246 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 16, NO. 2,APRIL 2001 Investigating the Impact of Embedded Generation on Relay Settings of Utilities’ Electrical Feeders Salman K. Salman, Senior Member, IEEE and Ibrahim M. Rida Abstract—Since the late 70s intensive efforts have been made to utilize renewable energy sources to generate electric power. On the other hand economical pressure experienced in recent years has led to the development of high efficiency combined heat and power schemes to use the normally wasted heat to generate electric power. Generators used under both schemes can be either synchronous or asynchronous types and when they are integrated into utilities’ dis- tribution networks (DNs) they are commonly known as “embedded generators (EGs).” Previous experience has shown that the integra- tion of EGs into DNs could create safety and technical problems. They may contribute to fault currents, cause voltage flickers, in- terfere with the process of voltage control, etc. This paper reports an investigation to determine the impact of the integration of EGs on the settings of protective devices of elec- trical feeders emanated from the substation to which EGs are con- nected. This study also covers the dynamic behavior of EG caused by disturbances on the host network. Results obtained from sev- eral case studies presented and discussed. Index Terms—Critical clearing time, distribution networks, em- bedded generation, EMTP, protection, simulation. I. INTRODUCTION S INCE the late 70s and after what is now known as the “energy crisis,” intensive efforts have been made to utilize renewable energy sources, such as wind, hydro, tidal, etc., to generate electric power. On the other hand economical pressure experienced in recent years has led to high-energy efficiency commitment. This in turn has led to developing high efficiency combined heat and power (CHP) schemes [1] to use the nor- mally wasted heat to generate electric power. Generators used under both schemes can be either synchronous or asynchronous types and when they are integrated into utilities’ networks at distribution voltage levels they are commonly known as “em- bedded generators (EGs).” Various investigations conducted by both industry and academia have revealed that EGs could affect the host distribution network in number of ways [2]–[8]. They may contribute significantly to the fault current, cause voltage flickers, interfere with the process of voltage control, affect the losses of the network etc. This paper reports an investigation related to the determination of the impact of the integration of em- bedded generators into distribution networks on the settings of protective devices of electrical feeders emanated from the substation to which EGs are connected. It particularly focused Manuscript received August 31, 1999. This work was supported by the Robert Gordon University and the Committee of Vice-Chancellors and Principals of the UK. The authors are with the School of Electronic and Electrical Engineering, The Robert Gordon University, Schoolhill, Aberdeen, AB10 1FR, Scotland, U.K. Publisher Item Identifier S 0885-8977(01)03415-X. on determining the critical clearing time (CCT) of integrated EG(s) when a disturbance occurred on the associated network. Such CCT is determined by the onset of an EG becoming unstable. The concept of stability is well defined for the case of synchronous machines but it is not so for asynchronous machines. Therefore, an attempt has been made in this paper to introduce a definition of the CCT which can be applied to asynchronous generator. The study is conducted on a simulated system consisting of a distribution network with integrated EG(s) using Electromagnetic Transient Program (EMTP). This study also covers the dynamic behavior of EG(s) caused by disturbances on the host network. Results obtained from several case studies are presented and discussed. In the first instance a definition of the stability criteria as far as asynchronous gener- ator is concerned will be introduced followed by presenting a comparison between the dynamic behavior of synchronous and asynchronous EGs. II. DEFINITION OF THE PROBLEM Prior to the introduction of embedded generation into distri- bution networks in large scale, such networks are considered passive, i.e., their role confined to transferring the electrical energy received from the transmission systems to consumers. Consequently the design and protection of DNs are based on the basis that such networks are passive. For example protec- tion schemes of distribution feeders are usually designed on the assumption of unidirectional power flow and are based on two basic principles; i) they must insure minimum disruption supply, i.e., the removal of a fault from the system should be achieved with minimum tripping of equipment and ii) they must be fast enough to minimize damage to system components [9]. The first principle is achieved by a proper coordination in the operating time of protective devices. This, however, may lead the opera- tion time of protective devices at the up- stream of a distribution network to be as high as 1.5 second [10]. When EG is connected into a DN, the operating time of protective devices installed on load feeders may exceed the CCT limit required to maintain the stability of EG which this paper tries to establish. III. MODELING ELECTRIC POWER SYSTEMS USING EMTP The Electro-Magnetic Transient Program (EMTP) has been used for several years for power system analyzes under transient and dynamic conditions. It consists of a library of models of network components such as electrical machines, transformers, lines, etc. that can be interconnected together to simulate any required electrical network [11]. 0885–8977/01$10.00 © 2001 IEEE