grid islanding and load shedding schemes

7/28/2019 Grid Islanding and Load Shedding Schemes

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Grid Islanding and Load shedding Schemes

1 Generators in parallel with grid :

DG sets / TG sets are finding more and more applications as captive power sources in modern day industrial plants. ADG set owner may decide to connect his DG to the local Electricity Grid supply (ie) the DG will be synchronized to thegrid. The main objective here is to feed the plant load will both by the grid as well as the DG set.

Whenever a DG is connected to local grid, there can be two modes of operation as shown in fig.-1 and fig.-2 shownbelow:

Please see fig.1. A DG is shown connected in parallel to EB power. Plant load is connected to the same bus where bothDG and EB are present.

Here the plant load is 9 MW. The DG capacity is 3 MW. The DG is connected to the bus where the grid supply is alsoconnected. The DG delivers 3 MW. Remaining 6 MW is drawn from the grid. Here the user has less dependence on EB,and better control on the power availability - even if grid fails, he has at least 3 MW available to run critical loads. In thiscase the plant is said to be importing power.

Please see fig.-2 . Here the plant load is 2 MW. The DG capacity is 3 MW. The DG is connected to the bus where thegrid supply is also connected. The DG delivers 2 MW to plant load. The DG has a spare 1 MW available from itsgeneration - it delivers this 1 MW back into the grid. In this case the plant is said to be exporting power.



In both cases , the DG owner should obtain special permission from the local Electricity board to connect his DG inparallel to the grid. The local EB will permit DG paralleling, subject to the condition that the DG owner installs a GridIslanding Scheme at the incomer of the plant.

2 What is Grid Islanding Scheme ?

Grid Islanding scheme is a set of protective relays, connected at the incomer bus - these relays will sense a disturbancein the grid and give a trip command to the incomer breaker whenever the grid disturbance exceeds a set limit. Byopening the incomer breaker, the plant is isolated from the grid. The plant as well as the DG is disconnected from thedisturbed grid by the grid islanding scheme. The process of disconnecting the DG from the EB is called Islanding of theDG.

3. Why is the Grid Islanding Scheme required ?

Grid Islanding scheme helps both the EB as well as the DG owner.

In our country, due to several reasons, the grid is not stable and is always fluctuating with respect to voltage andfrequency. In addition, Grid failures are frequent.

Benefits to EB :

The EB, who have the responsibility to maintain the grid, normally do not want an additional independent power source(like the DG) in their grid network, when the grid is disturbed. Since the power source is not in their control, it willcomplicate EB's methods of dealing with the disturbed grid. While the EB are in the process of solving the griddisturbance, they do not like to have another power source which they do not control and which may add to the

disturbance. Once the DG (non EB power source) is disconnected from the grid, it becomes easier for the EB to locatethe source of disturbance and rectify the same. The main idea is that the EB eliminates the possibility of the DG feedingthe disturbance. Hence the local EB will insist on installing the Grid islanding scheme in the DG owner's premises,before allowing the DG to be connected to grid.

Benefits to DG owner:

It is strongly advisable to disconnect the DG from the grid, when the grid is disturbed. The main reason is that the DG or the TG may get spoilt due to grid disturbances resulting in heavy repair costs and shut downs. It is necessary to protectthe DG from grid disturbances.

It is also better to disconnect the DG from EB whenever the EB fails, by opening the incomer. If not, the DG which is



running will experience a severe jolt when the EB comes back. This will cause extreme damage to DG.

4. What are grid disturbances ? A grid is said to be disturbed when :

a) there is an under voltage (U/V)b) there is over voltage (O/V)c) there is under frequency (U/F)d) there is over frequency (O/F)e) there is a rapid fall or rise in frequency ( +dF/dT or -dF/dT )f) there is a power failure in the gridg) there is a fault in the grid

It may also happen that the DG may experience a reverse power flow when operating in parallel with grid, without any of the above listed symptoms. This is due to changing PF conditions in the grid. This is not a major problem for the EB, butit is harmful to the DG. DGs may not be able to operate below a certain PF. If compelled to work with low PF, the DGwill experience a reverse reactive power flow - this can damage the DG.

