Grid Code Compliance Studies for Solar Power Plants - Key Parameters

A.   Load Flow

The load flow study of a Solar Power Plant is performed to compute the network losses, check the equipment feasibility i.e. all elements in the system including transformers, Inverters, Cables & Transmission line remain within their steady state limits and that the voltage regulation across the network remains within acceptable limits for all network contingency scenarios. In addition, it helps determine the number of Inverters needed to compensate the reactive power demanded by the Grid and optimize the network. The plant performance is studied at different power production levels, voltage variations and tap positions (preferably maximum and minimum).

Following are the key inputs needed to compute the results:

1.      Grid Code of the country or region

2.      P-Q Capability Curve to be complied at the POI

3.      Q-V Capability Curve to be complied at the POI

4.      Datasheet of the PV Module

5.      Datasheet of the Inverter including:

a.      P-Q capability curve

b.        Voltage dependent capability curve

6.      Plant Single Line Diagram including.

a.       DC Configuration

b.       All intermediate and evacuation voltage levels

c.      Plant configuration and interconnections.

d.      Ratings and design temperature

e.      Point of Interconnection (POI) & Metering

7.      Datasheet of Transformer including:

a.      Resistance of Each winding

b.      Full load & No Load loss

c.      Positive and zero sequence impedance

d.      X/R ratio

e.      Type of tap changer (Off load/ On load type) including no. of steps

8.      Cable datasheet, schedules & power loss limit (in line with PVSYST)

B.   Short Circuit Analysis

The short circuit study calculates the short circuit levels at each bus of the system in a PV plant in the event of a fault and checks if the equipment installed is rated to sustain such an event. The fault level contribution of the PV plant to the Grid is also calculated. Although it is well noted that such a contribution is usually small. Following are some of the key parameters required to compute the results:

·      Transformer impedance

·      Cable datasheet

·      Breaker ratings

·      Earthing Configuration

·      Isym at the POI for 3-phase faults and L-G faults

·      X/R ratio at the POI for 3-phase faults and L-G faults

C.   Power Quality

To determine the impact of the harmonic distortion of the PV Plant and verify if the same is less than the THD (Total Harmonic Distortion) allowed by the Grid at the Point of Interconnection, a detailed model of the PV plant needs to be created. Following are some of the key parameters required to compute the results:

·      Typical harmonic and waveform distortion spectrum of each Inverter

·      Direct Current Injection values at rated voltage

·      Flicker contribution of each Inverter

D.  Dynamic Studies including Frequency Response and Ride Through Capability

The PV plant should perform suitable active power adjustments in response to frequency variations of the Grid.

It should also have the capability to stay connected to the network when a fault occurs on the network and its effects are seen at the POI for a short duration. These are termed as the Ride Through capabilities of a PV Plant.

Following key parameters are required to compute the dynamic analysis:

·      Dynamic Controller Model of the Inverter

·      Detailed information about the Inverter impedance

·      Low Voltage and High Voltage Ride Through graphs of the Inverter

·      Sudden phase jump characteristic of the Inverter

E.   Electromagnetic Transient Studies (EMT Study)

Electromagnetic transient or EMT studies help evaluate the transformer inrush characteristics and determine if any specific switching sequence during energization may cause failures.

When the applied voltage to a transformer is sinusoidal, the core flux rises from -Φm to +Φm during the positive half cycle of the applied voltage. If the transformer is switched ON at an instant when the instantaneous value of applied voltage is at its positive peak, then the flux would rise from its natural zero value up to +Φm during the next quarter cycle (Magnetizing lags voltage by 90 deg). The magnetizing current required would remain normal and the switching of the transformer would be trouble free.

However, if the instantaneous value of applied voltage at switching instant is zero and going towards positive, then the core flux would rise from its natural zero value to +2Φm in the next half cycle. This phenomenon is also known as doubling effect.

This flux doubling is accompanied by a huge magnetizing inrush current which may reach 5 times the full load current or higher, leading to massive winding forces and a possible dip in the system. Magnetizing inrush is highly unsymmetrical and stays for quite a few cycles, decaying according to the time constant of the system. The inrush current is expected to delay quickly if the system is switched on resistive load or capacitive loads. However, it would delay slowly if switched on NO load or with inductive load (which is the case for most Grid connected Solar Power Plants).

Below parameters are required to perform successful EMT studies:

1)      Inrush current curve

Inrush current is a form of transient over current present during the energization of transformers. It depends on the residual flux of the transformer, magnetic characteristic of the core & voltage waveform at the time of switching.

2)      Inrush decay time constant (sec)

The Inrush current slowly decreases by the effect of oscillation damping due to winding and magnetizing resistance of the transformer as well as the impedance of the system it is connected to, until it reaches the normal current value.

3)      Air core/Saturated core reactance

To carry out the calculation of the energization inrush current, a transformer model including its saturation characteristics is required. The saturation provokes the magnetic core to lose the capacity of increasing its magnetization and the winding of the transformer to behave almost like an air core reactance. Due to the hysteresis of the magnetic core, once the transformer is disconnected from the electrical network it can remain magnetized (remanent magnetization), which can considerably affect the inrush current.

4)      Magnetizing/Excitation current (%)

Is equivalent to the no load current, which establishes the magnetic flux in a transformer.