Smart Power Management during islanding condition

With the expected growth of rooftop PV systems, net metering and the export of power to the grid, inverters will play a very crucial role in the future. My colleague,Giridaran Srinivasan wrote this article for the UK based magazine, Global Solar Technology, and it deals with the challenges in India for grid-tie inverters, especially during grid failures.
The article can be accessed here. The access is available only for the magazine subscribers. For others, we are reproducing the article below.
Abstract : In a grid tie solar PV system, no energy is wasted i.e. any excess electricity is exported to the electrical grid. Grid tie solar systems are particularly worthwhile during periods where there is absence of load, that is week-ends or holidays when your solar system continue to give out electrical power. Hence there is very good potential for grid tie inverters in India, but is Indian grid infrastructure strong enough for these latest distribution generation technologies? This article will discuss the issues associated with the above concern and explain the methods to overcome these conditions by integrating/synchronizing the grid connected inverter with diesel generator during a power cut/grid failure.
Power cuts are very common in developing countries like India. It is quite natural that under such circumstances, a solar rooftop owner will try to consume all the power generated through solar and reduce his dependence on diesel as fuel. However, the problem begins when the grid tie inverter fails to start without the reference voltage and frequency parameters that are non-existent during power cuts. As a result, the rooftop owner cannot use the solar power generated. This is disappointing because, they consider solar as a backup for power cuts and for payback through sale of excess power generated. The only existing solutions in the market today are off grid inverters that come with bulky battery banks which are very expensive compared to direct grid connected inverters. Similarly, hybrid inverters are also expensive compared to grid tie inverters hence payback or ROI gets prolonged, making it a financially less- viable option.using pc

Figure 1 Smart power management – Using PLC

What about installations with existing grid tie inverters? Most of the rooftop owners are not aware of these islanding conditions and most of the EPC vendors just install existing grid inverters without the alteration proposed below. Recently, MNRE has launched a pilot project scheme for large scale grid connected rooftop solar PV projects in six cities in India and SECI (Solar Energy Corporation of India) has been allotted as the implementing agency. The published RFS (Request for Selection) document by SECI has also stressed on synchronization of grid tie inverters with diesel generator during power cuts. “In case of grid failure, or low or high voltage, solar PV system shall be out of synchronization and shall be disconnected from the grid. Once the DG set comes into service PV system shall again be synchronized with DG supply and load requirement would be met to the extent of availability of power”.   
Proposal: The grid inverters are usually connected to a common distribution bus bar (India – 3phase/1phase, 415V/230V). In this proposal all sources such as grid, diesel generators, battery backup inverters and solar grid inverters can be connected along with the load on a common bus bar. What usually happens during power failure is, the grid inverters trips sensing loss of reference parameters and stops generation, now the DG sense this situation and switches ON to support the load automatically with help of AMF panels. Once the generator switches on, this creates a reference voltage and frequency on the common bus bar and this starts up the solar grid inverters (we actually deceive the GT inverters here).
Once the inverter is ON, the load will be supplied by both GT inverter and DG sets (at low RPM). Is it practically possible to synchronize a grid tie inverter with a diesel generator (DG sets) and make it work? That is during a power failure or when grid power is not available?
Yes, it is possible. Following are the conditions at which it is possible

  • We have a stable/good quality generator which gives good steady voltage and frequency.
  • The generator should not sag, dip or create any surge current when the inverter turns ON during the above mentioned condition.

