Power Quality

The evaluation of frequency deviation events is based on the 10s average values of the network frequency. The limits of the permitted frequency deviation distinguish between synchronous networks (±1 % = 0.5 Hz) and islanded networks (±2 % = 1 Hz).

Causes of large frequency deviations are sudden disconnections of large loads or generation plants. Excessive frequency deviation can lead to loss of synchronism of motors and generators and thus damage them.

Temporary drop in the effective value of the voltage below a specified initial dip threshold at a specific point in the electric utility grid. The voltage dip can be caused by the connection of loads.

Erklärung der Begriffe:
Extremwert: Tiefster Wert der 10ms-Spannung während des Ereignisses [V]
Dauer: Zeitunterschied zwischen Verlassen und Wiederkehr in den Nennbereich der Spannung [ms]

Temporary increase in the RMS value of voltage above a specified initial threshold at a particular point in the electric utility grid. The voltage swell can be caused by the disconnection of loads.

Erklärung der Begriffe:
Extremwert: Größter Wert der 10ms-Spannung während des Ereignisses [V]
Dauer: Zeitunterschied zwischen Verlassen und Wiederkehr in den Nennbereich der Spannung [ms]

The ratio of the sum of all harmonic rms values to the rms value of the fundamental. The value of THD indicates the total distortion due to non-linear loads.

Causes:

Due to nonlinear loads in the utility grid, distortion of the sinusoidal shape of the voltage in the grid can occur. Depending on the type of load, certain multiples of the grid frequency (harmonics) become particularly pronounced. For example, three-phase loads, such as synchronous/asynchronous motors or B6 rectifiers, experience particularly strong distortions of the 5th and 7th and 11th and 13th harmonics. For low-voltage single-phase loads, the multiples of 3 are often affected (15th, 21st, 27th). A distortion of a harmonic is recorded as a PQ event if the 10min average of the magnitude is above the limit defined by the standard. In the PQ event, the order of the harmonic as well as the exceeding of the limit are indicated in [%]. Excessively high levels of voltage harmonics can lead to malfunction, overheating and failure of electrical equipment as well as increased noise.

Rapid voltage change occurs relatively frequently in the electric utility grid and is mainly caused by load changes in customer facilities and switching operations in the grid. The power quality event of rapid voltage change is characterized by the grid voltage transitioning from one steady state to another steady state.

The duration of the transition, the maximum voltage deviation during the transition, and the steady-state change in voltage are recorded in the event. Rapid voltage changes usually do not have a large impact on electrical equipment. However, frequent RVC events can lead to the increase of the flicker level.

Flicker is a measure of the strength of periodic voltage fluctuations in the electrical network, which lead to a visually perceptible change in the luminance of electrical lamps. Using the flicker measurement method according to DIN EN 61000-4-15, these fluctuations are calculated as a dimensionless quantity and averaged over two time intervals using a weighted average method. The short-term flicker Pst (perceptibility short-term) is formed in the 10 minutes interval, the long-term flicker Plt (perceptibility long-term) in the 2h measuring interval.

Causes:
Causes for an increased flicker value are mostly periodic power fluctuations in the network with high network impedance (weak grid). Furthermore, high harmonic currents and interference between loads can cause the flicker limit to be exceeded. Power quality events such as momentary overvoltages or undervoltages (swell/dip) also very often violate the flicker limit of the affected voltages. In this case, the flicker events can be ignored.

Meaning for EN 50160 evaluation:
In the EN 50160 standard there are no limits for the number of Pst and Plt events. Only the weekly statistics of the long-term flicker Plt in the 2h interval is evaluated.

Voltage unbalance is a measure of the uneven loading of a three-phase system. This can be noticeable by different amplitudes ΔU of the individual phases or a phase angle difference Δϕ unequal to 120° between the phase-to-phase voltages. To calculate the voltage unbalance, the 10 min average values of the symmetrical components of the measured voltage system are used. The value for the unbalance [%] is calculated from the ratio of the negative sequence to the positive sequence.

Causes:
Problems with the symmetry of the three-phase system occur mainly in low-voltage grids and are caused by unevenly distributed single- and two-phase loads as well as generation plants. In medium and high voltage grids, unbalances can be caused by melting furnaces or twisted transmission lines.

Significance for EN 50160 evaluation:
In the EN 50160 standard, there are no limit values for the number of voltage unbalance events. Only the weekly statistics of the voltage unbalance measured values in the 10-min interval are evaluated. The limit value is usually 2%.

A power supply interruption is characterized by the voltage on all phases dropping to less than 5% of the nominal value. A distinction is made between scheduled or unscheduled interruptions of the supply in a given area, short-term interruptions (up to 3 minutes) and long interruptions (more than 3 minutes). Causes of interruptions can be external influences such as damage to transmission lines due to natural events or planned switching operations or circuit breaker response.