Mr. Marco Schenone Mr. Sandro Tuscano Mr. Alessandro Oldrati 1
Introduction Ansaldo Energia has recently completed the manufacture, installation and commissioning of two synchronous condenser sets for Terna Rete Italia in the preexisting substation of Codrongianos (Sardinia - Italy), in cooperation with ABB. 2
Introduction The synchronous condensers are connected to the EHV/HV grid of Sardinia (part of the Italian national grid) to increase the network s short-circuit power and inertia. This is particularly important for a grid like the one in Sardinia, where conventional generation plants fired by fossil fuels have been supplemented by significant capacity from renewable sources (wind and photovoltaic in particular). In addition, the links between Sardinian, Italian and Corsican grids, which use High Voltage Direct Current (HVDC) submarine cables, create the need for a continuous local supply of variable reactive power. At the instance of Terna the synchronous condensers have been made able to be remotely controlled by Terna substation located in Rondissone (Piedmont north of Italy) during the normal operation. 3
The supplied system The supplied system consists of two synchronous condenser sets, each mainly made up of: 250 MVAr synchronous condenser (SC) and relevant auxiliary systems 330 MVA step-up transformer for connection to EHV grid MV circuit breaker between SC and step-up transformer unit and auxiliary transformers MV and LV distribution panels protection, command, control and monitoring systems. These sets are not completely independent, because they have some common equipment, like the control system and the closed circuit cooling water system. 4
The synchronous condensers The two synchronous condensers are based on well proven standardized 2-pole air cooled turbogenerator design WY23Z-109RR, normally coupled to gas turbine in power plants, with some adjustments, which involve its auxiliary systems as well, introduced to satisfy the special operating conditions. 5
The synchronous condensers The main characteristics of this type of generator in standard version are: cylindrical rotor with 2 poles closed circuit cooling with 4 air-to-water coolers incorporated into the housing self-ventilating air cooling system for stator and rotor slip ring housing for static excitation class F insulation system for stator (resin rich technology) and rotor opposite collector end shaft grounding device and collector end bearing double insulation. 6
The synchronous condensers Main data Unit Value Rated power MVAr 250 Rated power factor - 0 over-excited Max. reactive power in under-excitation at rated voltage MVAr 125 Rated current / voltage A / kv 7597 / 19 Voltage variation range (continuous; exceptional) (*) % ± 5; ± 8 Rated frequency / speed Hz / rpm 50 / 3000 Frequency variation range (continuous; exceptional) (*) % ±2; -5 / +3 Max combined V/f variation (continuous; exceptional) (*) p.u. 1.05; 1.08 Phase number / Phase connection - 3/STAR Excitation current / voltage (at 110 C) at rated load A / V 1557 / 376 Cooling water / primary coolant rated temperature C 35 / 40 Ambient temperature range C / m a.s.l. -2 40 Site altitude m a.s.l. 315 Short circuit ratio - 0.6 Inertia moment (J) kg m 2 8575 (*) refer to figure on page 9 7
The synchronous condensers Voltage/frequency operation range 8
The synchronous condensers Peculiarities and auxiliary systems Some peculiarities, related to the lack of coupling to any turbine, has been added for the synchronous condensers with respect to the standard design: collector end thrust bearing phonic wheel, speed sensors and keyphasor on collector end of rotor. Moreover, the following main auxiliary systems, derived from the experience on gas turbines, have been adapted in size and characteristics in order to satisfy the project conditions: closed circuit cooling water system ventilation system lubrication/jacking oil system excitation system static frequency converters (SFCs). 9
The synchronous condensers Closed circuit cooling water system 10
The synchronous condensers Ventilation system 11
The synchronous condensers Lubrication/jacking oil system 12
Control philosophy The plant of synchronous condensers (ICS) is managed by a distributed control system (SC-ICS) which allows command, control and supervision of all equipment, power components and auxiliary systems. The command of these components is possible from three different levels: level 1 Local Peripheral : MV / LV local boards and control panels; level 2 Local Centralized : stations of SC-ICS with human-machine interface (HMI) in the local control room of synchronous condensers and in the control room of SA.Co.I. (Sardinia-Corsica-Italy) HVDC-LCC link, located in Codrongianos substation. level 3 Remote : remote control stations of SCCT (Terna Control and Conduction System) with HMI, located in Rondissone substation. These three levels are mutually exclusive, because the permission to command given to a specific level inhibits other levels from operating. 13