Let us now consider that the inverter has turned ON using the DG sets reference voltage and frequency from the generator. There are two possible conditions now,

  1. The load is always more than the PV generator supply
  2. Load can be less than PV generator supply

During the condition 1, no issues will occur, if everything works fine. But during the condition 2, the power generated from PV will be greater than the load requirements and hence there is a possibility that this excess power will be fed back to the DG sets (stator of DG sets). What will happen under this situation? This condition can damage the vital parts of the generator so the protection of generators becomes very critical and make it work as a motor (reducing its speed). We can expect an increase in AC line voltage above critical level. If this happens, the condition can trip the inverter due to over-voltage/frequency protection circuits. Now the rooftop owner cannot consume power from the solar and therefore he is forced to operate the DG sets.
How to custom design a smart power management system to use the solar generator during the condition 2 and also protect the DG sets? We also foresee very high requirement for an effective control system, which can handle all the above conditions such as DG protection

  • Using a dump load to remove the surplus power generated with help of an effective control system.
  • Effective control of loads (To increase or decrease the load as per requirements using PLC based controls)
  • And also include a reverse power relay to protect the diesel generators from back feeding/reverse currents.

A 150kWp rooftop grid connected system with synchronization of Diesel generators and solar inverters has been designed and successfully commissioned by Staten Solar India Private Limited (a leading solar EPC company in India) at Guru Nanak Dev Charitable hospital building at Punjab. Mr.Rakesh Singh, Co-founder and Director of Staten Solar says that “The system will operate without any problem if the load is at least two times greater than the solar power generated during the power failure period”.
This Solar installation was commissioned in less than a month and Staten Solar have used SMA string inverters which works upon synchronization either with the grid or DG set (3*750KVA). The generated electricity from the plant during the daytime can be consumed by the hospital load and there is no need for battery bank considering the huge loads. The grid and DG set are connected through auto-changeover panels (AMF Panel) and the Diesel generators start automatically during power cuts (Check SLD – figure 4).

staten 1

staten 2

Staten 3

Figure 2 – Staten Solar installation – 150kWp

Economics: There is less commercial benefit in running a generator just for reference voltage and current for the entire day time. But this can be economical for the above discussed condition i.e. just during the power cut period. The DG sets running at lower efficiency (20-30%) or less than its rated value can consume more fuel than during rated power (i.e. 85-100% efficiency), so operating it during the entire day just for reference parameters or in parts where there is no grid power (remote locations or forest regions) is not economical.
In such remote regions where there is no grid connection or very less grid power, off grid solar PV/wind systems with battery backup will be the best option, and even along with DG sets when the loads are huge. In regions of frequent power cuts, there is also an option of using a smaller DG set, to synchronize with the solar GT inverters just for the emergency load rather than the required full load size. But the disadvantage here is during power failure in the evenings or during the night, the load will be much higher than the generator system. So care must be taken care during system sizing, and we must make sure that the size of the generator is sufficient enough to supply required amount of energy during emergency or power cuts.
Developed counties like Germany and USA, have good potential for backup power models such as SMA island inverters.  These Island inverters can be connected to the common busbar during power outage with help of backup batteries. These backup batteries will be charged during excess power generation from solar system. This will also generate the necessary reference voltage and frequency for the GT inverters during power cuts. Unfortunately, this is   an additional expense and a huge investment on batteries. This additional cost on batteries and their maintenance, and also considering higher interest rates from local banks makes it unviable for developing countries like India and directly affects the payback. So existing /medium sized diesel generators will be the best option for rooftop owners and also more economical when compared to these island inverters.
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 Figure 3 SMA Sunny Island Inverters(Image credit – SMA)

Conclusion:  While considering a GT inverter the load pattern (early morning and evening peaks), grid availability (GT inverters can be tricky at times of frequent power failures), grid quality, feed in tariff (if available) have all to be reckoned and coordinated with. However, the challenge lies in designing an efficient power management control system for smart and economical power consumption.
Staten sld

 Figure 4 Punjab hospital building – SLD (Staten Solar India)

Glossary:

DG Sets – Diesel Generator Sets
MNRE – Ministry of New and Renewable Energy
GT Inverter – Grid Tie Inverter
SECI – Solar Energy Corporation of India
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23 thoughts on “Smart Power Management during islanding condition”

  1. Problem has been described very well but there is no proposed solution. There might be a condition where there is no backup generator, just a small inverter to support critical loads, what can be done in such scenario.

