CIRED Barcelona 12-15 May 2003
MV NETWORK OPERATION AND CONTROL
(2) ENEL DISTRIBUZIONE MV NETWORK
Nowadays, the Enel Distribuzione MV network extension is the following:
· primary (HV/MV) substations: 1,650, all remotely controlled;
· secondary (MV/LV) substation: 270,000 18 % of them remotely controlled
· 136,000 MV/LV pole mounted transformers;
· total length of MV lines: 332,000 km (37% cable and 63% overhead);
(3) MEDIUM TERM PROGRAM TO IMPROVE QUALITY OF SERVICE
At the end of 1998 ENEL Distribuzione launched a medium term program aimed at improving the quality of service, reducing the operating costs and updating the technology of network components.
This program is based on the following main projects:
· remote control of MV/LV substation;
· introduction of a new grounding system in medium voltage (MV) networks;
· automation of fault detection, isolation and service restoration procedures on MV network
At the end of 1999 Italian Regulator defined in the
Cumulative duration of outages in a year for LV customers
The parameter for electrical quality of service and fixed penalties/bonuses for values of indexes worse/better than the stated ones.
The planned activities are going ahead according to the first program and the requests of Italian Regulator : the obtained results are better than the foreseen ones.
(4) REMOTE CONTROL OF MV/LV SUBSTATIONS
A big project to improve the quality of service in a relatively short time period with respect to long-term structural intervention on the network: the remote control and supervision of Secondary Substations.
The main goal is the remote control of 80.000 secondary substations by the year 2004.
The main features of the System are :
· motorised switch disconnectors;
· low cost/high performance Remote Terminal Units(RTUs);
· telecom modules based on GSM system;
· 29 Control Centres for the all territory.
(5) RTUS for MV/LV substations
There are different kinds of Remote Terminal Units to operate signals, commands and, optionally, measurements.
Our RTUs are able to operate the switch disconnectors to localize the fault, isolate it and restore supply automatically, without the intervention of man.
RTUs are fully programmable through a PC based local configuration terminal, which can be used also for diagnostic. The application program and all the main important parameters are downloadable from the control centre: that is a fundamental function to support quick and reliable software maintenance.
(6) Telecommunication Subsystem
The key point of the project is the adoption of the existent GSM cellular network. ENEL Distribuzione developed and standardised by itself all procedures and criteria to measure the Electro-magnetic field level, choose the right antenna and the right mobile provider in the area, monitor and maintain the efficiency of all the connections.
A special integrated GSM module has been adopted, with a raw bit rate of 9600 bps and able to detect and establish the data link in both modes:
· V32 (analog);
· V110 (digital).
Even PSTN lines or leased lines can be used.
(7) Control Centres
Control centres are based on powerful servers for operation and network configuration.
Multiserial interface boards and ISDN modems implement the connections with RTUs.
As far as the connection with the field is concerned, the basic working mode is the following:
· spontaneous call originates directly from the peripheral when a severe alarm generates in the substation;
· at any moment the operator in order to acquire alarms, measurements or to send command on switches can launch a request of connection.
Of course, each alarm or command or change in open/closed status of the switches is displayed on the video diagrams and recorded in the log of the day
(8) MEDIUM VOLTAGE NEUTRAL EARTHING SYSTEM
The Italian choice, in the past, has been to operate the MV networks with isolated neutral in almost all the cases. This solution was adopted forty years ago since it assured simple operation together with earth fault currents reduced to acceptable values (MV networks were mainly overhead), even if some drawbacks like greater stress for insulation and intermittent arc phenomena were expected.
Recently, the issue of neutral to earth connection has been reconsidered, taking into account the raising requirements for power quality and the environmental constraints (cables instead of overhead lines).
At the end of 90’s Enel Distribuzione experimented the connection of neutral to earth through impedance in some of its MV networks (Petersen coil) . In 2000, the very interesting operation results brought Enel Distribuzione management to spread the application of the connection on MV neutral to earth through impedance : goal 2800 MV bars with neutral connected to earth trough impedence by 2007.
