Page 56 - Energize September 2022

TECHNICAL

• Q3: DG with capacitive kVAR: The active power flows from DG to Protection systems
the grid with θ < 0, |E|<|V2|, and the magnetizing current Im is Directional overcurrent (often abbreviated as DOC) protection
supplied by DG at the load terminal relays are often included in substations, to look at power flowing
• Q4: Normal load with capacitive kVAR: The active power flows in the reverse direction through a transformer, to detect faults
from grid to the load with θ>0, |E|<|V2|, and the magnetizing on the high voltage side of the transformer and to trip off the
current is supplied by DG at the load terminal. circuit breaker. However, these relays often can’t differentiate
between normal level load current and fault current, and so could
The magnetizing flux in the transformer core is established through spuriously trip if there was too much reverse power flow. As such,
the excitation voltage (E) which is different for all four quadrants of these relays often need to be changed when a generator causes
operation. For Q1/Q2, the core excitation is delivered by the grid reverse power flow.
and for Q3/Q4 excitation is delivered by the DER. Considering that
grid voltage is constant, excitation voltages for Q3/Q4 is always Hosting capacity 3
higher than design values which increases the core flux density. Considering the effect of PV in distribution networks raises the
Transformers are optimally designed closer to the knee following questions:
point of the magnetizing curve of the core; therefore, with small • How much PV can be added to an existing feeder without
increase in excitation voltage (E), the magnetizing current and alteration?
magnetic losses increase significantly. Usually magnetizing current • What parameters limit the amount of PV which could be
is very low for transformers, and a two-fold increase will not accommodated?
cause problems. Increased core losses will impact the thermal • What can be taken to increase the capacity of the feeder, and
and hence the life of transformers. Also, the three-phase core what is the cost involved?
type transformer has three limbs which does not provide a zero-
sequence flux path. Hosting capacity is defined as the maximum allowable penetration
If grid voltage is maintained, then the excitation current level of PV systems that will not violate any of the network’s
is increased, and the core could become saturated. Normally, operational constraints. (Hosting capacity may be increased by
infinite grid bus voltage V 1 at the grid terminal is maintained strengthening the grid). Note that PV hosting capacity is not a
resulting in further increase of magnetizing voltage. The shape limit on a single installation (such as a home), but for distributed
of the magnetising current becomes non-sinusoidal and contains residential-scale PV installations.
significant harmonic distortion which lead to higher core losses. The concept of hosting capacity estimation is based on the
The core also saturates if the voltage increases beyond the impact criteria, such as voltage limit violation. Grid code voltage
operational limits. If the power flow is maintained in Q3, the regulation states that under normal operating conditions, the
increase in temperature reduces the life of the transformer (up voltage level at any LV feeder on the circuit should not exceed
to 25% loss of life). If the life of the transformer is required to be 230 ± 10% (-0,9 to 1,10 p.u.). Since the effect of PV is overvoltage,
maintained, then the losses are to be maintained for all the four then 230 + 10% (253V) becomes the critical limit. Therefore,
quadrants as per the nominal design. additional PV can’t be integrated at any feeder if the voltage
Therefore, some utility companies impose certain restrictions at the feeder exceeds this threshold value. The PV generation
on developers, manufacturers, service providers and distributers, that corresponds to 1,10 p.u. voltage limit is called the hosting
for reverse power flow through the interconnect transformers capacity of the given distribution circuit. Similarly, the hosting
(substation transformers and distribution transformers). capacity of a distribution feeder can be evaluated in terms of
other impacts on the grid such as:
Effects of reverse power flow through transformers can be 1) Highest voltage deviation in primary and secondary wire
summarised as: voltages, and
• The transformer losses (core and harmonic) are significantly 2) Reverse power flow at the substation.
increased even at 15% higher excitations.
• For any design, reverse active and reactive power flow condition The adverse effects for each of the impact criteria are quantified
(Q3) results in maximum core losses for any load conditions. from the threshold specified by the grid code. Hosting capacity is
• If reverse power flow is not restricted, interconnect transformer the amount of PV generation that the circuit can accommodate
lifetime can be reduced by up to 25%. without violation of a threshold corresponding to the impact
• If the reverse power flow is greater than the rating of the criteria. Therefore, it is possible that varying amounts of hosting
transformer, the PV cannot be connected, or the network needs capacities can be obtained for the same distribution grid,
to be upgraded. corresponding to each impact criteria.
• Reverse power flow through a transformer can result in the
voltage on the high voltage side of the transformer increasing. Three hosting capacity classes are considered:
Sometimes this can increase to levels that are too high for the • Initial: The hosting capacity without any operational changes or
network equipment to cope with, or cause voltages to exceed grid upgrades
the statutory voltage for other customers. • Secondary: The hosting capacity with operational changes such

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