    1. Amit,
      There exists solution for your requirement. The off gird solution are available in two types: a) Commercial Off Grid & b) Semi – Commercial Off Grid.
      Most Commercial Off grid are basically Online UPS with MPPT Controller where in active power control is not available.
      But Semi – Commercial Off Grid solution, the same can be managed. Semi Commercial System consist of Smart Grid Inverter & On grid Inverter with which one can achieve Gird Management, Battery management & load management.
      The challenge in the aforesaid system is optimal designing & cost.
      Please feel free to write back.

    2. Hello Amit, Thanks for your feedback. This article is only based on commercial installations with solar and DG sets (most of the large commercial buildings in India have DG sets already installed).
      Thanks.

  2. Dear Madhavan,
    It has been long since we spoke. I appreciate the content in the article the work done by Staten. As a thumb rule, we don’t recommend solar system more than 20% of the installed DG capacity. With due respect to the author & Staten the underlining technology challenges are not understood.
    The challenges in integrating Solar + DG are:
    – DG Operation Mode
    – Spinning Reserve of the DG &
    – Of course Reverse Power Flow
    DG Operation Mode:
    DG’s have different 4 different operating mode and based on that their efficiency varies. The loading on the DG decides its Diesel Consumption, so need minimum 30% and some need minimum 60% loading. So, even if you have ability to generate solar power, the system should consider operation mode & efficiency in that operation mode before synchronizing solar power with DG.
    Spinning Reserve:
    DG’s ability to take up load, in case of sudden cloud drift or load variation. As you may know DG’s are very vulnerable to load variation and their frequency change as the load varies. The system should consider to what extent the DG and Solar can co-exist to have stable system.
    At present we have 3 probable solution to haev Solar integrate with DG’s
    Option 1: PLC + Contactors to switch off inverters. Applicable for smaller DG set installation up to 150 kVA
    Option 2: PLC = Power Reducer Box to control active power output of inverters. Applicable for installation between 200 – 800 kVA DG set
    Option 3: Fuel Save Controller Kit. Applicable for installation with more than 800 kVA DG Set. This is a unique solution.
    SMA has been working on this for last 2 years and our first reference site in India for a textile unit would be ready by middle of May
    Please feel free to call me.

    1. Hi Dilip,
      Thanks for the detailed comment. Will call you shortly. Do you have a whitepaper on the subject which will be useful for the end users?
      Madhavan

    2. Dear Dilip,
      Can you please share your contact no.
      We have faced similar scenario like EB gone and started with DG set and there was enough load in the DG
      1. Initially there was enough load part was tken by DG and then other by Solar.
      2. Then DG shared 30 % and Solar 70 % .
      3. There was a miscommunication suddenly a labor switched off main loads and reverse power scenario was seen which has now damaged the inverter.
      Need solution for the same for effective management of DG and Solar.
      Regards,
      Suresh kumar.S
      9535492927

      1. dear sir,
        we have a similar problem like Mr Suresh has faced, if 100% test solution is available please late as know, we required,
        Rajan jariwala
        09824751581

    3. I agree to Dillip’s suggestions. But if the solar inverters and ACDBs are located far away from the DG and main panel,how to shut down some inverters during high Solar power generation and low load conditions.

  3. Hi,
    We are designing 56kW Solar PV system using SMA string inverter with GRID/DG(40KVA).According to our design expert suggestion we cannot install solar PV system more than 8KVA , which is 20% of DG(40KVA).
    CASE-1-Maximum demand Load avaiable to connect is 30kW
    CASE-2-Maximum demand Load avaiable to connect is 50kW
    Please suggest.
    Regards
    Newlav

  4. Giridaran Srinivasan

    In this condition the DG minimum loading and load pattern need be to analyzed.
    It is better to have safe/low PV penetration to avoid damage of DG and if the customer still plans to increase the PV capacity it is better to add some protections like reverse current relay or power control modules which can reduce the solar power generation as per the load pattern and DG loading condition.

