The consequences of voltage sags

Starting currents

A known cause of small voltage sags are the starting or inrush currents for capacitors, motors and other devices. In the following diagram it can be seen that the current increases briefly when the motor starts up. The inrush current leads to a voltage sag across the impedances Z and Z1. However it leads to a smaller voltage sag at the low voltage bus bar (sag zone 1) and a somewhat larger voltage sag behind impedance Z1 (sag zone 2).

The "start-up" of large loads, e.g. motors, can lead to voltage sags

Possible improvement to this phenomenon lies in the optimisation of the system itself, i.e. the switching-on of electrical loads should not lead to critical voltage sags. Typical solutions are adequate starting equipment, e.g. capacitor contactors for PFC or soft starters for motors, but it can be as well the increase of short circuit power (reducing impedance), e.g. larger cable cross-section, changing the connection point to higher grid levels, stronger switchgear and transformer.

Short circuits in the low voltage network

A very high current flows in the event of a short circuit in the low voltage network. The peak of the short circuit current depends on the value of the impedances Z and Z3. In practice impedance Z3 is the larger and dominating one. The value of impedance Z3 is determined by the type (cross-section, material) and length of the cable amongst other things. The greater the length of the cable, the smaller the short circuit current due to a higher impedance. The short circuit current causes a voltage sag across impedance Z, whereby the voltage at the low voltage main distribution bus bar collapses briefly (sag zone 1).

In the event of a short circuit the breaker in group 3 should be tripped. If it takes more than 100 ms for the breaker to trip then the voltage sags deeply throughout the whole system for 100 ms.

Typical example for an operating situation where a voltage sag occurs due to a short circuit in the low voltage network

Short circuits occurring in the low voltage network are happening quite frequently. But as long as the selective short circuit protection is designed in a proper way across the network levels it can often be neglected in practice. However short circuits at the medium voltage side are much more critical.

Short circuits in the medium voltage network

Most often voltage sags are caused in the medium-voltage network. Typical root causes are as follows:

• Road works
• Digging and earthworks
• Flashover in a connection coupling
• Cable ageing
• Short circuit in the overhead transmission lines (storm damage, animals, etc.)
• Lightning strikes

The diagram shows a typical example of the design of a medium voltage network. The transformer substations / local secondary substations (green dots) are linked to one another in a ring and connected to a distribution main substation (blue dots). The ring is open at some point (see the lower right side of the green dot ring). If there is a short circuit a short circuit current will flow (red line). This will flow until the breaker in the distribution main substation switches the ring off. This can be seen in the left diagram (in the top left ring).

Thus during the short circuit, there will be a high current flowing briefly. Due to the network impedance, this results in a short-term lowering of the voltage throughout the whole network. This short-term lowering of the voltage is noticeable as a "voltage sag".

Most voltage sags are caused by short circuits in the medium voltage network

Around 75% of all voltage sags occur in the medium-voltage network. Often these cannot be avoided by the consumer.

Short circuits in the high voltage network

Short circuits in the high voltage network are not so common but in case they happen they are often caused by storms or (faulty) switchgear. The latter primarily in the areas at the end of a high voltage line.

Voltage sags can lead to the failure of computer systems, PLC systems, relays and frequency converters. With critical processes just a single voltage sag can result in high costs, continuous processes are particularly impacted by this. Examples of this are injection moulding processes, extrusion processes, cable and semiconductor factories, printing processes or the preparation of foodstuffs such as milk, beer or refreshments.

The costs for a voltage sag are comprised of:

• Loss of profits due to production stoppage
• Costs for catching up with lost production
• Costs for delayed delivery of products
• Costs for raw materials wastage
• Costs for damage to machinery, equipment and moulds
• Maintenance and personnel costs

The average costs of a single voltage sag vary greatly from one sector to another:

• Fine chemicals € 190.000
• Microprocessors € 100.000
• Metal processing € 35.000
• Textiles € 20.000
• Food € 18.000

Sometimes processes run in unmanned areas in which voltage sags are not immediately noticed. In this case, an injection moulding machine, for example, could come to a complete standstill unnoticed. If this is discovered later there will already be a large amount of damage. The customer receives the products too late and the plastic in the machine has hardened off. With publishers or in the paper industry paper can tear or even cause a fire.

Most voltage sags occur in the public electricity grid. These voltage sags cannot be prevented. Do you want to 'eliminate' voltage sags and keep their consequences to a minimum? Then you can consider using a UPS or Active Voltage Conditioners.

"A UPS is an uninterruptible power supply and supplies power to underlying equipment in situations where the usual primary power supply fails."

Static UPS systems

A static UPS is often used to absorb power outages. The system consists of a rectifier, energy storage by batteries followed by an inverter. In data centers and hospitals, for example, UPS systems serve as a bridge to the emergency power supplied by the generators. UPS systems are therefore versatile and also absorb short-term voltage dips. The disadvantages of static UPS systems are the efficiency (self-consumption), the physical space required for the systems, the purchase and the relatively high maintenance costs.

Active Voltage Conditioners (AVCs)

Dedicated systems are available for absorbing voltage dips. These can be distinguished into systems that work on the basis of supercapacitor and systems that obtain the required energy from the phase that is not subject to a dip. These systems are also called voltage stabilization systems. With its European power quality network, Fortop has experience in dimensioning, engineering, commissioning and maintenance of these types of systems. Contact us for more information.

Janitza offers a wide range of analysers that are able to identify short term interruptions and voltage sags. The UMG 604 network analyser continuously monitors more than 800 electrical parameters. All channels are sampled 20,000 times each second and this enables short term voltage interruptions and sags to be signalled and recorded. An email or an SMS can be sent on the basis of these events. A comprehensive report can be generated with the GridVis-Basic software package included. Read more about power analyzers.

By arranging the UMG 604 in the supply field one has a comprehensive and cost-effective solution for identifying, recording, alerting and reporting voltage sags. The measurement device is equipped with a WEB-browser that offers the facility to call up the most important parameters directly from the measurement device without great investment and without complex software programs. The interruptions and voltage sags can be analysed and compiled into reports with the integrated Event- Browser.

Voltage fluctuations are identified by a network analyser in the supply field

The GridVis basic license (GridVis-Basic) is provided along with the Janitza measurement devices free-of-charge.

Amongst other things the following are possible with this software package:

• Reading out real-time measurement values
• Retrieving historical measurement data in files and graphics
• Analysing short-term interruptions, transients and voltage sags
• Printing out complete EN 50160 reports at the push of a button
• Generating good/fault reports

You can carry out comprehensive analyses yourself with GridVis.

With the integrated report generator it is possible to compile concise reports yourself periodically providing an overview of voltage sags, short term interruptions and voltage spikes that have occurred by means of the ITI-curve (CBEMA). In the diagram below it is possible to see that three voltage sags occurred, resulting in a system failure.

Report regarding voltage sags and spikes by means of ITI curve

Read more about analyze and report software GridVis®

Most voltage sags occur in the public electricity grid. They are not always recognized, but they can lead to process failure. By signalling sags in time with a power analyzer, the consequential costs can be limited. With UPS systems or dedicated Active Voltage Conditioners, voltage sags can be eliminated.

• Voltage sags an explanation
  causes, effects and correction
  Ian K.P. Ross, MIEE
• Power Quality, over spanning, stroom en hun interactie
  Dr.ir. J.F.G. Cobben & Ir. J.N. Luttjehuizen