MV NETWORK AUTOMATION IN THE TOWN OF MILLION - ZAGREB (CROATIA)

MV NETWORK AUTOMATION IN THE TOWN OF MILLION - ZAGREB (CROATIA) Runjic Darijo Koncar Power Plant and Electric Traction Engineering Inc. - Croatia darijo.runjic@koncar-ket.hr Sporec Marko, Strmecki Goran HEP distribution - "Elektra" Zagreb - Croatia marko.sporec@hep.hr, goren.strmecki@hep.hr INTRODUCTION This paper describes the basic characteristics of MV network in the distribution area (DA) "Elektra" Zagreb and gives criteria for the selection of network points that are, due to their exceptional importance, required to have remote control capability. It also gives a description of procedure for remote control of switching devices built in medium voltage fields in MV/LV cable substations. Every element in the remote control system is described with all important tehnical data. 1. DA "ELEKTRA" ZAGREB POWER NETWORK Power network of DA "Elektra" Zagreb (that is a part of the state-owned Power Distribution Company that provides electric energy supply to the town of Zagreb and its neighbourhood) is a complex system of 0.4kV, 10kV, 20kV, 30kV and 110kV switchyards. Due to a relatively big surface of the distribution area, which has a variety of urbanisation degrees, as well as specific qualities of individual zones, during the process of planning and construction of power network, a range of parameters have to be taken into consideration. Since the construction of the first thermal power plant in Zagreb in 1907, the town power network has been widened from the centre outwards and together with a parallel development of neighbouring places have led to situation that today we practically do not have boundaries between the town and its suburbs. Circumstances under which the town was developing during the last century an absence of high quality and long term urbanisation plans that would be followed by long term plans for power system development, has resulted in insufficient capacities of existing supply points as well as distribution circulation the middle voltage power network in the centre of the town. Under today's conditions of the high degree urbanisation in the centre of the town, with existing complex public utilities network the great problem we are facing is resolving of property-rights relations bound to building sites for new substations, whether for new supply points subsations 110/10(20) kv, whether for new MV/LV cable substations and corridors for passing of new cables. 1.1. Supply points Supply points in power network of DA "Elektra" Zagreb are substations 110/10(20) kv and 30/10(20) kv. During a long period of time, building of new supply points in DA "Elektra" Zagreb has not followed a big increase in electric energy consumption, and the power load in some supply points has reached critical levels. A special problem represents switching off of some supply points due to ordinary maintenance during which supply of electric energy to consumers has to be accomplished through neighbouring supply points what causes their high power loading. In 2001 the DA "Elektra" Zagreb have maximum demand of 632,386MW. In 2002 preparations for construction of three new important supply points (substations 110/10(20)kV) in Zagreb have started. Making of investment and technical documentation for substations, as well as distribution of medium voltage cables from those stations, have entered into the final phase, while finishing of the construction and putting into operation is expected to be in a few years. By putting those supply points into operation, not only the reliability of DA "Elektra" Zagreb power system will be increased, but also preconditions for urbanisation and business development of some parts of the narrower and the wider centre of the town of Zagreb, that today are at a standstill due to overloading of some existing supply points, will be created. In development plans of DA "Elektra" Zagreb a tendency is to phase out supply points from the voltage 30kV, and to introduce middle voltage level 20kV. In accordance, transformations 110/30kV and 30/10kV are going to be abandoned, and the final aim is establishment of distribution power system with transformations 110/20kV and 20/0,4kV. For those reasons DA "Elektra" Zagreb has started the project of MV distribution network automation through installation of remotely monitored and controlled switching devices in the MV/LV cable substations. KON_Runjic_A1 Session 3 Paper No 16-1 -

During selection of MV/LV cable substations in which the equipment that enable remote control is being built-in, the following criteria have been taken into consideration: Substation supplies a consumer with a need of an increased reliability in power supply (consumer of an especial state or business importance); Substation on a long distance or on a difficult terrain a significant time is necessary for an operative team to access the substation, or the entrance into it is difficult; Substation in the area where a possibility of faults on middle voltage cables is high (pursuant existing statistics); Substation that represents a supporting point of a part of the middle voltage network; Substation in the middle of connective MV line (in connective network) and in the middle of a ring (in ring network) from the cable loading point of view; Substation in the