Power Quality

Frequency deviation events are evaluated using 10-second average values of the grid frequency.

The permitted deviation limits distinguish between synchronous networks (±1% = 0.5 Hz) and islanded networks (±2% = 1 Hz).

Large frequency deviations can be caused by sudden disconnections of major loads or generation plants. Excessive deviations may cause motors and generators to lose synchronism and can consequently damage them.

A voltage dip is a temporary decrease in the RMS voltage below a specified dip threshold at a particular point in the electricity supply network. It can be caused by the connection of loads.

Explanation of terms:
Extreme value: Lowest value of the 10 ms voltage during the event [V]
Duration: Time between leaving and returning to the nominal voltage range [ms]

A voltage swell is a temporary increase in the RMS voltage above a specified initial threshold at a particular point in the electricity supply network. It can be caused by the disconnection of loads.

Explanation of terms:
Extreme value: Highest value of the 10 ms voltage during the event [V]
Duration: Time between leaving and returning to the nominal voltage range [ms]

THD is the ratio of the combined RMS values of all harmonics to the RMS value of the fundamental component. It indicates the total distortion caused by non-linear loads.

Causes:
Non-linear loads in the supply network can distort the sinusoidal voltage waveform. Depending on the type of load, particular multiples of the grid frequency may become especially pronounced.

For three-phase loads such as synchronous or asynchronous motors and B6 rectifiers, the 5th, 7th, 11th and 13th harmonics can be particularly significant. For single-phase low-voltage loads, multiples of three are often affected, including the 15th, 21st and 27th harmonics.

A harmonic distortion is recorded as a PQ event when the 10-minute average value exceeds the limit defined by the applicable standard. The event indicates the harmonic order and the percentage by which the limit has been exceeded.

Excessive voltage harmonics can result in malfunction, overheating, equipment failure and increased audible noise.

Rapid voltage changes occur relatively frequently in electricity supply networks and are mainly caused by changes in customer loads and switching operations within the grid.

An RVC event is characterised by the grid voltage changing from one steady state to another. The event records the duration of this transition, the maximum voltage deviation during the transition and the resulting steady-state voltage change.

Rapid voltage changes usually have no major effect on electrical equipment. However, frequent RVC events may increase the flicker level.

Flicker is a measure of periodic voltage fluctuations in an electrical network that can cause visually perceptible changes in the brightness of electrical lamps.

Using the flicker measurement method according to DIN EN 61000-4-15, these fluctuations are calculated as dimensionless values and averaged over two time intervals using a weighted method. Short-term flicker Pst is calculated over a 10-minute interval, while long-term flicker Plt is calculated over a 2-hour measurement interval.

Causes:
Increased flicker values are commonly caused by periodic power fluctuations in networks with high impedance. High harmonic currents and interference between loads may also cause flicker limits to be exceeded.

Power quality events such as temporary overvoltages or undervoltages can also cause the flicker limit of the affected voltages to be exceeded.

Significance for EN 50160 evaluation:
EN 50160 does not define limits for the number of Pst and Plt events. Only the weekly statistics of long-term flicker Plt in the 2-hour interval are evaluated.

Voltage unbalance is a measure of uneven loading within a three-phase system. It may be indicated by different phase amplitudes ΔU or by a phase-angle difference Δϕ that is not equal to 120° between the phase-to-phase voltages.

The 10-minute average values of the symmetrical components of the measured voltage system are used to calculate voltage unbalance. The percentage value is calculated from the ratio of the negative-sequence component to the positive-sequence component.

Causes:
Three-phase system unbalance occurs mainly in low-voltage networks and can be caused by unevenly distributed single-phase and two-phase loads or generation plants. In medium- and high-voltage networks, unbalance may be caused by melting furnaces or transposed transmission lines.

Significance for EN 50160 evaluation:
EN 50160 does not specify limits for the number of voltage-unbalance events. Only the weekly statistics of the 10-minute voltage-unbalance measurement values are evaluated. The limit is usually 2%.

A power supply interruption is characterised by the voltage on all phases falling below 5% of the nominal value.

A distinction is made between scheduled and unscheduled interruptions within a particular area, short interruptions lasting up to 3 minutes and long interruptions lasting more than 3 minutes.

Interruptions may be caused by external influences such as damage to transmission lines during natural events, planned switching operations or the operation of protection devices.