Study on the relationship between LED lighting and power factor

When AC current is overloaded, the AC voltage applied to the load and the AC current passing through the load have a phase difference, from which people derive the concept of power factor. The electricity used for production and life comes from the power grid, which provides AC with a frequency of 50Hz or 60Hz. There are three types of AC loads: resistance, inductance, and capacitance:

1. When AC current passes through a pure resistive load, the AC voltage applied to the resistor is in phase with the AC current passing through the resistor, that is, the phase angle between them is ф = 0°. At the same time, active power is consumed on the resistive load, and the power grid needs to supply energy.

2. When AC current passes through a purely inductive load, the phase of the AC voltage on it leads the phase of the AC current by 90°. The angle ф between them is 90°, generating reactive power on the inductive load. The electric energy supplied by the power grid is converted into magnetic field energy in the inductor and stored briefly before being fed back to the power grid to be converted into electric energy. This cycle repeats itself cyclically. As a result, the power grid does not supply energy, so it is called "reactive power", but the "reactive current" that generates "reactive power" still exists.

3. When AC passes through a pure capacitive load, it is similar to this, except that the phase of the AC voltage lags behind the AC current by 90°, and the angle between them is ф = - 90°.

Here, the phase angle leading is defined as positive, and the phase angle lagging is defined as negative. The actual load is a complex of three types: resistance, inductive reactance of inductance, and capacitive reactance of capacitance. The composite is collectively called "impedance", which can be written as a mathematical formula: Impedance Z = R+j (XL – XC,. R is resistance, XL is inductive reactance, and XC is capacitive reactance. If (XL – XC, > 0, it is called "inductive load"; conversely, if (XL – XC, < 0, it is called "capacitive load".

When AC passes through an inductive load, the phase of the AC voltage leads the phase of the AC current (0°<ф<90°); when AC passes through a capacitive load, the phase of the AC voltage lags the phase of the AC current (-90°<ф<0°). Electrical engineering defines this angle ф as the power factor angle, and the cosine value of the power factor angle ф, i.e. Cosф, is called the power factor. For resistive loads, the phase difference between voltage and current is 0°, so the power factor of the circuit is 1 at most (Cos 0°=1, while for pure inductive circuits, the phase difference between voltage and current is 90°, and the voltage leads the current; in pure capacitive circuits, the phase difference between voltage and current is -90°, that is, the current leads the voltage. In the latter two circuits, the power factor is zero (Cos 90°= 0,. For general load circuits, the power factor is between 0 and 1. From the mathematical formula impedance Z= R+ j (XL – XC), if XL = XC, then Z= R, that is, the impedance Z becomes a pure resistor, and the power factor is equal to 1.

That is to say, inductive load and capacitive load can compensate each other. If the inductive reactance value of the inductive element in a circuit is exactly equal to the capacitive reactance value of the capacitive element, it can be completely compensated. The method of power factor compensation originates from this. When AC passes through an impedance load, the total power S generated is called "apparent power". Apparent power S includes two components: active power P and reactive power Q. Among them, active power P = S*Cosф, reactive power Q = S*Sinф. Only when the power factor Cosф value is equal to the maximum value 1, that is, ф= 0°, the reactive component Q is equal to zero, and the active power P is equal to the value of the apparent power S. But the actual working capacity of the load is only related to the active power.

Power Factor and LED Lighting

This article mainly describes the concepts of power factor and power factor compensation. Based on the capacitive load characteristics of LED lamps, it proves that there is no need to add power factor compensation circuits in LED lighting fixtures.

The harm of low power factor

(1) The load-carrying capacity of the power supply equipment is discounted, that is, the load-carrying capacity is reduced. For example, if a certain device can supply 100KVA of apparent power, if the power factor is 0.7, it can only supply 70KW of active power; if the power factor is 0.9, it can supply 90KW of active power. It can be seen that improving the power factor is very meaningful.

(2) The existence of reactive current in the transmission line increases the transmission line loss. For example, if the power factor is 0.7, to supply 70KW of active power, it is necessary to supply 100KVA of apparent power. As the current of the transmission line increases, the line loss will inevitably increase.

Power Factor Compensation Methods

The power supply department calculates the power supply based on "apparent power", but the electricity fee is calculated based on "active power". The user's "watt-hour meter" is actually an "active power meter". There is a "power factor" discount between the two, so the power factor is a data that the power supply department cares about very much. If the user does not achieve the ideal power factor, it is relatively consuming the power supply department's resources. At present, the power factor in China is required to be between 0.9 and 1 of the inductive power factor.

