Impacts of hidden failures on protection coordination in electrical distribution systems with distributed generation

In early days of electricity, electrical power generation, transmission and distribution were achieved by direct current (DC) generators and operated at a single voltage level. Load points had to be close to the power plants such as 800 meters because direct current power could not be easily transformed to higher voltages in order to minimize power loss during transmission for long distance. According to development in transformers, induction machines and polyphase systems, alternative current (AC) system has been started to use in electrical power generation, transmission and distribution. Using AC in generation, transmission and distribution systems provide low cost energy transfer to farther distance at higher voltages. In order to balance power between demand and generation is achieved by load shedding during the extreme situation. If there is, a fault occurred in power plant, supply of power provides from other power plants. Traditional distribution systems are designed with a radial structure. As a result, their protection systems are designed and set based on one utility source feeding the whole system. Demand for electricity is steadily increasing. In order to meet growth of electricity demand cause establishment of electricity interconnections between electrical power systems that's motivated by concerns with security of supply, more specifically by the need to satisfy domestic demand during periods of shortages in generation capacity. An interconnected electrical power system delivers electricity from suppliers to consumers. It consists of generating stations that produce electrical power, high-voltage transmission lines that carry power from distant sources to demand centers, and distribution lines that connect individual customers. Interconnected electrical power system has important advantages such as improvement in the reliability, quality of electricity supply. However, it has adverse effect on protection system and coordination. In interconnected electrical power systems, power flow is in multiple directions along distribution lines. As the grid matures and expands in complexity to accommodate multi-directional power flow, protection system and coordination are needed to revise for the direction of power flow along the distribution lines to ensure fast, selective and reliable relay operation to isolate the power system faulted sections. In Turkey, transmission system's voltage levels are 154 kV and 380 kV that steps down to distribution voltage level which is 4-34,5 kV for primary distribution system and 231/400 V for secondary distribution system, in distribution transformer centers. In recent years capacity of fossil fuels that use in electricity generation, have dramatically decreased. Another adverse impact of fossil fuels is on environmental concerns. These impacts lead to electricity generation from renewable energy sources such as wind power, solar photovoltaics, biomass, small hydro etc. Distributed generation (DG) is defined as small-scaled generation facilities that located nearby consumption area, connected to grid or operated in islanding mode. Considering reduction in gaseous emissions, more energy generation from renewable energy sources, energy efficiency, deregulation and competition in the market, high cost of transmission systems, generation closer to consumer and short construction time makes DG feasible to invest. Generally electrical power system with DG has lower transmission losses, improved voltage profiles tough short circuit currents gets higher because of the short circuit contribution of DG that has to be taken into account for protection system and coordination. Radial distribution systems, to which generally DG units are connected, are designed one-way power flow from a single source to the loads. Presence of DG in radial distribution not only changes system topology but also contribution to unconventional load flow patterns. DGs are classified into three major categories as: first one is resource that use in electricity generation such as renewable energy source, fossil fuels, second one is generation capacity micro scale (1 kW–5 kW), small scale (5 kW–5 MW), medium scale (5 MW– 50 MW) and large scale (50 MW–300 MW) and third one is the type of electricity that DG generates, like AC or DC. Photovoltaics, fuel cells produce DC while wind turbine, micro turbines produce AC. In power distribution systems, protection systems are installed to ensure continuity of supply, to limit damage to equipment and to prevent injury to people during the fault conditions. The protection system arrangement for electrical power system has to be satisfied the following basic principles: reliability, speed and selectivity. The reduction in the number and duration of the interruptions to consumers can improve the reliability of the electricity supply. Power quality can also be enhanced with a faster pick up time to minimize the possibility of voltage sags, voltage flicker etc. Protection systems generally consists of relays, circuit breakers, instrument transformers, auxiliary contacts, batteries. Protective relay is the most commonly used protection equipment and a main device in protection system. The main role of a protective relay is to detect system abnormalities such as faults and to execute selectively correct commands to circuit breaker in order to isolate only the faulty component while leaving the rest of the network in operation. Protective relays are connected to the electrical power systems over current transformer and voltage transformer. Voltage transformer is essential for directional and distance protection while it is optional