O&M Strategy for Grid Tied and Non Grid Tied PV system :JP Gupta

Professionally and specifically focuses for O & M of Grid Tied and Non Grid Tied PV system differ but target is ONE – “To maximise production of electricity while minimizing O & M expenses”.  In grid tied system which is in MW scale the  generation of solar power along with synchronisation with grid is cumpolsory to increase yield where as in non grid tied PV system owner must make sure their assets are managed in the best possible effective way and style.

            The evolving grid tied PV system is coming in forefront in last few years in major ways since there was no successfully benchmarked previous ways to perform operation and maintenance in this field.  Just lessons learned by others in different small pockets (mostly in KW range) from place to place were tried it before.  Now all is coming in a big way so the structure or governance in operation and maintenance will have exploring policies, procedures, sub-committee and service procedures all at the stage of commissioning of solar plant fully functional with explanation on how the operation and maintenance is built to last for life time of plant in most economical way.

            There are several risks in O & M of plant which has to be mitigated during life cycle.  First risk starts with EPC contractor.  It is very common that an EPC contractor is engaged for MW scale plant who gives certain warranty for the performance.  Here is the catch.  The warranty giver always make sure that project performance is as per agreement during pendency of warranty which does not include the full life time of plant in general.  So they have main interest in finding a solution for immediate performance problem avoiding at long term goal of project.  As a consequence to this, potential problem that probably impact performance later in the life time of project might not get noticed to the owner.  In fact the main focus of EPC agency is to commission the plant and provide O & M service as per agreement.  It is also noticed that O & M during initial period is much less and complexities occur after running of

plant for a considerable long time.  So it is advisable for hiring a competent third party to provide exhaustive observation cum report of all activities and issues post commissioning in view of O&M strategies.     This  will help a lot provided all issues are addressed as early  as possible.  It is worth to remember that each delay will adversely the ROI.  This becomes an important mile stone as it defines responsibilities at all stage from construction to commissioning and up to maintenance.  ABC analysis of this valuable report is made cumpolsory by owner of project which helps him for best O&M practices from Array to  recording meter at evacuation point of utility.  Before start of project the owner has to make sure for O&M strategy of the plant which will help him to keep finances under control.  There are variety of potential hiccups in entire game which needs to be analysed, forecasted and remedial action taken at the start itself.  This pays back and gives profit (ROI) in long run to owner.

            Let us take item wise O&M of PV MW plant for optimal performance and uninterrupted operation point of view.  Among all the top priority for stakeholder goes to regular monitoring of parameters of solar plant to ensure following:

1. Consistency and regularity
2. Repeatability
3. Data supremacy and quality
4. Reproducibility

            This must be for panels(Arrays), inverter and Balance of system as the case may be. The figures below shows the failure rate which are just guideline for grid tied system:

1. Inverter failure : 45%
2. Module failure like shading soiling etc: 10%
3. AC and DC system: 10%
4. Synchronisation with evacuation system : 15%
5. Tracker malfunction: 10%
6. Structure and racking failure include earth station and lightening  : 5%
7. B.O.S.  etc :  5%

            It is worth to note here that all products like transformers, inverters, CT, PT, batteries, modules, AJB, SJB, Trackers,  control and relay panel have their own manual for O&M and this be essentially followed for scheduled maintenance, preventive maintenance and corrective maintenance as per individual recommendation.

            Trouble shooting chart is provided by EPC and manufacturer for properly perusal by owner.  O&M is different for all equipment like some needs inspection and cleaning and greasing or topping with due quality oil and some needs tightening with certain torque force so this be adhered as required. Thermographs of entire area will save from electrical loose connection and experience of author says that these are over looked in majority of cases.  Such trouble shooting and O&M saves time, manpower, cost for spare parts inventory.  Depending upon the nature O&M is daily, weekly, bi-monthly, monthly, half yearly or yearly.  All this must be displayed in control room for both system.  The failure rate typically for non grid tied slightly vary as per guide line below:

1. Inverter failure                                                                                       35%
2. Battery system or energy storage                                                     20%
3. SCADA and MPPT                                                                                  10%
4. AC & DC system                                                                                     15%
5. Structure earth station and lightening                                             8%
6. B.O.S. etc.                                                                                                 12%

            International standard IEC 61724 has recommended procedure for :-

(Aa.)  Calculation of all key performance from beginning to end in PV plant of both types i.e. grid tied and non grid tied.