  5. Dear Giridaran,
    I need suggestion please help me out.
    One of our clients have 50KVA gent set in their factory & they wants to install the roof top solar power. So we propose them 10KW solar gridtie system & for this case during utility fails, to provide reference voltage to grid inverter we are planning to use the exciting Electronic fuel injection DG (ie common bus bar to all), as per experts articles & above comments it is understood that we need to put reverse current relay to protect DG (if Load can be less than PV generator supply). So to whom we approach for getting this reverse current replay & how to connect it (electrical connection diagram) wit our system.
    Thanks.
    WBR
    Thakur Prasad
    PV Engineer

  6. WHAT IS plc PROGRAMMING logic and how i can connect relay/Circuit breaker/switches
    Please help???

  7. Dear sir
    I am facing one problem in solar and DG gen set synchronise
    We have install 300 kW solar plant at bit ranchi Monday to Friday load is more then solar generation but Saturday and Sunday also holiday load 30 % less then solar generation when grid on then no issue but when grid out then DG set on but in that case the extra solar generation back fiil to DG set so how to resolve this problem plc control possibility nill

  8. When there is sudden change in load smooth changeover/takeover should happen in DG SET and SOLAR INVERTER.
    The system design should have power management functions like
    1. Smooth load shift
    2. Load shedding (outputs)
    3. Ready to load signal before switching on large loads (to Block loading)
    4. Switching off Solar string inv incase the load is less than 40% of the Plant capacity

  9. Dear Sir
    I am facing same problem while designing system
    load demand is 250 kva – 300 kVA
    DG set is 350 kVA
    solar system – 50 kW
    after power cut and reference Vltage is from DG and if load demand decreases to 50 – 40 KVA .power will feed to DG (Stator)
    how to solve this problem

  10. Ours is 300 KW Roof top on grid solar power plant and Main Load is around 396KW which is fed from one single connection(solar is connected with this connection. and around 150KW Which run from different connection in the same campus.
    But Full load gets transferred to Gensets in case of shut down.
    We have two gensets for this one is 625KVA which runs when full load or 80% of total load of both connections is running
    and second one is 250KVA which runs when Load is accordingly
    Condition-1 I have made the provision to connect my solar generation plant with genset to give it reference voltage and frequency,Whenever main supply is off and i have to run 625KVA Genset for fulfill my load requirements
    Please elaborate all Pros & Cons in running genset with Solar in this condition

  11. Dear sir,
    We have one site where we want to install 500 kW solar Grid tie Power plant .
    There is power cut problem frequently during day time , so they have to use DG set to continue to fulfill their power requirement . continous load is 260 KW. And Capacity of DG Set is 380 KVA .
    Here we want to Synchronize DG Set With Solar Grid Inverter.
    Kindly suggest if there is any solution
    Thanks & Regards
    Avinash Deshmukh
    9325422716

  12. Dear All,
    Thanks for sharing all your valuable information.
    Dear Dilip:
    Please confirm and provide your feed back and about as said above on April 4, 2013 at 3:15 pm
    “SMA has been working on this for last 2 years and our first reference site in India for a textile unit would be ready by middle of May”

  13. The problem will arise when load is less than Solar Generation during frequent variation of irradiation, since the load is not adjustable according to solar. What is the solution?

  14. At this point of time, there are several solutions available for this problem. One of them is what we have used for our 180kwp solar PV system in Goa for Konkan Railway. Here we have set 30% of dg capacity as a limit, and only when the load crosses this limit our Inverters starts generating. Our Inverters are capable of controlling the output power as per the load requirements using plc and that helps ensure that the total power generated never exceeds the load requirement (above the 30% capacity of load on dg).
    Note: Ensure that the power supply to your plc panel never goes down. In such circumstances the plc panel won’t be able to control the output power of inverter and excess power may go back to dg. We have also eliminated this possibility by using a simple, cost-effective yet reliable solution.
    Vishal Gawde
    Executive Engineer
    8788112958

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