How does a voltage dip occur?
One known cause of small voltage dips is the inrush currents of capacitors, motors or other devices. The figure below shows that the current increases briefly when the motor starts up. The voltage drop across impedances Z and Z1 leads to a slight voltage dip on the low-voltage distributor (dip zone 1) and a slightly larger voltage dip behind impedance Z1 (dip zone 2).
The solution to the problems caused by these dips lies in optimizing the system itself. Switching on devices should not lead to critical voltage dips. For example, a capacitor can be switched on using special capacitor contactors with inrush damping, thereby massively reducing the inrush current.
A short-circuit current flows in the event of a short-circuit in the low-voltage network. The amount of the short-circuit current depends on the size of impedances Z and Z3. In practice, impedance Z3 is the highest. The size of impedance Z3 is determined, among other things, by the type and length of the cable. The longer the cable length, the smaller the short-circuit current will be. The short-circuit current causes a voltage drop across impedance Z, causing the voltage at the low-voltage main distribution board to drop briefly (dip zone 1).
In the event of a short circuit, the fuse in group 3 should trip. If this takes 100 ms until the fuse blows, the voltage in the entire system experiences a deep dip for 100 ms.
Short circuits in the low-voltage network do occur, but can often be neglected in practice. Short circuits on the medium-voltage side are more critical.
Dips are most frequently caused in the MV grid. These can be caused by the following influences, for example:
- Earthworks
- Electrical breakdown in a connection sleeve
- Cable aging
- Short circuit in overhead line networks (storm damage, animals, etc.)
The figure below (Fig. 5) shows a typical example of the structure of a MV grid. The known transformer houses / distribution substations (green dots) are interconnected in a ring and connected to a distribution station (blue dots). The ring is always open somewhere (see ring in the green dots at the bottom right). If a short circuit occurs, a short-circuit current flows (red line). This flows until the fuse in the distribution station switches off the ring. This can be seen in the figure on the left (top left in the ring).
This means that a high current flows briefly during the short circuit. Due to the grid impedances, this leads to a brief drop in voltage across the entire network. This brief drop in voltage is experienced as a "voltage dip".
About 75% of voltage dips are caused in the medium voltage network. Often, these cannot be avoided by consumers.
Short circuits in the high-voltage network Short circuits in the high-voltage network are often caused by thunderstorms or (faulty) switching operations. The latter especially in areas at the end of a high-voltage line.
Problems due to voltage dips
Voltage dips can lead to the failure of computer systems, PLC systems, relays, and frequency converters. In critical processes, even a single voltage dip can result in high costs, and continuous processes are particularly affected by this. Examples include injection molding processes, extrusion processes, printing processes, or the processing of foodstuffs such as milk, beer, or soft drinks.
The costs of a voltage dip consist of:
- Lost profits due to production downtime
- Costs for making up for lost production
- Costs for late delivery of products
- Costs for lost raw materials
- Costs for damage to machines, devices, and molds
- Maintenance and personnel costs
The average costs of a voltage dip depend heavily on the sector:
- Fine chemicals €190,000
- Microprocessors €100,000
- Metal processing €35,000
- Textiles €20,000
- Food €18,000
Sometimes processes take place in unmanned areas where voltage dips are not noticed immediately. In this case, for example, an injection molding machine can come to a standstill unnoticed. If this is discovered later, major damage has already been done. Customers receive the products too late and the plastic in the machine has hardened. In print shops or in the paper industry, paper can tear or even cause a fire. Another well-known example is the damage caused to tire manufacturer Vredestein by voltage dips.
Susceptibility of IT systems to voltage dips and interruptions
IT systems in particular are susceptible to voltage interruptions and voltage dips. This means that all processes controlled by microprocessors are susceptible to such disturbances, for example
- PLC systems
- Frequency converters
- Machine controllers
- Servers, PCs, etc.
The ITI-CBEMA curve drawn up by the Information Technology Industry Council defines when a voltage dip leads to the failure of IT devices and when a voltage spike causes damage to IT devices. Although the model was developed for 120 V–60 Hz networks, it is also used for devices connected to 230 V–50 Hz networks. The model can be used by manufacturers as a design guideline.