for overcurrent protection. Protective relay directly linked with circuit breaker that receive trip signal to eliminate faulty area. Relay protection and coordination process is about choosing protective relays and its appropriate settings and determining the sequence of relay operations for each possible fault location and providing acceptable coordination margins without disproportionate time delay. Coordination margin or coordination time interval is the time delay between upstream and downstream devices. This value is generally between 0,20 s and 0,30 s. All protective devices settings have to be cautiously calculated in order to provide selective protection system. Protective relay and its circuit breakers only trips when fault occurred in own protection zone otherwise improper tripping causes power outages. Every equipment in electrical power system must be protected with protective devices. Inverse Define Minimum Time (IDMT) characteristic is the most extensively used that coordination range is possible in a wide range of currents and definite minimum time can be set for higher value of short circuit currents. IDMT relay has two main settings that are time multiplier setting (TMS) and pick-up current setting. Pick-up current setting is threshold value that shows the minimum operating current value. Pick-up current has to be bigger than full load current otherwise, protection function activated at fully load conditions. TMS is used to guarantee the coordination between protective devices. Coordination time interval is applied across the entire protection system. In radial electrical power system, fault current direction is from source to loads thus protective devices of source is the latest one operates. Protection coordination can be provided by setting different operating time for each relay. When DGs connected to radial distribution system, direction of power flow and system topology changes thus protection coordination has to be reconsidered. Under this circumstance, directional overcurrent relay usage is inevitable any other way selective protection system cannot be obtained. Especially when there is a fault on one of the parallel line, fault current will be supplied by the sources from different directions thus protective relays have to be detect and execute the proper signals for protection. The protection system provides reliable and safe operation of electrical power system. Integration of DGs make electrical power system more complicated since their contribution to the load and short circuıt currents. When the penetration level of DGs increase, detecting faults and executing proper protection functions are getting more difficult. Furthermore, coordination margin lost between the protective devices and causes protective system misoperation. In additional to that, hidden failure can arise in protective devices. Protective relay failure mode classified in two major categories as failure to operate which defined as related relay not working during a fault and undesired tripping which stated as tripping occurs in the absence of fault or out of the protection zone. Hidden failure is permanent fault, which is expose during the abnormalities in electrical power system such as short circuit faults, overloads, and reverse power flow etc., cause protection equipment to isolate incorrectly and inappropriately a circuit element as a direct consequence of another switching event. Hidden failures in protective devices cause cascading outage, which may lead to system collapse, in other words blackout. Hidden failures in protective devices does not frequently occur in electrical power system but its outcomes might be crucial. System collapse or blackout is defined as short or long time duration of electrical power outage in a wide area for disturbances. In this thesis, a distribution feeder, which, is operated by Bosporus Electricity Distribution Incorporation (BEDAŞ), is modelled in DigSilent Power Factory power system analysis software. Load flow and short circuit analysis according to the IEC 60909/2001 were carried on DigSilent for each different operations of distribution system such as radial, ring operation and presence of DG units. In the dissertation, DG's influences, overview of the protection coordination in electrical power systems and hidden failure in protective devices are presented. Protective relays settings and types which, are used in test system, have been determined to ensure selective protection coordination between main protective relays and back-up protective relays in consideration of load flow and short circuit analyses results. Coordination time interval between upstream device and downstream device is chosen as minimum 0,30 seconds using appropriate TMSs while overcurrent relays' characteristics choose as ANSI/IEEE normal inverse. By that way, same fault current seen by relays' tripping actions have been distinguished. After providing selective protection coordination overall the distribution system, different types of short circuit faults applied at different locations and related time-overcurrent plots and protective devices tripping times are given as well as relay settings that ensure protection coordination. At any time of distribution feeder topology has been changed, protection coordination is checked by a flowchart given in Section 4 and protective relay settings are altered in order to provide selective protection coordination. Finally, effect of hidden failures in protective devices is considered in terms of loss of load overall, the system and its results are given. In this study, new indices are defined to examine increasing amount of the loss of load between the protective devices proper operation and hidden failures in protective devices.