(Bb.)  Indicators for such PV installation

            Thus with help of above owner can easily come across the important evaluation criteria thru monitoring the required parameters like

(1a.)                             Aging of modules

(2b.)                           Reflection losses

(3c.)                            Soiling

(4d.)                           Thermal losses

(5e.)                           Wiring losses

 (6f.)                           Inverter and transformer losses

            All above help to calculate KPI(key performance indicator) and set threshold values.  PR (normal value is 0.8 to 0.85) also indicates the overall effect of losses on the array’s nominal energy output due to :-

(1.1)                             Array temperature

(2.2)                             Incomplete utilisation of irradiation

(3.3)                              Failure due to ineffencies

(4.4)                             System component

            So all PV project has to be examined on case to case basis for O&M to reach to favourite economic situation.   O&M cost in terms of percentage of income comes in the band of 0.75% to 2.5%.   This does not include insurance premium.  The variation attribute to factors like:

• local condition and location mean the area with good sunny days( 330 days in a year), table top means levelled land and often gradient land with presence of small or large trees or mix of above is available. These local condition effect much on type of O&M needed for the project.   Occasionally  open field in stretch of miles and or foot of hill pose variety of local situation.  Soil condition and saline atmosphere is equally responsible for variation.  Resistivity plays an important role.  More resistivity means more number of earth stations and proportionally increase in O&M.   Flooding during rainy days or dust particle like in desert area are all culprit for O&M variation.

Type of module, like Mono Csi, Poly Csi, thin film.  Module with wider power tolerance lead to mismatch i.e. lower over all output so only

• positive tolerance module help to reduce O&M. Aluminium alloy border or without border. Distance between them to avoid shading foundation design of structure and type of structure to withstand wind velocity of 150 miles/hour etc.
• Inverter make and specification. This alone has highest failure rate in whole system. For ventilation It is oil cooled or water cooled or fan cooled, it’s own running loss, in built MPPT, input DC voltage and current, Output AC voltage and efficiency, temperature related deration (reduction in efficiency), grading like “A to E” with transformer or transformer-less inverter, centralised (500 KW to 1667 KW) or string distributed, power clliping loss, harmonic content, power output of higher quality like pure sine wave or modified sine wave matters a lot on type of O&M as well as surge capacity etc.
•  AJB/SJB make and style, number of input and output of each AJB/SJB enclosure like IP65 or IP68 are directly linked with O&M because they all have to give satisfactory life of 25 years as per practise in the industry.
• Earth Station requirement for O&M is always on yearly basis but more earth station mean more expenses and it all depends on soil condition and design of system. This is also silent killer of system so O&M can not be avoided.
• A. requires replacement whenever there is lightening fall in that area and some areas are prone to thunder and lightening and some are free from this so depends on location.

DC and AC electrical net work demands thermography for O&M point of view.  DC SYSTEM demands larger attention for O&M.  Spark generation causing several type of faults creates disturbance in meeting the target.  This be done on regular interval of week to week basis.

• Transformer requires recommended scheduled maintenance and this keeps them fit for entire life of 25   Occasionally break-down maintenance is also done.
• Batteries are energy storage device for non grid ties system and they demand daily maintenance as per manufacturer manual
• Proper record of history of all equipments with the O&M done on it helps the owner in many ways. This is followed for entire life of plant.

      It is experienced that loss of annual performance is decreased with proper O&M.  Thus there is direct and indirect advantage to follow up best O&M practise.  So overall a strategy has to be chalked out by owner to make it economical on custom basis as explained above.  A typical preventive maintenance schedule for 50 MW AC solar farm project grid tied  of USA is prepared and GIVEN UNDER for perusal of reader.

More Information….O&M Master Schedule