At the end of 2002 406 busbars connected to earth trough impedence.
(9) COEXISTENCE OF NETWORKS WITH PETERSEN COILS WITH NEIGHBOURING ONES HAVING THE NEUTRAL POINT ISOLATED
In order to easily manage the coexistence of networks with Petersen coils with neighbouring ones having the neutral point isolated, a new generation of protections (called NCI) for MV lines has been developed to obtain the full protection against earth faults independently from neutral operation: isolated or compensated.
This feature is obtained by adding several programmable thresholds and angular sectors of intervention active at the same time (see fig. 1).
Intervention times associated to varmetric and wattmetric sectors are different and are typically 0.4 s for MV networks with isolated neutral point (called NI) and > 10 s in case of neutral point connected to earth through impedance (called NC).
(10) FEEDER PROTECTION RELAYS INTERVENTION SECTORS FOR ISOLATED (NI) AND COMPENSATED (NC) NETWORKS
(11) MV NEUTRAL CONNECTION TO GROUND CHOICE CRITERION
The MV neutral connection to ground choice (fixed coil or variable coil or simple resistor) criterion has been settled considering the phase to ground current (present and 5 years planning values) with isolated MV neutral according to the following table 1.
(12) OPERATION RESULTS
All the reported figures are relevant to 63 MV busbars operated with neutral connected to ground through impedance since the last quarter of 2001 and previously operated with isolated MV neutral.
The overall MV network length is 7348 km with an average length of 117 km. The cable percentage is 38%.
The time periods considered for operation performance comparison are the following:
- january to september 2001 with reference to operation with isolated MV neutral;
- january to september 2002 for MV networks operation with neutral connected to ground through impedance.
This corresponds to 567 months per busbar. It is worthwhile to mention that, in the time periods taken into account the considered networks didn’t change their length substantially.
Table 2 summarises the MV network characteristics.
In table 3 the operation performances obtained changing the MV neutral management from isolated to connected to ground through impedance are compared.
(13)
Table 4 shows the network performance related to cable percentage: all the networks, with cable percentage greater or lower than the mean value, have been grouped together.
With reference to transient interruptions (<1s), the compensated MV neutral operation eliminates almost totally (by 87%) the phase to ground ones and reduces substantially (by 57%) the total ones. The network performances don’t change with cable percentage.
Moreover the connection to ground of MV neutral through impedance brings a substantial reduction to both of phase to ground (by 53%) and total (by 40%) short interruptions (<180s). The analysis performed considering the cable percentage shows a bigger network performance improvement for networks with a higher cable percentage.
Even considering long interruptions (>180s), the MV network operation with compensated neutral allows for significant reduction of phase to ground (by 28%) and total (by 33%) ones.
For this kind of interruptions the network performance improvement is:
- inversely proportional to cable percentage for phase to ground faults;
- directly proportional to cable percentage for all faults.
(14)
The change of MV neutral earthing system from isolated to compensated by means of Petersen coil brought a sensitive reduction of MV feeder circuit breakers openings as a consequence of relays intervention.
The number of transient interruptions are strongly (by 57%) reduced as well as short ones (by 40%), at the same time a significant reduction of long interruptions (by 33%) has been noted.
MV networks with Petersen coils, being less stressed than those with isolated neutral, have better performances from continuity of supply point of view with substantial reduction of supply interruption due to faults.
The control systems used for MV network with compensated neutral allow also a better network monitoring increasing the number of “low” signals available for the network operator in order to focus on preventive maintenance.
MV networks operated with neutral point connected to ground through impedance allows for advanced fault clearing procedures by means of simple on load motorised disconnectors with further improvement of quality of supply of the customers.
(15) Basic concepts of automation
Automation on MV network is based on a group of automatons resident in the RTU memory.
