FACTS in distribution systems
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stant voltage on the secondary side. This is in large contrast with the normal situation where it decreases linearly with increasing load, since typically the tap changers of these distribution transformers are off-load tap changers, only being adjusted when the distribution system or the demand changes structurally. A similar idea was proposed in [104], where a thyristor switched tap changer is applied for the larger, discrete voltage steps and an additional PWM controlled series controller using GTOs or IGBTs ensures an overall continuous voltage control. 3.3.3.3
Transfer switches
A transfer switch is a device which is applied to select the supply path for the connected loads, see Figure 3.17. Typically, one of the supply Supply Path 2
Supply Path 1
Switches
Control
Loads
Figure 3.17: Transfer switch. paths is the public electricity network, while the other is formed by a local generator. Mechanical switches cannot be operated fast enough to perform this transition without a (short) power interruption, so instead, power electronic valves are used. The resulting device is the static transfer switch (STS), which has the capability to transfer loads within half a power frequency cycle from one source to the other, which is sufficiently fast for uninterrupted power supply [105]. This topology has been applied successfully on medium voltage levels to achieve uninterrupted power supply for the connected loads [94]. The topology of the STS has been extended with mechanical switches to take advantage of both the speed of power electronics and the low losses of mechanical switches, while still achieving uninterrupted supply, resulting in the hybrid transfer switch (HTS) [106]. The STS and HTS are, besides for uninterrupted supply, also applied to increase power quality, by switching from one source to the other if one source has a higher power quality level than the other.