place of important transversal connections between individual connection cables; 1.2. Middle voltage (MV) network development 1.2.1. MV network configurations In DA "Elektra" Zagreb, three types of MV power network configurations are comprised: Radial network Ring network Connective network Radial network model in which a range of cable substations 10(20)/0,4kV is radially supplied from a central supply point is present on margin parts of DA "Elektra" Zagreb, where a smaller number of consumers are placed on relatively big geographical surfaces. The terrain is often very difficult and the costs of establishing of a feedback connection that ensures higher reliability of electric energy supply are very high. Ring network model in which a cable output from one supply point supplies a certain number of MV/LV cable substations and it comes back to the starting supply point, exists in margin areas of the centre of the town of Zagreb where a relative high density of consumers exists, but it is significantly lower in comparison with the one in the centre of the town. Increasing of reliability of power supply in these areas is reached by transforming of ring network into connective network. point is present in the centre of the town of Zagreb, where the density of consumers is the highest, and where the need for increased reliability of power supply is expressed because a majority of state and social institutions, as well as important business subjects are placed there. Due to the limited possibility of building a new supply points, the connective network model with a support point is used, in which a direct cable from a neighbouring feeder point is led to the centre of the ring (in ring network) or to the centre of the connecting cable (in connective network), and in such a way transmitting capacity of cable outputs from supply points is significantly increased. 1.2.2. MV/LV cable substations remote control A continuous growth of industrial production, other economy branches and living standard results in the constant increase of power consumption in Croatia, especially in its capital, the town of Zagreb. On the other hand, the existing complex economic situation requires business rationalisation of all the subjects in the production chain, from generation to distribution of electric energy and exploitation of all existing capacities up to the highest level. Regardless of such conditions the distribution company task is to increase the reliability and quality of delivered electric energy (with minimal costs), in order to be, as much as possible, prepared for forthcoming process of energy market opening. 2. REMOTE CONTROL SYSTEM IN MV/LV CABLE SUBSTATIONS In the area of the town of Zagreb, in 2000 the total number of cable substations 10(20)/0.4kV was 1898, but neither station did have a possibility of remote monitor and control. At the beginning of 2001, starting form the defined criteria (pointed out in item 1.2.2.) the first 20 cable substations were selected in which MV switchgears with a possibility of remote control were built-in by the end of 2001. Low voltage switchgears do not have a possibility of remote control, but they are necessary for functioning of remote control system due to supply of some elements with 230V AC. A power transformer is seen as a source of 230V AC and as an element that has to be protected from faults. Besides MV switchgear with a possibility of remote control, other equipment necessary for realisation of all the foreseen remote monitor and control functions was built-in in each cable susbstation. Under term "other equipment" we consider devices placed in appropriate cubicles or independently, as shown in Fig.1, and described together with MV switchgear in items below. Connective network model in which each cable output from the individual supply point supplies a certain number of MV/LV cable substations and ends in some other supply KON_Runjic_A1 Session 3 Paper No 16-2 -

Fig.1: Elements of remote control system in MV/LV cable substation 2.1. Middle voltage switchgear manually also, as a reserve to the remote control. A middle voltage switchgear consists of one or more switching blocks that are equipped with supplementary components in comparison with blocks of classical manufacture for manual operation. 2.1.1. Motor drive The basic function of the motor is winding up of the spring (that is a part of driving mechanism) for manipulation of switching devices. During the process of manipulation of switching device a discharge of accumulated energy in the spring occurs. After that, the motor automatically receives the signal for switching on and winds the spring up. The process of winding up of a spring lasts 5-6 seconds and after that the driving mechanism of the switching block is ready to perform the following operation. A motor has demand of 350W and it is supplied by 48V DC. 2.1.2. Signalling switches Their main purpose is to determine switch position of switching devices. 