3 DG Auto Synchronising Scheme

LARSEN & TOUBRO LIMITED

TYPICAL SCHEME FOR AUTO SYNCHRONISING OF 3 DG SETS

R.Seshadri Larsen & Toubro Limited Mysore works

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Introduction : Following sections describe a typical scheme for auto synchronizing of 3 nos. DG sets to a common bus in a plant. The synchronizing scheme described can be envisaged for a building / hospital /hotel complex or any industry. The plant equipments are : a) b) c)

One no. incomer transformer + breaker three nos. DG sets + breakers Plant Load Feeders

The SLD is as per fig.1 shown below:

Under normal conditions the plant will be operating from the local SEB supply. When the SEB supply fails, the three DG sets will be automatically started and synchronized in a predetermined sequence . ………………….2

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Scope of work envisaged in the scheme are : a) b) c) d) e)

Mains failure detection Starting of all three Diesel engines after mains failure Sequential Synchronisation of generators to the bus with a single Auto Synchronising Relay SPM21. Interlocks required for safe operation of the plant Auto / Manual facility

Each of the above scope is explained more in detail in the following sections. 3.0

Components of the system ; The system is supplied in a floor mounting panel as per he GA diagram shown in fig. 2. The system consists of following components : Front panel mounted components : a) b) c) d) e) f) g) h) i) j) k) l)

1 no. Auto synchronizing relay SPM21 1 no. Auto / Manual Switch 1 no. PT switching module (PTSM) 18 nos. PBs – for inc / dec signals of Voltage & frequency, engine start/stop in manual mode ( 6 per DG) 4 nos. T-N-C switches for breaker control in manual mode ( 1 for incomer and 3 for the DG breakers) 1 no. 4 position selector switch – to select the master generator 1 no. 4 position selector switch – to switch PT in the manual mode 1 no. Double volt meter – for manual synchronization 1 no. Double frequency meter – for manual synchronization 1 no. Synchroscope – for manual synchronization 8 nos. Indicating lamps for breaker on/off status 4 nos. comprehensive measuring instrument QUASAR

Back Panel Instruments : a) b) c) d) e) f) g)

12 nos. MM00 contactors ( K1, K2, K3, K4) 3 nos. 3 attempt starter modules 1 no. DC MCB for switching the Auxiliary supply 2 nos. AC MCBs for switching the PTs Fuses as may be required Terminals as may be required Setoff plug in aux. Relays to trigger neutral switch closing/opening ………………….3

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System Operation : The system has Auto / Manual selection facility. If the A/M switch is manual, then, the complete synchronizing process , have to be done manually by front panel PBs, TNC switches and the indicating instruments like Double Voltmeter (DVM) , Double Frequency Meter (DFM) and Synchroscope. The following sections describe the auto synchronizing process, for which the A/M switch has to be kept in Auto position.

5.0 5.1

System Operation – Auto Mode : Auto Synchronising Relay SPM21 The heart of the system is the Auto Synchronising relay SPM21. Please see fig.3 for the connection diagram of SPM21.

The inputs to this relay are the two PT inputs – one from the bus and the other from the DG. The relay gives five contact outputs – two for frequency increase/decrease, two for voltage increase/decrease and one for breaker close. In addition, the relay can accept one contact as enable/disable trigger. The bus PT is permanently connected to the SPM21. Through this, the relay measures the bus voltage & bus frequency. ……………….5

