done because the mechanical vibration damage could occurred if the rotating machines spins at its resonant modes. However, this is not the main issue since the manufacturers will designs the resonant frequencies of the machines to be different from the operational frequency.
Additionally, the volts-per-hertz ratio is the ratio between voltage and frequency needed for a industrial generator to operate without any damages on the equipment. Generally, minimum volts-per-hertz ratio is required from the International Electrotechnical Commission (IEC) standard IEC 34-1. However, if the ratio is too high, the generator core will be overheated and ultimately fail to function. Thus, this ratio defines the flux density which is proportional to the heat loss in the system.
Fundamentally, the input and output energy of a particular generator must in equilibrium. If lesser electrical energy being consumed from the electrical terminal than the mechanical energy being delivered to the generator, the excess energy is then stored in the generator in the form of kinetic energy, which resulting in acceleration of the generator’s rotation and vice versa. At the end of the day, the rate of the frequency delivered is very much depending on the magnitude of the energy imbalance between the generators and loads in the system.
Therefore, the most important message to be convey here is that the importance of frequency control in a system cannot be neglected. Precise frequency regulation is needed to regulate the time on AC powered clocks whereby the system display time in proportion to the frequency of the power system. For instance, a minor frequency deviation as small as 0.1 Hz that lasted 8 hours would results in an apparent time error of 48 seconds which will then cause error in the system.
【Power Frequency Meter Logging System Concept:】
Frequency became the indication of the balancing between generation and load. For this reason, it is extremely important to check the frequency of any system consistently and with high accuracy and minimum cost. There must be a standard to follow to ensures all instrument and system are using the same definition and measuring techniques. In this research, IEC 61000-4-30 standard is being executed by using NI LabVIEW 2012 and KEIL Embedded Development Tool to implement a power frequency meter logging system.
【Logging System Implementation:】
By using Stellaris® LM3S8962 Evaluation Board and KEIL Embedded Development Tool that load the LabVIEW graphical programming to the evaluation board, the Power Frequency Meter Logging System is successfully implemented.
Real Time Clock DS1307 (RTCC) is used in this system to providing precise time for recording the frequency. In order for the Stellaris® LM3S8962 Evaluation Board to communicate with the RTC, Inter-Integrated Circuit (I2C) communication protocol is being applied. The advantages of using LabVIEW programming is more noticeable in the application of I2C. In the process of implementing I2C, NI LabVIEW 2012 provides block diagram to program and it is extremely convenience to use it. When comparing to this graphical programming to the conventional programming using any other programming language, NI LabVIEW 2012 is definitely the better choice. Imagine how powerful the NI LabVIEW 2012 can be when only three block used to replace the conventional programming of fifty programming lines.
With RTCC used, Stellaris® LM3S8962 Evaluation Board capture the rising edge of the input signal every 10 seconds precisely. Thus, minimising the error of timing in capturing signal. To calculate the frequency of the AC source, the source itself is first rectified into DC pulse. The DC pulse will eventually became the input signal.
Additionally, the frequency calculated is being displayed on the OLED and saved into memory card for future references. Traditionally, the implementation of memory card using other programming language is a lot more difficult to perform. But with the simple block available in NI LabVIEW 2012, which is referring to ”Open/Create/Replace File”, “Write to Text File” and “Close File”, the storing of data to the memory card is very much simplified.