2.1.3. Electromagnetic solenoids for switching on/off As a difference to switching blocks for manual operation where the command for opening/closing of a switching device is directed exclusively in mechanical way with the help of managing handles, switching devices in a remotely controlled switching block must have solenoids for switch-on/off that receive control signal from the remote terminal unit (RTU) and forward them to the driving mechanism. Each circuit breaker has a solenoid for switching on and switching off, while switch disconnector have one solenoid for grounding out and switching on, and the other solenoid for switching off and grounding (switch disconnectors are threeposition switches). The only device in switching block without a solenoid for switching on/off is a ground connector in transformer field that can be operated manually, only. Circuit breakers and switch disconnectors can be operated 2.1.4. Indication of voltage presence on the MV cables In order that the operator in the control centre is sure in performance of manipulation with switch disconnectors in MV cable fields, besides the information on switch position received via RTU, the operator has to know whether the MV cable is powered or not, That is enabled by a voltage indicator that forwards the signal of voltage presence to the RTU/control centre. 2.2. DC switchgear DC switchgear of the rated voltage 48VDC ensures uninterruptible power supply for all substation components. It consists of a battery produced in "dry fit" technology gel (dry battery), composed of four 12V blocks, of rated capacity 25Ah. The battery is charged over a rectifier produced in switching mode with input voltage 230V AC and output voltage 48V DC. 2.3. Power transformer protection Rated powers of power transformers in MV/LV substations are placed within the range from 400kVA to 1000kVA. With regard to power and importance in power system, these power transformers are protected from a short circuit (which is provided by digital relay), internal faults in the transformer (provided by Buchholz relay) while detection of transformer overheating is provided by contact thermometer. Buchholz relay and the contact thermometer have two levels of protection: the first level warns that it is necessary to perform certain measures for prevention of possible forthcoming fault, while the second level switches the circuit bracker off. In the case of activation of any protection, signal generated by auxiliary protection elements placed in a separate cubicle is forwarded to the RTU /control centre, and switched off the transformer circuit breaker in the following way: KON_Runjic_A1 Session 3 Paper No 16-3 -

Activation of the first level of the contact thermometer or the first level of Buchholz relay activates the following occurrence: The signal of activation of the protection is forwarded to the control centre. Activation of the second level of the contact thermometer or the second level of Buchholz relay or digital relay (short circuit protection) activates the following occurrence: Local signal - flashing lamp; The signal of activation of protection is forwarded to the RTU/control centre; Circuit breaker in the MV transformer field is switched off; As long as the signal of activation of the second level of protection or short circuit protection is present, the transformer circuit breaker control is blocked until the intervention team repairs the fault, and resets the protection activation signal. 2.4. Fault indication on the MV cables For realisation of the fault indication on the MV cables, a fault indicator is needed, placed in a separate cubicle and connected with signalling cables with three current sensors built on the end of the MV cable on the point where the cable enters the MV switchgear. Types of faults registered by the fault indicator are phase to phase and phase to earth faults, with a remark that the indicator does not distinguishes the type of the fault. After the fault detection, local fault signalization takes place and also, fault indicator forward signal to the RTU/control centre. The function of the fault indicator is to accelerate the procedure of MV network fault location and isolation. In the case that the remotely controlled cable substation 10(20)/0,4kV X is placed in a range of N stations counting from the supply point towards the end of the middle voltage cable, and a fault occurs "behind" the controlled cable substation X when we look at it from the feeding side, the fault current will flow through the controlled cable substation X towards the supply point where the appropriate protection will switch off circuit breaker. When the fault current flows through the controlled cable substation X the fault indicator will react and forward the signal towards the RTU/control centre and towards the local signalling blinking lamp. Based on the information on cable protection activation in the supply point and information on fault indicator activation in the substation X it can easily be concluded that the fault has happened "behind" the substation X and, in such a way, determine more precisely the location of the fault (Fig.2.). If the fault occurs "before" the substation X only the feeder protection in the supply point will be activated, while fault indicator in the substation X will not be activated what represents a sufficient number of information for making the decision that the fault has happened "before" the substation X (Fig.3.). 10(20)kV cable L FAULT Supply point substation 110/10kV 1 2 3 x n-1 n 1,2,3...x...n-1,n - MV/LV substations Fig. 2: Fault on the MV cable "behind" monitored MV/LV substation in respect with the supply point (L local flashing lamp is on). 10(20)kV cable FAULT L Supply point substation 110/10kV 1 2 3 x n-1 n 1,2,3...x...n-1,n - MV/LV substations Fig. 3: Fault on the MV cable "before" monitored MV/LV substation in respect with the supply point (L local flashing lamp is off) 2.5. Remote terminal unit DSSN 200 The main requirements regarding of MV/LV cable substations KON_Runjic_A1 Session 3 Paper No 16-4 -

