Reactive Capability Sizing of HPR1000 NPP in the UK Grid

To address the voltage profile, the reactive power requirements in the UK Grid Code have risen due to increasing power flows and the wind turbine system volume. This study strives to figure out the reactive capability of HPR1000 NPP in the UK grid and presents ETAP-based modeling and simulation of the demonstration. The paper first introduces the UK grid’s reactive capability requirements, then analyses the theory of synchronous generator reactive capability and unit transformer tap changer sizing, and finally demonstrates the sizing of reactive generator capability and unit transformer tap changer via load flow studies. The reactive capability requirements of the UK grid are more severe than China, causing the synchronous generator with a larger reactive capability [0.83PF(lagging), 0.98PF(leading)] and the unit transformer with a larger range of on-load tap changer[-10×1.25% / +4×1.25%], which should be concerned during HPR1000 NPP exportation.


Introduction
Hua-long Pressurized Reactor (HPR1000) Nuclear Power Plant (NPP) was developed as an advanced Pressurized Water Reactor (PWR) nuclear power technology in China.As the main brand of the Chinese nuclear power industry, HPR1000 is designed with a safety design concept that combines active and passive safety technologies [1] .Currently, HPR1000 is the most widely constructed in China.HPR1000 finished the Generic Design Assessment (GDA) in the United Kingdom (UK) in 2021 and will be applied in the following candidate sites in the UK.The reactive capability requirements of the UK Grid Code are different from Chinese requirements, so it is necessary to re-size the synchronous generator reactive capability and unit transformer tap changer, which is the bedrock of HPR1000 exportation.This paper focuses on the theory of reactive generator capability and unit transformer tap changer sizing, the different practices between China and the UK, and the load flow studies to determine the appropriate sizing parameters.HPR1000, as the business card of China, should comply with the local requirements in the UK.The power unit performances should be sized properly to underpin the grid, and the reactive capability requirements of the UK grid are compulsory, so this work is important to stabilize the grid and get permission to connect with the grid.This work is part of the Grid Code Compliance of HPR1000, which is the premise of its exportation.The practice in the UK is quite different from China.The severe requirements may cause challenges to the unit design, i.e., the generator capacity and transformer tap changer sizing of such a 1,200-megawatt-class unit.

Reactive Capability Requirements in the UK Grid Code
The detailed reactive capability requirements applied for HPR1000 in the UK Grid Code [2] are summarized as follows: a) When supplying the maximum capacity, the unit must be capable of continuous operation between the limits of 0.92 Power Factor (PF) lagging and 0.92 PF leading at the grid connection point.It should be fully available when the connection point voltage varies from 0.95 pu to 1.05 pu.The requirements are shown in Fig. 1.The power factor check-up point in the UK is at the connection point, while the point in China is at the outlet of the synchronous generator.If the connection point voltage is low, it indicates overfull inductive reactive power in the grid.The unit should provide capacitive reactive power (leading).If the connection point voltage is high, it indicates that the grid lacks inductive reactive power.The unit should provide inductive reactive power (lagging).Therefore, the points of (1.05Un, 0.92PF lagging) and (0.95Un,-0.92PFleading) in Fig. 1 should be assessed.Fig. 1 Reactive capability requirements b) When operating at an active power level other than rated MW, all synchronous generating units must be capable of continuous operation at any point between the reactive power capability limits identified on the synchronous generator performance chart.The following simulation study cases are carried out to demonstrate the synchronous unit's capability to meet the above requirements.
• Case RC1 -A load flow simulation study to demonstrate achieving the maximum lagging reactive power capability of the synchronous generating unit at the maximum capacity* when the grid entry point voltage is at 105% of nominal.• Case RC2 -A load flow simulation study to demonstrate achieving the maximum leading reactive power capability of the synchronous generating unit at the maximum capacity when the grid entry point voltage is at 95% of nominal.• Case RC3 -A load flow simulation study to demonstrate achieving the maximum lagging reactive power capability of the synchronous generating unit when operating at the minimum regulating level # when the grid entry point voltage is at 105% of nominal.• Case RC4 -A load flow simulation study to demonstrate achieving the maximum leading reactive power capability of the synchronous generating unit when operating at the minimum regulating level when the grid entry point voltage is at 95% of nominal.*Maximum capacity: The maximum continuous active power which a power generating module can produce, less any demand associated solely with facilitating the operation of that power generating module and not fed into the grid.
# Minimum regulating level: The minimum active power, as specified in the Bilateral Agreement or as agreed between the Grid Company and the generator, down to which the power generating module can control active power.