On line Power factor control of DG is a better solution than islanding in such cases. Please refer to a separatedocument from L&T on PF control of DG / TG sets.

5.What are the relays which can detect Grid disturbances ?

It should be noted that :

a) an over loaded grid will give rise to fall in frequencyb) a fault in the grid will give rise to fall in voltage.

It is possible to detect each of the above symptoms by individual relays. While detection of U/V , O/V and faults can bedone by simple electromechanical relays, the parameters like frequency, dF/dT have to be monitored by numericalrelays .

Numerical relays, with capability to perform mathematical algorithms and to offer very high accuracy & resolution (settings in terms of 0.01 Hz) which are normally required to detect dF/dT.

The EB supply failure condition ( the most frequent of the disturbances) is difficult to detect. It can not be detected by

under voltage relays - since the DG supply will be available on the bus. Normally the mains failure will be detected by adF/dT relay . As a back up a reverse power relay or a vector surge relay is also used to detect a EB failure.

If the DG is in export mode, a low forward power relay will be required to detect the EB failure.

Grid faults are detected by directional Over current + Directional E/F relays.

6. What else is required along with grid islanding scheme ?

A fast acting load management scheme and a reverse synchronization scheme are also required along with gridislanding scheme to ensure that DG operates properly after an islanding has occurred.

It should be noted that that there can be two conditions of grid paralleling of DG sets.

a) Case - 1 DG in parallel with grid, plant is importing power :

Refer fig.1. Let us say that the incomer is opened by grid islanding scheme, when the plant load of 9 MW was fed byDG (3 MW) and EB(6MW). The moment incomer opens, the DG will have all the plant load coming onto it - there wouldbe an overload of 6 MW. The generator protection relay will trip the DG, causing a total black out. This is not desirable -what we should have is that :

- the DG should continue to run and deliver its 3 MW to critical , pre selected loads- the over load of 6 MW should be removed from the DG bus, soon after islanding so that DG does not trip on overload.

This is achieved by a load shedding scheme.



b) a directional Over current + directional E/F relay DM30 - this is a numerical relay which senses high fault currentsflowing from the DG to the grid and issues trip command.

c) a high sensitivity reverse power relay MRP11 - this relay has possibility is set a reverse power of 0.5 %. This veryuseful to sense mains failure in import mode.

d) A power management relay MW33 - this will help sensing low forward power in export mode . At the same time user can set a max limit to power export. This relay also monitors the PF in the bus - another parameter which can bemonitored at the bus.

e) An under voltage relay MV12 is provided to detect re-appearance of voltage in grid.

f) A vector surge relay UM30-SV - this can be a back up for detecting mains failure.

Load shedding is carried out with following relays :

a) The UFD34 relay, which was used for islanding, can also be provide contacts for load shedding

Matrix module MX 7-5 - this is a logic module. This will accept 7 inputs and provide 5 out puts which can be configuredto follow a specific logical sequence of the 7 inputs.

Power management relay MW33 - this will help to have a load shedding scheme based on power levels.

Load shading is provided buy the relay RRS - this measures the power out of the DG - it also receives an auxiliarycontact from the incomer breaker. If incomer opens, this relay immediately give decrease pulses to exciter and governor to bring the power levels of the DG to the required levels of plant load.

Reverse synchronization is provided by Auto synchronizing relay SPM21. Please refer a separate document from L&Ton Auto Synchroninsing with SPM21 relay.

L&T offers all the above relays as a single source solution. The user can now select any of the relays to suit hisrequirements at a given site .

L&T has a team of full fledged application specialists to study, recommend, supply, install and commission a

comprehensive grids islanding, load shedding, load shading and auto synchronizing scheme.



Generator Protection Typical Schemes and L&T Solutions

1.0 With increasing complications in the power system, utility regulations , stress on cost reduction andtrend towards automation, Generator protection has become a high focus area. State of the art ,microcontroller based protection schemes from L&T offer a range of solutions to customers to address

the basic protections and control requirements depending upon the size and plant requirements.

Generators - size less than 300 KVA Normally these generators are controlled by MCCBs, which offer O/C and short circuit protections. It isadvisable to have following protections in addition to MCCB (Fig.1):

E/F protection (51N) : This will protect the generator from hazardous leakages and ensure operatorsafety. Many SEBs have already made E/F protection as mandatory. L&T Relay for this is MC12A.

3.0 Generators - size 300 to 1 MVA There are two major differences when compared with the small machines considered in section 2.0.

a) IDMT Over current + E/F relay will be required addition to normal MCCB or ACB releases - since thegenerator may need shorter trip times for faults in the range 100% to 400% level. L&T Relay MC61A willbe the ideal choice.

b) By virtue of larger power level, any faults inside the stator or fault between the neutral of themachine and the breaker terminals can reach very high intensity.

Such internal faults must be cleared instantaneously. Normal IDMT over current / E/F relays are notadequate to monitor this internal fault condition. A separate relay scheme is required to monitor this

internal fault status - otherwise the machine can circulate very high fault currents resulting in severedamage.

A high impedance differential relay scheme, with L&T relay SC14S (3 nos) is the best suited for thispurpose (Fig.2). If the neutral is formed inside the machine, the differential relay scheme will not bepossible - in this case a Restricted E/F scheme , with same SC14S relay (1 No.) is the solution. Careshould be taken to provide adequate no. of CTs as shown in the diagram.

c) Machines of this size are likely to have external controls for frequency and excitation - so that theycan be run in parallel with other power sources (other generators on the same bus or the local grid).This necessitates voltage and frequency related protections as well. L&T Relays UM30A is the best



solution for this purpose. Alternately Relay MV12A (4 Nos) can be used if frequency protection is built inthe engine.

4.0 Generators - Size 1 MVA to 10 MVA Being a medium sized generator, it will need more comprehensive protection both for the stator side andthe rotor side.

4.1 Stator side protections :

Voltage restrained Over Current Protection (50V / 51V) :

Normal IDMT O/C will not work here - when a over current fault occurs, due to higher current levels,there would be a drop in terminal voltage. For the same fault impedance, the fault current will reduce(with respect to terminal voltage) to a level below the pick up setting. Consequently normal IDMT maynot pick up. It is necessary to have a relay whose pick up setting will automatically reduce in proportionto terminal voltage. Hence the over current protection must be voltage restrained. Two levels of Overcurrent protection is required - low set and highest ( for short circuit protection).

Thermal Overload (49) :

This protection is a must - i t monitors the thermal status of machine for currents between 105% to thelow set O/C level ( normally 150%).

Current Unbalance (46) :Generators are expected to feed unbalanced loads - whose level has to be monitored. If the unbalanceexceeds 20%, it may cause over heating of the windings. This heating will not be detected by thethermal overload relay - since the phase currents will be well within limits. A two level monitoring forunbalance is preferred - first level for alarm and the second level for trip.



Loss of excitation(40) :

Loss of excitation(40) : When excitation is lost in a running generator, it will draw reactive power fromthe bus and get over heated. This condition is detected from the stator side CT inputs - by monitoringthe internal impedance level & position of the generator.

Reverse Power (32) : Generators of this size may operate in parallel with other sources, which may

cause reverse power flow at certain times ( during synchronization or when there is a PF change due toload / grid fluctuation or when there is a prime mover failure). When reverse power happens, thegenerator along with prime mover will undergo violent mechanical shock - hence reverse powerprotection is absolute must.

Under power (37) : It may not be economical to run generators below a certain load level. Thisprotection will monitor the forward power ……………..4 -4- delivered by the machine and give alarm whenthe level goes below a set point.

Under / Over Voltage (27 / 59 ) : This will protect the machine from abnormal voltage levels,particularly during synchronization and load throw off conditions.

Under / Over frequency (81) : This will protect the machine from abnormal frequency levels,particularly during synchronization and load throw off conditions. This will also help in load sheddingschemes for the generator.

Breaker Failure Protection : This protection detects the failure of breaker to open after receipt of tripsignal. Another trip contact is generated under breaker fail conditions , with which more drasticmeasures (like engine stoppage, opening of bus coupler etc) can be taken.

Multifunction relay IM3GV from L&T, as a single unit protects the generator for all above listed faults(a) to (h)- giving at the same time the benefit of small panel space, simple wiring and user friendlyoperation.

Stator Earth fault (64S): L&T relay MC12A will do this job.

Differential Protection (87G) : This protection is very important - since the machines of this size haveto be protected for severe damages that may occur due to internal faults. Considering the large powerlevels, it is necessary to have a percentage biased, low impedance differential relay - L&T's relay MD32Gis the ideal solution. MD32G has following advantages : - it provides percentage biased differentialprotection with dual slope characteristics - it has a built in REF protection element (87N), which willmonitor the generator for internal earth faults. - It has a built in O/C protection, as a back up

PT Fuse Failure Protection : This relay will detect any blowing of PT secondary fuse - and give acontact which can be used to block the under voltage trip. L&T relay PTF03, with less than 7 msresponse, will be ideal for this purpose.

4.2 Rotor side protections : Generators of this size, will need rotor side protections listed below :

a) Rotor Excitation Under Current : This is a DC under current relay ( UBC/A/37), which will monitorthe excitation current.

b) Rotor Excitation U/V(80) : This is a DC under voltage relay (UBC /80), which will monitor rotorvoltage.

c) Diode failure Relay : Brushless excitation systems will have rotor mounted diodes, which canbecome short or open during operation. Diode Failure relay (RHS) will monitor the condition of thesediodes , for both open circuit and short, and give alarm.



d) Rotor Earth Fault(64R) : Relay UBO/CR will monitor the rotor winding status for the Earth fault.

Please see Fig 3 for the scheme with relays as above.

5.0 Generators above 10 MVA

For large generators above 10 MVA size, the philosophy of main protection and back up protection has to be followed. In addition to the protections listed in Section 4.0, following extra protections are to be considered:

a) 100% Earth Fault Protection : This will help in sensing earth faults close to neutral.

b) Inadvertent Breaker Closure : This will avoid closing of generator to bus during coasting tostop, or when stand still or before synchronism.

c) Under Impedance : This will be required as a back up protection for the whole systemincluding the generator transformer and the associated transmission line. If the distance relay failsto pick for some reason, this under impedance function will pick up and save the generator.

d) Over Excitation : This will protect the generator from Over fluxing conditions.

Please see Fig. 4 for the SLD.



6.0 Generator connected in parallel to grid : Whenever generators are running parallel to grid,a comprehensive Auto Synchronising & Grid Islanding Scheme will be required. This schemewill help in synchronizing the generator to the bus and opening the incomer breaker of the plantwhenever there is a severe grid disturbance, thus protecting the generator from ill effects of disturbed grid. L&T Relays SPM21, UFD34 , MW33 and MRP11 are ideal for this scheme.

Please see Fig. 5.



R.Seshadri

7.0 Generators connected in parallel on a common bus : Whenever more than one generator isoperating in parallel , it is necessary to see that the plant load is equally shared by the generatorsin parallel. If there is unequal sharing, there would severe hunting amongst the generators andeventually this will lead to cascaded tripping of all generators, causing a total black out. L&Trelays RRS (two relays per generator) provide the most effective, online load sharing system for

generators in parallel. Please see Fig 6 for a representative scheme of load sharing for threegenerators.

8.0 Summary : The sections above describe the actual requirements and various options available

grid islanding and load shedding schemes

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