2018 International Conference on Computer, Communications and Mechatronics Engineering (CCME 2018)

ISBN: 978-1-60595-611-4

Research on Relay Protection Schema and Protection Setting of Large Capacity Synchronous Condenser

Yan-jun LI1, Xiao-yang WANG2,* and Rui YU3

1China Electric Research Institute, Beijing 100085, China

2Beijing Sifang Automation Co., LTD, Beijing 100085, China

*Corresponding author

Keywords: Large capacity synchronous condenser, Protection schema, Protection setting.

Abstract. Now, the large capacity synchronous condensers are needed in the State Grid. However,

the research on its protection is rare. Based on its application in AC/DC converter stations, this paper

summarizes the studies of the relay protection function and schema about large capacity synchronous

condensers and transformer group, furthermore analyzes the main protection principle and setting

principle of relay protection (differential protection, inter-turn protection, stator grounding

protection, etc). At last an example of setting value of synchronous condenser and transformer group

protection in converter station is given.

Introduction

The characteristic of “strong DC and weak AC” grid requires fast and effective reactive power

compensation devices. The first, to quid or reduce grid voltage fluctuations, the reactive power

compensation device should quickly generate a large amount of reactive power in a very short time at

the moment of the fault [1]

. The second, in the case of fault recovery (transient process), the reactive

power compensation device should continuously provide a large amount of reactive power, that is, a

strong overload capability is required. The last, it should provide reactive compensation for the

system during steady state operation.

Since the 1990s, the development of power grids is not very fast, the SVC and SVG technologies

have been fully developed. The synchronous condenser has been greatly weakened in the grid, and

the research on the operation and maintenance technology (especially the principle of relay

protection) has been greatly weakened too.

Recently, the State Grid Corporation of China has carried out researches on the impact of large

capacity synchronous condenser including relay protections [2]

. Though the design structure of the

new synchronous condenser has been improved there are still some failure possibilities. Especially

the stator turn-to-turn fault will pose a serious threat to the condenser and the grid if not installed

short-circuit protection [5]. The operation of the large capacity synchronous condenser will also

affect the current grid relay protection which based on electrical quantities such as impedance,

voltage, phase angle, etc. And it may threaten the safe and stable operation of the power system.

Therefore, it is necessary to carry out systematic research on the relay protection of the large capacity

synchronous condenser.

Synchronous Condenser Protection Schema and Setting

The latest developed synchronous condenser transformer protection (including excitation transformer

protection, without rotor grounding protection) is shown in Table 1 below [3]

.

410

Table 1. Protection schema of synchronous condenser.

No1 Protection

1 Pilot differential protection

2 Stator winding inter-turn short circuit protection

3 Voltage controlled time overcurrent

4 Stator winding grounding protection

5 Stator winding overvoltage protection

6 Stator winding overload protection

7 Rotor surface (negative sequence) overload protection

8 Excitation winding overload protection

9 Excitation circuit grounding protection

10 Over excitation protection

11 Power-on protection

12 Under voltage decoupling protection

13 Loss of field protection

14 Low frequency overcurrent protection

15 Non-electricity protection

This paper focuses on the characteristics of differential protection and turn-to-turn short circuit

protection, as well as characteristics of zero-voltage stator grounding protection and tuning

principles.

1) Pilot differential protection

The main protection reflects the phase-to-phase short-circuit fault of the internal stator winding and

its lead-out line, which is basically similar to the generator longitudinal differential protection and

consisting of an instantaneous differential protection differential and a ratio differential protection as

shown in figure 1.

Idz

Izd

Idiff

IB

Trip area Restraint

area

Iins

Instantaneous differential

Percent differential

Figure 1. Differential protection operation characteristics.

Instantaneous differential protection regardless of the magnitude of the restraining current, a trip

signal will be issued as soon as the differential current exceeds the threshold. The purpose of this

stage of differential protection is extremely fast operation in case of high magnitude internal fault

currents. The setting value should avoid maximum unbalance current while unsynchronized close.