Literature Review
The reactive capability sizing of units has been widely studied, and IEEE Std C57 -116TM -2014 [3] is applied in those studies.Typically, a weak grid indicates more severe reactive capability requirements.The Chinese national grid is relatively robust, so its exportation project will have more severe reactive capability requirements.
Ping Chen [4] presented the transformer tap changer selection process and the sizing practice in the Philippines, which can be referred to for the Chinese aboard the project.Meanwhile, there are some shortcomings, i.e., the paper lacks theory analysis, and the grid requirements in the Philippines are directly for the generator capability (0.85PF lagging, 0.9PF leading) differing from the UK requirements.Liying Liu [5] also presented the practice in the Vietnam project similar to the Philippines project.
TOJO [6] studied the loading guide for individual transformers with an on-load tap-changer, and Nan C [7] provided a new hybrid power electronics on-load tap changer for the transformer.Those studies form the base foundation of this paper.
Currently, no literature compares the different practices between China and the UK.The designer must approach the practices for transnational projects.This work is worthy of figuring out the different practices and further theoretical analysis of the reactive capability sizing.

Generator Reactive Capability
The manufacturer of the synchronous generator can provide the performance chart, while a typical generator performance is quoted as follows, shown in Fig. 2, from IEEE Std C57 -116TM -2014.

Fig. 2 Generator performance chart
The generator voltage deviation is usually controlled within 5% Un.If the MW and voltage are constant, the max MVAR will be constrained by the armature and excitation windings' heat limits.The apparent power is confined when the outlet voltage and the armature current (considered the maximum value defined by the armature winding heat) are constant.The following Equation (1) approaches the relationship of MW and MVAR as a circle.Where, V a = generator outlet voltage; I a = armature current; P= MW; Q=MVAR Similarly, considering the excitation current as the maximum value confined by the excitation windings heat limit and the outlet voltage, the relationship of MW and MVA is expressed in the following equation.The following Equation (2) approaches the relationship of MW and MVAR as a circle with the center at Q=-( 2 a V / S X ) as shown in Fig. 3.
Where, X s =synchronous reactance; E af = the inductive voltage of the armature windings Excitation winding heat limit Armature winding heat limit Rated value(MVA) Fig. 3 Generator performance derivation Some scenarios cause the generator to operate under excitation, e.g., the leading operator of the generator, the over-voltage of the grid caused by disturbances, or the fault of the exciter.There will be some concerns if the generator operates at the under-excitation zone.Firstly, the magnetic leakage of the stator end-iron core will increase, causing the core temperature to rise; secondly, if the underexcitation condition is deep, the excitation current will be small, which may break through the static stability of the generator; thirdly, the risk of losing excitation protection of generator will increase, so an under-excitation limiter should be added.