Control philosophy The normal operation is managed at level 3 Remote, in which the availability of macro-commands and main information on the plant conditions is exclusively present, in order to simplify the work of operators and face adequately the constraints of exchanging data between SCCT in Rondissone and SC-ICS in Codrongianos via IEC 60870-5-104 protocol. The local SC-ICS permits the management of all operative conditions at levels 2 and 3, included the start-up and the stop of a single unit or both of them. Since the plant is functionally divided in different systems and subsystems, for all of them a start-up sequence and a stop sequence have been arranged; moreover each system is managed by automatic logics during normal operation and in case of malfunctions. In order to simplify the management of units on the part of operators both from level 2 and 3, a main start-up sequence and a main stop sequence of the single unit have been arranged: these main sequences activate progressively the sequences of single systems to perform all possible transitions among the different states of the condensers. 14
Control philosophy From STILL to READY From READY to CONNECTED STILL UNIT READY UNIT CONNECTED UNIT From READY to STILL From CONNECTED to READY Still unit : the synchronous condenser (SC) is still and disconnected from the grid, its dedicated auxiliary systems are still, the auxiliary systems in common with the other unit can be in operation. Ready unit : the SC is disconnected from the grid, its dedicated auxiliary systems and the auxiliary systems in common with the other unit are in operation. This is the condition of operational rest in which the condenser is maintained in order to be put into service quickly and reliably. Connected unit : the synchronous condenser is connected to the grid. 15
Malfunctions In case of malfunctions, the logics in the SC-ICS automatically disconnect the unit from the grid in the following ways, depending on the type of protection activated: mechanical trip : disconnection from the grid and consequent electrical braking by means of SFC up to the complete stop of the unit; electrical trip (electrical problem inside the condenser): disconnection from the grid and consequent natural inertial slowing down to the complete stop of the unit; electrical disconnection (electrical problem outside the condenser): disconnection from the grid and consequent electrical braking by means of SFC up to the turning gear speed. 16
Starting system A Static Frequency Converter (SFC) is used to accelerate (or brake) the condenser, by driving the rotating machine as a synchronous motor. 3000 kva 2250 kw 17
Starting data By turning on the thyristors at the right time, the inverter is able to anticipate (or delay) the stator magnetic field and to accelerate (or brake) the rotor, that in the meantime must be supplied by the excitation system. The SFC controls the excitation system by a field current reference signal and regulates the stator voltage at 1500 V, after an initial ramp. The SFC is able to start the condenser from zero speed or to grab the condenser at higher speed, also next to the rated speed of 3000 rpm. The SFC size of 2250 kw allows to reach the final speed value within 7 minutes, starting from zero speed. 18
Main SFC circuit 19
Overspeed The control of the static starter has a speed regulator, that brings the condenser frequency over the actual grid frequency, to allow the synchrocheck to close the main circuit breaker during the natural speed fall down. The maximum speed reached by the SFC must allow to synchronize the unit at the higher grid frequency of 51,5 Hz (1.03 p.u.), that is an exceptional condition foreseen less than 10 times per year: in this case the maximum peak speed of the condenser can arrive at about 53 Hz. In order to avoid stressing the condenser rotor with unnecessary overspeed at each starting, the final SFC frequency is not a fixed value, but it is calculated each time on the base of the actual grid frequency. 20
Synchronization At the maximum speed the SFC is switched off, while the excitation system takes the lead and performs a ramp to increase the condenser voltage from 1500 V to the actual value of the grid voltage: this ramp takes about 4 seconds. In the meanwhile the speed is passively slowing down. When the speed and the voltage fulfill the synchronizing criteria, the main circuit breaker closing command is given. Legend: Violet trace = stator voltage Green trace = frequency Yellow trace = active power max ΔV 2% max Δf 1 % max Δ angle 40 deg 21
