OLTC Transformer Model Connecting 3-Wire MV with 4-Wire Multigrounded LV Networks Evangelos E. Pompodakis a , Georgios C. Kryonidis a , Minas C. Alexiadis a a Power Systems Laboratory, School of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Thessaloniki GR 54124, Greece Abstract: This short communication presents a comprehensive model of on-load tap-changer (OLTC) transformers that connect 3- wire medium voltage (MV) with 4-wire multigrounded low voltage (LV) networks. The proposed model enables the inclusion of the 3-wire MV network and the 4-wire multigrounded LV network into a single Y BUS matrix without any assumption or simplification. Its distinct feature is that the tap changer of the transformer is simulated outside the Y BUS matrix, thus a refactorization of the Y BUS matrix is not required in every tap change. The proposed transformer model has been validated in a 4- Bus network, while its performance has been tested in the IEEE 8500-Node and IEEE 906-Bus test networks. Keywords: Implicit Z BUS power flow, Multi-grounded networks, OLTC Transformer. 1. Introduction Technological advances over the last years have made the OLTC MV/LV transformers a viable solution in distribution networks [1]. OLTC transformers allow LV network operators to 1) integrate economically renewable energy sources, 2) optimize grid topologies by reducing secondary substations, 3) stabilize industrial processes in volatile grids, and 4) complying economically with grid codes [1]. OLTC transformers have been studied in power flow literature assuming single-phase balanced [2] or 3-wire unbalanced configurations [3]. However, the accurate simulation of OLTC MV/LV transformers necessitates a precise transformer model that can be effectively combined with existing power flow algorithms, considering the distinct configurations that the MV and LV networks have. Although some transformer models have been proposed in power flow literature for integrating 3-wire MV and 4-wire multigrounded LV networks, they ignore the effect of tap changer [4]. Ideally, the tap changer should be included in the power flow with the tap variations not affecting the structure of the Y BUS matrix. In this way, the factorization or inversion of the Y BUS matrix, which is the most time-consuming action in power flow is executed only once and not at every tap change. Thus, several power flow applications that require sequential tap variations e.g. voltage stability analysis, optimal power flow (OPF), Volt/Var control (VVC), optimal feeder reconfiguration (OFR) [2], [3], heuristic optimization are significantly accelerated. This paper proposes an OLTC transformer model with the following distinct features: It has a Dyn configuration, which is by far the most common type of transformer supplying LV feeders. The model represents realistically the 3-wire nature of MV and the 4-wire multi-grounded nature of LV networks. The tap changer is considered in the form of current sources and it is not included in the Y BUS matrix of the network. Thus, any variation of the tap does not affect the structure of the Y BUS matrix. This property is very important «This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (ΙΚΥ)». in Z BUS -based power flow methods since the factorization and inversion of the Y BUS is realized only once and not in every tap change, reducing in this way dramatically the computation time of the algorithm. This short communication is an extension of [5], in which the authors implemented implicit Z BUS method to solve the power flow in 4-wire multigrounded LV networks operated in either grid-connected or islanded mode. By applying the proposed OLTC transformer model, the 3-wire upstream MV network can be also considered into the power flow. Thus, the estimation of several issues related to the interaction between the MV and LV networks e.g. overvoltages, unbalances and tap regulation is enabled. 2. Network description A typical representation of a distribution network is shown in Fig. 1. It consists of a three wire MV network, which supplies a 4-wire multigrounded LV network through a Dyn11 transformer. More specifically, node 1 is the slack bus of the network with a constant reference voltage. The tap-changing MV windings of the transformer are connected to bus 2 in delta configuration. The neutral point of the LV side of the transformer is grounded though the impedance Z gr2' , which represents the grounding impedance at the MV/LV substation. The LV nodes 3 and 4 are grounded with the impedances Z gr3 and Z gr4 , respectively, which represent the grounding impedances at the customer side. The 3-wire MV line can be represented through a 3x3 admittance matrix consisting of the self- and mutual- admittances between the phases as follows: (1) Similarly, the 4-wire multigrounded line can be represented through a 5x5 admittance matrix consisting of the self- and mutual-admittances between the phases, neutral and ground. For instance, the line between the nodes 3-4 is represented as follows: (2)
4
Embed
OLTC Transformer Model Connecting 3-Wire MV with 4-Wire ...
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
OLTC Transformer Model Connecting 3-Wire MV with 4-Wire
Multigrounded LV Networks Evangelos E. Pompodakis
a, Georgios C. Kryonidis
a, Minas C. Alexiadis
a
a Power Systems Laboratory, School of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Thessaloniki GR 54124, Greece
Abstract: This short communication presents a comprehensive model of on-load tap-changer (OLTC) transformers that connect 3-
wire medium voltage (MV) with 4-wire multigrounded low voltage (LV) networks. The proposed model enables the inclusion of
the 3-wire MV network and the 4-wire multigrounded LV network into a single YBUS matrix without any assumption or
simplification. Its distinct feature is that the tap changer of the transformer is simulated outside the YBUS matrix, thus a
refactorization of the YBUS matrix is not required in every tap change. The proposed transformer model has been validated in a 4-
Bus network, while its performance has been tested in the IEEE 8500-Node and IEEE 906-Bus test networks.
Keywords: Implicit ZBUS power flow, Multi-grounded networks, OLTC Transformer.
1. Introduction
Technological advances over the last years have made the
OLTC MV/LV transformers a viable solution in distribution
networks [1]. OLTC transformers allow LV network
operators to 1) integrate economically renewable energy
sources, 2) optimize grid topologies by reducing secondary
substations, 3) stabilize industrial processes in volatile grids,
and 4) complying economically with grid codes [1].
OLTC transformers have been studied in power flow
literature assuming single-phase balanced [2] or 3-wire
unbalanced configurations [3]. However, the accurate
simulation of OLTC MV/LV transformers necessitates a
precise transformer model that can be effectively combined
with existing power flow algorithms, considering the
distinct configurations that the MV and LV networks have.
Although some transformer models have been proposed in
power flow literature for integrating 3-wire MV and 4-wire
multigrounded LV networks, they ignore the effect of tap
changer [4].
Ideally, the tap changer should be included in the power
flow with the tap variations not affecting the structure of the
YBUS matrix. In this way, the factorization or inversion of the
YBUS matrix, which is the most time-consuming action in
power flow is executed only once and not at every tap
change. Thus, several power flow applications that require
sequential tap variations e.g. voltage stability analysis,
optimal power flow (OPF), Volt/Var control (VVC), optimal