**2.1. System configuration**

Figure 1 shows the configuration of Taipower 3rd nuclear power plant. The NPP has two 951 MW generators which are connected to the local 345kV gas-insulated substation (GIS) in oneand-half breaker configuration as shown in Fig. 1. The NPP is then connected to the power grid via four 345 kV overhead power lines (Darpen 1, 2 and Lunchi Sea/Mountain) to the Darpen and Lunchi 345kV EHV (Extra High Voltage) substation and two 161kV overhead lines (Kengting and Fengkang) to Kenging and Fenkang 161kV HV (High Voltage) substation.

It is important to note that there are three 13.8 kV buses (in the middle) and four 4.16 kV buses (at the bottom) for plant utility. Among these buses, the two 4.16 kV buses in the middle are responsible for feeding the safety-critical equipment such as cooling pumps and are designated as "essential buses". (The 2nd 4.16kV bus from the left where "DGA (Diesel Generator A)" is connected is designated as "Essential Bus A". The one next to it, where "DGB" is connected to, is "Essential Bus B".)

Another notable but subtle feature of this configuration is the use of 3-phase gas-insulated line (GIL) design with the 3 phases enclosed in a single duct of approximately 340 meters for the connection of the generation units and auxiliary systems, (located at the foot of a hill) to the 345kV GIS (on the top of the hill) due to topography feature of the location. This feature has implication on the generation and propagation of switching transients which will be explained in later sections.

### **2.2. Event sequence**

On 18 March, 2001, a Level 2 event occurred at Taipower's 3rd NPP, and the whole plant went into blackout from 00:45 to 02:58. The event started at 00:45 when EHV CB3510 (see Fig. 1, highlighted in red) was closed to energize the then-offline 345 kV/13.8 kV/4.16 kV start-up transformer (X01). Upon CB3510 closure, medium voltage (MV) CB#17 on "Essential Bus A" exploded damaging not only CB#17 but also CB#15. CB#15 formed a permanent ground fault keeping "Essential Bus A" at ground potential thus Essential Bus A became useless. CB #3 and #5 on "Essential Bus B" was then opened hoping DGB will start and supply power to those critical loads. However, DGB failed to start and the whole plant went into blackout. The only hope remained at that time was DG5 (on the far right in Fig. 1) which, however, needs to be started locally and manually. After 2 hour since the first problem occurred, DG5 was finally started and started to supply power to the critical loads via "Essential Bus B".

**Figure 1.** System Configuration of the NPP

section of this chapter.

**2. Taipower "318 Event"** 

**2.1. System configuration** 

"DGB" is connected to, is "Essential Bus B".)

explained in later sections.

**2.2. Event sequence** 

via "Essential Bus B".

In the following sections, we will examine Taipower's "318 Event" in detail to demonstrate the various possibilities that could lead to NPP plant blackout. Moreover, as these possibilities are not mutually exclusive, we will use this example to illustrate how multiple or cascaded problem can present further challenges to the overall NPP power system protection design. Recommended preventive measures are then summarized in the final

Figure 1 shows the configuration of Taipower 3rd nuclear power plant. The NPP has two 951 MW generators which are connected to the local 345kV gas-insulated substation (GIS) in oneand-half breaker configuration as shown in Fig. 1. The NPP is then connected to the power grid via four 345 kV overhead power lines (Darpen 1, 2 and Lunchi Sea/Mountain) to the Darpen and Lunchi 345kV EHV (Extra High Voltage) substation and two 161kV overhead lines (Kengting and Fengkang) to Kenging and Fenkang 161kV HV (High Voltage) substation. It is important to note that there are three 13.8 kV buses (in the middle) and four 4.16 kV buses (at the bottom) for plant utility. Among these buses, the two 4.16 kV buses in the middle are responsible for feeding the safety-critical equipment such as cooling pumps and are designated as "essential buses". (The 2nd 4.16kV bus from the left where "DGA (Diesel Generator A)" is connected is designated as "Essential Bus A". The one next to it, where

Another notable but subtle feature of this configuration is the use of 3-phase gas-insulated line (GIL) design with the 3 phases enclosed in a single duct of approximately 340 meters for the connection of the generation units and auxiliary systems, (located at the foot of a hill) to the 345kV GIS (on the top of the hill) due to topography feature of the location. This feature has implication on the generation and propagation of switching transients which will be