Consecutive starting attempts Since the condenser is not equipped with a flywheel for additional inertia (that would give additional time during the slowing down), the time available to align the vector of the condenser voltage to the grid after the voltage ramp is quite short (about 8 seconds), so during this time there are only one or two chances to get the synchronizing conditions. For this reason, in case of failed parallel the automatic sequence will try immediately another attempt, by activating again the static starter when the condenser speed has dropped to about 2900 rpm. After the third consecutive starting attempt, the SFC is switched off and the speed goes down naturally to 0 rpm, without electrical braking; further starting commands are up to the operator. The statistics observed during the commissioning and first operation show that the chance to fail the parallel at the first attempt is very unlikely. 22
Electrical braking Green = speed Grey = stator voltage Black = SFC DC current Red = excitation current The absence of a mechanical brake leads the condenser to slow down in about half an hour, by only mechanical friction. For this reason, the electrical braking is normally carried out in the automatic shutdown sequences by means of the SFC, with energy recovery to the grid. In this way the condenser can be stopped in about 4 minutes. 23
Crossed starting Unlike the excitation system, the SFC internal components are not redundant, but the complete starting system of each unit can also start the other condenser (crossed starting), by a set of disconnecting switches and the relevant logics. This solution guarantees a 100% redundancy. 24
Turning gear function As the condenser is not equipped with a mechanical turning gear, the static starter takes care of this duty. To avoid bending of the rotor, especially during the cooling phase after shut down, suitable cycles of periodical turning are accomplished by the logics of the control system and the static starter. For example, a typical cycle is 2 hours of rotation at 120 rpm every 12 hours. In case of starting after a long maintenance stop without periodical turning, at least 20 minutes rotation is requested before starting the synchronization sequence, to avoid anomalous vibrations during ramp up. 25
SFC tasks - Priority policy The following tasks can be requested to each SFC: Start condenser 1 Start condenser 2 Brake condenser 1 Brake condenser 2 Turning gear for condenser 1 Turning gear for condenser 2 Simultaneous operations of the 2 SFCs are allowed, until no crossed operation is involved. To handle all possible situation of simultaneous requests, the following priority order is assigned (from the highest to the lowest priority): 1) Braking command for mechanical trip 2) Starting command 3) Braking command, except in the case of mechanical trip 4) Turning gear function If the same operation is requested simultaneously to the same SFC for the two condensers, the priority is given to the command that first comes. 26
Condenser operative modes The synchronous condenser connected to the grid can operate in the following modes, that can be chosen by the operator at level 2 and 3, acting on the AVR (Automatic Voltage Regulator) of the excitation system: Control of the stator voltage Control of the reactive power Control by ASRV (Automatic System for Regulating Voltage). In the first two operative modes the voltage at the stator terminals of the SC or the reactive power exchanged by the SC are maintained constant by the AVR, that receives UP and DOWN commands from the SC-ICS to change the setpoint reference. In the ASRV mode, which is the default regulation, the AVR adjusts the voltage at the stator terminals on the basis of the reactive power reference coming from the System Operator. Alternatively the ASRV can regulate the voltage at the HV busbar. The ASRV (included in the SC-ICS) also balances the reactive power between the two condensers. 27
Conclusion On completion of testing, the first condenser was connected to the grid on September 28, 2014, bringing this event forward 4 months with respect to the initial scheduled date: this result was obtained also thanks to the choice of using a standardized turbogenerator as condenser. The second condenser was connected to the grid on December 4, 2014. During the commissioning and the first months of operation the 2 condensers were controlled from level 2 by Ansaldo Energia and ABB personnel, with a gradual handover to the level 3 in charge of Terna personnel. Nowadays the plant is in full operation controlled at level 3 and the condensers are actively supporting the local network. An event occurred on November 11th 2014 is worth to be mentioned: due to a grid fault, 70% of the Sardinian power production was lost in about 1 minute and SC1 operated at its maximum reactive power in under-excitation, avoiding the complete black-out in the island. 28
Thanks for Your attention 29