-5The second input to SPM21 is from the DG PT. Since the scheme uses one SPM21 for three DG sets, three PT secondaries are to be connected to the relay. At any time only one DG will undergo synchronization. Consequently, the DG PT input terminals of SPM21 is sequentially switched by MM00 contactors to the respective PT secondary of the DG being synchronized. When DG1 is to be synchronized, Contactor K1 gets energized - it connects the DG1 PT to SPM21 & SPM21 outputs to DG1 controls. When DG2 is to be synchronized, Contactor K2 gets energized - it connects the DG2 PT to SPM21 & SPM21 outputs to DG2 controls. When DG3 is to be synchronized, Contactor K3 gets energized - it connects the DG3 PT to SPM21 & SPM21 outputs to DG3 controls. When SPM21 receives the two PT input signals, it initiates the synchronization process automatically . It gives 4 contact outputs to give increase / decrease pulses for voltage and frequency controls ( the exciter and governor of the DG set) so that the DG’s voltage and frequency are made equal to bus voltage and frequency. After matching the V & F , the SPM21 generates small kicker pulsed to match the phase angle of the two PT inputs and finally gives a breaker close trigger. The user can select any one of the 3 DGs as the master DG. When the mains has failed, the scheme will synchronise the master DG first on to the dead bus and the remaining two DGs in succession. It should be noted that the PT switching is through the MM00 contactors which in turn are controlled by the PT Switching Module PTSM. The PT switching logic is explained in section 5.2 . 5.2

PT Switching Module (PTSM): PTSM is a small logic module – its functions are : a) b) c)

detect mains failure and energise contactor K1 energise contactors K2 to K4 as per sequence selected by Master DG selector give enable / disable signal to SPM21

The inputs to PTSM are : Breaker auxiliary contacts, A/M switch, Master selector switch and PT input for Mains failure Trigger. ……………..6

-6The outputs from PTSM are : Triggers to contactors K1 – K4 and SPM21 enable / Disable signal. Please see fig. 4 for connection diagram of PTSM.

PTSM logic is as follows : a)

detect the mains failure – energise contactor K4 . The contacts of K4 are used for alarms and further processing.

b)

read the master DG selector : if in position 1, select synchronizing sequence as DG1 – DG2 – DG3 . In this case the contactor switching sequence is K1 – K2 – K3. -

if in position 2, select synchronizing sequence as DG2 – DG1 – DG3 . In this case the contactor switching sequence is K2 – K1 – K3.

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if in position 3, select synchronizing sequence as DG3 – DG1 – DG2 .In this case the contactor switching sequence is K3 – K1 – K2 …………………….7

-7It should be noted that : if K1 is on, DG1 is synchronized. if K2 is on, DG2 is synchronized if K3 is on, DG3 is synchronized Example : Master DG selector is in postion 1 . PTSM will now do as follows : -

close the neutral of DG1 energise contactor K1 to start synchronizing DG1. Enable SPM21 to start Auto synchronizing read DG1 breaker contact to know if DG1 breaker has closed, after SPM21 has given breaker close command. wait for 2 sec time delay disable SPM21 energise contactor K2 to start synchronizing DG2 Enable SPM21 to start Auto synchronizing read DG2 breaker contact to know if DG2 breaker has closed, after SPM21 has given breaker close command. wait for 2 sec time delay disable SPM21 energise contactor K3. Enable SPM21 to start Auto synchronizing read DG3 breaker contact to know if DG3 breaker has closed, after SPM21 has given breaker close command. wait for 2 sec time delay disable SPM21

The above process would complete auto synchronization of all 3 DG sets. Based on the position of master selector, the other sequences are implemented. 5.3

3 attempt starter modules (AMFR) : This is a stand-alone hardware , used mainly for implementation of 3 attempt starter scheme . The functions of this module are : -

provide a maximum of 3 pulses to the starting motor of the diesel engine, after receipt of trigger provide a start failure signal , if engine has failed to start within the three attempts provide permissive to enable SPM21 ………………..8

-8Thus the AMFR helps in starting the diesel engines satisfactorily. Please see fig 5 for connection diagram of AMF relay.

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System Operation - Manual Mode : In this case the modules SPM21, PTSM and AMFR are out of picture. The operator has to recognize mains failure and synchronise each DG set, one after the other by pressing the inc / dec PBs and the T-N-C switches on the panel . The check synchronizing has to be done by observing the double voltmeter, double frequency meter and the synchroscope.

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Control Circuit wiring diagram : The above sections explain the general guidelines, control actions and hardware required for autosynchronisation of 3 DG sets. There can be minor variations , additional requirements based on the specific needs of an actual installation. On receipt of an order, L&T will be pleased to provide all customers an elaborate control circuit wiring diagram for implementing the entire scheme. oooooOOOOO&$&OOOOOooooo