The following methods can be used to compensate the power factor:

(1) Semi-centralized and centralized compensation methods require that each power distribution room of the power-consuming enterprise must be equipped with a power factor automatic control device to detect the power factor in real time and automatically switch on or off the number of compensation power capacitors for motor operation compensation (because the main power load of the enterprise is motors, etc.), so that the power factor of the local power network meets the standard. This method has been enforced since the late 1970s and early 1980s, and it has been at least 20 years since then. In addition, each power supply station also installs a power factor automatic control device to further compensate for the power supply area under its jurisdiction.

(2) The decentralized compensation method requires that each electrical appliance be designed with advanced technology to meet the power factor standard, so that the power factor can be guaranteed to meet the standard no matter when and where the electricity is used. However, this will increase the cost and the size of the appliance, and some appliances have strict restrictions on size, which increases the difficulty of design.

Review of electric light source lighting fixtures and power factor compensation Electric light sources started with incandescent bulbs, which are pure resistive loads and do not have the problem of power factor compensation. After the 1950s, fluorescent lamps quickly became popular and became the main lighting fixtures. The ballast used silicon steel sheet inductors, which are highly reliable and have a long life. A small number of them are still used today, and most of them do not have any power factor compensation measures. This may be due to cost factors, or people do not know much about power factor compensation and have a weak awareness of energy saving. There are also capacitors with appropriate capacity added for power factor compensation, which are mostly used in 30W and 40W high-wattage fluorescent lamps, and are rarely used below 20W. After the 1990s, people's awareness of environmental protection and energy saving increased, and three-primary color fluorescent powder energy-saving lamps were developed, which have higher light efficacy. Electronic ballasts were also introduced later, and with three-primary color fluorescent powder lamps, the energy-saving effect is more significant. Some integrated circuit manufacturers at home and abroad have launched lamp chips with active power factor compensation for electronic ballasts. They have excellent performance, but they increase the cost and size of the electronic ballasts. Ordinary people cannot afford their prices, so they are only used in high-end lighting products. A large number of popular electronic ballasts, including those for energy-saving lamps, do not have any power factor compensation measures, which can be seen everywhere in popular energy-saving lamps and fluorescent lamps on the market. In other words, previous lamps basically had no power factor compensation measures, but everyone was using them.

LED consumes less electricity, and the power of the lamp is even lower than that of energy-saving lamps. LED lighting is of course more advanced and more suitable for environmental protection, energy conservation and emission reduction. The author's opinion on whether LED lamps should add power factor compensation is:

(1) According to expert analysis, LED is a capacitive load. There are many inductive loads in the power grid, such as motors, transformers, etc. It is often necessary to connect a capacitive load for compensation, and the power factor automatic control device is used for this purpose. LED is a capacitive load, which just compensates for the problem of low power factor caused by the large number of inductive loads in the power grid, which is exactly what is used. Based on this understanding, the author believes that LED lighting fixtures do not need to add power factor compensation measures in principle.

(2) Single LED lamps for indoor lighting are all low-power, and the power will not exceed 30W. The low power of the lamps will have little impact on the power grid. The author believes that such lamps can completely avoid power factor compensation measures. Adding them is not good, but will lose the function of LED lamps as capacitive loads that can compensate for the low power factor of the power grid due to the large inductive load. Most of these low-power lamps are small and compact, and the internal space is very limited. For example, MR16, PAR30, and PAR38 lamp cups cannot be placed after the power PCB board is enlarged. Even if you want to add power factor compensation measures, you can't add them. In addition, adding power factor compensation will bring about the side effect of reduced efficiency, which may be not worth the loss. In addition, the cost increase will affect sales. Moreover, the power supply department has taken countermeasures to compensate for the power factor of the power grid, so there is no need for lamp manufacturers to add unnecessary measures.

(3) For power above 100W, power factor compensation measures can be considered. Loads with high power have a greater impact on the power grid, such as LED street lights with power from 100W to several hundredW. Street lights are public welfare undertakings, so a slight increase in cost is not a big deal, and there is space for the power PCB board to be slightly larger. Adding power factor compensation measures can help the power supply department reduce some of the regulation burden and prevent over-compensation caused by excessive capacitive loads.