The percent differential protection uses a dual-slope operating characteristic. And the differential

start up current should avoid maximum unbalance current while the synchronous condenser running

in rated load, which is: Is ≥ Krel(Ker + ∆m)I𝑛

Krel - Reliability factor, value 1.5～2.0;

Ker - CT bias, value 0.1;

In - Rated secondary current of synchronous condenser;

∆m - Unbalance current factor caused by sampling bias, value 0.02.

2) Stator winding inter-turn protection

411

There is a Longitudinal zero sequence voltage while the inter-turn is short circuited. And interturn

protection is designed on this voltage.

The protection criterion is 3U0 >UZJ, and with a delay.

3U0 - Fundamental wave zero sequence voltage;

UZJ - Longitudinal zero sequence voltage setting.

Transformer Protection Schema and Setting

The transformer protection of synchronous condenser transformer set is shown in Table 2 below[3]

.

Table 2. Protection schema of transformer.

No1 Protection

1 Pilot differential protection

2

Voltage controlled time

overcurrent

3 Neutral point zero overcurrent

4 Over excitation protection

5 Over load protection

6 Open phase protection

1) Instantaneous differential protection

The instantaneous differential which entails an overcurrent protection is provided for fast tripping

on heavy internal faults. It will issue a trip signal as soon as differential current of any phase exceeds

the threshold. And the instantaneous differential protection should avoid maximum excitation current

and maximum unbalance current when an external fault occurs. Generally the threshold is K ∗ Ie

K - Reliability factor，value 1.5~2.0;

In- Rated current of the transformer.

2) The percent differential protection

The percent differential protection uses a treble-slope dual break-point operating characteristic

with magnetizing inrush and overexcitation and CT failure detection inhibits integrated, as shown in

figure 2.

Kb1

Kb3

Idz

Idiff0.6In

IIns

Trip area

Restraint

area

5In0.4In

K2

Idiff

Instantaneous differential

K1

Kb2

IT

Variable percentdifferential

Percentdifferential

Figure 2. Differential protection operation characteristics of transformer.

Differential and restraint current calculation are all with automatic ratio compensation and vector

group compensation. The differential current is Idiff = | ∑ IiNi=1 | . And he restraint current is

Ires =1

2|Imax − ∑ Ij

N−1j=1 |.

Ii - The current vector of side I, corresponding to HV, MV and LV windings.

N - Total current inputs of the IED.

Imax - Maximum current vector among the N current inputs of the IED.

412

∑ IjN−1j=1 - The sum of the other current inputs of the IED, not including Imax, suppose it is side j.

Differential startup current setting value should avoid unbalance current while working with rated

load, shown as IdiffStar = Krel(Ker + ∆U + ∆m)IT2n.

Krel - Reliability factor, value 1.3~1.5;

Ker - CT bias, value 0.032 for 10P CT and 0.012 for 5P/TP CT;

∆U - Regulation bias;

∆m - Unbalance current factor caused by CT difference, value 0.05;

IT2n- Rated secondary current of transformer.

Excitation Transformer Protection Schema and SETTING

The main protection of excitation transformer includes differential protection and overcurrent

protection. Differential startup current setting value of excitation transformer should avoid unbalance

current while working with rated load. As forced excitation lasts longer time when the synchronous

condenser is working, big difference between CTs in two sides causes big unbalance current. And

0.5In is recommended for startup value.

An Example of Synchronous Condenser Protection Setting

A typical application of the large capacity synchronous condenser is shown in figure 4.

Synchronous Condenser

Excitation Transformer

Transformer

CB1 CB2 CB3

Figure 4. Typical diagram of the large capacity synchronous condenser.

The parameters of the system are shown as table 3.

413

Table 3. Parameters of the system.