These automatons which can be programmed, activated, deactivated and trimmed by suitable messages from the Control Centre, operate fault detection and selection through simple actions and delays triggered locally by two signals:
· Presence/absence of voltage on the input line;
· Intervention of fault detector.
The recloser unit, installed in correspondence with each MV line inside the line protection panel, masters the timing sequence of the operations.
(16) Fault detectors (RGDAT)
Apart from the right design of the automatons in the RTUS, the core of the system is the fault detector.
This device detects earth faults and short circuits activating different outputs and has the following features :
· directional sensitivity;
· remote changing of the working direction;
· high sensitivity in both grounding configuration of the MV network (isolated/compensated neutral)
(17) Description of FRG and FNC automation techniques
FRG and FNC are two different methods of automatic fault location, isolation and service restoration on the healthy sections located upstream from the faulty ones.
These methods are based on special logic programmed inside the RTU and applied to one or more of the switches connected to the secondary substation MV bus bars and on the automatic reclosing device installed in correspondence with the circuit breaker at the line departure in Primary substations.
The operator in the Control Centre can:
· select the appropriate procedure in depending on the type of the line (overhead, cable, mixed) and the status of the neutral in Primary substation (isolated, compensated);
· configure the chosen procedure for each line departing from the Primary subs;
· enable/disable the procedure for each line, each secondary subs and each switch;
· receive the spontaneous call from RTUs in order to be alerted and complete the procedure, re-supplying the healthy sections of the line located downstream from the faulty section.
(18) FRG technique
It can be applied to cables and overhead lines (only time parameters are differently chosen) and with both configurations – isolated/compensated - of the neutral point.
The logic utilizes fault (FD) and voltage detectors (VD) integrated into the RGDAT installed in correspondence with each automated MV switch.
Usually, in a secondary substation (S.S.) , only the energy input switch disconnector is automated but the system allows to automate more than one switch.
The simple rules performed by each automated switch are the following:
1. The switch is opened if there is a lack of voltage on the VD lasting more that a certain time and the FD has been activated by the fault current;
2. the switch is closed (there is a priority order in case of more automated switches) when the VD detects the voltage restoration due to the reclosing of the circuit breaker or the switch located upstream;
3. the switch is opened and locked in the open position when, executing the closing operation according to the rule n.2, there was a lack of voltage (VD) and at the same time the activation of FD, both within a time window starting from the closed position of the switch.
The lock in the open position of a switch causes a spontaneous call towards the Control centre so that the operator is alerted and can proceed to re-supply the other feeder sections.
(19) FNC technique
It can be applied to cables and overhead lines, but only on networks with neutral compensated configuration of the neutral point.
The adopted logic is different depending on the type of fault so as detected by RGDAT:
· for short-circuits (overcurrent detector) the logic acts as in the FRG procedure;
· for earth faults (earth detector) the automatons act in such a way as to isolate the fault without any tripping of the circuit breaker at the departure line.
The second case is the peculiarity of the procedure and deserves a more detailed description.
The simple rule performed by each automated switch is the following:
· the switch is opened and locked, after a programmed delay, if there is the intervention of the earth fault detector and the intervention itself lasts until the end of the calculated delay;
the delay programmed for each automated switch is calculated according to the position of the substation along the feeder so as to open the farthest switch from the Primary substation among those having detected the fault.
(20) TYPICAL ENEL MV FEEDER
The typical ENEL MV feeder has :
Lenght: 16.8 kM 63 % overhead 37% cable
Substations 21 (7 pole mounted trasformers)
MV Customers 5
Power subscript MV Cust 1,355 kW
LV Customers 1,400
Trasformers MV/LV No 18
Trasformers MV/LV PW 3,304 kVA
(20) “Comparison of times for the selection of a faulty section”
(Manual selection, selection by remote commands, automatic selection)
Eugenio Di Marino Italy Session 3 Alpha1 Block1 10/1 29/04/03