Impact on electrical system if the Transformer is given 100 % Solar Load

    

The electrical system on the solar load

 Let us consider the case first - 

    I have a 100 KVA distribution transformer installed near my house (considering PF 0.9). The total solar load applied by households in that area is 90 kW, which will be fed through this 100 kva transformer. The parameters are -

1. Transformer rating = 100 kVA  
2. Power Factor = 0.9 (lagging, assumed typical for residential loads)  
3. Rated real power = 100 × 0.9 = 90 kW
4. Total solar load on system= 90 kW

    As per the condition, let's say all 90 kW solar generation is connected to feed into the transformer (i.e., export to the grid or supply local loads).

What Happens Technically When 90 kW Solar Is Connected?

1. Loading of Transformer - The transformer is rated 90 kW at PF 0.9. If all 90 kW solar starts generating at peak - 

1.  The transformer is operating at full real power capacity.
2.  But in real life, transformers also handle reactive power (kVARs), so apparent power can exceed 100 kva.

Risk Involved - If the local load is low (at night or on holidays), this full 90 kW might be pushed back to the grid, causing reverse power flow.

2. Reverse Power Flow - Transformers are usually not designed for continuous reverse flow unless specially specified.

- If generation > local consumption, surplus power flows back from LV to HV side, i.e., from 415 V to 11 kv in India.

Impacts -

• Overheating of the transformer due to reverse magnetisation.
• Protection malfunctions (relay settings typically designed for forward load).
• Can lead to overvoltage on the 11 kv feeder, especially if other transformers also have high solar.
• Utilities may limit solar penetration per transformer (usually 30-50 % of kva capacity); however, in JdVVNL, AVVNL, JVVNL, Rajasthan  norms its 80 % of the Transformer capacity.) 

3. Voltage Rise on LT Side -

• The LT line's voltage increases as a result of solar power.
• With 90 kW connected and minimal local consumption:
• LT voltage can exceed permissible limits (say > 240 V phase-to-neutral).
• May trip inverters due to overvoltage (anti-islanding protection).
• Uneven PV output can cause voltage fluctuations, especially if single-phase inverters are used.

4. Thermal Overload Risk -

• The transformer is designed for typical residential loads, with diversity and non-peak coinciding.
• 90 kW solar = non-coincident, simultaneous injection → actual load on transformer may exceed thermal limit.
• Temperature rise in windings, insulation degradation → shortens transformer life.

5. Power Quality Issues -

• If many inverters operate simultaneously - 
• It may introduce harmonics and voltage flicker.
• Also results in unbalanced loads (if solar is not equally spread across 3 phases), neutral heating, and voltage imbalance.

Solutions & Measures -

1. Limit Solar Capacity on DT -

• Follow solar hosting capacity guidelines and respective acts and norms to be followed with discipline. 
• Generally, max 30–50% of transformer capacity or as applicable by various discom norms of different states.
• For 100 kVA DT → limit solar to 30–50 kW (or as decided by the distribution company norms across states).

2. Upgrade Transformer -

- If demand justifies -

•  Upgrade to a 100 kVA or 160 kVA transformer.
•  Ensure it supports bi-directional power flow.

3. Install LT Side Voltage Regulation -

• Use on-load tap changers (OLTC) or line voltage regulators.
• Monitor and stabilise the voltage rise due to solar.

4. Install Reverse Power Relays or Limiters -

• Prevent dangerous export by tripping or curtailing excess generation.
• Or install net metering + export limiters.

5. Phase Balancing -

• Ensure solar connections are equally distributed across the 3 phases.
• Prevents overloading of a single phase and neutral heating.

6. Real-time Monitoring System -

IoT or SCADA monitoring of ----

•  Transformer loading
•  Voltage at the LT side
•  Solar injection
•  Reverse power

7. Energy Storage (Optional but Effective) -

• Use battery storage to store excess daytime generation.
• Reduces grid injection and stabilises voltage.