Studies On Voltage Sags For Distribution Systems

Voltage sag is probably the most important power quality (PQ) problem affecting industrial and large commercial customers. Most faults occur in distribution system, and fault clearance time is longer, so voltage sags are more severe in distribution system than that in transmission system. Issues of voltage sags in distribution system are discussed in this dissertation. It includes the analysis of voltage sag state estimation, propagation of sags in distribution system and sag mitigations with relay protection. Main contents are shown in following.1) In response to the need for feeder-level PQ performance characterization, this dissertation introduces the concept of 'voltage sag state estimation' and proposes a least-square algorithm, Improved Sag State Estimation (ISSE) method, to estimate the sag levels for all customers in a feeder. The proposed method has the following characteristics: 1) it makes use of the radial connection characteristic of a distribution feeder; 2) it employs a least-square method to predict the sag profile along a distribution line. Case studies show that the method is with high precision and rapid estimation speed.2) The dissertation proposes a new fault path-searching algorithm, division point tree searching method. It is accelerated in searching speed with division point as a searching objective and feeder section as a searching radius in the new searching method, and it is with good anti-noise capability. Every metered data will also be checked in the last feeder section for the accurate location of the fault point.3) Sag state estimation observability analysis is modeled and correlative modification method is proposed from the point of fault current superimposing firstly.4) It is pointed that estimation error will increase as the form of a 2^nd order polynomial function with the decreasing of Signal Noise Ratio (SNR) through a mass of simulations. Then bad data revision method is introduced to improve the sag estimation accuracy at the situations with low SNR.5) The severity of sags received by LV customers is mainly decided by two crucial factors. One is the transition characteristic of sags caused by different transformer connections. Another is the fault status at fault point with different neutral grounding modes. In the 4^th chapter of the dissertation, the above two factors is analyzed and it pointed that there is an 'exceeding state' for line-line fault for 110kV system. In this state, voltage drop reaches its extremum state, but fault current is weak. So I zone protection may be not tripped with less fault current, and the fault should be cleared with backup protection, which causes longer sag durations and disturbed the customers badly. The opinion that neutral grounding modes should also be evaluated from the point of transient PQ for medium voltage distribution system is proposed in the dissertation. And the transient PQ level with all kinds neutral grounding modes in medium-voltage distribution system are analyzed. It is resulted that neutral-point-insulated mode and resonance-grounding mode are good at transient PQ level than others. The most severe sags will occur in the neutral point grounding with small resistor modes for SLGF.6) Practical PQ monitoring schemes are mainly implemented from power system side up to now. After introductions of correlative PQ indices, the dissertation proposed a scheme of PQ monitoring based on LV customers and discussed the network configurations and key technologies.7) By introducing the protection operation characteristic into the CBEMA curve, safe work region of SE controlled by current protection under voltage sag is modeled. The Sag Safe Region Percentage Index (SSRPI) and Sag Improvement Region Percentage Index (SIRPI) are introduced to quantize the size of the valid and improvable work regions. For other regions, a new evaluation model is proposed to detect the capability of sag mitigation for current protection from the point of the energy, and correlative indexes, Sag Protection Usability Degree Index (SPUDI) and Feeder Sag Protection Usability Degree Index (FSPUDI) are also introduced. Then the most sag protection capabilities for every protection stage and every feeder can all be achieved.8) After the evaluation of sag mitigation capability for current protection, the new protection schemes, 'whole-line fast faults clearance without selectivity protection scheme' and 'pilot unit protection scheme', are proposed. Since the first new scheme is losing selectivity, the rationality of the scheme is evaluated from the cost opinion, and some project problems for the second new scheme, such as communication modes selection and protection coordination, are also discussed at last.