Abstract
A distribution system was designed and operated by considering unidirectional power flow from a utility source to end-use loads. The large penetrations of distributed generation (DG) into the existing distribution system causes a variety of technical problems, such as frequent tap changing problems of the on-load tap changer (OLTC) transformer, local voltage rise, protection coordination, exceeding short-circuit capacity, and harmonic distortion. In view of voltage regulation, the intermittent fluctuation of the DG output power results in frequent tap changing operations of the OLTC transformer. Thus, many utilities limit the penetration level of DG and are eager to find the reasonable penetration limits of DG in the distribution system. To overcome this technical problem, utilities have developed a new voltage regulation method in the distribution system with a large DG penetration level. In this paper, the impact of DG on the OLTC operations controlled by the line drop compensation (LDC) method is analyzed. In addition, a generalized determination methodology for the DG penetration limits in a distribution substation transformer is proposed. The proposed DG penetration limits could be adopted for a simplified interconnection process in DG interconnection guidelines.
Highlights
It is well known that the interconnections of distributed generation (DG) to an existing distribution network cause unfavorable technical problems, which include power quality problems such as slow/fast voltage changes, harmonic distortions, protection coordination, and exceeding the rating of power equipment
In the line drop compensation (LDC) method, the on-load tap changer (OLTC) regulates the sending-end voltage (SEV) or the distribution bus voltage to maintain the voltage of the high-voltage (HV) distribution line within the designated ranges by maintaining the voltage on the center point at which a load concentrates at an optimal voltage level
Generalized formulations were introduced to evaluate the reasonable interconnection capacity of the DGs at the distribution substation transformer level. These formulations can be used for a detailed interconnection study process for distribution systems which adopt OLTC transformers and LDC control methods
Summary
It is well known that the interconnections of distributed generation (DG) to an existing distribution network cause unfavorable technical problems, which include power quality problems such as slow/fast voltage changes, harmonic distortions, protection coordination, and exceeding the rating of power equipment. The OLTC transformers in a distribution substation adopt the line drop compensation (LDC) method to compensate the distribution voltage drop and to maintain the customers’ terminal voltage within permissible limits. The voltage profiles at the HV distribution line are dominated by the main transformer OLTC tap control using the LDC method with respect to the load change. In the LDC method, the OLTC regulates the sending-end voltage (SEV) or the distribution bus voltage to maintain the voltage of the high-voltage (HV) distribution line within the designated ranges by maintaining the voltage on the center point at which a load concentrates at an optimal voltage level. The LDC control method employs time delay and dead band to prevent frequent and unnecessary tap movements [15,16]. A value of the dead band larger than 3% results in poor voltage regulation performance of the distribution lines [17]
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