**Abstract**

In a Nuclear Power reactor, safety loads are backed by standby battery system. The healthiness of the battery is very essential requirement and prominent attention is given to availability and reliability of battery supply in nuclear plants. Hence regular monitoring and testing the performance of the battery is a prime requirement. The capacity and load cycle discharge testing of the battery is done annually and the current system employed is to discharge the battery current through resistor banks, which results in unusable power consumption and is uneconomical. The growing trend in power electronics field has given the new technology of regenerating the dissipated power to grid. This paper proposes a high power electronic regenerative technology with high efficiency, low harmonics to pump the dc power to the grid. Though, it is available at lower rating in industry, the paper proposes a high power regenerative discharge system. The topology selected is interleaved boost converter interfaced to a three phase grid connected inverter. The challenges involved are high power operation, steep current discharges with a minimal interference to the normal plant operation power supplies during the regeneration. This paper also presents the system design and simulation results.

**Keywords:** Regenerative battery discharger, Interleaved Boost Converter, Grid connected Inverter, harmonics reduction, nuclear safety load

#### **1. Introduction**

In nuclear power plants, the batteries and DC power system plays a prominent role in the reactor safety and shutdown. Hence the preventive maintenance and testing the capability of battery to cater the safety loads during power failure are done regularly as per nuclear standards. There is requirement of performing C10 and HRD discharging the batteries. The growing trend of large capacity NPP's being installed worldwide has increased the battery ratings substantially, resulting the discharge testing requirements to high current and voltage levels [1]. In conventional plants, resistor banks employed for discharging battery has resulted in power loss, hence there is a need to use the regenerate power electronics system in the battery application. Even though Low power discharge systems are available in the current scenario, the discharging of high power battery systems for higher currents and shorter duty cycles is the motivation behind this paper [2, 3]. The system employs two phase interleaved boost converter to step up the connected battery

**Figure 1.** *Block diagram of grid connected regenerative discharger system.*

#### **Figure 2.**

*Schematic of regenerative battery discharge system.*

voltage level from (48–360)V DC to 560 V DC as shown in **Figure 1**. The boost converter topology is selected to ensure minimum DC ripples in the inverter dc bus.

The boost converter is modularized into two independent 50% power units and they are independently connected to 2 X 50% two level 415 V Inverter.

Accordingly the boost converter is designed in the modular approach, with two sets of 3 X 100A boost converter to provide required current rating as shown in **Figure 2**. The Grid connected inverter discharges the power to Grid by converting DC to AC. The output isolation transformers in the inverter reduce the third harmonic component and also provide isolation between grid and the plant system.

#### **2. Regenerative power electronics topology**

#### **2.1 High power load cycles of batteries**

The plant consist of the following battery banks for the reactor safety loads in the range of 48 V DC, 220 V DC, 360 V DC. Following are the various battery bank capacities with various DC voltage ranges as shown in **Table 1**.

In the nuclear power plant the load cycle is more complex when compared with other conventional plants [4]. A typical example of load cycle is given in **Table 2**.

## **2.2 DC-DC converter topology**

The topology selected for our application is dual phase Interleaved Boost Converter (IBC). Interleaving is a method of multi-phasing in which two converters are connected in parallel. In interleaved boost converters, the number of phases has a significant impact on the current ripple [5, 6]. Though ripple content reduces with increase in the number of phases, the power circuit, on the other hand, the complexity of the circuit and triggering signals will be increase [7].

In this paper, a two-phase interleaved boost converter is selected as DC-DC Converter topology [8]. In a two-phase converter, there are two Output stages that are driven 180 degrees out of phase as shown in **Figure 3**. By splitting the current into two parallel paths, conduction losses can be reduced, leading to improved efficiency compared to a single-phase converter. The ripple generated from switch S1 and complimentary switch S2 cancels each other [9, 10]. Employing coupled inductors in this topology adds to the advantage of the input current-ripple cancelation from magnetic coupling between the phases. The frequency of the current ripple is twice for two phase IBC than the conventional boost converter [11]. The


#### **Table 1.**

*Various batteries with different voltage rating and capacity available in the plant.*


#### **Table 2.**

*The load demanded with required time duration of all battery banks available in the plant.*

converter must be able to operate over a wide input-voltage range (40-400 V) to accommodate batteries of different voltages (48 V, 220 V, 360 V). Because of the wide input range, the converter also must be able to operate with a wide inputvoltage to output-voltage ratio [12].

The controller sets the pre-set duty cycle as input for converter switching as the input voltage to converter is selected by the operator. The output voltage of the boost converter is fixed to 560 VDC for providing required DC bus voltage for the inverter module. The main design consideration of this converter is done with respect to the battery's end bank voltage [13]. During discharging of batteries, as the battery reaches the end bank voltage, the voltage boosting has to be done by the converter for the reduced input voltage to maintain a steady DC bus at inverter input and has to supply the rated current at the output. This design consideration is implemented through dynamic duty cycle variation based on the input DC voltage feedback to the converter [14, 15]. Switching frequency of the converter is selected nominally at 10 kHz and the duty cycle for the switching is selected as per the input-voltage equations of a traditional boost converter. The inductors selected for the converter is uncoupled type.