remote control is to apply the control unit of acceptable price and appropriate capacity. In that sense the remote terminal unit (RTU) DSSN 200, shown in Fig.4, has been developed. It represents a central unit of the secondary system in the MV/LV cable substation and allows remote monitoring and control. Switching commands: opening and closing of switching devices in MV switchgear Measuring: DC supply voltage, load current of low voltage switchgear Indications: Double: open/close positions of switching devices in MV switchgear Single: activation of short circuit protection of the transformer activation of thermal protection of the transformer activation of Buchholz protection of the transformer. fault current (for each MV cable) charger fault battery fault operating mode local or remote the position on entrance door of the substation voltage presence (for each MV cable) Fig. 4. Remote terminal unit DSSN 200 RTU DSSN 200 includes 64 digital optically insulated inputs, 8 analogous inputs and 16 double digital outputs. Coloured LED's display the flow of communication with control centre and communication errors, while sets of LED's besides digital inputs, also have the function of a local indication of the states of switching devices and the other equipment in the MV/LV cable substation. The remote terminal unit comprises the integrated modem RM 1200 as well. RTU communication interface consists of 4 communication ports. The communication port COM, beside the local programming of the RTU processor, also enables working with an external modem. TEST port is used for local configuration of the modem that is performed, the same as programming of the processor, by portable PC. LINE port is used for connecting of the RTU to the communication line towards the control centre, while EXP port enables extending of the RTU. Remote programming of processor is possible from the control centre through the regular communication line. RTU DSSN 200 incorporates within a single unit all the functions necessary for remote MV/LV cable substation monitor and control, namely: collecting and processing of signals generated by equipment in substation, collecting and processing of analogous measuring, communication with control centre, executing of switch devices opening and closing commands from the control centre. Data exchanged between RTU DSSN 200 and control centre may be, in respect to their character, divided into: 3. CONTROL CENTRE Software and hardware for remote monitor and control of the MV/LV cable substations are placed in the control centre of DA "Elektra" Zagreb. The computer that consists of an industrial PC with a monitor performs the function of realtime remote control. SCADA software system PROZA R/F based on operation system UNIX and Windows environment has been implemented. The software system WINARH enables searching of SCADA archive based on different criteria and export of data to Windows platform, with no influences on SCADA system performance and operation function. Remote searching of SCADA archive via Web (Internet) is also possible. 4. COMMUNICATION SYSTEM During selection of a communication system that is used for communication between SCADA system and remote terminal units in MV/LV cable substations, the following facts have been considered: The plan is to control remotely about 100 substations Geographically speaking the substations are very dispersed Communication should be reliable, safe and highly available Communication with RTU's should be permanent Considering fundamental advantages and disadvantages of possible communication systems, the following have been selected: Utility owned telecommunication cable network and Radio communication system with a simplex channel (441,975MHz). KON_Runjic_A1 Session 3 Paper No 16-5 -

Utility owned telecommunication network has been used in the centre of the town, where it was built for needs of connection of primary substations (transformer stations 110/30kV, 110/10(20) kv, 30/10kV) and business buildings of DA "Elektra" Zagreb. All the locations that are not covered by the cable system, and that are in the first line locations on the periphery, are connected by radio connection with the control centre. It is important to mention that remote terminal units DSSN 200 are able to perform the function store and forward which make them to be digipitors for substations that are optically invisible from the control centre. Communication protocol is made in accordance with IEC standard 60870-5-101. 5. CONCLUSION With realisation of the first phase of MV network automation precious knowledge and experience have been obtained and good bases for a wider application of distribution automation have been built. A more detailed financial analyses and optimisation algorithm will determine in which scope it would be necessary to realise MV network automation in the end. KON_Runjic_A1 Session 3 Paper No 16-6 -