The definition of power quality: voltage, current, or frequency deviation that causes electrical equipment to malfunction or malfunction, including steady-state power quality and transient power quality. Steady state power quality: voltage deviation, frequency deviation, three-phase imbalance, harmonics, voltage fluctuations and flicker; Transient power quality: temporary overvoltage and transient overvoltage, temporary voltage rise, temporary voltage drop, and short-term voltage interruption.
(1) Voltage deviation: The deviation of the supply voltage from the nominal voltage.
The problem of voltage deviation belongs to the category of fundamental reactive power, mainly related to factors such as wire diameter, power supply distance, power flow distribution, voltage regulation methods, reactive power compensation methods and capacity, load electricity characteristics, etc. It is a power quality indicator that has been recognized earlier, researched more maturely, and controlled more comprehensively in the power system.
The essence of voltage deviation is the voltage drop caused by the current flowing through the transmission network on its internal resistance.
(2) Frequency deviation: The difference between the actual and nominal values of the system frequency.
Frequency deviation belongs to the problem of active power balance, mainly related to the system's active power reserve and generator speed regulation methods. It is also a power quality indicator that has been recognized earlier, researched more maturely, and controlled more effectively in the power system. In the steady-state operation mode of the power system, although the voltage of the entire system may vary at different points, the frequency is basically the same.
(3) Three phase imbalance: The degree of imbalance of three phases in a three-phase power system, expressed as the percentage of the root mean square of the negative and positive sequence components of voltage or current.
This problem is mainly determined by the electricity consumption characteristics of the load and is also related to the load balancing strategy. Although the asymmetry of network topology parameters is also one of the factors, it has little impact on the steady-state operation of the power grid. The commonly referred three-phase imbalance mainly refers to the degree of voltage imbalance, and the degree of three-phase current imbalance can also be analyzed when necessary.
(4) Harmonic: Fourier series decomposition of periodic AC components to obtain components with frequencies greater than one integer multiple of the fundamental frequency.
Firstly, the harmonic frequency is related to the fundamental wave, therefore the high-frequency harmonic components in various transient phenomena do not belong to the category of harmonics; In addition, interharmonics and subharmonics do not belong to the current category of harmonics.
(5) Voltage fluctuation: a series or continuous change in the root mean square value of voltage.
Voltage fluctuations are a collection of voltage changes. Voltage variation is different from voltage deviation. It is not directly compared with the rated value of the voltage, but rather the difference between two adjacent extreme values on the voltage effective value variation curve with respect to the variation of the rated value. In addition, voltage fluctuations should not only be evaluated based on amplitude, but also take into account their frequency of fluctuation.
Flicker: Visual perception caused by unstable lighting intensity.
Flashing is a characteristic of voltage fluctuations characterized by changes in the illuminance of incandescent lamps based on their voltage variations. Flicker is a product of voltage fluctuations, which is essentially the result of subharmonic effects.
(6) Temporary overvoltage: An oscillation overvoltage that persists for a long time at a given installation point and does not decay or if it decays.
Transient overvoltage: a type of overvoltage with a duration of several milliseconds or less, typically oscillating or non oscillating with strong damping.
From the analysis of the location of the excitation source, overvoltage can be divided into internal overvoltage and external overvoltage.
From the analysis of direct causes, overvoltage can be divided into power frequency overvoltage, resonant overvoltage, operational overvoltage, lightning overvoltage, etc.
(7) Voltage dip (rise): A temporary drop (rise) in voltage at a certain point in the power system, which returns to normal after a brief duration of half a cycle to a few seconds. Voltage sag is similar to a power (mainly reactive) surge phenomenon, which occurs with certain transient faults in the system or transient operating conditions of electrical equipment. Its process is generally described using state parameters expressed in differential equations.
Short term voltage interruption: The power supply voltage disappears for a period of time, generally not exceeding 1 minute. Short term interruption can be considered as a voltage dip of 90% to 100% amplitude.