how-many-monitoring-systems-on-a-pv-power-plant

IEC 61724-1:2021 

The 61724-1 standard for PV system performance monitoring has been revised. The latest version, released July 2021, contains a recommendation for the number of monitoring stations at a PV power plant. The numbers suggested are too low. Here is what we have found interviewing Hukseflux users.

Figure 1 Two SR30 Class A pyranometers measuring GHI (Global Horizontal Irradiance) and POA (Plane of Array) in a PV performance monitoring system. How many systems should there be on such a power plant? 

Introduction

IEC 61724-1: Photovoltaic system performance monitoring – Guidelines for measurement, data exchange and analysis –, includes recommendations for the number of monitoring stations at a PV power plant. See below the IEC recommendations and Hukseflux’s observations of common practice in tables 1 and 2. Hukseflux sees many more stations at PV power plants than recommended by IEC.

In some cases, Class A and Class B monitoring systems are mixed. Class B systems, in such case, contain only the most important sensors; POA, GHI and PV module temperature.

Table 1 How many monitoring systems per PV power plant? Copied from  IEC 61724-1: 2021 table 3. The number suggested by IEC is not a requirement, but “guidance” (IEC 61724-1 Clause 7). 

PV SYSTEM SIZE (AC) 

 

NUMBER OF STATIONS: SUGGESTED BY IEC COMMENT HUKSEFLUX
[MW][ # ]Although not specifically mentioned in IEC 6172401, the suggested numbers are generally understood to be a minimum
 < 40 2 
 40 to 100 3 
 100 to 300 4 
 > 300 + 1 / every additional 200 MWThe number of 1 / 200 MW is generally considered much (at least a factor 4) too low. See next table for common practices

 

Table 2 What number of monitoring stations to use? Common practices at utility scale solar power plants as observed by Hukseflux.

PV SYSTEM SIZE (AC) 

 

NUMBER OF STATIONS COMMON PRACTICES AND CONSIDERATIONS AS OBSERVED BY HUKSEFLUX
[MW][ # ] 
all

 + 1 / offtaker

+ 1 / VPP source

+ 1 / terrain type

In case of a large power plant with multiple offtakers (buyers of electricity): at least 1 station per offtaker

In case a large power plant consists of smaller sources grouped together (together forming a VPP or Virtual Power Plant): at least 1 station per smaller source

The topography of the terrain may require deployment of additional systems to attain good representativeness.

all+ mixing Class A and Class B systemsIn some cases, some of these added systems are not all Class A.
Class B or “secondary systems” may be added to Class A systems to reduce costs. IEC 61724-1 Class B  systems contain a limited set of sensors only, at least as POA, GHI and panel temperature. In such cases, the Class B systems are equipped with the same “high quality” sensors POA, GHI and panel temperature as used in the Class A systems.
all+ soiling systemsAll systems contain soiling measurement, however, in case non-uniform soiling is expected over the power plant: add separate systems only equipped with soiling measurements.
all+ albedo / reflectedThe optional albedo or rearside reflected irradiance measurement is often seen when bifacial systems are used. Typically at every station, however at least at 1 station / 50 MW.
all+ diffuse irradianceThe optional diffuse measurement is often seen when 1 axis trackers are used. Typically at 1 station / 50 MW.
 > 10 MW minimum number: 2Users consider use of Class A systems above 10 MW.
20 to 40/50 MW

 minimum number: 3

 

 

Above 20 MW Class A systems are commonly required as part of asset management.
Many – but not all – users prefer a minimum of 3 systems because this guarantees:
– spatial coverage
– redundancy of data (possibility for cross checks)
– redundancy in case of system malfunction
– redundancy in case sensors are sent away for recalibration
> 40/50 MW

 + additional systems

Simple rules of thumb that we see used in practice as the starting point for design:

– minimum 3 Class A, above 40 MW + 1 / 20 MW, all Class A
– minimum 3 Class A, above 40 MW + 1 / 40 MW, all Class A
– minimum 3 Class A, above 50 MW + 1 / 50 MW, all Class A
– minimum 2 Class A, above 50 MW + 2 / 50 MW, all Class A
– minimum 3 Class A, above 50 MW + 2 / 50 MW, (1 Class A + 1 Class B)
– minimum 2 Class A, above 60 MW + 1 / 30 MW, all Class A

Where can I order the standard?

The standard can be purchased from the IEC Webshop

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