On 18 March, 2001, a Level 2 event occurred at Taipower's 3rd NPP, and the whole plant went into blackout from 00:45 to 02:58. The event started at 00:45 when EHV CB3510 (see Fig. 1, highlighted in red) was closed to energize the then-offline 345 kV/13.8 kV/4.16 kV start-up transformer (X01). Upon CB3510 closure, medium voltage (MV) CB#17 on "Essential Bus A" exploded damaging not only CB#17 but also CB#15. CB#15 formed a permanent ground fault keeping "Essential Bus A" at ground potential thus Essential Bus A became useless. CB #3 and #5 on "Essential Bus B" was then opened hoping DGB will start and supply power to those critical loads. However, DGB failed to start and the whole plant went into blackout. The only hope remained at that time was DG5 (on the far right in Fig. 1) which, however, needs to be started locally and manually. After 2 hour since the first problem occurred, DG5 was finally started and started to supply power to the critical loads

**Figure 2.** 345kV and 161kV Overhead Line Switching Event Log

Figure 2 shows the switching event log of the four 345kV and two 161kV lines connecting to the NPP. After reviewing the event log, it was found that CB#17 on Essential Bus A broke down when one GIS switching operation was occurring. The event log also showed that there were 37 EHV switching operations during the 48-hour period prior to the event due to salt-fog influence in the plant area. Because of the unstable offsite power, the GIS switched between different offsite power to acquire the stable power sources.

Power System Protection Design for NPP 5

**2.3. Electrical stress in plant power system** 

even bigger overvoltage.

neutral voltage transfer.

local power network [8,9].

rated line-to-ground peak voltage!

*2.3.3. Switching surges on both EHV and MV systems* 

transfer.

*2.3.2. Neutral voltage transfer* 

now operated as induction generator after loss of external power.

*2.3.1. Line conductor overvoltages due to over-excitation and nonlinear resonance [6,7]* 

The transient recorder in Fig. 3(a) recorded 2 abnormal overvoltages (at 56 Hz and 45 Hz, respectively) after the last 345 kV-transmission line connecting to the NPP was tripped on the remote end which turned the NPP into an electrical island. As will be explained in the next section, the 1st overvoltage was caused by the over-excitation of the motors (e.g. recycle water pump) in the plant who, with terminal voltages supported by large line capacitance,

The 2nd overvoltage is caused by a different mechanism. After a few cycles the low voltage relays tripped many of the plant motors leaving only 2 biggest motor (now operating as induction generator) still connected and were supported by a comparatively much larger capacitance leading to not only over-excitation but also magnetic saturation of both the motors and transformers. This created a condition very close to ferroresonance resulting in

It can be seen from Figure 3 that overvoltage were observed not only on the line conductors of phase A, B, and C but also on the neutral. As will be explained in the next section, neutral voltage transfer can occure through 2 different mechanisms: electromagnetic and capacitive

In the presence of transformer core saturation, 3rd harmonic neutral voltage will be present on the windings through electromagnetic transfer as long as the neutrals of the respective windings are not grounded. In the presence of neutral voltage on any of the transformer windings, the stray capacitance among the windings and earth will result in capacitive

From Fig. 2, it can be seen that there were 37 switching operations during the 48-hour period prior to the breakdown. This unusually high number of switching operation can create lots of switching surges (with magnitude of around 7 times the rated line-to-ground peak voltage in the medium voltage system) which, when propagating through the NPP local power network, can degrade the insulation level or even cause breakdown of CB's in the

It should be noted that while there were 37 switching operations on the EHV side, none of the switching surges were captured by the transient recorder in the 345kV GIS in Fig. 3(a) due to insufficient bandwidth of the transient recorder. In a follow-up field test [9] after the event, it was found that such switching often causes switching surges of around 7 times the

Figure 3 shows the transient recording of overvoltages for both the 345kV bus and one MV (medium voltage) bus at 20:38 in March, 2001. At t0, the flashover on the 345kV line occurred leading to its subsequent tripping at t1. The tripping took place on the remote end of the 345kV line thus overvoltage can still be observed at the NPP between t1 and t2 due to "motor-generating effects" to be explained in the following section of this Chapter. The overvoltage on the 345kV line eventually caused flashover from Phases A and B to ground pulling down the line voltage and all motors on the 4.16kV bus were tripped by their respective under voltage relays at t3. At t4, the flashover from Phases A and B to ground was cleared and the "motor-generating effects" start to build up the voltages again with the two remaining motors on the 13.8kV bus.

(a) Transient Recording at 345 kV GIS

(b) Line-to-Line Voltage Transient Recording of one 13.8 kV motor, Recycle Colling Pump B

**Figure 3.** Overvoltages at 20:38 in March 17, 2001
