Active Power Filter Introduction
Non-linear loads have been widely used in recent years. Yet, the quality of power supply is seriously affected due to their non-linear, impactive and unbalanced characteristics. Before the application of power electronic components, the main harmonic sources are transformer excitation current, followed by generators. But after large scale application of power electronic components, these power electronic devices become the main harmonic sources. Typical ones include arc furnace, electrified railway, UPS, rectifier, frequency inverter, computer, fluorescent lamp, household appliance, etc.
Conventional passive filters are mainly used for harmonic suppression and reactive power compensation due to their advantages of low cost and simple structure. Nevertheless, the passive filter is large in size, and its compensation performance is easily affected by system impedance and its operation state. It is easy for passive filters to resonate with the system. Therefore, it is not enough to use only passive filters to manage harmonic; we must use more advanced and smarter electronic filter -- active power filter (APF) for improving power quality in power distribution system.
The main topological structure of APF is as follows according to modes of connection with compensation objects.
At present, shunt active power filter is the most popular type of active filter. It is like a compensation current generator. It tracks harmonic components of harmonic current automatically and controls PWM converter to inject compensated current into power grid. The compensated current has the same amplitude with the harmonic current, but with opposite polarity. As a result, the harmonic current is filtered. In addition, the active power filter can also compensate reactive power and three-phase unbalance according to different requirements.
Multiple active power filters, at most 10 pieces for our products, can be connected in parallel to compensate large harmonic current, so our range of linear filter is widely applied in industrial areas. However, since the power supply voltage is input directly to the converter, the voltage level of switching components is higher. When the harmonic current contents are higher, the required capacity of APF is also higher. Consequently, the investment is very large. Therefore, the shunt APF is only suitable for current-source harmonic loads.
The active power filter is necessary in power supply system due to the following harmonic hazards.
1. Harmonic will cause extra loss in power grid. It will reduce the power usage efficiency of generation, transmission and power loads. Excessive 3rd harmonics can easily cause wiring and transformer overload, generate heat and, in extreme cases, cause fire.
2. Harmonic will affect the normal running of all kinds of power equipment. It will cause mechanical vibration, noise and over-voltage of motors in addition to extra loss. Meanwhile, it will make power transformers, capacitors, and cables overheat, and then result in insulation aging as well as life shortening, and finally cause damage.
3. Harmonics can interfere with adjacent communication systems, making these devices generate noise, reducing communication quality, and even resulting in data lost.
4. Harmonics will cause maloperation of relay protection device and automation equipment, and result in inaccurate measurement.
5. Parallel resonance and series resonance may also be caused by harmonics in power grid. They will in turn magnify harmonics, so serious power accidents may be resulted in.
According to the national standard of PRC, "Quality of Electric Energy Supply - Harmonics in Public Supply Network", GB/T14549-1993, the limit value of harmonic voltage is listed in table 1.
Maximum Permissible Harmonic Current Injected into the PCC (Table 2)
When the minimum short circuit capacity at the PCC is different from the baseline short circuit capacity given in the above table, the maximum permissible harmonic current can be calculated according to the following formula:
Sk1: Minimum short circuit capacity at the PCC, MVA;
Sk2: Baseline short circuit capacity, MVA;
Ihp: Maximum permissible hth harmonic current, A, in table 2;
Ih: Maximum permissible hth harmonic current when the short circuit capacity is Sk1.