**Figure 3.** *Schematic of boost converter.*

**Figure 4.** *Schematic of two level inverter systems.*

*Design of High Power Regenerative Battery Discharger System for Nuclear Power Plant DOI: http://dx.doi.org/10.5772/intechopen.98534*

#### **2.3 Grid connected inverter topology**

2 X 100kVA IGBT (Insulated Gate Bipolar Transistor) based inverter at the DC boost converter output operates in synchronism with Grid supply.

Two power stacks for each phase is designed for effective load sharing and thereby reduced heat dissipation [16]. The switching frequency of inverter is selected at 1 kHz. Sinusoidal PWM algorithm is implemented for generating inverter switching pulses. There are two inverters each rated 50% capacity (100KVA) connected parallel sharing the load. Inverter-1 is fed from Group-1 DC Boost converter and Inverter-2 is fed from Group-2 Boost converter. In case of failure of one inverter, the 50% load can be taken up by the second inverter. The topology of two level inverter is selected to reduce the complexity in inverter design. The output harmonics primarily 5th and 7th harmonic components are reduced with the help of LC Filter as shown in **Figure 4**. The 415/415 V output isolation Δ-Δ transformer eliminates third harmonics in the output. The inverter is in synchronized to grid during operating conditions. The battery power is delivered to grid with the help of inverter synchronized to grid supply.

### **3. Circuit parameter design**

The design of interleaved boost converter is very similar to traditional boost converter design.

### **3.1 Duty cycle (D)**

Generally output voltage equation of any conventional boost converter is given in (1), Duty cycle for any input and output voltages can be represented as (2),

$$\mathbf{V\_O} = \frac{\mathbf{V}d}{(\mathbf{1} - D)} \tag{1}$$

$$\mathbf{D} = \frac{(\text{Vo-Vd})}{\text{Vo}} \tag{2}$$

Boost converter to work with three different input voltages 48 V, 220 V, and 360 VDC respectively as represented below,

D48V = Dmax = (560–48) / 560 = 0.914. D220V = (560–220) / 560 = 0.607. D360V = Dmin = (560–360) / 560 = 0.357

#### **3.2 Current ripple (ΔIO)**

Each Boost Converter is designed for 30KW Power rating. Load current IO = Output power /Output voltage IO = 33 x 10<sup>3</sup> /560 = 59A. For D < 0.5

$$\text{Jorms} = \frac{\text{Io}\sqrt{\text{D}(\mathbf{1} - \mathbf{D})}}{2(\mathbf{1} - \mathbf{D})} \tag{3}$$

For D > 0.5

$$\text{Jorms} = \frac{\text{Io}\sqrt{\frac{1}{2}(2D-1)(2-2D)}}{2(1-D)} \tag{4}$$

By using (3) and (4), Output rms current is arrived for all modes as below. *Iorms* (48 V) = 91.52 A.

*Iorms* (220 V) = 21.72 A.

*Iorms* (360 V) = 21.98 A.

Considering ΔIO load current ripple to be 8% of output current. For evaluation, maximum duty cycle, in this case for input 48 VDC Input mode, current ripple (ΔIO) is arrived to be 7.2 A. For other duty cycles also, ΔIO can be arrived in similar lines [17].

#### **3.3 Inductor value**

For device switching frequency set at 10 kHz. The inductance parameter can be calculated as below.

Switching Time Ts = 1/fs. = 1/10 kHz = 100 μs.

$$\mathcal{L} \ge \frac{(V\_{in} D\_{\text{max}} T\_s)}{2\Delta I\_o} \tag{5}$$

For Vin = 48 V & D max = 0.914. Substituting the values of Vin & D max in (5), L ≥ (48)(0.914)(10�<sup>4</sup> ) / 14. L ≥ 313 μH.

The inductance parameter is selected to be greater than 313 μH, so the optimized design value is 368 μH considering design tolerances.

#### **3.4 Capacitance value**

The capacitor selection is decided based on voltage ripple at output. Considering ΔVO output voltage ripple to be 1 V.

For D = 0.914.

$$\mathbf{C} \ge \frac{(V\_O \mathbf{D} \mid)}{\mathbf{R} \, \Delta V\_O F\_s} \tag{6}$$

Substituting the values of Vo *& D* max in (6),

C = (560) (0.914) / (100\*1\*10<sup>5</sup> ). C ≥ 102 μF. For D = 0.357. C ≥ 199 μF. For D = 0.607. C ≥ 45 μF.

The capacitance parameter is selected to be greater than 200 μF. considering 5% margin, capacitance value arrived at 210 μF.
