LARSEN & TOUBRO LIMITED

TYPICAL SCHEME FOR AUTO LOAD SHARING OF

2 DG SETS

R.SeshadriLarsen & Toubro LimitedMysore works

Rev.1: P K Sharma

-1-1.0 Introduction :

Following sections describe a typical scheme for auto Load sharing of 3 nos. DG sets connected in parallel to a common bus in a plant. The load sharing scheme described can be envisaged for a building / hospital /hotel complex or any industry. The plant equipments are :

a) One no. incomer transformer + breakerb) three nos. DG sets + breakersc) 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 synchronized to the plant bus and they will start feeding the plant load.

When the three DGs are in parallel, and are feeding the same load, any of the following events can happen – if the load sharing scheme is not in place :

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a) Cascaded Tripping : One of the DGs , can get over loaded and trip – causing the remaining DGs to get over loaded and trip . This happens due to mis-match in DG makes, age, and size. The most efficient / largest DG will try and swallow the entire plant load – will get over loaded in the process- will trip on over load and cause cascaded tripping of remaining DGs.

b) Hunting of DGs : It should be noted that the plant load may not remain constant – it would normally be fluctuating. The operator may stabilize the DGs ( by setting the kW & kVAR outputs) at one level of plant load – if the load changes, the operator is required to stabilize the DGs again.As can be seen this is a continuous process – the operator will be busy handling the DG controls with every fluctuation in plant load. If the fluctuations are frequent, then the DGs may under go hunting – rapid rise/ fall in speed till the operator arrives at correct out put settings. If the operator does not respond quickly , there can be reverse power into DGs and may result in heavy damage to DGs.

c) Heavy circulating current (reactive current) within the DGs : If the power output levels are not same, then one DG will feed current into the next DG – resulting in a situation where there is a heavy circulating current among DGs. The plant load can be zero – but one can see full laod currents flowing in the ammeters of the DGs. This condition will reduce the life of the DG sets.

d) Poor power factor : One or more generators may run on a poor PF, depending upon the ratio of its kw output to kVAR output.

Consequent to the above, it is important to install an Automatic Load Sharing scheme , whenever more than one DG is run in parallel with another DG on the same bus. This will control the power out put of of each DG such that the % loading with respect to its rating.(ie) all DGs will share the plant load equally . For example, all DGs will run 60% if the plant load is 60% of the total capacity of all DGs. If the plant load goes to 70% of the total capacity, each DG will start delivering 70% of its rated power. This change in the output levels of the DGs are achieved automatically by providing control signals to the exciter ( for kVAR sharing) and to governor ( for kW sharing).Since the percentage loading is maintained same, there will be no hunting , there will be no over load ( unless the plant load goes beyond the cumulative capacities of all three DGs , which is rare), there will be no circulating current and there will be no cascaded tripping.

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2.0 Scope of work envisaged in the scheme are :

a) On line measurement of power output of each DG set for a given plant load

b) Monitoring of over power condition of each DG setc) Providing control pulses (on line) for the governor & exciter of each

DG as required to ensure that all DGs run at same percentage loading with respect o their ratings.

d) Cut in / cut out logic for the DG sets – this will determine the optimum number of DG sets to be run , to feed a given plant load.

e) Interlocks required for safe operation of the plantf) Auto / Manual facility

Following sections give a more detailed idea on how the proposed system achieves the above scope of work.

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) 3 nos. Load sharing Relays , for active power kW ( one per DG) b) 3 nos. Load sharing Relays, for reactive power kVAR ( one per DG)c) 1 no. Auto / Manual Switch d) 9 nos. PBs – for inc / dec signals of Voltage & frequency of each DG

set e) 2 nos. comprehensive measuring instrument QUASAR

Back Panel Instruments :

a) 10 nos. plug in auxiliary relaysb) 1 no. DC MCB for switching the Auxiliary supplyc) Fuses as may be requiredd) Terminals as may be requirede) One no. mini PLCf) 5 nos. aux. contactors

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4.0 Load sharing process (Auto Mode) :

The heart of the system is the Auto load sharing relay (LSR) type RRS. This relay has the following features (please refer fig.3).

a) RRS continuously measures the power out put of each DG. If set in kW mode , it will measure active power. If set in kVAR mode, it will measure reactive power.

b) RRS gives three relay out put contacts. One is increase pulse . The second is decrease pulse. The third is over power contact. If RRS is used as kW controller, the inc / dec pulses are given to the governor controls of DG set. If RRS is used as kVAR controller, the inc / dec pulses are given to the exciter controls of the DG set. The over power contact is used as a over power monitor / protection.

c) RRS gives a DC voltage output , proportional to the measured power. This DC voltage is made available at terminals 14 & 15, and it will be equal to 3 V if the measured power is equal to the rated power (100% ) of the DG. The rated power can be set on the front panel of the RRS relay.

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Please see fig. 4 which shows typical connection of two RRS relays, one for each DG. Both RRS relays are for kW control. Please note that the DC voltage out put terminals of each RRS (14 & 15) are connected as a DC bus. Under this condition, the status of the RRS out puts will be as shown in the table in fig .4. Points to be noted are :

- RRS will give increase pulse if internal DC current is from A to B- RRS will give decrease pulses if internal DC current is from B to A- RRS will not give any pulse if there is no current (ie) when both RRS

voltages are equal. ……………….7

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In other words, each RRS will give inc / dec pulses so as to bring its DCvoltage equal equal to the DC voltage of other – which inturn will happen only if each RRS is measuring same percentage of power output from their respective DGs. This is the process by which the RRS relays ensure proper load sharing.

Load sharing can be enabled / disabled by means of a contact at terminals at terminals 21/22 of RRS. This will help in Auto/Manual mode selection.

5.0 Frequency Control: The system also ensures that load sharing is performed at 50 Hz( + or – 0.5Hz), when the frequency control is enabled. The master DG will run at 50 HZ and the slaves DGs will follow the the master, to maintain the speed of the system at 50Hz equivalent.

6.0 Load sharing process - Manual Mode :

In this case all RRS relays are totally disabled. The relays will not give any pulse out puts. The operator has to maintain the output levels of each DG by means of front panel inc/dec push buttons.

7.0 Control Circuit wiring diagram :

The above sections explain the general guidelines, control actions and hardware required for auto load sharing scheme. 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.

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