Tap Changer Sizing
The international consensus is that the tap changer of the unit transformer should be sized according to IEEE Std C57 -116TM -2014.Load flow studies should size the tap changer of the transformer via the following Equation (3).
Where, V s = system voltage, kV V g = generator voltage (assumed to be at zero angles for reference) per unit on Vgbase V gbase =generator nameplate voltage rating, kV V THV = high-voltage winding tap voltage rating of UT, kV V TLV = low-voltage winding tap voltage rating of UT, kV

Novelties in the UK Project
The UK grid is an island system; off-shore wind power has contributed a greater share of the grid in recent years.Therefore, the reactive capability requirements become more severe than China and even European to underpin the grid stability.The novelties in the UK project are summarized as follows: a) The practices in China and the UK are different.The generator has direct, reactive capability requirements (0.9 PF lagging and 0.95PF leading).The rated voltage set-point of the unit transformer from the grid company and no-load tap changer (±2×2.5%) is typically adopted in China, while in the UK, there are reactive capability requirements (PF) at the connection point, and the reactive generator capability and the transformer tap changer should be sized accordingly.The theory in Section 4.1 and 4.2 are applied in the sizing process; b) The load flow studies in the UK practice have been simplified according to the reactive capability requirements in the UK grid and theory analysis.Although there are four points for each unit condition in Fig. 1, only two points are the limitations according to the analysis above in Section 2. The study cases in the UK project are simplified as two for each unit condition, i.e., two cases for the maximum capacity and two cases for the minimum regulating level.

Load Flow Studies
According to the assessment, the points of (1.05Un, 0.92PF lagging) and (0.95Un,-0.92PFleading) in Fig. 1 should be assessed.The generator power of HPR1000 differs slightly from site to site, so the generator power in the study is typically set as 1200MW.The rated voltage of the grid is set as 400kV.
The Grid Code requires that the minimum regulating level of the generating unit is 55% of the registered capacity, which refers to the capacity at the connection point, so the figure is assumed as 1096MW less the house consumption.The generator is roughly set to produce 700MW at the minimum regulating level.Some calculations should be performed before the load flow simulation for a rough estimation.When the generator operates at registered capacity (lagging), besides the plant auxiliaries' and transformers' reactive power consumption (total about 310MVAR), the unit should supply reactive power of about 467MVAR into the grid.Therefore, the generator should supply at least 777MVAR with a power factor of 0.83(lagging).Similarly, when the generator operates at registered capacity (leading), considering the plant auxiliaries' and transformers' reactive power consumption (total about 310MVAR), the unit should absorb reactive power 467MVAR from the grid.Therefore, the generator should absorb at least 157MVAR.
The following constraints are considered in the sizing: a) The PF at the entry point should be at least 0.92(lagging, leading, respectively); b) As long as the plant meets its auxiliaries consumption and the PF requirement at the entry point, to reduce loss, the plant should not inject or absorb more reactive power; c) The generator voltage deviation should be controlled within 5% Un.

Case RC1
The simulation results of Case RC1 are shown in Table 1 and Fig. 4.  The following points can be derived from the simulation: a) The generator with a power factor of 0.84 (lagging, 744MVAR) can not meet the requirement in the simulation; b) Maintaining the PF at the entry point at least 0.92(lagging), the lower the generator PF, the higher the positive tap changer; c) When the generator PF is set as 0.83, the tap changer of the unit transformer should be sized at 5%.When the generator PF is set as 0.82, the tap changer of the unit transformer should be sized at 11.25%.In summary, the generator PF should be sized as 0.83(lagging), and the tap changer of the unit transformer should be sized at 5% in the RC1 scenario.

Case RC2
The simulation results of Case RC2 are shown in Table 2.The following points can be derived from the simulation: a) The generator with 160MVar (leading) can not meet the requirement in the simulation; b) Maintaining the PF at the entry point at least 0.92(leading), the higher the generator MVAR, the higher the negative tap changer; c) If the generator MVAR is 180MVar, the tap changer of the unit transformer should be sized at -12.5%.If the generator PF is sized as 0.95(leading, Chinese practice), the tap changer of the unit transformer should be sized at-16.25%.

Table 1
reduce its reactive capability if the power factor at the entry point meets 0.92PF.Therefore, the tap changer of the unit transformer should be sized at 5%.
NoteThe tap changer of the unit transformer should be sized at 11.25% if the generator delivers 837Mvar.