Synchronous condenser

Rated capacity 300Mvar

Xd 148%

Xq 145%

Rated rotating speed 3000 round/min

Stator rated current 8kA

Rated no-loaded Excitation voltage 100V

Rated no-loaded Excitation current 900A

Rated on-load Excitation voltage 300V

Rated on-load Excitation current 2.4kA

Transformer

Rated capacity 360MVA

Wiring type Y /Δ-11

Excitation transformer

Rated capacity 5.3MVA

Rated voltage 20/6.3kV

For the large capacity synchronous condenser, instantaneous differential current setting is 3~5 In.

In this case the value is 4In . And, startup differential current setting applies the formula Is ≥Krel(Ker + ∆m)In, while Is = (0.2~0.3)I𝑛 and Krel = 2, the setting value is 0.3In .

For the transformer, instantaneous differential current setting is 5In . And, the startup differential

current setting value is 0.5In

The setting value of the main protection is shown as table 4.

Table 4. The list of settings for a synchronous condenser transformer group protection.

No. Setting Value

1 synchronous condenser instantaneous differential current (A) 4In

2 synchronous condenser startup differential current (A) 0.3In

3 synchronous condenser differential over limit current (A) 0.1In

4 transformer instantaneous differential current (A) 5In

5 transformer startup differential current (A) 0.5In

6 excitation transformer instantaneous differential current (A) 8In

7 excitation transformer startup differential current (A) 0.5In

8 excitation transformer differential over limit current (A) 0.2In

Summary

The large capacity synchronous condenser improves the short-circuit ratio of DC transmission system

access, enhances the dynamic voltage support capability of the AC side grid, and improves the system

stability by using the force excitation.

It is of great significance not only to the reliable operation of the power system, but also to the

important and expensive equipment to reduce the damage caused by various short circuit and

abnormal operation, and it also has significant economic benefits. In this paper, the principle and

setting value of main protection of the synchronous condenser transformer group are customized and

detailed verification. Combined with the example, a feasible setting list is proposed, which can be

used as an example of the synchronous condenser protection settings.

Acknowledgement

This research was financially supported by Science and Technology Project of State Grid Corporation

of China: Research on Fault Simulation, Protection and Control of Synchronous Condenser.

414

References

[1] Ya-ting Wang, Yi-chi Zhang. Study on Application of New Generation Large Capacity

Synchronous Condenser in Power Grid [J]. Power System Technology, 2017, 41(1) (in Chinese).

[2] Lin Gui. Inernal Falult Analysis and Optimization Design of Main Protection Scheme for

Shanghai Electric Phase Modifier [J]. Electirc Power, 2017, 50(12):33-37(in Chinese).

[3] Dongxia Zou, et. SFC start and relay protection of 300 Mvar large synchronous condenser, Power

System Protection and Control, 2016, 44(20): 160-164(in Chinese).

[4] Yan-wei Wang, Yun Qin, Xian-fu Gong, et al. Optimizing Configuration of STATCOM Under

Asynchronous Networking Mode of Guangdong Power Grid[J]. Guangdong Electric Power, 2017,

30(7):53-59(in Chinese).

[5] Qing-yu Zeng. Techno-economic analysis of UHVAC and UHVDC power transmission

systems[J]. Power System Technology, 2015, 39(2):341-348(in Chinese).

[6] Shun Zhang, Zhi-ping Ge, Guo Tao, et al. Research on relationship between the capacity of

systematic peak regulation and conventional power startup mode after access to large-scale new

energy[J]. Power System Protection and Control, 2016, 44(1): 106-110

[7] Chao Liu, Yong-ming Yang, Xiao-fu Xiong, et al. Analysis of the impact of fixed SVC on fault

component directional protection[J]. Power System Protection and Control, 2011, 39(19): 110-114.

[8] Qing Liu, Zeng-ping Wang, Yuan Zhang. Research of a new method of eliminating the effect of

STATCOM on distance protection[J]. Power System Protection and Control, 2010, 38(24): 93-98.

[9] Yong Li, Han-xiang Cheng, Wei-ming Fang, et al. Review on Application of Reactive Power

Compensation Device in Power System[J]. Guangdong Electric Power, 2016, 29(5):77-81.

415