Mitigation of Voltage Sags

Mitigation of Voltage Sags:

^ü   Different power quality problems would require different solution. It would be very costly to decide on mitigate measure that do not or partially solve the problem.

^ü   These costs include lost productivity, labor costs for clean up and restart, damaged product, reduced product quality, delays in delivery and reduced customer satisfaction.

^ü   When a customer or installation suffers from voltage sag, there is a number of mitigation methods are available to solve the problem.

^ü   These responsibilities are divided into three parts that involves utility, customer and equipment manufacturer.

^ü   Different mitigation methods are

o  Dynamic voltage restorer

o  Active series Compensators

o Distribution static compensator (DSTATCOM) o Solid state transfer switch (SSTS)

o  Static UPS with energy storage

o Backup storage energy supply (BSES) o Ferro resonant transformer

o  Flywheel and Motor Generator set

o   Static Var Compensator (SVC)

1. Dynamic Voltage Restorer: (DVR)

^ü   Dynamic Voltage Restorers (DVR) are complicated static devices which work by adding the ‘missing’ voltage during a voltage sag. Basically this means that the device injects voltage into the system in order to bring the voltage back up to the level required by the load.

^ü   Injection of voltage is achieved by a switching system coupled with a transformer which is connected in series with the load.

^ü   There are two types of DVRs available; those with and without energy storage. Devices without energy storage are able to correct the voltage waveform by drawing additional current from the supply.

^ü   Devices with energy storage use the stored energy to correct the voltage waveform. The difference between a DVR with storage and a UPS is that the DVR only supplies the part of the waveform that has been reduced due to the voltage sag, not the whole waveform.

^ü   In addition, DVRs generally cannot operate during interruptions. Figure 10 shows a schematic of a DVR.

^ü   As can be seen the basic DVR consists of an injection/booster transformer, a harmonic filter, a voltage source converter (VSC) and a control system.

^ü   For readers who are interested in further knowledge of DVR systems, the article in gives a thorough description of the design and operation of DVRs. DVR systems have the advantage that they are highly efficient and fast acting. It is claimed in that the DVR is the best economic solution for mitigating voltage sags based on its size and capabilities.

^ü   In the case of systems without storage, none of the inherent issues with storage are relevant. Another advantage of DVR systems is that they can be used for purposes other than just voltage sag mitigation.

2. Ferro Resonant Transformer:

^ü   A Ferro resonant transformer, also known as a constant voltage transformer (CVT), is a transformer that operates in the saturation region of the transformer B-H curve.

^ü   Voltage sags down to 30 % retained voltage can be mitigated through the use of Ferro resonant transformers.

^ü   Figure shows a schematic of a Ferro resonant transformer. The effect of operating the transformer in this region is that changes in input voltage only have a small impact on the output voltage.

^ü   Ferro resonant transformers are simple and relatively maintenance free devices which can be very effective for small loads. Ferro resonant transformers are available in sizes up to around 25 KVA.

^ü   On the down side, the transformer introduces extra losses into the circuit and is highly inefficient when lightly loaded. In some cases they may also introduce distorted voltages.

^ü   In addition, unless greatly oversized, Ferro resonant transformers are generally not suitable for loads with high inrush currents such as direct-on-line motors.

3. Uninterrupted Power Supply (UPS):

^ü   Uninterruptible power supplies (UPS) mitigate voltage sags by supplying the load using stored energy. Upon detection of voltage sag, the load is transferred from the mains supply to the UPS.

^ü   Obviously, the capacity of load that can be supplied is directly proportional to the amount of energy storage available.

^ü   UPS systems have the advantage that they can mitigate all voltage sags including outages for significant periods of time (depending on the size of the UPS).

^ü   There are 2 topologies of UPS available; on-line and off-line. Figure 1 shows a schematic of an off-line UPS while Figure 2 shows a schematic of an on-line UPS.

^ü   Comparison of the figures shows that the difference between the two systems is that for an on-line UPS the load is always supplied by the UPS, while for off-line systems; the load is transferred from the mains supply to the UPS by a static changeover switch upon detection of voltage sag.

^ü   The lack of a changeover switch renders the on-line system more reliable as any failure of the changeover switch will result in the off-line UPS being ineffective. UPS systems have disadvantages related to energy storage components (mostly batteries) which must be maintained and replaced periodically.

^ü   Small UPS systems are relatively simple and cheap. However, large units are complex and highly expensive due to the need for large energy storage capacities.

Block Diagram of Offline UPS:

Block Diagram of Online UPS:

4. Static Var Compensator(SVC):

^ü   A SVC is a shunt connected power electronics based device which works by injecting reactive current into the load, thereby supporting the voltage and mitigating the voltage sag.

^ü   SVCs may or may not include energy storage, with those systems which include storage being capable of mitigating deeper and longer voltage sags.

Block Diagram of SVC: