Allow a small percentage of leeway in the comparator algorithms otherwise the PLC will be constantly hunting for the perfect conditions which it will never find. With three variables it's going to be quite a juggling act.

I've fallen in to the same hole when I tried to tie two inputs via a comparator far to closely.

Best of luck!

yes sir,

I am going to put tolerance of +/- 1% for voltage comparision and +/- 0.15Hz for frequency. PLC will turn the ACB On only if values fall in this band.

Comparison of Relay scheme v/s PLC scheme for

Auto synchronizing (AS) and Load Sharing (LS)

Introduction:

Normally AS(Auto-Synchronising) & LS (Load Sharing) functions are envisaged in DG/TG sets installations where more than one generator will be operating in parallel on a common bus.

The process involves measurement of analog signals like CT currents, PT voltages, frequency, phase angle difference, comparisons with set points, initiation of increase/decrease pulses to governor/exciter & breaker closing.

Traditionally this job has been performed manually by synchroscope & dark lamps. The modern trend is to do it by dedicated relays. Off late, many PLC manufactures are configuring stand alone PLCs for this application.

While PLC is a very good device for logic control, it may be inadequate in many respects for job of AS & LS. Following sections explain various areas where a dedicated relay scheme is a better choice than a PLC based scheme.

1. Architecture related issues:

The PLC is a device which has got certain no. of I/O modules ( analog & digital), CPU, Programmer hardware and interfaces as may be required. The CPU and I/O size is dependent on the job to be handled. For a typical installation of 4 DG sets, the total inputs and outputs may be 100.

The CT/ PT signal inputs, which are critical for synchronizing / Load sharing operation, will be a total of 9 analog inputs out of this 100 I/Os. The PLC has to do the A/D conversion for measurement of these analog inputs. Normally, PLC follows a scanning method of all I/Os to register the value or status. For this purpose the PLC may be required to dwell at each I/O for a minimum of 5-10 milliseconds. In other words it means that PLC takes about 1second to complete one scan of all the I/Os.

This would also mean that the CT/PT signals are updated only once in a second. For an application like synchronizing, it means a delay of 50 cycles which is a very long time. Such mismatch of reading which are 1 second apart may result in wrong synchronization and will result into high degree of mechanical stresses to DG sets resulting in progressive deterioration of mechanical life.

In case of load sharing a time gap of 1 second between 2 measured values will result in improper correction & may lead to instability of the connected DG sets and hence defeating the very purpose of DG sets & load sharing.

On the other hand a relay scheme handles maximum 2 signals at a time

Which are processed simultaneously. The correcting outputs are on-line (almost instantaneous following the input values) resulting in accurate synchronizing with least mechanical shocks and stable load sharing.

2. Resolution of measurement:

The measuring accuracy of dedicated relays is better than the system used in PLC as the relays deal with maximum 2 analog inputs which are processed parallel. The relay uses minimum of 12 bits for ADC function giving a higher accuracy due to better resolution of measurement.

The PLC normally has an 8-bit resolution for ADC conversion and serial processing of analog signals resulting in poor resolution which also results in improper synchronizing and load sharing.

3. Auxiliary Power supply for the system

Our country has highly unstable power supply. Most of the electronic equipment suffer damage due to spikes, rapid variation and sustained under voltage/under frequency condition of auxiliary supply.

Dedicated relay schemes are designed to conform to Electromagnetic interference / electromagnetic compatibility standards whereas industrial PLC are not always obliged to meet this requirement. Consequently, the power supply unit of PLC system may fail frequently resulting in total shut-down of the plant whereas in relays even if one supply fails then corresponding unit only gets shut down.

Additionally, dedicated relays come with a facility of accepting a wide range of power supply (e.g. 20-120Volts or 80-270volts) & thus they are immune to large voltage variation.

4. Issues related to Governor/ AVR response time

A typical ASLS scheme may involve dealing with multiple DG sets where the makes may be different, age of machines may be different, technology (e.g. electronic governor v/s pneumatic governor) may be different.

A dedicated relay scheme will have facility to match the relay outputs to the prevailing response time of the frequency/ voltage controllers. Since each machine will have a set of dedicated relays, it is possible to tune each relay to its own DG perfectly.

On the other hand PLC will have outputs whose characteristic will be same for all the DGs which may result in over-correction of faster responding machines & under correction in slower responding machines.

5. Concept of Frequency control:

During load sharing the system continuously raise / lower the governor control. In this process, there is a danger that the largest machine can reach a speed proportional to 45 HZ or below. Normally, smaller machines tend to follow the larger machines- i.e. they will also run at speed proportional to 45Hz. This would mean that the load sharing would carry on but at a frequency of 45Hz which is undesirable.

To avoid this situation, it is necessary to identify one of the DG sets as master during the process of load sharing and ensure that the master generator is run at the frequency of near 50 Hz. Consequently, other generators will follow the master & run at a speed near to 50HZ which is a happy situation.

Dedicated relays for load sharing have a specific facility to enable the frequency control to ensure master DG set to run at 50Hz.

The frequency control can be enabled/ selected in any of the relay by a remote contact- this feature will help in incorporating frequency control in any of the DG set on line while they are already running.

The PLC scheme may not have facility of this frequency control. The PLC scheme may have indirect frequency control only on a pre-fixed machine- can not be shifted to other running machine on line. If pre-fixed machine is off, there is no frequency control.

6. System Expansion:

Normally, any DG set installation will undergo progressive expansion. In such a case, a PLC system will need a total re-structuring not only in terms of modular hardware but also in programming software. Both of these are expensive and time consuming and may be not expandable every time. It will also involve tampering with existing wiring, hardware and software. Whereas, a relay scheme is normally add-on & its per unit cost is already established and tampering/modification in the existing system is totally avoided.

Additionally, new add-on unit based on relay scheme is always compatible with existing relay scheme even if it is from different supplier. The same is not true for PLC based add on scheme. This ensures that the customer is not exploited whenever expansion is taking place

7. Troubleshooting and maintenance:

In the case of relay scheme, relays are hard-wired and with little training and experience the operator are able to trouble shoot and replace the faulty part.

This ensures that the cost, downtime and dependence on supplier are minimal. The same cannot be said about PLC since trouble shooting may expand to software debugging for which special skills are required and also for replacement of faulty parts, the customer is dependent on the supplier.

8. Local Display of parameters/ physical status:

Most of the relays have on line value and output status display. This facility makes the operator aware and confident of what is happening in the system. Additionally, the parameters can be captured on PC also thru RS485 link.

In PLC based system the user is totally dependent on information logged on PC which is normally remote. The operator will be totally blind and helpless to the system status

9. PC connectivity:

It is normally believed that only through PLC, a PC can control the system centrally.

Conclusion:

Technically, the relay scheme is superior to PLC based scheme in the area of on line measurement and control. The relay scheme can be fine-tuned with the physical requirement of each individual DG set with respect to governor and exciter response time. The relay scheme requires fewer skills to operate and maintain. Relay scheme offers more flexibility for future expansions.

All this at a lower package cost than the PLC scheme.

Sir,

Thank you very much for your detailed and very useful information. I know that micro controller based relays are preferable for DG synchronising as they are specially designed for it. But, here we have to go with PLC only as it is demanded by client.

1) As the voltage and frequency I/ps will be taken from MFM directly, neither any analogue input will be occupied nor will be proceed by PLC. Hence the proceed time for analogue to digital conversion will be saved.

2) The PLC which I am using has 12 bit resolution. which is equal to the relays.

3) I have devloped the ckt which uses 24V DC as aux. supply. External 24V DC supply contiunously available at site. Also it can be taken from DG batteries.

4) All DG sets are of equal make and capacity and all are new. Hence, the response time of all will be same.

Future expansioin and maintenance can be some what difficult for client but as it is selected by them, it may be taken care.

However, i am confused for the programming of load sharing. Is it that only to increase or decrease the speed?

I must admit my first reaction to most new projects is put a PLC in and then found myself having to juggle several variable that just do not want to play to my rules.

I just hope our OP hasn't painted him self in to a corner by saying the PLC is the way forward.

In my opinion PLC is not required for this installation for synchronising.The cummins PCC 2100 will do the sync operation.

Further taking analog signal from multifunction meter and feeding it to PLC and comparing with other machine parameters will take much time .It all depends on scan time of the PLC. Further load sharing also not possible.

PCC 2100 it self will do all the functions.

PLC can be used for logical function only.

for your information I have designed and commissioned many installations where in synchronising of DG sets and and grid is involved

Sir,

thanks for looking in to the matter. It seems there is a small miss understanding. PCC 3100 can do the synchronising itself and not PCC 2100. Pcc 2100 does not have analogue inputs. It can be added after attaching the aux cards. however, attachment of aux cards also do not provide synchronising function as they dont have analogue outputs.

Thanks for your observation.

I am sorry i did not notice PCC 2100.

If the controller is PCC 3100 then there is no need for PLC.

For PCC 2100 ,you may try Woodward controllers like EGCP or EasyGEn.These controllers have both speed bias as well as voltage bias signals for proper synchronizing and load sharing.

There is no need for any interface modules

Sir as my concept In EGCP-2 is oldest version

Synchronising of DG set you use Woodward EasyGen 3200 controller in that controller

1)load sharing

2)load dependent

3) KVAR sharing

4)Reverse power protection &also Reverse KVAR protection Avaible

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